PNNL

Abstract

Tropical forests represent the warmest and wettest of Earth’s biomes, but with additional anthropogenic warming these ecosystems will be pushed into climate states that have no current analog. We are likely on a path to create a new climate – the hypertropics – that has been absent on Earth for millions of years. Droughts are already becoming more intense in the tropics as they occur at successively higher temperatures as the planet continues to warm. Here we synthesize and integrate across numerous datasets to explore the impacts of a series of droughts on a Central Amazon forest. First, a >30 year record of annually-resolved forest demographics data from a selective logging experiment demonstrated higher tree mortality rates during intense droughts, particularly among fast growing pioneer tree species with low wood density. Second, analysis of ecophysiological field measurements for the intense 2015 and 2023 El Niño droughts identified a soil moisture content threshold beyond which transpiration rates rapidly declined. As rainless days past this threshold continued, drought conditions intensified, increasing the risk of hydraulic failure, and driving higher tree mortality rates. Third, data analyses from a suite of Earth system climate models in the Coupled Model Intercomparison Project Phase 6 (CMIP6) demonstrated that a large area of tropical forest will shift to a hotter, no-analog state by mid-century. Lastly, under a hypertropical forest climate, temperature and moisture conditions during typical annual dry season months will more frequently pass identified drought-mortality thresholds, increasing the risk of forest die-back. Present-day hot droughts are harbingers of this emerging climate, providing windows of opportunity to better understand tropical forest responses to increasingly extreme future conditions.