Intact tropical forests have a crucial role in mitigating climate change as they store and sequester substantial amounts of carbon. However, the impact of climate-driven changes on the carbon balance of these forests remains poorly understood and a significant source of uncertainty in climate models. South America, with its vast and diverse tropical forests, is especially important in the context of climate change. The record heat and drought of the 2015-2016 El Niño event provided a unique opportunity to study the impacts of extreme climate anomalies on a network of South American forests that have been monitored since the 1980s. In this blog post, I delve into our article which sheds light on the vulnerability of these forests and the implications for climate change.

Sensitivity of South American Tropical Forests to Climate Anomalies

Our study investigated the impact of the 2015-2016 El Niño on South American tropical forests using data from 123 long-term monitoring plots. These structurally intact forest plots in Amazonian and Atlantic forests included areas transitional with dry forests and Cerrado. During the El Niño event, most forests experienced higher temperatures and increased drought stress. The long-term biomass carbon sink, or carbon absorption by forests, halted during the El Niño. 

Sensitivity to Temperature and Drought

We found that higher temperature anomalies led to increased carbon losses through tree mortality, so more trees died where it was hotter. On the other hand, stronger drought anomalies not only increased carbon losses but also reduced carbon gains from tree growth and recruitment, further impacting aboveground biomass carbon. 

Vulnerability of Drier Forests

We reveal that drier forests were more vulnerable to the El Niño event. Forests in regions with a drier long-term climate experienced more substantial carbon losses from mortality and reduced carbon gains during the period of extreme climate. This suggests that pre-adaptation to seasonal climates might not confer resistance to novel climate extremes.

Increased Tree Mortality

The El Niño event increased tree mortality rates for all tree sizes, but the relative risk differed across size classes. Larger and medium-sized trees were more affected than smaller trees, indicating that larger trees faced greater vulnerability due to increased hydraulic stress.

Implications for the Carbon Sink

While South American forests were a carbon sink before the El Niño, this sink ceased during the event. Records show that the impact of the 2015-2016 El Niño on forest carbon dynamics was not stronger than previous droughts in the region.

Conclusion

Our analysis of the 2015-2016 El Niño event offers valuable insights into the vulnerability of South American tropical forests to extreme climate anomalies. Drier forests, perhaps contrary to expectations, appear to be more susceptible to climate change impacts. Our findings emphasise the significance of intact tropical forests in mitigating climate change as their conservation is a key defence against future climate challenges. Understanding the responses of tropical forests to climate extremes is critical for predicting and preparing for the potential consequences of climate change on a global scale. Further research and continued monitoring of these forests are essential for developing effective strategies for climate change mitigation and adaptation.