A tipping point for tropical tree longevity (Keynote presentation)
How old are tropical trees? This fundamental question has long driven the curiosity of laymen and scientists. But only recently, a great number of studies conducted by many brave dendrochronologists resulted in a significant tree-ring-based knowledge that allows us to start accurately estimating tree ages across the globe. As science goes, not only knowing the longevity of tropical trees is essential to understanding forest dynamics and its role in biogeochemical cycles, but one must also know how it compares to the extratropics and what drives the variability in tropical tree longevity and growth rate. Our literature survey combined with the ITRDB datasets show that the longevity and growth rate of trees covary negatively across world biomes, and that tropical biomes hold trees that are on average half of the age of the trees outside the tropics while growing twice as fast. Within the tropics, the longevity of trees is higher in the wet than in the dry biomes challenging the common sense that trees tend to be older under water limiting conditions as long observed in temperate forests. Our Bayesian model shows that not only does longevity of tropical trees seems to be limited in the driest sites, but it is strongly limited by temperature above an estimated threshold of 25.4°C. About 37% of the whole tropics is already under the influence of temperatures above this threshold, and this area is expected to increase to 60% by 2050. These independent effects of water availability and temperature on tropical tree longevity are in accordance with observed changes in the dynamics of forests and carbon pools in the Amazon and Western Africa impacting the role of tropical vegetation in the carbon cycle.