Trees in both wet and dry forests are vulnerable to drought conditions

Published: Thursday January 24 2013, 16:07:02

Stress caused by droughts has long term implications for forest coverage according to a new study in Nature. Changes in rainfall patterns (both timing and location) coupled with higher temperatures can lead to a reduction in forested areas in locations where droughts are forecast to increase. It is known that sensitivity to drought is a factor driving the distribution of individual plant types. There is recent evidence that rising temperatures are enhancing chances in forests brought on by drought conditions. One outcome of longer droughts and higher temperatures is rapid forest collapse which has a knock on effect on global forests and their carbon storage ability. Indeed parts of the Amazon forest continues to suffer from the effects of a drought which started in 2005 (for details see here ). Using satellite data, a change in forest canopy has suggested widespread dieback of older, larger trees in the SW Amazon rainforest. Despite rainfall levels returning to more normal conditions, these old growth trees did not appear to recover. The persistence of damage caused by drought could be explained by the new study of Choat et al. that details the physiological mechanisms which govern drought stress in individual tree species. This new study describes the harm caused by the development of gas bubbles in the water vessels transporting water throughout the plants/trees. Understanding how this transport system functions is important for predicting how forests will respond to changes in future climates.

All plants transfer water from root to leaf, however if air enters this pathway the delivery system is interrupted. This pathway is made up of many thin tubes that carry the water upwards to the tree leave, in mature trees if air embolisms form in the vessels they cannot be restored. The structure of this water delivery system varies between tree species making some more resilient than others. It is possible for trees to acclimatise its delivery system to cope with different conditions, but that is mainly possible during the growth stage and mature trees cannot “re-grow” new pathways once existing ones are blocked. As water is a key ingredient for photosynthesis (the plants growth process), any long term disruption in the delivery of water to plant leave has consequences on its survival. This recent work describes how air embolisms in the vessels delivering water through the plant can develop and result in these vessels becoming dead, as they can no longer transport water. If this decline in water delivery persists, it can ultimately lead to the death of the tree. In some flower trees the existing embolism can be reversed if a drought is followed by sufficiently high rainfall events, but more work is needed to understand this better.

Improved knowledge of gas embolisms in the water transporting vesicles and their development is beneficial in our ability to predict forest dynamics and their ability to cope with changes in rainfall patterns. This work sheds new light on explaining the impact of drought stress on mature trees and the persistence of drought symptoms even after resumption of more normal rainfall patterns.

Additional information
The paper: “Global convergence in the vulnerability of forests to drought” Brendan Choat et al. was published in Nature on 21 November 2012 doi:10.1038/nature11688
Science news article on this paper

For further detail on the Amazon study