El Niño impacts in the Australian tropics
Associate Professor Susan Laurance, James Cook University, Australia
Insights from long-term drought experiments in the tropics
Professor Patrick Meir, Australian National University, Australia and University of Edinburgh, UK
El Niño events tend to impact Amazonian forest with positive temperature anomalies and strong negative rainfall anomalies. The net effect is to impose drought and heat stress on the forest. The frequency of strong drought events in parts of Amazonia may increase in the future, and indeed elsewhere across the global tropics, with potentially large impacts on climate services in relation to carbon storage and transpiration, and in relation to tree mortality and biodiversity. Understanding and predicting how tropical rainforest responds to drought extremes requires field measurements to provide information about the underlying mechanisms. This information can be used to enable advances in predictive land-surface models. These data have historically been vary sparse. Rainfall manipulation treatments provide unique scientific leverage by experimentally imposing a drought treatment, reducing soil moisture availability. We report on results from the world’s only long-term, ecosystem-scale drought experiment in tropical rainforest, led through a 20 year collaboration between UK and Brazilian scientists. We consider evidence from the effects of short and long term soil moisture reduction (equivalent to a 50% reduction in rainfall over >15 years). We address uncertainty in how resistant eastern Amazon rainforest is to drought, and whether we can expect recovery from drought impacts. We consider how the evidence from this experiment can inform understanding of short-term El Niño impacts on vegetation and climate, and over the longer term, as drought stress may increase in both punctuated and secular patterns, partly related to a potential increase in the frequency of El Niño events.
The role of drought in modulating forest transpiration and altering water cycling in tropical forests
Dr Lucy Rowland, University of Exeter, UK
Transpiration from the Amazon rainforest generates an essential water source at a global and local scale. However, changes in rainforest function during drought events can disrupt this process, causing significant reductions in precipitation across Amazonia, and potentially at a global scale. Using a long-term (>10 year) experimental drought treatment in Amazonian forest as a case study we explore how drought events can alter transpiration and forest water cycling in tropical rainforests. We find that after 15 years of receiving half the normal rainfall forest transpiration decreased by 30%, driven mostly by drought-induced tree mortality. The droughted trees which survived, actually maintained or increased transpiration despite the continued drought, because of reduced competition for water and increased light availability, which is consistent with increased growth rates. Consequently, the amount of water supplied as rainfall reaching the soil and directly recycled as transpiration increased to 100%. This value was 25% greater than for adjacent non-droughted forest. If these drought conditions were accompanied by even a modest increase in temperature and VPD, such as those which typically occur during El Niño events water demand would dramatically exceed supply, making the forest more prone to increased tree mortality. Considering future predictions of drying climates across many tropical regions, the combined effects of long-term changes in climate and El Niño style drought events on water cycling must be considered when making future predictions of tropical forest mortality.
The ecophysiological response of a Bornean rainforest to El Niño drought
Dr Terhi Riutta , University of Oxford and Imperial College London, UK
This presentation assesses the response of a South East Asian moist tropical forest to the 2016-2017El Niño drought, at the tree and forest ecosystem scale. On a typical year, the region does not have clear dry and wet seasons, but experiences high levels of rainfall throughout the year. Thus, occasional but severe droughts may cause significant changes in the functioning these forests. We monitored the tree physiology and forest ecosystem carbon dynamics in an intensive carbon plot and with an eddy covariance tower at the SAFE Project site in Sabah, Malaysian Borneo. We observed a clear decline in above-ground woody productivity and sap flow rates during the peak drought. The ecosystem gross primary productivity showed a marked decrease during the drought, as a result of a decrease in both leaf-level photosynthesis and ecosystem-level leaf area index. Ecosystem respiration decreased, mainly due to reduced autotrophic respiration, while heterotrophic respiration was less sensitive. The ecosystem was resilient, with the processes returning to pre-drought levels in two to six months post-drought. Intensive monitoring of multiple processes at different spatial scales increases our understanding on the forest ecosystem functioning during water stress.