Global warming is hurting the western United States especially hard. In critical mountain areas, more of the late winter precipitation is falling as rain, helping the mountain glaciers melt.
In the long term, glacial melting will severely disrupt water supplies to many major western US population areas. This is because the region is already water-limited, and depends heavily on these glaciers for water.
Another consequence of global warming in this region has been the reduction of forest health. This has helped an extremely destructive pest, the bark beetle, to do even more damage, because the trees are in a weakened state.
While it is known that such home-grown threats are killing trees, it is unknown whether global environmental changes are having a more direct negative impact on forest health. One reason why this is an important issue is that forests contribute greatly to atmospheric carbon dioxide consumption, and therefore may help counteract the human-induced dumping of greenhouse gases into the atmosphere.
A team of US and Canadian scientists, led by Phillip van Mantgem at the US Geological Center (Three Rivers, California), has reported reason for concern. They have shown that the rate of tree death in western United States forests is accelerating, and widespread.
The forests.
The scientists chose to study seventy-six geographically dispersed, unmanaged forests (elevations 130 meters to 3353 meters) that are greater than 200 years old (on average, 450 years old). This was to ensure that any observed persistent changes in tree mortality would be likely due to environmental changes.
Plot sizes ranged from 3/5 of an acre to 39 acres. 58,736 trees were included in these plots, of which 11,095 (19%) died over the course of the study (starting in 1955, ending in 2007).
Tree mortality rates.
The scientists found that mortality rates increased in 87% of the plots over the course of the study. In fact, mortality rates are doubling at a rate of every 29 years in the interior of the country, up to a rate of every 17 years in the Pacific northwest.
This trend held for all altitude ranges, and all tree diameter ranges. It was observed for all of the tree species in the plots.
The three most common trees in these plots, comprising 77% of all the trees, have different life and growing histories, and they have all been dying more quickly. The remaining 23% of the trees have also been dying more quickly.
Controlling for a damaging fungus that attacks one of the common tree species in these forests did not negate the observed enhanced mortality results. This further suggests that the accelerated death rates the scientists have noted is general to western US forests.
In contrast, only 52% of the plots saw on increase in new tree growth rates. This suggests that the dying trees are not being replaced, and the forests are dying overall.
Why are the trees dying?
This leaves a queston as to what is causing the trees to die. Neither methodological biases, forest density and area, forest fire suppression, large dead trees crushing smaller trees, forest fragmentation, nor air pollution were the cause of these observations.
The scientists propose that climate changes are the culprit. The average annual temperature of these forests has been increasing at a rate of 0.3°C to 0.4°C per decade, from the 1970s to 2006.
This warming has had several consequences:
The scientists have found that tree mortality rates are positively correlated with these temperature increases and precipitation deficits. They propose that the climate changes may contribute to tree mortality by limiting their water supply and/or rendering the trees more susceptible to pests.
Why is this noteworthy?
The scientists propose four reasons to be concerned with this observed accelerated tree death in western US forests.
This is even more reason for US government officials to take global climate warming seriously. It is already having a long-term impact, one that may not correct itself.
for more information:
van Mantgem, P. J.; Stephenson, N. L.; Byrne, J. C.; Daniels, L. D.;
Franklin, J. F.; Fulé, P. Z.; Harmon, M. E.; Larson, A. J.;
Smith, J. M.; Taylor, A. H.; Veblen, T. T.
Widespread increase of tree mortality rates in the western
United States.
Science 2009, 323, 521-524.