Impacts of elephants (Loxodonta africana africana) on woody vegetation has attracted substantial ... more Impacts of elephants (Loxodonta africana africana) on woody vegetation has attracted substantial attention for decades, but plant-level responses remain a gap in the understanding of savanna ecology. Marula (Sclerocarya birrea caffra) forms an important part of elephant diets. We investigated the relationships between browsing intensity and shoot/leaf size, nitrogen (N) and condensed tannin (CT) concentrations in upper and lower canopies of male and female marula individuals in Hluhluwe-iMfolozi Park, South Africa. Browsing intensity (54%) did not differ between sexes, suggesting no preference by elephants for either sex. Females had higher [CT] than males and tannin decreased with increasing browsing intensity in both sexes. In lightly or moderately browsed trees, [CT] was controlled by unmeasured factors such that within-tree impacts of browsing were more variable in lightly/moderately browsed than heavily browsed trees. There was little change in [N] up to 60% browsing intensity, but [N] increased dramatically at higher intensity. Shoots and leaves on broken branches in the lower canopy were larger (2.5 and 1.2 times, respectively) than those on unbroken branches in either upper or lower canopies. Chemical responses were systemic and potentially influence browsing among trees, while growth responses were strongly localised and potentially influence browsing within trees. Although marula trees are able to compensate vigorously for browsing at the scale of individual organs, trees may become progressively carbon-deficient and have their lives shortened if total plant growth is negatively affected by chronic browsing, e.g. near permanent water.
Genetic differentiation in 20 hierarchically sampled populations of wild barley was analyzed with... more Genetic differentiation in 20 hierarchically sampled populations of wild barley was analyzed with quantitative traits, allozymes and Random Amplified Polymorphic DNAs (RAPDs), and compared for three marker types at two hierarchical levels. Regional subdivision for both molecular markers was much lower than for quantitative traits. For both allozymes and RAPDs, most loci exhibited minor or no regional differentiation, and the relatively high overall estimates of the latter were due to several loci with exceptionally high regional differentiation. The allozyme and RAPD-specific patterns of differentiation were concordant in general with one another, but not with quantitative trait differentiation. Divergent selection on quantitative traits inferred from very high regional QST was in full agreement with our previous results obtained from a test of local adaptation and multilevel selection analysis. In contrast, most variation in allozyme and RAPD variation was neutral, although several allozyme loci and RAPD markers were exceptional in their levels of regional differentiation. However, it is not possible to answer the question whether these exceptional loci are directly involved in the response to selection pressure or merely linked to the selected loci. The fact that QST and FST did not differ at the population scale, that is, within regions, but differed at the regional scale, for which local adaptation has been previously shown, implies that comparison of the level of subdivision in quantitative traits, as compared with molecular markers, is indicative of adaptive population differentiation only when sampling is carried out at the appropriate scale.
Reciprocal introduction of seeds and seedlings was used to test for local adaptation and to ident... more Reciprocal introduction of seeds and seedlings was used to test for local adaptation and to identify a set of coadapted traits of Mediterranean and desert ecotypes of wild barley Hordeum spontaneum. Evidence for local adaptation was found in seedling introductions into intact environments and from ecotype colonization success in the first generation after seed dispersal. Estimates of fitness were obtained at particular stages of the life cycle (seed, seedling and adult). Experiments that manipulated the environment (vegetation removal, different plant density) demonstrated the intensity and direction of natural selection in different life history episodes, but there was no strong evidence for local adaptation under these circumstances. The observed genetically determined differences between Mediterranean and desert ecotypes can be summarized as the following: Reproductive output was higher in desert plants, with smaller seeds than in Mediterranean plants. There was a higher competitive ability of Mediterranean than desert plants. Plants of desert origin had significant reductions in yield when grown in mixed stands with Mediterranean plants; no such effect was observed for plants of Mediterranean origin. Seed germination and seedling survival was lower in seeds of desert origin. This was due to both: genetically determined higher dormancy of desert seeds and a trade-off between number of seeds and their size (directly related to seed/seedling vigor).
We examined the adaptive importance of allozyme variation in wild barley, Hordeum spontaneum. The... more We examined the adaptive importance of allozyme variation in wild barley, Hordeum spontaneum. The test involved: 1) a nested sampling design with four population groups each representing a different environment; and 2) a comparison of observed allozyme variation with that expected under the assumption that allozymes are not neutral. Measurements of plant fitness in indigenous and alien environments in reciprocal introductions of seeds and seedlings in the four environments provided a guideline for the expected pattern of allozyme variation. The results showed a great variation in both degree of regional and population subdivision, and pattern of the subdivision among loci. The observed pattern of variation was ambiguous. Although two alleles exhibited pattern of distribution that can not be explained by genetic drift as a function of geographic distance, we failed to detect either a significant relationship between genetic distance and environmental similarity or any favored epistatic allele combinations across the four environments. Our results suggest that interpretation of allozyme variation in wild barley as adaptive and directly related to local environment still needs justification. Although we could not reject the null-hypothesis, a proposed methodology seeking a concordance between observed and "adaptive" (i.e. expected under hypothesis that allozymes are not neutral) allozyme variation may prove to be effective in resolving the neutralist-selectionist debate when applied to other species.
We compared intra- and inter-specific competitive responses of wild barley, Hordeum spontaneum, p... more We compared intra- and inter-specific competitive responses of wild barley, Hordeum spontaneum, plants from four populations originating in distinct environments in Israel. The environments ranged along two parallel gradients of rainfall amount and predictability from low (desert) to moderate (semi-steppe batha) to high (Mediterranean grassland and mountain, the latter also experiencing frost stress). The target barley plants grew under one of five densities (0, 4, 8, 16 and 32 surrounding plants per bucket) of either barley from the same population or oats (Avena sterilis) from a neutral population. The traits examined included estimates of fitness, reproductive and resource allocation.
The effect of intra-specific competition was stronger than inter-specific competition at a high increment of neighbor density (from 4 to 32 neighbors). There was no difference in interspecific competitive responses of plants originating in the four environments at any neighbor density increments, but intraspecific competitive responses of the four ecotypes consistently differed at low competitive intensity (4 neighbors). The superior competitors were the plants originating from Mediterranean grassland, the most favorable in respect to rainfall and abiotic stress (i.e. drought or frost) environment. The plants from the mountain environment, which is highly productive and predictable in respect to rainfall but experiences severe frost stress, were the poorest competitors. Our results are inconsistent with the hypothesis that there is no relationship between competitive ability and environmental favorability. High competitive ability appears to be a distinct property of plants living in favorable environments (i.e. productive and predictable and without abiotic stress) corresponding to the 'competitive' strategy of the C-S-R model. However, in less productive and/or predictable environments, or under conditions of severe abiotic stress, plant features other than ability to tolerate low water or nutrient levels may be more important, with reduced competitive ability as a trade-off.
