From leaf to ecosystem level: controls of transpiration in seasonal tropical forests across a rainfall gradient

Abstract:

Evapotranspiration is a major component of the hydrological balance strongly mediated by vegetation. To better understand the role of environmental drivers in regulating gas exchange, the responses of plants to climatic variation and their effects on the hydrological cycle, we need high quality environmental and plant physiological measurements on a broad range of temporal and spatial scales, which span leaf to ecosystem levels and climate gradients.

Central American moist tropical forests are characterized by a distinct dry season and a rainfall gradient from the dry Pacific coast to the wet Atlantic coast. Interannual variability, caused by ENSO events, generally produces warm/dry conditions over this part of the tropics.

We take advantage of this climatic gradient and a strong ENSO event to assess how plant transpiration responds to environmental drivers in three experimental sites located in central Panama. We assess the impact of vegetation control on the hydrological cycle on a small experimental catchment which discharges into the Panama Canal.

Meteorological variables and soil moisture were monitored by automated stations. A microclimatic station equipped with an eddy covariance system measured energy and CO₂/H₂O exchanges. Plant level measurements included sapflow, canopy temperature and, through access with canopy cranes, leaf water potential and leaf gas exchange. Run-off was monitored on a 10ha catchment since 1972.

At ecosystem scale, water losses were approximatively constant across the year, consistent with low water supply and high atmospheric demand which characterized the dry seasons. Vegetation responded to climatic variation with structural and physiological adjustments. Species level transpiration exhibited interspecific variability, with different species response to environmental stimuli. Similarly, we observed a large phenological signal at the species level, with some species increasing in leaf area, while others became deciduous, and these effects tended to cancel at the ecosystem level. Across the rainfall gradient, large differences in canopy temperature were detected, with the dry site showing higher temperatures and larger diurnal fluctuations.

We show that vegetation play a central role in regulating water losses and the hydrological cycle of tropical forests.