The Ecophysiology of Leaf Lifespan in Tropical Forests: Adaptive and Plastic Responses to Environmental Heterogeneity
Leaf lifespan, the time from leaf expansion to shedding, exhibits wide variation and is a key integrator of relationships with photosynthetic rate, leaf mass per area (LMA), and leaf nitrogen among coexisting tropical tree species. We present a hierarchical view of sources of variation in leaf lifespan in tropical forests, emphasizing the importance of substantial within-species variation, which has rarely been addressed. Interspecific variation in leaf lifespan is positively correlated with LMA, varying from short-lived, low-LMA leaves to long-lived, high-LMA leaves of species associated with resource-rich versus resource-depleted habitats, respectively. Phenotypic responses of leaf lifespan and LMA to light show counter-gradient variation: with acclimation to shade, leaf lifespan increases, and LMA decreases, but both increase with adaptation to shade. In contrast, phenotypic responses to soil fertility are predicted to show co-gradient variation: both leaf lifespan and LMA increase with declining fertility both inter- and intraspecifically. We present new data analyses supporting these predictions, but the interactive effects of light and soil resources can produce complex phenotypic responses. Future studies of leaf lifespan should devote more attention to within-species variation to better quantify and explain how leaf lifespan is central to trade-offs generating the contrasting ecological strategies of tropical tree species.