A vast survey of millions of trees covering dozens of different species across North America has concluded that both short and tall trees are at greater risk of dying during droughts than mid-sized trees—a finding that will affect how forests are managed as the globe warms.
A small number of previous studies had shown increasing mortality in the woods over time, in step with increases in the frequency and severity of local droughts. However, scientists did not fully understand the upper limits of trees’ ability to transport water upward through their roots, branches and leaves. What was expected is that it depends very much on the length of the path the water takes inside the tree, and the force of gravity.
As a result, until now, we haven’t had a good idea how increasing drought affects trees of varying heights amongst different species, and without taking this into account, current models of forest productivity (annual change in the amount of biomass) are potentially blind to how this key factor can contribute to catastrophic die-offs.
Researchers lead by Natural Resources Canada, in part funded by PICS, explored field plot observations of 4.5 million individual trees covering 64 dominant North American tree species across 46,000 unique sites throughout the continent. For each tree, at least two measurements at different dates had to have been taken in order to form a measurement interval for each. The tree data were then compared to climate data and a model of water balance (the flow of water in and out of an area).
They found that sensitivity to soil water deficits was greatest in short trees and decreased as trees grew taller, but then increased once again during late height development. The scientists believe that the reason for this is that it is more difficult for taller, older trees to transport water upwards. At the other end of the spectrum, they hypothesize that because water deficits in the soil begin to form at the surface and then spread downward, vulnerability increases for young trees with less deeply developed root systems.
When the models were applied, they revealed that changes in the water balance had indeed caused mortality to increase from 1.1 percent a year in 1951 to 2.0 percent a year in 2014, with mortality spikes during the increasingly severe droughts of recent years: 1988, 1998, 2006, 2007 and 2012.
The researchers suggest that their finding has practical implications for forest management. Beyond taking into account the risk of wildfire and insect outbreaks, as well as the effects of different forestry practices, forest managers need to be aware that catastrophic die-offs may be more frequent and severe in both old-growth forests (with accompanying significant release back into the atmosphere of greenhouse-effect exacerbating carbon dioxide) and in young, regenerating stands.
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