Non-linear effects of drought under shade: reconciling physiological and ecological models in plant communities

Abstract

The combined effects of shade and drought on plant performance and the implications for species interactions are highly debated in plant ecology. Empirical evidence for positive and negative effects of shade on the performance of plants under dry conditions supports two contrasting theoretical models about the role of shade under dry conditions: the trade-off and the facilitation hypotheses. We performed a meta-analysis of field and greenhouse studies evaluating the effects of drought at two or more irradiance levels on nine response variables describing plant physiological condition, growth, and survival. We explored differences in plant response across plant functional types, ecosystem types and methodological approaches. The data were best fit using quadratic models indicating a humped-back shape response to drought along an irradiance gradient for survival, whole plant biomass, maximum photosynthetic capacity, stomatal conductance and maximal photochemical efficiency. Drought effects were ameliorated at intermediate irradiance, becoming more severe at higher or lower light levels. This general pattern was maintained when controlling for potential variations in the strength of the drought treatment among light levels. Our quantitative meta-analysis indicates that dense shade ameliorates drought especially among drought-intolerant and shade-tolerant species. Wet tropical species showed larger negative effects of drought with increasing irradiance than semiarid and cold temperate species. Non-linear responses to irradiance were stronger under field conditions than under controlled greenhouse conditions. Non-linear responses to drought along the irradiance gradient reconciliate opposing views in plant ecology, indicating that facilitation is more likely within certain range of environmental conditions, fading under deep shade, especially for drought-tolerant species.

Fig. 1

Mean drought effect size (lnOR for survival, lnRR for continuous variables) along irradiance gradients for a survival, b biomass, c relative growth rate (RGR), d root:shoot ratio, e specific leaf area (SLA), f maximum net photosynthetic rate (A _max), g stomatal conductance (gs), h light compensation point (LCP), and i maximum photochemical efficiency of photosystem II (F_v/F_m). Regression lines are shown for significant models only. Dotted lines indicate zero effect sizes

Fig. 2

Comparison of drought effects on survival along the light gradient between groups of studies that differed in a their approach (field vs. greenhouse) and b species shade tolerance (intolerant vs. tolerant). Only those grouping variables for which differences among groups were detected are shown (see Appendix S11). Dotted lines indicate zero effect sizes

Fig. 3

Comparison of drought effects on whole-plant biomass along the light gradient between groups of studies that differed in a type of ecosystem (semiarid vs. cold temperate vs. wet tropical), b species drought tolerance (intolerant vs. tolerant), and c species shade tolerance (intolerant vs. tolerant). Only those grouping variables for which differences among groups were detected are shown (see Appendix S11). Dotted lines indicate zero effect sizes

Fig. 4

Comparison of drought effects on photosynthetic rate (A _max) along the light gradient between groups of studies that differed in a their approach (field vs. greenhouse), b type of ecosystem (semiarid vs. cold temperate), c species leaf habit (evergreen vs. winter deciduous), and d species shade tolerance (intolerant vs. tolerant). Only those grouping variables for which differences among groups were detected are shown (see Appendix S11). Dotted lines indicate zero effect sizes

Fig. 5

Comparison of drought effects on stomatal conductance (gs) along the light gradient between groups of studies that differed in a their approach (field vs. greenhouse), b species leaf habit (evergreen vs. deciduous), and c species shade tolerance (intolerant vs. tolerant). Only those grouping variables for which differences among groups were detected are shown (see Appendix S11). Dotted lines indicate zero effect sizes