The effect of variation of rainfall across different time scales on subcanopy CO2 storage and flux in a Costa Rican rainforest

Precipitation intensity, amounts and timing regimes in the tropics can vary greatly over time – the classic “wet season-dry season” distinction is only one important example of rainfall variability. Vegetation response to these changes in hydrologic input can have significant impacts on ecosystem cycling rates and storage pools. We present here results on CO₂ storage and fluxes from a mature tropical rainforest site in Costa Rica, taken during the transition from an abnormally dry season to an abnormally wet season.

During 2014, the site underwent an atypical drought over the course of 4 months, followed by a rapid transition to an abnormally wet period. In 2014, precipitation during the dry season (Days 24-98) was 85% lower than in 2013, while it nearly doubled for the first part of the wet season (Days 99-192). This atypical set of precipitation regimes allowed a clear distinction of the effects of the hydrologic forcing on ecosystem behavior with respect to CO₂ and H₂O land-atmosphere fluxes. Notable differences in ecophysiological drivers were detected between the dry and the wet season. Average photosynthetically active radiation decreased by 35%; however, seasonal differences were only apparent in the top 10 m of the canopy. Average leaf wetness increased by 55%, a trend that was consistent throughout the canopy profile.

While variations in these drivers resulted in detectable differences in average subcanopy water vapor concentrations (+7%), carbon dioxide concentrations remained nearly identical (-0.4%). This finding implies that in this system assimilation and evapotranspiration may be decoupled.