Tropical Dry Forest Response to Climate Variability

Primary and secondary Tropical Dry Forests (TDF) still cover extensive portions of the tropics, but are largely underrepresented in the networks of sites where the role of terrestrial vegetation in the sequestration of CO2 is being studied around the world. Estimates of biomass increments and CO2 emissions to the atmosphere during conversion to other land uses suggest that TDF are sites of substantial CO2 exchange, but its actual magnitude is unknown. In order to learn about the environmental variables that regulate the daily to annual flux of CO2, water and energy in these markedly seasonal systems, we installed an eddy covariance system above a pristine TDF in the Chamela-Cuixmala Biosphere Reserve, in the coast of Jalisco, Mexico. The occurrence of a strong El Niño event in 2009 and 2010 has allowed us to compare the flux of CO2 (Fc) under the influence of such environmental phenomenon vs. a normal year. During the normal water year of 2007-2008, the wet-season (June to October) rainfall in the region of Chamela totaled 1032 mm, with no rain occurring in the dry season. In contrast, El Niño of 2009-2010 was associated with a decreased precipitation (551 mm) during the wet season, but an increased dry-season (November to May) rainfall (230 mm). The ecosystem showed a quick response to this change in water availability, with most trees showing a second leafing event in January and February 2010, after having shed almost all leaves in November and December 2009. Accordingly, the CO2 uptake for the period of November 2009 to March 2010 was more than double the magnitude of that recorded for the same period during the dry season of 2007-2008 (Fc=-228 and -530 g CO2 m-2, respectively). It was still smaller than the Fc for the wet-season period of June-October 09 (-641.3 g CO2 m-2). Our results constitute a first accounting of the CO2-sequestration potential in a Mexican TDF and its response to cyclical and transient weather phenomena, which will ultimately enable us to predict the future of TDF under climate change scenarios.