The structure of whole plant communities emerges in part from local interactions, which in turn are determined by the physiology of the interacting species. For example, a common explanation for why diverse communities are more productive than monocultures is that diverse communities are better able to partition limiting resources that are crucial for physiological processes like photosynthesis. Another common explanation is that some species may relieve stress in other species, resulting in net facilitation. Both explanations are grounded in physiological mechanisms that explain variation in productivity.
Tree communities often have complex canopies that influence physiological processes at the leaf level. A crucial challenge for trees is to allocate scarce resources to optimize net carbon assimilation rates across their entire crown. The structure of the canopy influences the local light and climatic microenvironment, which may in turn affect optimal decisions about allocation.
We performed a series of measurements to investigate the physiological mechanisms underpinning positive diversity effects in the Forests and Biodiversity (FAB) experiment, a tree diversity experiment at Cedar Creek Ecosystem Science Reserve in Minnesota. In order to determine whether complementary uptake of nitrogen could enable greater productivity in this experiment, we measured both leaf area index (LAI) and leaf nitrogen content (LNC), allowing us to investigate how trees allocated nitrogen. We also used gas exchange and chlorophyll fluorescence to investigate how canopy environments affected light and climatic stress.
Results/Conclusions
We found that more diverse plots display an overyielding-like effect resulting in larger-than-expected total nitrogen pool sizes, calculated as a product of LAI and LNC on an area basis. Most of this variation was driven by LAI, especially in low light environments. Furthermore, high LNC enabled higher leaf-level assimilation, and high total nitrogen pool size caused higher plot-level relative growth rates.
Finally, chlorophyll fluorescence and gas exchange measurements show that for several species, photosynthetic light-response (A-I) curves are often hump-shaped. Coupled with evidence that growth rates for these species peak under partial shade produced by their neighbors, these data support the conclusion that species vying for shared resources may facilitate one another by relieving stress associated with high light exposure. This conclusion is also reinforced by the fact that some species display more vertical leaf angles in high-light environments, a plausible mechanism for light avoidance.
Together, these results suggest that leaf-level physiology must be understood in terms of the environment created by the canopy.