Could leaf phenology help the wet season recommence in Amazonia?

Thursday, 9 June 2016
Loren Albert1, Jin Wu1, Neill Prohaska1, Marielle Smith1, Valeriy Yu Ivanov2, Plinio Barbosa de Camargo3, Raimundo Cosme Jr.4, Travis E Huxman5 and Scott R Saleska1, (1)University of Arizona, Tucson, AZ, United States, (2)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (3)University of Sao Paulo, Sao Paulo, United States, (4)EMBRAPA Brazilian Agricultural Research Corportation, Campinas, Brazil, (5)University of California, Irvine, Irvine, CA, United States
Abstract:
By transpiring water and taking up carbon, leaves act at the interface between the biosphere and atmosphere. Few studies have investigated how leaf age impacts transpiration activity in tropical trees, and whether leaf phenology could impact hydrology at the scale of tropical ecosystems.

We conducted a case study in the Tapajos National Forest KM67 site, near Santarém, Brazil, to investigate whether leaf aging and seasonal shifts in leaf demography could impact gross primary productivity or water fluxes during the dry season. In a series of fieldwork campaigns beginning in August 2012, we monitored leaf demographic composition (leaf age categories) from 1-m branches collected from 20 trees representing abundant species, and we assessed how photosynthesis and stomatal conductance varies with leaf age for a subset of these trees.

Under similar environmental conditions, we found that stomatal conductance reaches a peak when leaves are mature (fully expanded and green) and then declines in old leaves. For many trees, leaf demography shifted during the dry season such that recently matured leaves replaced old leaves. These results suggest that shifts in leaf demography together with the effects of leaf age on leaf physiology could increase late dry season transpiration more than would be expected from constant leaf phenology.

It has been proposed that that evapotranspiration during the dry season in Amazonia causes the wet season to commence, as surface latent heat flux causes a sequence of events that ultimately launch the wet season every year. This hypothesis relies upon an increase in evapotranspiration during the late dry season. Our results suggest that dynamic conductive activity of leaves, due to leaf phenology, could contribute to this dry season increase in evapotranspiration. Thus the timing of large-scale leaf phenology may play a critical role in hydrological cycles that allow the forest to persist.

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