Plants from four populations of Hordeum spontaneum originating in distinct environments of Israel... more Plants from four populations of Hordeum spontaneum originating in distinct environments of Israel were compared for stress induced phenotypic plasticity. The environments ranged along a gradient of increasing rainfall amount and predictability from low (desert) to moderate (semi-steppe batha) to high (Mediterranean grassland and mountain, the latter also experiencing frost stress). The plants were exposed to a set of four treatments: no stress (optimum water and nutrients), water, nutrient and both water and nutrient stress. Plants from the four populations (or ecotypes) exhibited different patterns of plasticity in response to the different stresses (water and nutrients) and in different trait categories (reproductive, fitness and resource allocation). The importance of plasticity in response to water stress appears to decrease, and to nutrient stress appears to increase along the increasing rainfall gradient. The mountain ecotype, growing in an area with high potential productivity (amount of rainfall) but experiencing periodic frosts, was the most plastic among ecotypes in resource allocation under both water and nutrient stress, but exhibited low plasticity in other trait categories. In contrast, the desert ecotype had low plasticity in resource allocation under water stress and the lowest plasticity among the 4 ecotypes in all trait categories in response to nutrient stress. The ecotype originating in Mediterranean grassland, a predictable and most favorable environment, was highly plastic in fitness and allocation traits in response to low nutrient levels which is likely to occur due to competition in productive environment. We discuss the observed differences in ecotype plasticity as part of their environmentally induced adaptive 'strategies'. We found no support for the hypothesis that plants originating in environments with greater variation and unpredictability are more plastic.
Plasticity of the phenotypic architecture of wild barley, Hordeum spontaneum, was studied in resp... more Plasticity of the phenotypic architecture of wild barley, Hordeum spontaneum, was studied in response to water and nutrient stress. Direct and indirect selection on several vegetative and reproductive traits was estimated and path analysis used to reveal how regulating pathways via maternal investment differed between environments. Vegetative traits displayed differential regulating effect on fitness across experimental environments: 1) increase in size was selected for under optimal conditions and under water stress, but not under nutrient stress; 2) allocation to root biomass was selected for under optimal conditions, but it had no effect under nutrient stress and was strongly selected against when water was limiting; 3) delayed onset of reproduction was selected under nutrient limitation whereas earlier onset was selected under water stress.
The regulating effect of reproductive traits on final reproductive output also differed across treatments, operating either at the “early” stage of plant development through varying the number of initiated spikelets per spike (no stress and water stress treatment) or at the “late” developmental stage adjusting the fertile spikelet weight (no stress and nutrient stress treatment). Reproductive output was regulated via seed abortion under no stress and water stress treatments. Although the underlying mechanism of the regulation through abortion has yet to be discovered, the specific mechanism of abortion under water stress appears to be different from that under optimal conditions. Our results demonstrate that not only is the character architecture in wild barley plastic and sensitive to changing availability of water and nutrients, but the regulating mechanism of maternal investment is also environmentally sensitive.
We compared the demography of two populations of wild barley, Hordeum spontaneum, that occupy env... more We compared the demography of two populations of wild barley, Hordeum spontaneum, that occupy environments of different aridity (desert and Mediterranean) and represent distinct parts of the species' distribution (periphery and core) in Israel. Our demographic study included a survey of survival and fecundity of adults in quadratee plots marked at two sites in natural vegetation (one site per population), and the creation of experimental seed banks examined over three years. We measured variability in population growth rate and investigated the role of the seed bank in the demography of two populations. The latter employed construction of two-stage, three age-class transition matrices and usage of elasticity analysis. At both sites, population dynamics appear to be transient with high fluctuations of population growth rate due to variability in annual rainfall and, most likely, intra- and inter-specific density dependence. The importance of the seed bank differed among two populations. Seed dormancy had almost no demographic effect on the Mediterranean population, while it was of paramount importance in the desert population. The two populations also differed in seed and seedling survival and per adult fecundity, presumably due to the difference between two sites in (i) relative impact of seed predation, (ii) effects of density dependence on population demography, and (iii) lower vigor of seeds of desert vs. Mediterranean origin. Our study highlights the importance of life history adaptations that may evolve under specific selective forces in different parts of a species' range, which, in certain circumstances, may be critical for the persistence of a species.
Reciprocal introduction of seeds and seedlings of wild barley, Hordeum spontaneum, originating in... more Reciprocal introduction of seeds and seedlings of wild barley, Hordeum spontaneum, originating in four different environments of Israel, was used to: 1) test for local adaptation; 2) make inferences about environmental effects on life history and reproductive traits, and 3) identify trait combinations with already recognized 'strategies'. The four populations examined represented the following environments: (1) desert, low productivity and predictability, drought stress; (2) semi-steppe batha, moderate productivity and predictability; (3) grassland, high productivity and predictability; and (4) mountain, high productivity and predictability but with severe frost stress.
Significant genotype by environment interactions were observed for yield and reproductive biomass, seedling biomass and percent germinated and survived seeds, suggesting local ecotype adaptation. Increasing productivity and predictability of environment in respect to rainfall, without concomitant frost stress, was found to select for high reproductive biomass and large seeds, high fraction of germinating seeds and high vigor of seedlings. The optimal strategy changes with increasing productivity and predictability and involves a trade-off between seed size and number, with reduced yield but increased seed mass, which is expected under K- selection. No specific life history adaptations to predictable frost stress were detected for the mountain ecotype, but there was higher survival of seedlings in their indigenous (mountain) environment as compared with other ecotypes. The latter appears to be a physiological adaptation to frost, which is consistent with stress (or adversity) selection type. The other stress factor, drought, which is very unpredictable in deserts, was associated with high seed dormancy, small seed size and low vigor of seedlings, but relatively high yield, which is consistent with a stress-escape bet-hedging strategy.
Woody plant encroachment is frequent in dry savannas. Grazing is often considered to be a major c... more Woody plant encroachment is frequent in dry savannas. Grazing is often considered to be a major cause of encroachment in dry savannas because grasses are removed by livestock, leaving bare areas for trees to colonise in wetter years. Earlier experiments conducted in the Kimberley area of the Northern Cape showed that neither fire nor grazing was important for woody plant encroachment. We used aerial and fixed-point repeat ground photographs, including historical photographs taken at the time of the Second Anglo-Boer War of 1899–1902, to assess the scale and timing of woody plant encroachment in the dry savannas near Kimberley in South Africa (mean annual rainfall = 300–400 mm). There were large increases in woody plant encroachment in most areas. Even at the battlefield of Magersfontein, where grazing has been virtually absent since its protection in 1960, we found that encroachment by trees and shrubs has occurred. Using aerial photographs, we found that the rate of encroachment has increased substantially since 1993. However, repeated photographs at certain sites indicate that encroachment produced cohorts of trees. We show that global drivers are perhaps of greater importance than local drivers such as heavy grazing and absence of fire.
