Non-linear effects of drought under shade

reconciling physiological and ecological models in plant communities. Oecologia (2012).

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.

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