Untangling intraspecific and interspecific changes in leaf nitrogen across nitrogen availability gradients in forests

Background/Question/Methods

We present existing data from across a forested landscape of increasing soil nitrogen availability in which intraspecific leaf nitrogen increases, but community composition shifts to more shade tolerant, low leaf nitrogen species. Combined, these two contrary effects cause community-weighted mean leaf nitrogen to change little, despite a large change in soil nitrogen availability. 

Results/Conclusions

Using a simple, game-theoretic, physiologically-based community model of height-structured light competition for late-successional tree species, we consider two hypotheses to explain these patterns: (1) species are defined by leaf nitrogen per se or (2) species are defined by the rate at which photosynthesis saturates at high leaf nitrogen. Under either hypothesis, the understory stage’s low-light requirements largely determine traits at the canopy stage. However, only the second hypothesis simultaneously predicts the patterns described above and explains the physiological mechanisms by which more- and less-shade tolerant species differentially optimize canopy leaf nitrogen on a common soil.