PNNL

Abstract

Tropical forests play a disproportionately large role in the regulation of the Earth’s climate due to their influence over the carbon, water, and energy cycles (e.g. Pan et al., 2011). Questions exist regarding the future contribution of tropical forests to climate regulation, however, due to uncertainties surrounding both chronic and discrete changes in the environment and their subsequent effects on the terrestrial system (Friedlingstein et al., 2006; Friend et al., 2014). Among the most spatially and temporally pervasive threats to the structure and function of tropical forests are those of stronger, more frequent droughts, atmospheric warming, and their combination (Davidson et al., 2012). There is already evidence of these changes. Two ‘100-year’ droughts occurred in theAmazon in 2005 and 2010 (Lewis et al., 2011), and climate models consistently suggest that the length of the dry season will increase (reviewed by McDowell et al., in this issue of New Phytologist, pp. 851–869). Simultaneously, the atmospheric demand for water (vapor pressure deficit; VPD) increases with temperature (Trenberth et al., 2014), placing greater stress on the hydraulic supply capacity of plants (McDowell & Allen, 2015). Finally, a lengthening of the dry season and the warming of the atmosphere are superimposed over the El Ni~no Southern Oscillation (ENSO) pattern – such as the 2015–2016 ENSO– which creates drier-thannormal conditions throughout much of the tropics (IPCC, 2014). Thus, there is a confluence of large predictive uncertainty with the high-importance of the future of the tropical carbon cycle.