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• We studied five selected calcareous grassland species across 18 sites in Europe. • Photosynthesis and plant functional traits were influenced by abiotic site conditions. • Plant functional traits had a big direct impact on photosynthesis rates. • Abiotic site conditions influence plant traits stronger than photosynthesis rates. Photosynthesis is a main driver of plant performance and varies between and within species. This study investigates the effects of plant functional traits as well as abiotic site conditions on the intra- and interspecific variability of photosynthetic performance measured via maximum carboxylation capacity (V cmax) in five widespread species (Campanula glomerata, Centaurea jacea, Plantago media, Salvia pratensis and Trifolium montanum) and on 18 dry calcareous grassland sites across Europe. In addition to that we assessed plant traits associated with plant performance like specific leaf area, leaf nitrogen and carbon status and stable nitrogen isotope content in parallel on each individual. Climate variables, site characteristics and soil nutrients were recorded to test whether abiotic conditions had a direct impact on photosynthesis rates, or whether that influence was mitigated by their impact on the leaf functional traits measured. Leaf functional traits and abiotic site conditions had an influence on V cmax both, within and between species. However, the results differed between these scales with differences between species, where mainly T. montanum responded differently than the other species. Leaf nitrogen content had the strongest link of all parameters analysed to V cmax and was positively related to it both, intra- and interspecifically. Slope, soil nitrogen, irradiation and temperature influenced V cmax yet we found that mainly leaf traits had direct effects on V cmax when we analysed all traits and site conditions together using structural equation models. However, the indirect effects of abiotic site conditions via changing leaf functional traits were strong. We thus conclude that abiotic site conditions change V cmax mainly via affecting leaf functional traits, thus representing an indirect effect. This effect has to be considered when using abiotic site conditions and leaf functional traits for forecasting and measuring photosynthesis. [ABSTRACT FROM AUTHOR]

Abstract: The co-dominance of trees and grasses is a much-debated topic in savanna ecology. One of the proposed mechanisms facilitating the co-dominance of these two different life-forms is root niche partitioning. Specifically, the rooting zone is hypothesized to have two layers: grasses are thought to be the superior competitors for water in the upper soil horizons, whereas trees avoid competitive exclusion by sourcing water from soil layers beyond the vertical reach of grasses. We examined this two-layer hypothesis through a field experiment in which we severed deep and shallow roots of Terminalia sericea saplings, 6 weeks before the onset of the new growing season, using a novel technique. During the course of the growing season, physiological responses to the root severing were measured. Severing lateral (shallow) roots resulted in rapid abscission of the old leaves, smaller leaves after the onset of the following growing season and lower xylem pressure potentials – indicating higher water stress – compared to the other treatment groups. Effects of severing deep-growing taproots were small, demonstrating the importance of lateral roots for maintaining the trees’ water balance, notably in drier times. We have shown that saplings of the common and locally dominant savanna tree T. sericea manage to coexist with grasses without avoiding competition through spatial root separation. The results of our study therefore do not support the two-layer hypothesis for explaining tree–grass coexistence in this mesic savanna. [Copyright &y& Elsevier]

Abstract: Perennial grasses are a dominant component of many vegetation formations and provide important ecosystem services including supporting herbivores and preventing soil erosion. Despite their importance, our understanding of the processes that influence their mortality rates is surprisingly limited. This study explores the effects of local and landscape-scale processes on mortality of a perennial grass (Stipagrostis uniplumis) in semi-arid savanna. We focussed on three local-scale factors: self-shading by the standing dead biomass of a tuft, plant size, and neighbour abundance as a measure of intra-specific competition. Three indices of neighbour abundance were calculated: number of neighbours, sum of the neighbours’ basal area, and sum of the neighbours’ living basal area. At the landscape scale, we explored the influence of fire on tuft mortality. The amount of standing dead biomass increased the mortality rates of tufts. Neighbour abundance, indexed as the sum of the living basal area of neighbours, was also associated with higher mortality rates, whereas the other indices of neighbour abundance had no influence on mortality rates. On a landscape level, fire significantly increased tuft mortality rates, from up to 31% for unburned tufts, to 73% for burned tufts. Fire, on the other hand, indirectly reduces the risk of future mortality by reducing self-shading and competitive pressure. Our results imply that the timing and frequency of fires is crucial for their positive indirect effects on plant fitness. As the onset of local effects on plant mortality is highly dependent on grazing pressure and stochastic rainfall, fire management should flexibly take into account the accumulation of dead plant material on a site. [Copyright &y& Elsevier]

