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Is shade tolerance a fixed species trait? Evidence for shifting tolerance with herbivory.
Collins, Rachel1, Carson, Walter1, 1
ABSTRACT- Successional models for eastern deciduous forests use species specific shade tolerances (i.e. the ability to survive in low light conditions) to predict forest dynamics. These models assume that species specific shade tolerances are fixed across biotic gradients. To date, few empirical studies have examined this assumption. Here, we test the hypothesis that shade tolerance rankings do not change with biotic context. To that end, we have experimentally manipulated fire, browsing by deer and gap formation in 64, 400m2 plots where we are following survival, growth and recruitment of 17,000 permanently marked individuals. Shade tolerance theory predicts that under an intact canopy shade tolerant species will have higher survival than intolerant species. Indeed, Fagus grandifolia, a very shade tolerant species, had consistently high survival (70-80%) across all non-fire treatments. However, Betula lenta, which is intermediate in shade tolerance, was ranked last in shade tolerance (0% survival) in controls but jumped to second in the shade tolerance ranking (70% survival) in the absence of herbivory. Acer saccharum, a very shade tolerant species, consistently ranked among the poorest survivors (0-30%) across treatments. Hence models based solely on shade tolerance may fail to predict forest dynamics, or may succeed but be based on the wrong mechanism (i.e. shade tolerance and not herbivore tolerance). Taken together, our results suggest that some metrics of shade tolerance vary based on biotic conditions, in particular, levels of herbivory.
KEY WORDS: shade tolerance, succession, fire, herbivory