Woody plant encroachment is a common consequence of disturbance in savannas. Grazers and browsers... more Woody plant encroachment is a common consequence of disturbance in savannas. Grazers and browsers interfere with sapling establishment dynamics by direct consumption of plant tissue, changing soil nutrient status (through fertilization and trampling) and grass competition. Studies evaluating the effects of herbivory on sapling establishment have mostly been extrapolated from single species. In a controlled field experiment, we studied the effects of clipping (simulating grazing and browsing), nutrients, grass competition, and their interactive effects on sapling survival and growth of four dominant humid and four dominant mesic savanna species. We conducted this experiment in a humid South African savanna. We found no effects on sapling survival by the treatments provided. However, clipped saplings of all species increased their investment in relative growth rate of stem length (RGRL). Clipping had a greater negative impact on relative growth rate of more humid than mesic species in terms of stem diameter (RGRD), total dry biomass and proportion of leaf biomass. Nutrients had a positive effect on the RGRL and sapling biomass of three mesic species. Positive effects of nutrients on RGRL of one humid and two mesic species were observed in their clipped saplings only. Grass competition had a strong negative impact on all growth parameters measured. Clipped saplings of one humid and two mesic species had lower RGRL with grass competition whereas intact saplings showed no significant response. After clipping, humid savanna species were more vulnerable to grass competition than mesic species, with reduced ability to use nutrients. In conclusion, herbivory increases sapling vulnerability to grass competition, with humid species being more susceptible than mesic species, indicating that woody-plant control strategies are more likely to be effective in humid savannas.
The pods of many woody plants form an important part of the diet of livestock during the dry seas... more The pods of many woody plants form an important part of the diet of livestock during the dry season due to their high nutritive value. However, the dispersal of seeds that remain intact and can potentially germinate after excretion is of particular concern when animals consume seeds of encroaching or invasive woody plants. The objectives of this study were to determine the effects of animal species in two experiments (experiment 1: goats, sheep; experiment 2: goats, cattle), diet quality (Medicago sativa hay, Digitaria eriantha hay) and seed characteristics (size, hardness) on the effectiveness of animal seed dispersal and germination of Dichrostachys cinerea and Acacia nilotica seeds. Owing to a limitation on the availability of seeds, the two experiments were done separately at different times. Each animal in both experiments received 1 000 A. nilotica seeds and 1 000 D. cinerea seeds mixed with either a low-quality diet (D. eriantha hay) or a high-quality diet (M. sativa hay). In experiment 1, we found a significant interaction effect of animal species (goats, sheep), diet (high-quality hay, low-quality hay), and seed species (A. nilotica seeds, D. cinerea seeds) on germination (P, 0.0001). There was also a higher seed recovery (P,0.009) when animals were offered high-quality hay (47.4%64.65) compared to low-quality hay (30.2%63.24). In experiment 2, animal species affected seed recovery (P,0.0325; goats 32.0%66.44; cattle 50.3%64.27) and germination percentage (P,0.055; goats 14.1%61.48; cattle 9.3%60.94). The diet quality fed to the animals may affect dispersal and germination. However, animal species and seed characteristics also had important effects on germination of D. cinerea and A. nilotica seeds. Thus, all three of these factors play a major role in dissemination of viable seeds.
The quality of the landscape is declining in many grassland and savanna areas of Africa as a cons... more The quality of the landscape is declining in many grassland and savanna areas of Africa as a consequence of woody plant encroachment. We investigated the changes in vegetation at selected sites on the battlefields of the Anglo-Zulu War of 1879 in KwaZulu-Natal. We used fixed-point repeat photographs to compare the landscape at the time of the Anglo-Zulu War with the same landscape in 2011. Nine photo-pairs were analysed, covering three specific sites: Isandlwana, Fugitives’ Drift and Rorke’s Drift. A further eight photographs from these sites, taken during the intervening years, were matched and compared with the present landscape. All sites we analysed showed an increase in woody cover since 1879. We used the Whittaker plant diversity sampling method to determine current woody and grass species composition on the sites. We also investigated the effect of catena, aspect, rockiness and land use on the bush encroachment. Rainfall and temperature data were also analysed. The observed large increases in woody cover cannot be explained by land use or by local climatic conditions only. The changes are also consistent with global climate change effects, most likely due to increasing atmospheric CO2.
Remote sensing, together with geographic information systems (GISs), provide a powerful tool for ... more Remote sensing, together with geographic information systems (GISs), provide a powerful tool for monitoring environmental processes. Aerial and satellite photographs enable the analysis of an entire landscape and, using multi-temporal sets of photographs, one is able to follow processes over time. In our study, we used multi-temporal sets of aerial photographs, taken between 1944 and 2005, to observe the phenomenon of woody plant encroachment in four neighbouring study sites in northern KwaZulu-Natal, South Africa. We also analysed rainfall and temperature trends. Woody plant encroachment was evident in all sites. We suggest possible drivers behind the bush encroachment; in particular, climatic trends. However, encroachment did not relate directly to rainfall and temperature trends. We also speculate on the effects of herbivory and fire, as well as rising atmospheric CO2 concentrations and its effect on C3 and C4 plants. We suggest that increasing atmospheric CO2 concentrations could be the overriding driver of woody plant encroachment in our study area, with other drivers, such as rainfall, temperature, fire, herbivory, and land use, playing a modifying role.
Questions: What is the effect of neighbour competition on the survival, growth and biomass of mes... more Questions: What is the effect of neighbour competition on the survival, growth and biomass of mesic and humid savanna tree species? Can competitive effect and response be linked to plant functional traits? Location: Neil Tainton Arboretum, University of KwaZulu-Natal, Pietermaritzburg, ZA. Methods: Using a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability. Results: We found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to showthat the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource-acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition. Conclusions: Competitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high- or low-resource environments. Our results show significant relationships between plant traits and competitive ability, which may be used to predict competitive interactions between tree seedlings fromhumid and mesic savannas.
Impacts of elephants (Loxodonta africana africana) on woody vegetation has attracted substantial ... more Impacts of elephants (Loxodonta africana africana) on woody vegetation has attracted substantial attention for decades, but plant-level responses remain a gap in the understanding of savanna ecology. Marula (Sclerocarya birrea caffra) forms an important part of elephant diets. We investigated the relationships between browsing intensity and shoot/leaf size, nitrogen (N) and condensed tannin (CT) concentrations in upper and lower canopies of male and female marula individuals in Hluhluwe-iMfolozi Park, South Africa. Browsing intensity (54%) did not differ between sexes, suggesting no preference by elephants for either sex. Females had higher [CT] than males and tannin decreased with increasing browsing intensity in both sexes. In lightly or moderately browsed trees, [CT] was controlled by unmeasured factors such that within-tree impacts of browsing were more variable in lightly/moderately browsed than heavily browsed trees. There was little change in [N] up to 60% browsing intensity, but [N] increased dramatically at higher intensity. Shoots and leaves on broken branches in the lower canopy were larger (2.5 and 1.2 times, respectively) than those on unbroken branches in either upper or lower canopies. Chemical responses were systemic and potentially influence browsing among trees, while growth responses were strongly localised and potentially influence browsing within trees. Although marula trees are able to compensate vigorously for browsing at the scale of individual organs, trees may become progressively carbon-deficient and have their lives shortened if total plant growth is negatively affected by chronic browsing, e.g. near permanent water.