Highlights • The aDGVM2 model was updated to distinguish annual and perennial grasses based on traits. • A new scheme for selective grazing was implemented and tested for South African sites. • Increasing grazing intensity reduced productivity and perennial grass abundance. • Heavy grazing and drought had similar effects on simulated grass communities. • Simulated rangeland recovery after grazing exclusion took between 2 and 15 years. Abstract Semi-arid tropical rangelands substantially contribute to livelihoods of subsistence farmers, but are threatened by undesired vegetation shifts due to climate change and overgrazing. Grazing-induced shifts of the grass community composition are often associated with rangeland degradation. To identify sustainable management strategies, a process-based understanding of grass functional diversity and rangeland dynamics is required. We present a new scheme for aDGVM2, a dynamic vegetation model for tropical ecosystems, that distinguishes annual and perennial grasses based on trait trade-offs to improve the representation of rangeland communities. Additionally, the model includes a new scheme that describes selective grazing and grazing effects on grass-layer composition. We tested the new model version for various grazing intensities along a precipitation gradient in South Africa. Mean annual precipitation below 500 mm constrained rangeland productivity and carrying capacity. Increasing grazing intensity reduced rangeland productivity and increased annual grass abundance. Heavy grazing resulted in annual grass dominance. Livestock preferred perennial over annual grasses at low grazing intensities at all except the two driest sites; preference switched to annual grasses at intermediate intensities, and became non-discriminating at high grazing intensities. Rangeland recovery after removal of grazers required 2–15 years. We conclude that management intervention reducing or eliminating grazing pressure during and after stress years is crucial to allow rangeland recovery and avoid permanent degradation. [ABSTRACT FROM AUTHOR]

Stomata enable plants to balance uptake of CO 2 with water loss via transpiration. Previous studies have shown that stomatal size and density trade-off with one another and are related to a suite of environmental factors (light, atmospheric CO 2 concentrations, moisture and temperature) which influence gas exchange. We examined the hypothesis that the habitat preferences of species are reflected in stomatal traits. Thus, we examined the relation of stomatal size and density of 36 herbaceous species along elevational gradients in relation to Ellenberg indicator values for light and moisture. The light value is a proxy for the transpiration demand due to exposure to radiation and air mass exchange, the moisture value reflects water supply. Additionally we measured carbon isotope discrimination (Δ 13 C), a proxy for intrinsic water use efficiency and internal CO 2 concentration. Stomatal size changed in parallel on both sides of a leaf, whereas changes in density differed between sides depending on species identity. There was an increase in absolute variation of sizes and densities with increasing mean size and density respectively, but not in relative variation. Species with few but large stomata tended to adjust stomatal size across environmental gradients, whereas species with small but many stomata mainly adjusted stomatal density. A higher Ellenberg indicator value for light and a lower value for moisture was associated with equal distribution of stomata between leaf sides. The carbon isotope discrimination data indicated that amphistomatic species, which illustrated a preference for high radiation and high air mass exchange as well as for dry habitats, had higher water use efficiency. We conclude that stomatal traits such as size, density or the distribution between the two sides of the leaf are indicators of how species optimize carbon uptake and balance water loss and radiation gain. [ABSTRACT FROM AUTHOR]