Resprouting is an important means of plant regeneration especially under conditions that do not favour regeneration through seeding such as frequent disturbances, low productivity, unfavourable soil conditions, extreme cold and limited... more
Resprouting is an important means of plant regeneration especially under conditions that do not favour regeneration through seeding such as frequent disturbances, low productivity, unfavourable soil conditions, extreme cold and limited understorey light availability. Sprouts may be advantageous over seedlings because they have higher survival and growth rates than seedlings, since they use resources from parent plants unlike seedlings that have to acquire their own resources. Resprouting is well documented for ecosystems that experience severe disturbances that damage aboveground biomass. For example, resprouting is important for plant persistence against fire in fire-prone savannas and Mediterranean shrub-lands, and hurricanes and cyclones in tropical forests. In these ecosystems, resprouting often results in multi-stemming, because this dilutes the risk of damage among many stems, improving the chances of individual survival. This study was conducted in coastal dune forest at Cape...
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Research Interests: Reproduction, Plant Biology, Pollination, Bees, South Africa, and 3 moreAnimals, Flowers, and Acanthaceae
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The regeneration niche is commonly partitioned along a gradient from shade-tolerant to shade-intolerant species to explain plant community assembly in forests. We examined the shade tolerance of tree seedlings in a subtropical coastal... more
The regeneration niche is commonly partitioned along a gradient from shade-tolerant to shade-intolerant species to explain plant community assembly in forests. We examined the shade tolerance of tree seedlings in a subtropical coastal forest to determine whether the ecological filtering effect of a dominant, synchronously monocarpic herb (Isoglossa woodii) selects for species at either end of the light response continuum during the herb’s vegetative and reproductive phases. Photosynthetic characteristics of seedlings of 20 common tree species and the herb were measured. Seedlings were grown in the greenhouse at 12–14% irradiance, and their light compensation points measured using an open-flow gas exchange system. The light compensation points for the tree species were low, falling within a narrow range from 2.1 ± 0.8 μmol m−2 s−1 in Celtis africana to 6.4 ± 0.7 μmol m−2 s−1 in Allophylus natalensis, indicating general shade tolerance, consistent with a high and narrow range of apparent quantum yield among species (0.078 ± 0.002 mol CO2 mol−1 photon). Rates of dark respiration were significantly lower in a generalist pioneer species (Acacia karroo) than in a forest pioneer (C. africana), or in late successional phase forest species. We argue that the general shade tolerance, and phenotypic clustering of shade tolerance, in many tree species from several families in this system, is a result of ecological filtering by the prevailing low light levels beneath the I. woodii understorey, which excludes most light-demanding species from the seedling community.
Research Interests: Botany, Conservation Biology, Photosynthesis, Adaptation, Coastal Dunes, and 19 moreEcology, Medicinal Plants, Community, Forests, Light, Tree, Trees, Assimilation, Oecologia, Regeneration niche, Seedling, Shade tolerance, Plant Community, Tropical Climate, Low Light, Quantum Yield, Gas Exchange, Oceans and Seas, and Plant Leaves
South African coastal dune forests are young, highly-disturbed subtropical communities where conventional models of forest dynamics may be challenged. We tested predictions from the gap-phase regeneration model by comparing seedlings of... more
South African coastal dune forests are young, highly-disturbed subtropical communities where conventional models of forest dynamics may be challenged. We tested predictions from the gap-phase regeneration model by comparing seedlings of three common species representing contrasting regeneration strategies: Acacia karroo as a ruderal, Celtis africana as a coloniser of forest gaps, and Diospyros natalensis as a late-successional species. We grew seedlings under contrasting light and nitrogen levels in a greenhouse and in the field for one year to compare their growth and survival rates, allocation and photosynthetic traits.
Species' growth rates generally followed the expected order: Acacia > Celtis > Diospyros, but Acacia responded strongly to light and Celtis responded strongly to nitrogen, leading to cross-overs in growth rates. The plasticity of allocation and photosynthesis did not clearly differentiate the strategies, although it was greater in the light-demanding species. Acacia and Celtis tended to survive better in Acacia stands than in forest plots. Leaf-level light compensation points (LCPs) were similar for the three species in most conditions, but auxiliary data suggest Diospyros has a lower whole-plant LCP than Acacia. Growth rates and LCPs were lower than most of those reported for primary-forest species in the literature, suggesting an unusual degree of shade-tolerance in this habitat. We discuss reasons why variation in shade-tolerance may be less important here than in the prevailing model for forest regeneration and suggest other biotic factors that may help differentiate regeneration niches.