Genetic differentiation in 20 hierarchically sampled populations of wild barley was analyzed with... more Genetic differentiation in 20 hierarchically sampled populations of wild barley was analyzed with quantitative traits, allozymes and Random Amplified Polymorphic DNAs (RAPDs), and compared for three marker types at two hierarchical levels. Regional subdivision for both molecular markers was much lower than for quantitative traits. For both allozymes and RAPDs, most loci exhibited minor or no regional differentiation, and the relatively high overall estimates of the latter were due to several loci with exceptionally high regional differentiation. The allozyme and RAPD-specific patterns of differentiation were concordant in general with one another, but not with quantitative trait differentiation. Divergent selection on quantitative traits inferred from very high regional QST was in full agreement with our previous results obtained from a test of local adaptation and multilevel selection analysis. In contrast, most variation in allozyme and RAPD variation was neutral, although several allozyme loci and RAPD markers were exceptional in their levels of regional differentiation. However, it is not possible to answer the question whether these exceptional loci are directly involved in the response to selection pressure or merely linked to the selected loci. The fact that QST and FST did not differ at the population scale, that is, within regions, but differed at the regional scale, for which local adaptation has been previously shown, implies that comparison of the level of subdivision in quantitative traits, as compared with molecular markers, is indicative of adaptive population differentiation only when sampling is carried out at the appropriate scale.
Reciprocal introduction of seeds and seedlings was used to test for local adaptation and to ident... more Reciprocal introduction of seeds and seedlings was used to test for local adaptation and to identify a set of coadapted traits of Mediterranean and desert ecotypes of wild barley Hordeum spontaneum. Evidence for local adaptation was found in seedling introductions into intact environments and from ecotype colonization success in the first generation after seed dispersal. Estimates of fitness were obtained at particular stages of the life cycle (seed, seedling and adult). Experiments that manipulated the environment (vegetation removal, different plant density) demonstrated the intensity and direction of natural selection in different life history episodes, but there was no strong evidence for local adaptation under these circumstances. The observed genetically determined differences between Mediterranean and desert ecotypes can be summarized as the following: Reproductive output was higher in desert plants, with smaller seeds than in Mediterranean plants. There was a higher competitive ability of Mediterranean than desert plants. Plants of desert origin had significant reductions in yield when grown in mixed stands with Mediterranean plants; no such effect was observed for plants of Mediterranean origin. Seed germination and seedling survival was lower in seeds of desert origin. This was due to both: genetically determined higher dormancy of desert seeds and a trade-off between number of seeds and their size (directly related to seed/seedling vigor).
We examined the adaptive importance of allozyme variation in wild barley, Hordeum spontaneum. The... more We examined the adaptive importance of allozyme variation in wild barley, Hordeum spontaneum. The test involved: 1) a nested sampling design with four population groups each representing a different environment; and 2) a comparison of observed allozyme variation with that expected under the assumption that allozymes are not neutral. Measurements of plant fitness in indigenous and alien environments in reciprocal introductions of seeds and seedlings in the four environments provided a guideline for the expected pattern of allozyme variation. The results showed a great variation in both degree of regional and population subdivision, and pattern of the subdivision among loci. The observed pattern of variation was ambiguous. Although two alleles exhibited pattern of distribution that can not be explained by genetic drift as a function of geographic distance, we failed to detect either a significant relationship between genetic distance and environmental similarity or any favored epistatic allele combinations across the four environments. Our results suggest that interpretation of allozyme variation in wild barley as adaptive and directly related to local environment still needs justification. Although we could not reject the null-hypothesis, a proposed methodology seeking a concordance between observed and "adaptive" (i.e. expected under hypothesis that allozymes are not neutral) allozyme variation may prove to be effective in resolving the neutralist-selectionist debate when applied to other species.
We compared intra- and inter-specific competitive responses of wild barley, Hordeum spontaneum, p... more We compared intra- and inter-specific competitive responses of wild barley, Hordeum spontaneum, plants from four populations originating in distinct environments in Israel. The environments ranged along two parallel gradients of rainfall amount and predictability from low (desert) to moderate (semi-steppe batha) to high (Mediterranean grassland and mountain, the latter also experiencing frost stress). The target barley plants grew under one of five densities (0, 4, 8, 16 and 32 surrounding plants per bucket) of either barley from the same population or oats (Avena sterilis) from a neutral population. The traits examined included estimates of fitness, reproductive and resource allocation.
The effect of intra-specific competition was stronger than inter-specific competition at a high increment of neighbor density (from 4 to 32 neighbors). There was no difference in interspecific competitive responses of plants originating in the four environments at any neighbor density increments, but intraspecific competitive responses of the four ecotypes consistently differed at low competitive intensity (4 neighbors). The superior competitors were the plants originating from Mediterranean grassland, the most favorable in respect to rainfall and abiotic stress (i.e. drought or frost) environment. The plants from the mountain environment, which is highly productive and predictable in respect to rainfall but experiences severe frost stress, were the poorest competitors. Our results are inconsistent with the hypothesis that there is no relationship between competitive ability and environmental favorability. High competitive ability appears to be a distinct property of plants living in favorable environments (i.e. productive and predictable and without abiotic stress) corresponding to the 'competitive' strategy of the C-S-R model. However, in less productive and/or predictable environments, or under conditions of severe abiotic stress, plant features other than ability to tolerate low water or nutrient levels may be more important, with reduced competitive ability as a trade-off.
