Tree-Grass Interactions

The spatially explicit and mechanistic model TREEGRASS was used to test the effects of tree density, tree spatial distribution, and tree size distribution on radiation absorption, net primary production (NPP), and, water fluxes for a West African savanna . The model has previously been shown to adequately predict radiation absorption, production, and soil water balance in humid savannas . Using field measurements quantifying the spatial variations in grass NPP, the model was found to predict correctly the effect of tree abundance on grass NPP. Model simulations showed that annual photosynthetically active radiation (PAR), absorption efficiency, NPP, and transpiration of the tree layer increased with increasing tree density and decreased with increasing tree aggregation ; the grass layer showed the opposite trends . Total NPP (tree plus grass) remained stable for all tree densities and decreased slightly for aggregated tree distributions . Total transpiration increased with tree density and was not affected by tree aggregation. Changing the tree size distribution had no effect on radiation absorption or on carbon and water fluxes . This study showed that fine-scale vegetation structure can influence NPP and water fluxes. As such, it should be taken into account when assessing the functioning of tree- grass systems. However, the detailed fine-scale structure of the tree layer is rarely available for savanna ecosystems. Thus we assessed the reliability of two common descriptors of savanna vegetation, leaf area index (LAI) and tree canopy cover, to predict PAR absorption, NPP, and evapotranspiration. Neither of these descriptors could adequately predict all the simulated effects. However, tree canopy cover can be used to reasonably estimate radiation absorption and carbon fluxes, whereas LAI is a good predictor of water fluxes. Therefore, mapping and monitoring approaches incorporating both LAI and tree canopy cover appear to be suitable alternatives to quantifying the fine-scale vegetation spatial structure when assessing ecosystem function in savanna-like ecosystems .

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties1–3. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover1,2,4,5, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than ,650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered ‘stable’ systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of ,650mm, savannas are ‘unstable’ systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation6 may considerably affect their distribution and dynamics.

A key question in savanna ecology is how trees and grasses coexist under N limitation. We used N stable isotopes and N content to study N source partitioning across seasons from trees and associated grasses in a semi-arid savanna. We also used (15)N tracer additions to investigate possible redistribution of N by trees to grasses. Foliar stable N isotope ratio (δ(15)N) values were consistent with trees and grasses using mycorrhiza-supplied N in all seasons except in the wet season when they switched to microbially fixed N. The dependence of trees and grasses on mineralized soil N seemed highly unlikely based on seasonal variation in mineralization rates in the Kruger Park region. Remarkably, foliar δ(15)N values were similar for all three tree species differing in the potential for N fixation through nodulation. The tracer experiment showed that N was redistributed by trees to understory grasses in all seasons. Our results suggest that the redistribution of N from trees to grasses and uptake of N was independent of water redistribution. Although there is overlap of N sources between trees and grasses, dependence on biological sources of N coupled with redistribution of subsoil N by trees may contribute to the coexistence of trees and grasses in semi-arid savannas.

The function and dynamics of savanna ecosystems result from complex interactions and feedbacks between grasses and trees, involving numerous processes (i.e. competition for light, water and nutrients, fire, and herbivory). These interactions are characterised by strong relationships between vegetation structure and function. Given the heterogeneous structure of savannas, modelling appears as a convenient approach to study tree – grass interactions. Most current models that describe carbon and water fluxes are not spatially explicit, which restricts their ability to simulate plant interactions at small scales in heterogeneous ecosystems. We present here a new 3D process-based model called TREEGRASS. The model aims at predicting, in heterogeneous tree – grass systems, plant individual radiation, carbon and water fluxes at a local spatial scale. It is run at a daily time-step over periods ranging from one to a few years. The model includes (i) a 3D mechanistic submodel simulating radiation and energy (i.e. transpiration) budgets; (ii) a soil water balance submodel, and (iii) a physiologically based submodel of primary production and leaf area development. The ability of TREEGRASS to predict the seasonal courses of grass dead and leaf mass, soil water content and light regime as observed in the field has been tested for grassy and shrubby areas of Lamto savannas (Ivory Coast). Simulations showed that the spatial distribution of primary production can be strongly affected by the spatial vegetation structure. Potential applications involve predicting net primary production and water balance from the individual to the ecosystem and from the day to the annual vegetation cycle (e.g. effects of tree spatial patterns on carbon and water fluxes at the ecosystem level).

