It's not the best time to be a tropical tree, as rising average temperatures risk impacting their lifespan.
A new study explains that the longevity of trees at the tropics is shortened by higher temperatures. The findings help further our understanding of how climate change will impact ecosystems in the area and its effects on the rest of the planet. The team argues this is the first direct evidence that tropical trees experience shorter lives in hotter environments, and that forests all around the world will be affected.
"Many regions in the tropics are heating up particularly rapidly and substantial areas will become warmer, on average, than approximately 25 °C," says Professor Manuel Gloor at the University of Leeds, a co-author of the paper.
"Our findings - which are the first to demonstrate that there is a temperature threshold - suggests that for trees in these regions, their longevity is likely to be negatively affected."
The temperature above which trees become affected is 25 °C, the paper explains. This result is based on four years' worth of tree ring data recovered worldwide. Roughly 100,000 trees from 400 species in 3,000 sites across the planet formed the dataset. All in all, the team reports that although tropical trees grow twice as fast as those in cold areas, they also live shorter lives (186 years vs 322 years on average).
Average temperatures in tropical forests today sit between 21 °C and 30 °C depending on location. These averages will rise alongside the rest of the world to around 2.5 °C above pre-industrial levels over the next 10 to 20 years. The effect this will have on trees varies depending on exactly how much hotter it gets. Changes in precipitation patterns (another effect of climate change) are going to exacerbate this ever further.
Substantial areas of today's rainforests will see significantly lower tree longevity. They only cover 7% of the Earth's surface, but harbor around 50% of its species of plants and animals, and a corresponding 50% of the planet's carbon stocks. Any change here will have strong, global echoes for habitats, air quality, and carbon scrubbing ability.
"These results are a warning sign that, along with deforestation, global warming adds extra stress on the Earth's tropical forests," says Dr Roel Brienen from Leeds, paper co-author.
"If tropical trees die earlier, this will affect how much carbon these forests can hold, raising concerns about the future potential of forests to offset CO2 emissions from fossil fuel burning. It could also cause changes in biodiversity and a decrease in the number of species on the planet."
Tropical forests in South America are closest to this threshold, but they're not the only ones at risk. Even the Congo Forest in west Africa, the world's second largest but with lower average temperatures, will be affected.
The saddest finding here, in the words of co-author Marcos Buckeridge, Director of the Biosciences Institute of the University of São Paulo, is that it's "unavoidable". It's too late to stop average temperatures from passing this threshold "even if we were to take drastic emissions reductions measures".
The paper "Global tree-ring analysis reveals rapid decrease in tropical tree longevity with temperature" has been published in the journal Proceedings of the National Academy of Sciences.