Plants from four populations of Hordeum spontaneum originating in distinct environments of Israel... more Plants from four populations of Hordeum spontaneum originating in distinct environments of Israel were compared for stress induced phenotypic plasticity. The environments ranged along a gradient of increasing rainfall amount and predictability from low (desert) to moderate (semi-steppe batha) to high (Mediterranean grassland and mountain, the latter also experiencing frost stress). The plants were exposed to a set of four treatments: no stress (optimum water and nutrients), water, nutrient and both water and nutrient stress. Plants from the four populations (or ecotypes) exhibited different patterns of plasticity in response to the different stresses (water and nutrients) and in different trait categories (reproductive, fitness and resource allocation). The importance of plasticity in response to water stress appears to decrease, and to nutrient stress appears to increase along the increasing rainfall gradient. The mountain ecotype, growing in an area with high potential productivity (amount of rainfall) but experiencing periodic frosts, was the most plastic among ecotypes in resource allocation under both water and nutrient stress, but exhibited low plasticity in other trait categories. In contrast, the desert ecotype had low plasticity in resource allocation under water stress and the lowest plasticity among the 4 ecotypes in all trait categories in response to nutrient stress. The ecotype originating in Mediterranean grassland, a predictable and most favorable environment, was highly plastic in fitness and allocation traits in response to low nutrient levels which is likely to occur due to competition in productive environment. We discuss the observed differences in ecotype plasticity as part of their environmentally induced adaptive 'strategies'. We found no support for the hypothesis that plants originating in environments with greater variation and unpredictability are more plastic.
Plasticity of the phenotypic architecture of wild barley, Hordeum spontaneum, was studied in resp... more Plasticity of the phenotypic architecture of wild barley, Hordeum spontaneum, was studied in response to water and nutrient stress. Direct and indirect selection on several vegetative and reproductive traits was estimated and path analysis used to reveal how regulating pathways via maternal investment differed between environments. Vegetative traits displayed differential regulating effect on fitness across experimental environments: 1) increase in size was selected for under optimal conditions and under water stress, but not under nutrient stress; 2) allocation to root biomass was selected for under optimal conditions, but it had no effect under nutrient stress and was strongly selected against when water was limiting; 3) delayed onset of reproduction was selected under nutrient limitation whereas earlier onset was selected under water stress.
The regulating effect of reproductive traits on final reproductive output also differed across treatments, operating either at the “early” stage of plant development through varying the number of initiated spikelets per spike (no stress and water stress treatment) or at the “late” developmental stage adjusting the fertile spikelet weight (no stress and nutrient stress treatment). Reproductive output was regulated via seed abortion under no stress and water stress treatments. Although the underlying mechanism of the regulation through abortion has yet to be discovered, the specific mechanism of abortion under water stress appears to be different from that under optimal conditions. Our results demonstrate that not only is the character architecture in wild barley plastic and sensitive to changing availability of water and nutrients, but the regulating mechanism of maternal investment is also environmentally sensitive.
We compared the demography of two populations of wild barley, Hordeum spontaneum, that occupy env... more We compared the demography of two populations of wild barley, Hordeum spontaneum, that occupy environments of different aridity (desert and Mediterranean) and represent distinct parts of the species' distribution (periphery and core) in Israel. Our demographic study included a survey of survival and fecundity of adults in quadratee plots marked at two sites in natural vegetation (one site per population), and the creation of experimental seed banks examined over three years. We measured variability in population growth rate and investigated the role of the seed bank in the demography of two populations. The latter employed construction of two-stage, three age-class transition matrices and usage of elasticity analysis. At both sites, population dynamics appear to be transient with high fluctuations of population growth rate due to variability in annual rainfall and, most likely, intra- and inter-specific density dependence. The importance of the seed bank differed among two populations. Seed dormancy had almost no demographic effect on the Mediterranean population, while it was of paramount importance in the desert population. The two populations also differed in seed and seedling survival and per adult fecundity, presumably due to the difference between two sites in (i) relative impact of seed predation, (ii) effects of density dependence on population demography, and (iii) lower vigor of seeds of desert vs. Mediterranean origin. Our study highlights the importance of life history adaptations that may evolve under specific selective forces in different parts of a species' range, which, in certain circumstances, may be critical for the persistence of a species.
Reciprocal introduction of seeds and seedlings of wild barley, Hordeum spontaneum, originating in... more Reciprocal introduction of seeds and seedlings of wild barley, Hordeum spontaneum, originating in four different environments of Israel, was used to: 1) test for local adaptation; 2) make inferences about environmental effects on life history and reproductive traits, and 3) identify trait combinations with already recognized 'strategies'. The four populations examined represented the following environments: (1) desert, low productivity and predictability, drought stress; (2) semi-steppe batha, moderate productivity and predictability; (3) grassland, high productivity and predictability; and (4) mountain, high productivity and predictability but with severe frost stress.
Significant genotype by environment interactions were observed for yield and reproductive biomass, seedling biomass and percent germinated and survived seeds, suggesting local ecotype adaptation. Increasing productivity and predictability of environment in respect to rainfall, without concomitant frost stress, was found to select for high reproductive biomass and large seeds, high fraction of germinating seeds and high vigor of seedlings. The optimal strategy changes with increasing productivity and predictability and involves a trade-off between seed size and number, with reduced yield but increased seed mass, which is expected under K- selection. No specific life history adaptations to predictable frost stress were detected for the mountain ecotype, but there was higher survival of seedlings in their indigenous (mountain) environment as compared with other ecotypes. The latter appears to be a physiological adaptation to frost, which is consistent with stress (or adversity) selection type. The other stress factor, drought, which is very unpredictable in deserts, was associated with high seed dormancy, small seed size and low vigor of seedlings, but relatively high yield, which is consistent with a stress-escape bet-hedging strategy.
Woody plant encroachment is frequent in dry savannas. Grazing is often considered to be a major c... more Woody plant encroachment is frequent in dry savannas. Grazing is often considered to be a major cause of encroachment in dry savannas because grasses are removed by livestock, leaving bare areas for trees to colonise in wetter years. Earlier experiments conducted in the Kimberley area of the Northern Cape showed that neither fire nor grazing was important for woody plant encroachment. We used aerial and fixed-point repeat ground photographs, including historical photographs taken at the time of the Second Anglo-Boer War of 1899–1902, to assess the scale and timing of woody plant encroachment in the dry savannas near Kimberley in South Africa (mean annual rainfall = 300–400 mm). There were large increases in woody plant encroachment in most areas. Even at the battlefield of Magersfontein, where grazing has been virtually absent since its protection in 1960, we found that encroachment by trees and shrubs has occurred. Using aerial photographs, we found that the rate of encroachment has increased substantially since 1993. However, repeated photographs at certain sites indicate that encroachment produced cohorts of trees. We show that global drivers are perhaps of greater importance than local drivers such as heavy grazing and absence of fire.
