A common view in ecology is that light is often a limiting resource for plants, and competition for scarce light shapes the fate of plant species. While this is true in many settings—for example, in the understory of a dense forest—plant physiologists have also shown that, as other factors begin to limit photosynthesis, plants may absorb more light than they can use, especially under otherwise stressful conditions. Chronic exposure to excess light can cause photoinhibition, a long-lasting decline in photosynthetic efficiency that, when severe enough, can reduce carbon uptake. This phenomenon creates the potential for some species to facilitate others through shading.
Here, we estimated these light-mediated interactions using physiological measurements and woody biomass surveys in the early-successional Forests and Biodiversity (FAB) experiment at Cedar Creek LTER (East Bethel, MN). We aimed to see if fast-growing conifer species can facilitate the carbon uptake of slow-growing broadleaf species. We measured photosynthetic light-response curves of broadleaf trees in monoculture (full sun), in a biculture with fast-growing conifers (shade), and in twelve-species plots (intermediate). We also assessed the severity of photoinhibition using chlorophyll fluorescence, and measured concentrations of photoprotective xanthophyll pigments.
Results/Conclusions
We found that three out of four broadleaf species tested were photoinhibited in monoculture. The exception, Betula papyrifera, is an early-successional species adapted to high light levels. In one late-successional species (Tilia americana), photoinhibition in full sun was so strong that it reduced net carbon assimilation by up to 25%. We observed a corresponding decrease in woody biomass in T. americana in less dense plots. The other species, Quercus ellipsoidalis and Acer rubrum, were photoinhibited in full sun, but not enough to decrease net carbon assimilation. All species showed greater allocation to photoprotective pigments and more steeply inclined leaf angles at higher light levels, indicating a greater need to avoid or tolerate excess light. Finally, we found that in T. americana, leaf abscission in the fall occurred an average of one month earlier in full sun, reducing the period for carbon gain.
We show that plant species can facilitate each other by ameliorating light stress. In the context of the literature on plant biodiversity and ecosystem function, these results provide a plausible explanation for how positive interactions in diverse communities might arise. Our results demonstrate that physiological measurements can often provide a mechanistic basis to explain species interactions.