Species' growth rates generally followed the expected order: Acacia > Celtis > Diospyros, but Acacia responded strongly to light and Celtis responded strongly to nitrogen, leading to cross-overs in growth rates. The plasticity of allocation and photosynthesis did not clearly differentiate the strategies, although it was greater in the light-demanding species. Acacia and Celtis tended to survive better in Acacia stands than in forest plots. Leaf-level light compensation points (LCPs) were similar for the three species in most conditions, but auxiliary data suggest Diospyros has a lower whole-plant LCP than Acacia. Growth rates and LCPs were lower than most of those reported for primary-forest species in the literature, suggesting an unusual degree of shade-tolerance in this habitat. We discuss reasons why variation in shade-tolerance may be less important here than in the prevailing model for forest regeneration and suggest other biotic factors that may help differentiate regeneration niches.
Dispersal or recruitment limitation may arrest succession after disturbance. In north-eastern South Africa the Acacia karroo successional pathway is used to facilitate coastal forest recovery after strip-mining. However, although A.... more
Dispersal or recruitment limitation may arrest succession after disturbance. In north-eastern South Africa the Acacia karroo successional pathway is used to facilitate coastal forest recovery after strip-mining. However, although A. karroo establishes naturally, it forms monospecific stands, arresting forest succession for decades. This casts doubt on the efficacy of this restoration pathway. We investigated the causes of arrested succession. The seed and seedling banks of A. karroo stands and of forest at Cape Vidal, and three A. karroo stands (7–27 years old) on rehabilitated strip-mined dunes at nearby Richards Bay were examined. The establishment and growth of seedlings at Cape Vidal were also considered. The seed bank was larger and more diverse in forest, but the seedling bank was larger in Acacia stands. At Richards Bay, the size of the seed bank increased and the seedling bank decreased with Acacia stand age. Excluding mammalian herbivores in Acacia stands at Cape Vidal resulted in greater species richness and survival of naturally established seedlings, as well as two experimentally planted species. Neither seed dispersal nor seedling establishment limited recruitment of tree species in Acacia stands. Herbivory arrested forest succession by causing the differential mortality of seedlings. In contrast, at Richards Bay where there were few mammalian herbivores, the advanced regeneration in A. karroo stands converged on the diversity of nearby forests 29 years after restoration. Controlling herbivore access and seeding Acacia stands with forest species are site-specific options for preventing arrested succession when using the A. karroo successional pathway.
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Dispersal or recruitment limitation may arrest succession after disturbance. In north-eastern South Africa the Acacia karroo successional pathway is used to facilitate coastal forest recovery after strip- mining. However, although A.... more
Dispersal or recruitment limitation may arrest succession after disturbance. In north-eastern South Africa the Acacia karroo successional pathway is used to facilitate coastal forest recovery after strip- mining. However, although A. karroo establishes naturally, it forms monospecific stands, arresting forest succession for decades. This casts doubt on the efficacy of this restoration pathway. We investigated the causes of arrested succession. The seed and seedling banks of A. karroo stands and of forest at Cape Vidal, and three A. karroo stands (7–27 years old) on rehabilitated strip-mined dunes at nearby Richards Bay were examined. The establishment and growth of seedlings at Cape Vidal were also considered. The seed bank was larger and more diverse in forest, but the seedling bank was larger in Acacia stands. At Richards Bay, the size of the seed bank increased and the seedling bank decreased with Acacia stand age. Excluding mammalian herbivores in Acacia stands at Cape Vidal resulted in greater species richness and survival of naturally established seedlings, as well as two experimentally planted species. Neither seed dispersal nor seedling establishment limited recruitment of tree species in Acacia stands. Herbivory arrested forest succession by causing the differential mortality of seedlings. In contrast, at Richards Bay where there were few mammalian herbivores, the advanced regeneration in A. karroo stands converged on the diversity of nearby forests 29 years after restoration. Controlling herbivore access and seeding Acacia stands with forest species are site-specific options for preventing arrested succession when using the A. karroo successional pathway.
