Perhaps this isn’t surprising, but a new piece of research shows that the ability of trees to survive drought is reduced when temperatures are higher. A species of pine that grows in dry conditions was exposed to temperatures 4.3 degrees higher than a control group. Both sets of trees were kept without water. The trees in the hotter atmosphere typically died in 18 weeks compared to the 25 weeks of the control.
Extraordinarily little is known about what causes tree mortality. The researchers say in this case that the cause of death seems to be ‘carbon starvation’. When conditions are dry, trees close their pores, reducing photosynthesis to almost zero and cutting water loss. No new carbon dioxide is being absorbed by the tree. But the tree still needs carbon-based molecules to provide the food to keep its living processes going. So eventually a tree exposed to drought, and therefore not taking in new CO2, runs out of food. Death results. The new paper suggests that higher temperatures cut the lifetime of trees because higher temperatures increase the typical metabolic rate of the organism, meaning it uses up its available food faster.
In recent years some experts have noted the unexpectedly quick death of trees in droughts around the world. Woodlands that survived long water shortages in the past succumbed to shorter periods of drought. The new paper gives us a hypothesis that higher temperatures are to blame.
Although these conclusions are not unexpected, this is an important result because it allows us to estimate roughly the increase in the risk of catastrophic forest death. The researchers say that ‘regional scale tree die-off events’ in southwestern USA would be five times as frequent with 4 degree higher temperatures. Put another way, the 18-week long droughts necessary to kill the pine species under investigation are very much more frequent than the 25-week droughts it takes for trees to die at present temperatures. We shouldn’t be surprised by this – trees won’t generally have successfully colonised an area that has droughts of sufficient frequency to cause death because they wouldn’t have lived long enough to be reproductively mature.
The implications of the new research for the world’s carbon balance are severe. At the moment tropical trees are substantial absorbers of CO2 through the photosynthesis process. Higher temperatures could make them net emitters as the trees succumb to less severe, and therefore more frequent, droughts.
This isn’t the complete story of course. Most climate scientists suggest that dry areas are generally likely to become drier as climate change progresses. So not only will forests be stressed by higher temperatures they will also be vulnerable to higher frequencies of periods of catastrophic drought. And woodlands may also be more vulnerable to insects that attack trees. Recent experience in western Canada shows that bark beetles can devastate enormous areas within a few years. Part of the cause may be the higher temperatures of recent decades, combined with drought stress.
The conclusion to the new research glumly states that the unexpectedly high temperature sensitivity for the species of pine that was investigated ‘could profoundly alter assessments of climate change impacts, which continue to reveal increasingly dangerous risks’.
Footnote  Henry D. Adams and others, 'Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global change-type drought', Proceedings of the National Academy of Sciences, 13 April 2009 (Early Edition) http://bit.ly/Adams2009 (doi:10.1073/pnas.0901438106).