Tropical forests can handle the heat, up to a point

The world’s tropical forests store a quarter-century worth of fossil fuel emissions in their trees alone. There are fears that global heating can reduce this store if tree growth reduces or tree death increases, accelerating climate change.

An international research team, including four researchers from Wageningen University & Research, measured over half a million trees in 813 forests across the tropics to assess how much carbon is stored by forests growing under different climatic conditions today. The team reveals that the amount of carbon stored and sequestered by tropical forests is reduced in warmer forests, especially beyond a threshold of 32 degrees Celsius. Forests release carbon dioxide into the atmosphere when the amount of carbon gained by tree growth is less than that lost through tree mortality and decay.

Time to adapt

Projections of changes in forest carbon storage under an expected 2 degrees Celsius warming, suggest substantial carbon losses, particularly in the Amazon region. Yet, the World’s tropical forests can continue to store substantial amounts of carbon under climate change if they have time to adapt, if they remain intact, and if global heating is limited. Pieter Zuidema, co-author of the study: “Our study issues a warning that tropical forests may lose their role as carbon sinks in a warmer world. This situation may be worsened if forests also experience more frequent and severe droughts.”

Biodiversity

Using repeated tree measurements in 24 countries, the study suggests temperature has the greatest effect on forest carbon stocks by reducing growth, followed by drought, which reduces tree survival. The researchers conclude that tropical forests have long-term capacity to adapt to some climate change, in part because of their high biodiversity, as tree species better able to tolerate new climatic conditions grow well and replace less well-adapted species. Eric Arets, co-author of the study, adds: “Such adaptation is only possible if tree seeds can move through landscapes and forests are connected. Fragmentation hampers the ability to withstand global warming.”

Tree-ring analyses

The negative effect of heat and drought on carbon stocks in tropical forests may partially be mitigated by the stimulatory effect of CO₂ on tree growth. In another study (Global Change Biology), Pieter Zuidema and colleagues use measurements of >5000 tree rings to evaluate whether recent CO₂ rise has allowed trees to grow faster during hot or dry years. For warm Asian forests (>25 degrees Celsius), they found the opposite: CO₂ rise led to stronger heat and drought stress. In cooler forests, stimulatory effects were found. Pieter Zuidema: “Together, these studies suggest that the future role of tropical forests in the global climate system will be determined by their response to heat, rather than to CO₂ rise.”