Leaf traits combinations may explain the occurrence of savanna herbaceous species along a gradient of tree encroachment

In savanna environments, plants have specific leaf traits to deal with high irradiance. These traits allow plants to show high carbon assimilation capacity. However, under encroachment, reduced light availability may act as a filter on traits of plants established under typical savanna conditions. Here we studied morpho-physiological traits of species exclusively found in typical and forested savanna conditions to evaluate how encroachment selects for specific leaf traits in such environments. We also evaluated if species occurring in distinct encroached situations would show plasticity to deal with light variations. We studied two species exclusively found in typical savanna (TS, open condition), two species exclusively found in forested savanna (FS, encroached condition) and two species growing along a gradient of tree encroachment (typical, dense and forested savanna). We measured specific leaf area (SLA), maximum photosynthetic rate in an area basis (Amax), stomatal conductance (gs), water use efficiency (WUE), leaf carbon (C) and nitrogen (N) concentrations. We found that herbaceous species exclusively found in TS possess higher Amax, gs, WUE and C in comparison with plants from forested savanna. Such strategies are necessary to thrive under environments with elevated irradiances. In turn, species from FS showed elevated SLA and foliar N concentration, strategies linked to capture diffuse light in forested environments. Species capable of thriving in sites with distinct degrees of encroachment changed their leaf traits according with light availability. We conclude that differences in leaf traits between typical and forested savanna species may explain the non-occurrence of typical savanna species when their environment become encroached. Only those species capable of showing a certain degree of plasticity may survive under such distinct encroached states.