Woody plant encroachment is a common consequence of disturbance in savannas. Grazers and browsers... more Woody plant encroachment is a common consequence of disturbance in savannas. Grazers and browsers interfere with sapling establishment dynamics by direct consumption of plant tissue, changing soil nutrient status (through fertilization and trampling) and grass competition. Studies evaluating the effects of herbivory on sapling establishment have mostly been extrapolated from single species. In a controlled field experiment, we studied the effects of clipping (simulating grazing and browsing), nutrients, grass competition, and their interactive effects on sapling survival and growth of four dominant humid and four dominant mesic savanna species. We conducted this experiment in a humid South African savanna. We found no effects on sapling survival by the treatments provided. However, clipped saplings of all species increased their investment in relative growth rate of stem length (RGRL). Clipping had a greater negative impact on relative growth rate of more humid than mesic species in terms of stem diameter (RGRD), total dry biomass and proportion of leaf biomass. Nutrients had a positive effect on the RGRL and sapling biomass of three mesic species. Positive effects of nutrients on RGRL of one humid and two mesic species were observed in their clipped saplings only. Grass competition had a strong negative impact on all growth parameters measured. Clipped saplings of one humid and two mesic species had lower RGRL with grass competition whereas intact saplings showed no significant response. After clipping, humid savanna species were more vulnerable to grass competition than mesic species, with reduced ability to use nutrients. In conclusion, herbivory increases sapling vulnerability to grass competition, with humid species being more susceptible than mesic species, indicating that woody-plant control strategies are more likely to be effective in humid savannas.
The pods of many woody plants form an important part of the diet of livestock during the dry seas... more The pods of many woody plants form an important part of the diet of livestock during the dry season due to their high nutritive value. However, the dispersal of seeds that remain intact and can potentially germinate after excretion is of particular concern when animals consume seeds of encroaching or invasive woody plants. The objectives of this study were to determine the effects of animal species in two experiments (experiment 1: goats, sheep; experiment 2: goats, cattle), diet quality (Medicago sativa hay, Digitaria eriantha hay) and seed characteristics (size, hardness) on the effectiveness of animal seed dispersal and germination of Dichrostachys cinerea and Acacia nilotica seeds. Owing to a limitation on the availability of seeds, the two experiments were done separately at different times. Each animal in both experiments received 1 000 A. nilotica seeds and 1 000 D. cinerea seeds mixed with either a low-quality diet (D. eriantha hay) or a high-quality diet (M. sativa hay). In experiment 1, we found a significant interaction effect of animal species (goats, sheep), diet (high-quality hay, low-quality hay), and seed species (A. nilotica seeds, D. cinerea seeds) on germination (P, 0.0001). There was also a higher seed recovery (P,0.009) when animals were offered high-quality hay (47.4%64.65) compared to low-quality hay (30.2%63.24). In experiment 2, animal species affected seed recovery (P,0.0325; goats 32.0%66.44; cattle 50.3%64.27) and germination percentage (P,0.055; goats 14.1%61.48; cattle 9.3%60.94). The diet quality fed to the animals may affect dispersal and germination. However, animal species and seed characteristics also had important effects on germination of D. cinerea and A. nilotica seeds. Thus, all three of these factors play a major role in dissemination of viable seeds.
The quality of the landscape is declining in many grassland and savanna areas of Africa as a cons... more The quality of the landscape is declining in many grassland and savanna areas of Africa as a consequence of woody plant encroachment. We investigated the changes in vegetation at selected sites on the battlefields of the Anglo-Zulu War of 1879 in KwaZulu-Natal. We used fixed-point repeat photographs to compare the landscape at the time of the Anglo-Zulu War with the same landscape in 2011. Nine photo-pairs were analysed, covering three specific sites: Isandlwana, Fugitives’ Drift and Rorke’s Drift. A further eight photographs from these sites, taken during the intervening years, were matched and compared with the present landscape. All sites we analysed showed an increase in woody cover since 1879. We used the Whittaker plant diversity sampling method to determine current woody and grass species composition on the sites. We also investigated the effect of catena, aspect, rockiness and land use on the bush encroachment. Rainfall and temperature data were also analysed. The observed large increases in woody cover cannot be explained by land use or by local climatic conditions only. The changes are also consistent with global climate change effects, most likely due to increasing atmospheric CO2.
Remote sensing, together with geographic information systems (GISs), provide a powerful tool for ... more Remote sensing, together with geographic information systems (GISs), provide a powerful tool for monitoring environmental processes. Aerial and satellite photographs enable the analysis of an entire landscape and, using multi-temporal sets of photographs, one is able to follow processes over time. In our study, we used multi-temporal sets of aerial photographs, taken between 1944 and 2005, to observe the phenomenon of woody plant encroachment in four neighbouring study sites in northern KwaZulu-Natal, South Africa. We also analysed rainfall and temperature trends. Woody plant encroachment was evident in all sites. We suggest possible drivers behind the bush encroachment; in particular, climatic trends. However, encroachment did not relate directly to rainfall and temperature trends. We also speculate on the effects of herbivory and fire, as well as rising atmospheric CO2 concentrations and its effect on C3 and C4 plants. We suggest that increasing atmospheric CO2 concentrations could be the overriding driver of woody plant encroachment in our study area, with other drivers, such as rainfall, temperature, fire, herbivory, and land use, playing a modifying role.
Questions: What is the effect of neighbour competition on the survival, growth and biomass of mes... more Questions: What is the effect of neighbour competition on the survival, growth and biomass of mesic and humid savanna tree species? Can competitive effect and response be linked to plant functional traits? Location: Neil Tainton Arboretum, University of KwaZulu-Natal, Pietermaritzburg, ZA. Methods: Using a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability. Results: We found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to showthat the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource-acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition. Conclusions: Competitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high- or low-resource environments. Our results show significant relationships between plant traits and competitive ability, which may be used to predict competitive interactions between tree seedlings fromhumid and mesic savannas.
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Papers by David Ward
The effect of intra-specific competition was stronger than inter-specific competition at a high increment of neighbor density (from 4 to 32 neighbors). There was no difference in interspecific competitive responses of plants originating in the four environments at any neighbor density increments, but intraspecific competitive responses of the four ecotypes consistently differed at low competitive intensity (4 neighbors). The superior competitors were the plants originating from Mediterranean grassland, the most favorable in respect to rainfall and abiotic stress (i.e. drought or frost) environment. The plants from the mountain environment, which is highly productive and predictable in respect to rainfall but experiences severe frost stress, were the poorest competitors. Our results are inconsistent with the hypothesis that there is no relationship between competitive ability and environmental favorability. High competitive ability appears to be a distinct property of plants living in favorable environments (i.e. productive and predictable and without abiotic stress) corresponding to the 'competitive' strategy of the C-S-R model. However, in less productive and/or predictable environments, or under conditions of severe abiotic stress, plant features other than ability to tolerate low water or nutrient levels may be more important, with reduced competitive ability as a trade-off.
The regulating effect of reproductive traits on final reproductive output also differed across treatments, operating either at the “early” stage of plant development through varying the number of initiated spikelets per spike (no stress and water stress treatment) or at the “late” developmental stage adjusting the fertile spikelet weight (no stress and nutrient stress treatment). Reproductive output was regulated via seed abortion under no stress and water stress treatments. Although the underlying mechanism of the regulation through abortion has yet to be discovered, the specific mechanism of abortion under water stress appears to be different from that under optimal conditions. Our results demonstrate that not only is the character architecture in wild barley plastic and sensitive to changing availability of water and nutrients, but the regulating mechanism of maternal investment is also environmentally sensitive.
