Impacts of a strong El Niño event on leaf phenology and carbon dioxide exchange in a secondary dry dipterocarp forest

Abstract

This study evaluates the impacts of the 2015/2016 El Niño event on the phenology and carbon dioxide (CO2) exchange in a secondary dry dipterocarp forest (DDF). In western Thailand, El Niño events significantly reduced rainfall and increased air temperature, leading to prolonged dry season with low soil water content and high vapor pressure deficit (VPD). During 2014–2017, leaf phenology was monitored and CO2 exchanges in this forest were measured by an eddy covariance method. During El Niño year, dry dipterocarp species were shed their leaves earlier when compared to neutral years (i.e. with no El Niño event). The species Shorea siamensis Miq. and Sindora siamensis Teijsm.&Miq. showed complete deciduousness during the dry season 2015/2016 as typically observed for these species, but the leafless period was longer than during non-El Niño years. Shorea roxburghii G.Don and Shorea obtusa Wall. showed untypically complete deciduousness during the El Niño year. For the species Sindora siamensis, however, the timing of first leaf expansion was not affected by the strong El Niño. In addition, El Niño resulted in a decline of the annual gross primary productivity (GPP) and evapotranspiration (ET) by 9.6 and 14.1%, respectively. The net ecosystem productivity (NEP) was only affected by El Niño during the dry months, but not during the following wet season therefore the total annual NEP was not significantly affected. Our result demonstrates that El Niño events have a strong impact on canopy duration of DDF ecosystem. However, our study suggests that the carbon loss during this prolonged dry season of 2015/2016 can be compensated in this young secondary DDF during the following wet season, leading to increased NEP on a yearly basis.