Repertoire of traits in the sapling of a dwarf Mediterranean shrub confers withstanding the combined stress of drought and shade

Abstract

ABSTRACTThe ability to cope with combinations of stresses is crucial for the establishment and development of young plants and determines processes of population dynamics. Seedlings and saplings of Mediterranean woody species in woody ecosystems often experience both drought and shade stresses. Our study aims at understanding the effect of the combined stresses of drought and shade on young sapling survival and development. We tested three alternative hypotheses: (1) Drought decreases plant performance in shade. (2) The impact of drought is lessened in shade or shade may even ameliorate plant performance under drought. (3) Plant response to shade is independent of plant response to water availability. The first and second hypotheses imply an interaction between drought and shade, while the third implies an orthogonal impact of drought and shade. We also ask what are the functional morphological traits that are involved in tolerating shade and drought stresses. We tested the hypotheses and questions focusing on Sarcopoterium spinosum as a model species in a fully factorial container experiment with three shading levels and two levels of water availability. The results demonstrate that moderate shading (50%) ameliorates seedling performance (measured by final biomass) under dry conditions. In contrast, when water was available, shading had a monotonic negative effect on seedling performance. Conversely, shade and drought had an independent effect on biomass partitioning and under drought – allocation to roots increased, while under shade allocation to leaves increased. The dry conditions did not cause a reduction in shade tolerance, and both shade avoidance and tolerance morphology were evidenced under moderate (50%) and deep (70%) shading (stem elongation, increased leaf area). Deep rooting – a drought-avoidance trait – was similar in high and low water availability. Our overall results support the hypothesis that shade ameliorates drought stress. Moreover, S. spinosum demonstrates that woody species inhabiting water-limited ecosystems may have a repertoire of traits that allows simultaneous tolerance to both drought and shade.