Significant genotype by environment interactions were observed for yield and reproductive biomass, seedling biomass and percent germinated and survived seeds, suggesting local ecotype adaptation. Increasing productivity and predictability of environment in respect to rainfall, without concomitant frost stress, was found to select for high reproductive biomass and large seeds, high fraction of germinating seeds and high vigor of seedlings. The optimal strategy changes with increasing productivity and predictability and involves a trade-off between seed size and number, with reduced yield but increased seed mass, which is expected under K- selection. No specific life history adaptations to predictable frost stress were detected for the mountain ecotype, but there was higher survival of seedlings in their indigenous (mountain) environment as compared with other ecotypes. The latter appears to be a physiological adaptation to frost, which is consistent with stress (or adversity) selection type. The other stress factor, drought, which is very unpredictable in deserts, was associated with high seed dormancy, small seed size and low vigor of seedlings, but relatively high yield, which is consistent with a stress-escape bet-hedging strategy.
photographs, including historical photographs taken at the time of the Second Anglo-Boer War of 1899–1902, to assess the scale and timing of woody plant encroachment in the dry savannas near Kimberley in South Africa (mean annual rainfall = 300–400 mm). There were large increases in woody plant encroachment in most areas. Even at the battlefield of Magersfontein, where grazing has been virtually absent since its protection in 1960, we
found that encroachment by trees and shrubs has occurred. Using aerial photographs, we found that the rate of encroachment has increased substantially since 1993. However, repeated photographs at certain sites indicate that
encroachment produced cohorts of trees. We show that global drivers are perhaps of greater importance than local drivers such as heavy grazing and absence of fire.
interfere with sapling establishment dynamics by direct consumption of plant tissue, changing soil nutrient
status (through fertilization and trampling) and grass competition. Studies evaluating the effects of
herbivory on sapling establishment have mostly been extrapolated from single species. In a controlled
field experiment, we studied the effects of clipping (simulating grazing and browsing), nutrients, grass
competition, and their interactive effects on sapling survival and growth of four dominant humid and
four dominant mesic savanna species. We conducted this experiment in a humid South African savanna.
We found no effects on sapling survival by the treatments provided. However, clipped saplings of all
species increased their investment in relative growth rate of stem length (RGRL). Clipping had a greater
negative impact on relative growth rate of more humid than mesic species in terms of stem diameter
(RGRD), total dry biomass and proportion of leaf biomass. Nutrients had a positive effect on the RGRL and
sapling biomass of three mesic species. Positive effects of nutrients on RGRL of one humid and two mesic
species were observed in their clipped saplings only. Grass competition had a strong negative impact on
all growth parameters measured. Clipped saplings of one humid and two mesic species had lower RGRL
with grass competition whereas intact saplings showed no significant response. After clipping, humid
savanna species were more vulnerable to grass competition than mesic species, with reduced ability to
use nutrients. In conclusion, herbivory increases sapling vulnerability to grass competition, with humid
species being more susceptible than mesic species, indicating that woody-plant control strategies are
more likely to be effective in humid savannas.
value. However, the dispersal of seeds that remain intact and can potentially germinate after excretion is of particular concern
when animals consume seeds of encroaching or invasive woody plants. The objectives of this study were to determine the effects
of animal species in two experiments (experiment 1: goats, sheep; experiment 2: goats, cattle), diet quality (Medicago sativa hay,
Digitaria eriantha hay) and seed characteristics (size, hardness) on the effectiveness of animal seed dispersal and germination of
Dichrostachys cinerea and Acacia nilotica seeds. Owing to a limitation on the availability of seeds, the two experiments were
done separately at different times. Each animal in both experiments received 1 000 A. nilotica seeds and 1 000 D. cinerea seeds
mixed with either a low-quality diet (D. eriantha hay) or a high-quality diet (M. sativa hay). In experiment 1, we found a
significant interaction effect of animal species (goats, sheep), diet (high-quality hay, low-quality hay), and seed species (A.
nilotica seeds, D. cinerea seeds) on germination (P, 0.0001). There was also a higher seed recovery (P,0.009) when animals
were offered high-quality hay (47.4%64.65) compared to low-quality hay (30.2%63.24). In experiment 2, animal species
affected seed recovery (P,0.0325; goats 32.0%66.44; cattle 50.3%64.27) and germination percentage (P,0.055; goats
14.1%61.48; cattle 9.3%60.94). The diet quality fed to the animals may affect dispersal and germination. However, animal
species and seed characteristics also had important effects on germination of D. cinerea and A. nilotica seeds. Thus, all three of
these factors play a major role in dissemination of viable seeds.
woody plant encroachment. We investigated the changes in vegetation at selected sites on the battlefields of the
Anglo-Zulu War of 1879 in KwaZulu-Natal. We used fixed-point repeat photographs to compare the landscape
at the time of the Anglo-Zulu War with the same landscape in 2011. Nine photo-pairs were analysed, covering
three specific sites: Isandlwana, Fugitives’ Drift and Rorke’s Drift. A further eight photographs from these sites,
taken during the intervening years, were matched and compared with the present landscape. All sites we analysed
showed an increase in woody cover since 1879. We used the Whittaker plant diversity sampling method to
determine current woody and grass species composition on the sites. We also investigated the effect of catena,
aspect, rockiness and land use on the bush encroachment. Rainfall and temperature data were also analysed. The
observed large increases in woody cover cannot be explained by land use or by local climatic conditions only. The
changes are also consistent with global climate change effects, most likely due to increasing atmospheric CO2.
Location: Neil Tainton Arboretum, University of KwaZulu-Natal, Pietermaritzburg, ZA.
Methods: Using a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability.
Results: We found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to showthat the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource-acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition.
Conclusions: Competitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high- or low-resource environments.
Our results show significant relationships between plant traits and
competitive ability, which may be used to predict competitive interactions between tree seedlings fromhumid and mesic savannas.
The effect of intra-specific competition was stronger than inter-specific competition at a high increment of neighbor density (from 4 to 32 neighbors). There was no difference in interspecific competitive responses of plants originating in the four environments at any neighbor density increments, but intraspecific competitive responses of the four ecotypes consistently differed at low competitive intensity (4 neighbors). The superior competitors were the plants originating from Mediterranean grassland, the most favorable in respect to rainfall and abiotic stress (i.e. drought or frost) environment. The plants from the mountain environment, which is highly productive and predictable in respect to rainfall but experiences severe frost stress, were the poorest competitors. Our results are inconsistent with the hypothesis that there is no relationship between competitive ability and environmental favorability. High competitive ability appears to be a distinct property of plants living in favorable environments (i.e. productive and predictable and without abiotic stress) corresponding to the 'competitive' strategy of the C-S-R model. However, in less productive and/or predictable environments, or under conditions of severe abiotic stress, plant features other than ability to tolerate low water or nutrient levels may be more important, with reduced competitive ability as a trade-off.
