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News Article

Increasing tree mortality in moist tropical forests

Empirical and simulated data used to unravel drivers and mechanisms behind tree mortality

Globally, forests have a baseline mortality that is the result of numerous biotic and abiotic factors and which ensures a relatively constant level of carbon storage. As environmental conditions continue to change globally, the impact on tree mortality, especially in tropical regions will be of importance in understanding how the global carbon balance may shift.

Increasing mortality has been observed in some areas, including the amazon, but not others, with researchers at a loss as to why. In a bid to increase understanding, a recent study has used a mixture of empirical and simulated data to better understand the drivers and mechanisms responsible for this mortality rise in moist tropical forests. Moist tropical forests are defined as those which receive in excess of 1500mm of rain a year with only intact, non-harvested examples being used in the study to remove the mortality effects caused by human intervention. The geographical scope of the paper covered South America, Africa and Southeast Asia though some findings were more widely applicable.

The results were broadly grouped into those with a global relevance and those with only a regional impact. Temperatures are increasing globally and have already been linked to increasing mortality in multiple ways; through impacts on respiration and photosynthetic metabolism, both leading to carbon starvation, and through increasing vapour pressure deficit levels which impacts stomata closure and evaporative demand. A related driver often compounded by increased temperatures is drought; as a global effector drought can increase mortality through physiological stress and greater risk of biotic attack. It is often the largest trees which suffer greatest from drought, creating a disproportionate impact on carbon storage. The steady rise of global carbon dioxide levels will also have wide reaching impacts though these will not be quite so easy to predict; higher CO2 levels have the capacity to increase productivity and water use efficiency, therefore reducing mortality, however, increased productivity can lead to greater competition for resources and eventually successional thinning. Faster growth may also mean more trees reach a height whereby they are more vulnerable to abiotic damage such as wind.

Regionally specific drivers included lianas and fire. The former are considerably more common in tropical forests as opposed to temperate and whilst are known to reduce productivity and increase mortality in their hosts, quantitative studies of this impact are rare. Fire was significantly impacted by the presence of other drivers for instance, drought and therefore temperature. Other regional drivers identified included wind, biotic agents and shading.

Better understanding of the drivers and causative mechanisms behind tree mortality will allow improved forecasting of the effects of environmental changes and therefore more accurate predictions of the knock-on effects for the carbon cycle globally. This is especially important as many drivers included in this study are predicted to continue increasing. The review was not able to include resilience or recovery within its coverage, though it did note that higher species diversity can help buffer forests against mortality, and these processes are both key to gaining a full picture of forest decline.


Read the original paper here:

McDowell, N., Allen, C.D., Anderson‐Teixeira, K., Brando, P., Brienen, R., Chambers, J., Christoffersen, B., Davies, S., Doughty, C., Duque, A. and Espirito‐Santo, F., 2018. Drivers and mechanisms of tree mortality in moist tropical forests. New Phytologist.


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