February 16, 2018
A Challenging Future for Tropical Forests
Tree death shuts off photosynthesis, increases carbon release (from dead wood), leaving more carbon dioxide in the atmosphere.
Moist tropical forests are the largest terrestrial carbon sink in the world and house most of Earth’s terrestrial biodiversity. However, intact moist tropical forests face the threat of increasing tree mortality due to environmental and biotic changes. Researchers determined the risks of increasing tree mortality. They created a conceptual framework. The framework links the drivers, mechanisms, and interactions that may underlie rising mortality rates of moist tropical forests. The team identified the next steps to refine the framework.
Mortality rates of trees in moist tropical forests are increasing. Why? The drivers and mechanisms of tree mortality—such as temperature, drought, and carbon dioxide—continue to rise. These effects are expected to continue increasing under future environmental conditions, with serious consequences to Earth’s carbon cycle.
Tropical forests absorb a significant amount of atmospheric carbon dioxide. Tree death reverses this process by shutting off photosynthesis and increasing carbon release (from dead wood), leaving more carbon dioxide in the atmosphere. Increasing tree mortality rates observed over the past few decades in moist tropical forests are associated with rising temperature, vapor pressure deficit, liana (woody vine) abundance, drought, wind events, fire, and possibly carbon dioxide fertilization-induced increases in stand thinning. Most of these mortality drivers ultimately kill trees in part through carbon starvation and hydraulic failure, though the relative importance of each driver is unknown. Ecosystems with greater diversity may buffer tropical forests against large-scale mortality events, but recent and expected trends in mortality drivers are likely to continue or increase. Model predictions of tropical tree mortality are rapidly improving, but they require more empirical knowledge regarding the most common drivers and their subsequent mechanisms. This study identified critical hypotheses, data sets, and model developments required to quantify the underlying causes of increasing mortality rates and to improve predictions of future mortality and carbon storage consequences under environmental change.
Pacific Northwest National Laboratory
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
This manuscript is the product of the workshop “Tropical forest mortality” held in Santa Fe, New Mexico, in 2015. The Department of Energy Office of Science supported the workshop and the writing of the manuscript as part of the Next Generation Ecosystem Experiment-Tropics project.
N. G. McDowell, C. D. Allen, K. Anderson-Teixerira, and P. Brando, et al. "Drivers and mechanisms of tree mortality in moist tropical forests." New Phytologist 219 (3), 851 (2018). https://doi.org/10.1111/nph.15027.