The regulating effect of reproductive traits on final reproductive output also differed across treatments, operating either at the “early” stage of plant development through varying the number of initiated spikelets per spike (no stress and water stress treatment) or at the “late” developmental stage adjusting the fertile spikelet weight (no stress and nutrient stress treatment). Reproductive output was regulated via seed abortion under no stress and water stress treatments. Although the underlying mechanism of the regulation through abortion has yet to be discovered, the specific mechanism of abortion under water stress appears to be different from that under optimal conditions. Our results demonstrate that not only is the character architecture in wild barley plastic and sensitive to changing availability of water and nutrients, but the regulating mechanism of maternal investment is also environmentally sensitive.
Significant genotype by environment interactions were observed for yield and reproductive biomass, seedling biomass and percent germinated and survived seeds, suggesting local ecotype adaptation. Increasing productivity and predictability of environment in respect to rainfall, without concomitant frost stress, was found to select for high reproductive biomass and large seeds, high fraction of germinating seeds and high vigor of seedlings. The optimal strategy changes with increasing productivity and predictability and involves a trade-off between seed size and number, with reduced yield but increased seed mass, which is expected under K- selection. No specific life history adaptations to predictable frost stress were detected for the mountain ecotype, but there was higher survival of seedlings in their indigenous (mountain) environment as compared with other ecotypes. The latter appears to be a physiological adaptation to frost, which is consistent with stress (or adversity) selection type. The other stress factor, drought, which is very unpredictable in deserts, was associated with high seed dormancy, small seed size and low vigor of seedlings, but relatively high yield, which is consistent with a stress-escape bet-hedging strategy.
photographs, including historical photographs taken at the time of the Second Anglo-Boer War of 1899–1902, to assess the scale and timing of woody plant encroachment in the dry savannas near Kimberley in South Africa (mean annual rainfall = 300–400 mm). There were large increases in woody plant encroachment in most areas. Even at the battlefield of Magersfontein, where grazing has been virtually absent since its protection in 1960, we
found that encroachment by trees and shrubs has occurred. Using aerial photographs, we found that the rate of encroachment has increased substantially since 1993. However, repeated photographs at certain sites indicate that
encroachment produced cohorts of trees. We show that global drivers are perhaps of greater importance than local drivers such as heavy grazing and absence of fire.
interfere with sapling establishment dynamics by direct consumption of plant tissue, changing soil nutrient
status (through fertilization and trampling) and grass competition. Studies evaluating the effects of
herbivory on sapling establishment have mostly been extrapolated from single species. In a controlled
field experiment, we studied the effects of clipping (simulating grazing and browsing), nutrients, grass
competition, and their interactive effects on sapling survival and growth of four dominant humid and
four dominant mesic savanna species. We conducted this experiment in a humid South African savanna.
We found no effects on sapling survival by the treatments provided. However, clipped saplings of all
species increased their investment in relative growth rate of stem length (RGRL). Clipping had a greater
negative impact on relative growth rate of more humid than mesic species in terms of stem diameter
(RGRD), total dry biomass and proportion of leaf biomass. Nutrients had a positive effect on the RGRL and
sapling biomass of three mesic species. Positive effects of nutrients on RGRL of one humid and two mesic
species were observed in their clipped saplings only. Grass competition had a strong negative impact on
all growth parameters measured. Clipped saplings of one humid and two mesic species had lower RGRL
with grass competition whereas intact saplings showed no significant response. After clipping, humid
savanna species were more vulnerable to grass competition than mesic species, with reduced ability to
use nutrients. In conclusion, herbivory increases sapling vulnerability to grass competition, with humid
species being more susceptible than mesic species, indicating that woody-plant control strategies are
more likely to be effective in humid savannas.
value. However, the dispersal of seeds that remain intact and can potentially germinate after excretion is of particular concern
when animals consume seeds of encroaching or invasive woody plants. The objectives of this study were to determine the effects
of animal species in two experiments (experiment 1: goats, sheep; experiment 2: goats, cattle), diet quality (Medicago sativa hay,
Digitaria eriantha hay) and seed characteristics (size, hardness) on the effectiveness of animal seed dispersal and germination of
Dichrostachys cinerea and Acacia nilotica seeds. Owing to a limitation on the availability of seeds, the two experiments were
done separately at different times. Each animal in both experiments received 1 000 A. nilotica seeds and 1 000 D. cinerea seeds
mixed with either a low-quality diet (D. eriantha hay) or a high-quality diet (M. sativa hay). In experiment 1, we found a
significant interaction effect of animal species (goats, sheep), diet (high-quality hay, low-quality hay), and seed species (A.
nilotica seeds, D. cinerea seeds) on germination (P, 0.0001). There was also a higher seed recovery (P,0.009) when animals
were offered high-quality hay (47.4%64.65) compared to low-quality hay (30.2%63.24). In experiment 2, animal species
affected seed recovery (P,0.0325; goats 32.0%66.44; cattle 50.3%64.27) and germination percentage (P,0.055; goats
14.1%61.48; cattle 9.3%60.94). The diet quality fed to the animals may affect dispersal and germination. However, animal
species and seed characteristics also had important effects on germination of D. cinerea and A. nilotica seeds. Thus, all three of
these factors play a major role in dissemination of viable seeds.
woody plant encroachment. We investigated the changes in vegetation at selected sites on the battlefields of the
Anglo-Zulu War of 1879 in KwaZulu-Natal. We used fixed-point repeat photographs to compare the landscape
at the time of the Anglo-Zulu War with the same landscape in 2011. Nine photo-pairs were analysed, covering
three specific sites: Isandlwana, Fugitives’ Drift and Rorke’s Drift. A further eight photographs from these sites,
taken during the intervening years, were matched and compared with the present landscape. All sites we analysed
showed an increase in woody cover since 1879. We used the Whittaker plant diversity sampling method to
determine current woody and grass species composition on the sites. We also investigated the effect of catena,
aspect, rockiness and land use on the bush encroachment. Rainfall and temperature data were also analysed. The
observed large increases in woody cover cannot be explained by land use or by local climatic conditions only. The
changes are also consistent with global climate change effects, most likely due to increasing atmospheric CO2.
Location: Neil Tainton Arboretum, University of KwaZulu-Natal, Pietermaritzburg, ZA.
Methods: Using a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability.
Results: We found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to showthat the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource-acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition.
Conclusions: Competitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high- or low-resource environments.
Our results show significant relationships between plant traits and
competitive ability, which may be used to predict competitive interactions between tree seedlings fromhumid and mesic savannas.