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Wesurveyed the community composition of trees thathost the mistletoe Agelanthus natalitius (Loranthaceae) at two sites (Highover and Mtontwane) in South Africa.We recorded a total of 1464 trees (Acacia karroo and A. caffra) hosting 1202... more
Wesurveyed the community composition of trees thathost the mistletoe Agelanthus natalitius (Loranthaceae)
at two sites (Highover and Mtontwane) in South Africa.We recorded a total of 1464 trees (Acacia karroo and A. caffra)
hosting 1202 mistletoes in the 64 surveyed plots (20 × 50 m). There were almost four times as many A. karroo as A.
caffra at Highover and three times as many A. karroo as A. caffra at Mtontwane. There was no significant difference
in prevalence (percentage of infected trees) at Highover (A. karroo = 22% and A. caffra = 26%), but a significantly
greater percentage of A. caffra trees were parasitized at Mtontwane (A. karroo = 25% and A. caffra = 34%). Intensity
of infection (number of mistletoe infections per tree) was higher for A. karroo (0.73 ± 0.04 and 1.03 ± 0.64) than
for A. caffra (0.66 ± 0.01 and 0.89 ± 0.035) at Highover and Mtontwane, respectively. Prevalence and intensity
of infection showed a significant positive relationship with tree size for both host species at both sites. We tested the
genotype-by-environment interaction effects in this mistletoe by conducting reciprocal transplant experiments (64
individual trees each received 20 seeds). Initial germination was not site-, substrate- or host-sensitive. However, a
general pattern was found that hypocotyls of the germinated seeds grew longer when seeds were placed on the same
host species as the parent plant within their own source locality. Consistent with this observation, mistletoes placed
on their source host species generally had higher survival than those transferred to non-source host species after 6
mo. Overall, mistletoe seeds from parent plants on A. karroo and mistletoe seeds placed on A. karroo had the highest
survival. This could be the result of an adaptation of the mistletoe to the most frequently encountered host species.
at two sites (Highover and Mtontwane) in South Africa.We recorded a total of 1464 trees (Acacia karroo and A. caffra)
hosting 1202 mistletoes in the 64 surveyed plots (20 × 50 m). There were almost four times as many A. karroo as A.
caffra at Highover and three times as many A. karroo as A. caffra at Mtontwane. There was no significant difference
in prevalence (percentage of infected trees) at Highover (A. karroo = 22% and A. caffra = 26%), but a significantly
greater percentage of A. caffra trees were parasitized at Mtontwane (A. karroo = 25% and A. caffra = 34%). Intensity
of infection (number of mistletoe infections per tree) was higher for A. karroo (0.73 ± 0.04 and 1.03 ± 0.64) than
for A. caffra (0.66 ± 0.01 and 0.89 ± 0.035) at Highover and Mtontwane, respectively. Prevalence and intensity
of infection showed a significant positive relationship with tree size for both host species at both sites. We tested the
genotype-by-environment interaction effects in this mistletoe by conducting reciprocal transplant experiments (64
individual trees each received 20 seeds). Initial germination was not site-, substrate- or host-sensitive. However, a
general pattern was found that hypocotyls of the germinated seeds grew longer when seeds were placed on the same
host species as the parent plant within their own source locality. Consistent with this observation, mistletoes placed
on their source host species generally had higher survival than those transferred to non-source host species after 6
mo. Overall, mistletoe seeds from parent plants on A. karroo and mistletoe seeds placed on A. karroo had the highest
survival. This could be the result of an adaptation of the mistletoe to the most frequently encountered host species.
