This is a story that passes for good news at a time of global warming. A new study based on data from 21 broadleaf forests in northern latitudes over a 20 year period reports that forests in some places seem to be capable of achieving normal rates of growth while using less water. For the moment, the assumption is that increased levels of carbon dioxide are essentially acting as a fertilizer, promoting growth with less water. This suggests that at least in some locations, it might be possible for forests to survive through the droughts caused by climate change.
Like most changes in the environment, there are pros and cons to forests using less water because forests recycle the water into the atmosphere where it becomes rain clouds. If the forests take up less water, they will probably supply less moisture to agricultural areas downwind of the forests.
Where forests exist on the perimeters of their climatic ranges, they are not faring as well. In the American West, for example, there are massive tree die-offs caused by less rainfall and snow as well as beetle infestations where temperatures are no longer cold enough to kill them in the winter.
Scientists had predicted some growth benefit from higher levels of carbon dioxide, but this study found the benefit to be far greater than previously predicted. Higher growth rates also predict that forests will be capable of absorbing more carbon dioxide because carbon storage is mainly proportionate to biomass.
Trees absorb carbon dioxide through the pores in their leaves, called stomata. Scientists hypothesize that trees don’t need to open their pores as wide when carbon dioxide levels are higher. Since moisture is lost when the pores open, less moisture is lost if the pores don’t open as wide. That’s the working theory of this new research.
The forest at the Harvard arboretum was one of the forests included in this study. It has the longest continuous record of forest growth in the world.
Many questions remain. Which species are becoming more efficient in their water use? Are there intervening factors that are reducing water use? Will this trade-off between water use and carbon dioxide levels have an upper limit?
“Some Trees Use Less Water Amid Rising Carbon Dioxide, Paper Says,” New York Times, 7/11/13