Whereas leaf gas exchange properties are important to assess carbon and water fluxes in ecosystems worldwide, information of this type is scarce for savanna species. In this study, gas exchange characteristics of 2 C4 grass species Andropogon canaliculatus and Hyparrhenia diplandra and 2 C3 tree species Crossopteryx febrifuga and Cussonia arborea from the West-African savanna of Lamto Ivory Coast were investigated in the field. Measurements were done in order to provide data to allow the parameterization of biochemically-based models of photosynthesis for C4 and C3 plant metabolic types and stomatal conductance ; and to compare gas exchange characteristics of coexisting species. No systematic difference was found between grass and tree species for reference stomatal conductance, under standard environmental conditions, or stomatal response to incident light or vapour pressure deficit at leaf surface. Conversely, grass species displayed higher water 1.5-2 fold and nitrogen 2-5 fold photosynthetic use efficiencies WUE and NUE, ratio of net photosynthesis to transpiration and leaf nitrogen, respectively. These contrasts were attributed to the CO2 concentrating mechanism of C4 plants. When looking within plant life forms, no important difference was found between grass species. However, significant contrasts were found between tree species, Cussonia showing higher NUE and reference stomatal conductance than Crossopteryx. These results stress the need to account for functional diversity when estimating ecosystem carbon and water fluxes. In particular, our results suggest that the tree/grass ratio, and also the composition of the tree layer, could strongly affect WUE and NUE at the ecosystem scale in West African savannas.

Despite their overwhelming influence on (1) variations in the physiological activity of vegetation (2) plant phenology, and (3) potential competitive interactions between coexisting species, comprehensive information on water relations and soil water uptake patterns is still lacking for grasses and shrubs in African humid savannas. Over 2 years, seasonal variations in plant water status were measured with the pressure bomb technique for two deciduous shrub species (Cussonia barteri and Crossopteryx febrifuga) and a perennial grass species (Hyparrhenia diplandra) in a humid savanna. Concurrently, soil moisture was surveyed with the neutron probe technique in grassy areas and under shrub clumps. Absence of transpiration at dawn was assessed by surveys of the leaf water isotopic signal, and plant water relations were characterized during rainy and dry periods with the pressure-volume curve technique. The seasonal changes in predawn and minimum shoot water potentials were pronounced for the grass species and the shrub C. febrifuga, but weak for the shrub C. barteri. Relationships between plant and soil water status showed that both the grass species and the shrub C. febrifuga did not maintain high water potentials when soil moisture in the upper soil layers decreased. In contrast , the shrub C. barteri exhibited a predawn water potential around A0.5 MPa when the 0 to 60-cm soil layer was at the permanent wilting point. During the early dry season, leaf dehydration was observed for C. febrifuga, but not for C. barteri. We conclude that the grass H. diplandra and the shrub C. febrifuga had very poor and limited access to deep soil layers (below 60 cm), respectively while the shrub C. barteri was able to access this water resource. The relatively low osmotic potential and tissue elasticity measured for C. febrifuga as compared to C. barteri were consistent with the use of water from upper soil layers by this species, since these characteristics could help water withdrawal and promote turgor maintenance at lower shoot water potentials. Di€fferent water uptake patterns could explain the contrasted leaf shedding patterns reported for the two shrub species. These results emphasize that the water economy of deciduous shrub species of African humid savannas can di€er significantly and that both grasses and some shrub species acquire water from the upper soil layers even during dry spells. These findings will aid understanding of interspecific competition and modelling ecosystem function in this kind of humid savanna.