Impact of the 2015-2016 El Niño on tropical forests

Tropical forests comprise an important long-term carbon sink. Loss of carbon and biomass following severe droughts across Borneo in 1997-98 and Amazonia in 2005 and 2010 suggest this sink is sensitive to drought, but recent evidence also identifies a shift in species composition to more drought tolerant species and thus the exact climate sensitivity of the tropical carbon sink remains unclear. Uncertainty in how the world’s most biodiverse and productive ecosystems will respond to increasing temperatures and altered precipitation is a major limitation for predicting future climate. The El Niño Southern Oscillation causes anomalies of high temperature and drought in tropical regions and the responses of tropical forests to these climate anomalies gives us an insight into whether and how key carbon pools, dynamics and species might endure the hotter and periodically drier conditions predicted for the end of the century. Therefore, the record hot and dry climate caused by the 2015-16 El Niño offers a novel opportunity to understand the response of the tropical sink to temperature and drought as it was the first very strong El Niño to be captured by the African Tropical Rainforest Observation Network (AfriTRON) and the Amazon Forest Inventory Network (RAINFOR) that monitor structurally intact forests. This thesis combines climate data with measurements from long-term forest monitoring plots across tropical Africa (100 plots) and South America (137 plots) over the El Niño climate anomaly to investigate the impacts of the unprecedented temperatures and droughts on tropical forests. For the first time, the resistance of intact African tropical forests to short-term drought is assessed and despite record temperatures and drought, the El Niño was insufficient to reverse the longstanding biomass carbon sink which reduced but was still a sink of 0.52 ± 0.20 Mg C per hectare per year in the El Niño. South American tropical forests were a carbon sink prior to the El Niño and became a small carbon source of -0.1 ± 0.40 Mg C per hectare per year in the El Niño. The negative responses of African and South American tropical forests to high temperatures were similar, but the negative impacts of drought differed between the continents’ forests. Overall, the results indicate some resistance to environmental change in the short-term and potential resilience in the long-term in African forests, although hotter drier forests in South America are vulnerable to carbon sink reversal. The results in this thesis provide evidence that much of the world’s intact tropical forests have the capacity to resist climate anomalies only if conservation efforts succeed in keeping forests intact and global temperature increases are limited.