Phenology of an evergreen moist tropical forest: Shifts in leaf demography and physiology during the dry season of the Tapajós National Forest, Brazil

Background/Question/Methods

Tropical forests are a significant component of the earth’s carbon budget, yet how these forests respond to seasonal changes in the environment, and the extent to which tree biology synchronizes with seasonal cycles, are poorly understood.  Most global vegetation models assume no seasonality in LAI (leaf area index) or photosynthetic parameters in tropical evergreen forests, yet gross primary productivity estimated from the eddy covariance method shows clear seasonal patterns for a number of seasonally dry forests in the Amazon basin.  In the Tapajós National Forest, our study site, eddy covariance data suggest that the ecosystem photosynthesis increases throughout the dry season.  Hypothesized drivers for this trend include the following: 1) increases in photosynthesis due to environmental changes, 2) an increase in LAI throughout the dry season, and 3) an increase in leaf-level photosynthetic capacity, perhaps associated with shifts in leaf demography. In this study, we investigated shifts in leaf demography and leaf-level photosynthesis during the dry season for trees belonging to abundant species in the Tapajós National Forest.  We divided leaves into age classes and counted the number of leaves in each age class for one-meter branches collected from the canopy using arborist rope-access techniques.

Results/Conclusions

The maximum carboxylation rate, V_cmax, is higher for leaves that matured during the current dry season than ‘old’ leaves from previous flushing events.  Leaf demographic composition appears to shift towards younger cohorts during the dry season, a consequence of the flushing of new leaves and the dropping of old leaves; for instance, analysis of leaf demography of an Erisma uncinatum, the most abundant emergent tree species at our site, showed significantly more recently matured leaves, and significantly fewer old leaves, late in the dry season (after mid-October) than early in the dry season (prior to mid-September).  These results suggest that, in combination, the shifts in leaf demography and leaf physiology are important drivers of increasing gross primary productivity during the dry season in the Tapajós National Forest.