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We studied avian dispersal of seeds from the hemiparasitic mistletoe Plicosepalus acaciae (Loranthaceae) to its tree hosts Acacia raddiana and A. tortilis in the Syrian–African Rift (Arava) valley, Israel. The Yellow-vented Bulbul... more
We studied avian dispersal of seeds from the hemiparasitic mistletoe Plicosepalus acaciae (Loranthaceae) to its tree hosts Acacia raddiana and A. tortilis in the Syrian–African Rift (Arava) valley, Israel. The Yellow-vented Bulbul (Pycnonotus xanthopygos) was the sole avian visitor observed feeding on mistletoe fruits. Bulbuls consumed mistletoe fruits whenever they were available, but the fruits only constituted a significant portion of the diet (71% of foraging attempts) when they were most abundant. These birds are potentially good dispersal vectors of P. acaciae because they swallowed the fruit whole and defecated viable seeds that were covered in a viscid pulp, which allowed the seeds to adhere to substrates when voided. In addition, bulbuls spent a large proportion (66–93%) of total observation time perched in Acacia trees, allowing for directed dispersal. Ephemeral river valleys (wadis) with high mistletoe infection were adjacent to those containing no infections, demonstrating that mistletoe dispersal is common within, but not among wadis. This is consistent with the flight behaviour in bulbuls, which do not typically move among wadis. We combined data on bulbul movements between Acacia trees with transit times of mistletoe seeds to create a hypothetical seed shadow as a function of distance from the parent mistletoe plant. Because they are directed dispersers, the movement patterns of bulbuls may explain the current distribution of P. acaciae in the Arava valley.
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North American coastal sandplain heathlands are unique in species composition and vegetation, but the extent to which edaphic factors influence the structure of these communities is currently debated. It was hypothesized that salt spray... more
North American coastal sandplain heathlands are unique in species composition and vegetation, but the extent to which edaphic factors influence the structure of these communities is currently debated. It was hypothesized that salt spray and edaphic factors maintain the dwarf stature and community composition of heathlands by limiting plant growth and excluding competitively dominant woody species close to the ocean. Field surveys were carried out to investigate the spatial patterns of salt spray accumulation, soil salt and soil moisture. High salt spray correlated significantly with increased leaf necrosis and water stress in Myrica pensylvanica and with decreased plant height. Plant community composition changed across a salt spray and soil gradient, as well. Distinctive sub-communities were identified that separated according to soil salt and soil moisture but salt spray was the main factor affecting sites occupied only by heathland vegetation. Results from this study suggest that salt spray suppresses the growth of heathland plants in close proximity to the ocean, and therefore maintains the low stature in these dwarf shrublands. This research also demonstrates that the physical environment influences the community structure in heathlands, particularly by limiting tree species from growing in high salt spray, low water availability sites.
Research Interests: Plant Ecology, Plant Biology, Water Availability, Ecology, Soil moisture, and 12 moreField Survey, Water Stress, Plant growth, Community Structure, Species Composition, Spatial Pattern, Physical Environment, North American, Plant Community, Plant Water Relations, Community Composition, and Plant Height
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Dominant understorey species influence forest dynamics by preventing tree regeneration at the seedling stage. We examined factors driving the spatial distribution of the monocarpic species Isoglossa woodii, a dominant understorey herb in... more
Dominant understorey species influence forest dynamics by preventing tree regeneration at the seedling stage. We examined factors driving the spatial distribution of the monocarpic species Isoglossa woodii, a dominant understorey herb in coastal dune forests, and the effect that its cover has on forest regeneration. We used line transects to quantify the area of the forest understorey with I. woodii cover and with gaps in the cover. Paired experimental plots were established in semi-permanent understorey gaps with I. woodii naturally absent and in adjacent areas with I. woodii present to compare plant community composition, soil, and light availability between the two habitats. Isoglossa woodii was widespread, covering 65–95% of the understorey, while gaps covered the remaining 5–35% of the area. The spatial distribution of this species was strongly related to tree canopy structure, with I.␣woodii excluded from sites with dense tree cover. Seedling establishment was inhibited by low light availability (<1% of PAR) beneath I.␣woodii. When present, I. woodii reduced the density and species richness of tree seedlings. The tree seedling community beneath I. woodii represented a subset of the seedling community in gaps. Some species that were found in gaps did not occur beneath I. woodii at all. There were no significant differences between the sapling and canopy tree communities in areas with I. woodii gaps and cover. In the coastal dune forest system, seedling survival under I. woodii is dependent on a species’ shade tolerance, its ability to grow quickly during I. woodii dieback, and/or the capacity to regenerate by re-sprouting and multi-stemming. We propose a general conceptual model of forest regeneration dynamics in which the abundant understorey species, I. woodii, limits local tree seedling establishment and survival but gaps in the understorey maintain tree species diversity on a landscape scale.