Leaf quality, rather than leaf abundance, drives seasonal fluxes of carbon dioxide in tropical regions, a new study reveals. These findings may help explain why previous observations of forest carbon cycles have been so disparate. To date, models of seasonal forest-atmosphere interactions in tropical regions have assumed that lower precipitation means less water for plants and thus less photosynthesis and carbon uptake. However, by monitoring the photosynthesis capacity of plants across four sites in the Amazon, Jin Wu et al. have shown that plant productivity remains constant, or even increases, in the dry season. Increasing leaf losses (litterfall) during dry seasons were more than compensated by coinciding increases in new leaf production. These changes not only result in an increase in overall leaf coverage, but also causes a shift in the age composition of canopies toward younger leaves, which should have better photosynthesis quality than the older leaves they replace, the authors note. Thus, they created a "leaf demography-ontogeny model," which partitions leaves based on their age class and stage of development, rather than their overall surface area. Using these parameters to analyze one of the driest sites in this study revealed a correlation between plant productivity and the maturation of leaves, which was evident in a second, wetter site as well. Further analysis measuring the sensitivity of these data suggests that varying leaf quality could explain about twice as much seasonal variation in ecosystem productivity than leaf quantity alone.
