Dr. Lori D. Daniels
I established my permanent research plots in 1992 and have been monitoring them for 17 years. In a recent collaboration with scientists from the US, we have made an alarming discovery…
WIDESPREAD INCREASE OF TREE MORTALITY RATES IN THE WESTERN UNITED STATES
Tree death rates have more than doubled over the last few decades in old-growth forests of the western United States and southwestern British Columbia, and the most probable cause of the worrisome trend is regional warming, according our research published in Science on January 23 (http://www.sciencemag.org/cgi/content/short/323/5913/447).
Our study compares population changes in forests in southern British Columbia, Washington, Oregon, California, Idaho, Colorado and Arizona. My collaborators and I have monitored 76 permanent plots that include 58,736 trees. During that time 11,095 trees died and tree mortality rates have more than doubled in recent decades.
Tree death is a natural part of old-growth forest dynamics. Each year we expect a small number of trees to die. But our long-term monitoring of many types of old forests shows that tree mortality has been increasing, but the establishment and growth of replacement trees has not. As a result, the forests are losing trees faster than they are gaining them.
Mortality rates, the rate at which trees die, have increased from just under 1% to almost 2% per year. These numbers may seem small, but tree death rates are like interest on a bank account – the effects compound over time. So, a doubling of death rates eventually could reduce average tree age in a forest by half, thus reducing average tree size.
The increase in dying trees has been pervasive. Tree death rates have increased across a wide variety of forest types, at all elevations, in trees of all sizes, and in pines, firs, hemlocks, and other kinds of trees.
Ultimately increased mortality rates could lead to substantial changes in western forests. As trees die, they change the composition and structure of the forest, which can have cascading effects, such as altering habitat for wildlife species. Additionally, increasing tree mortality rates mean that western forests could become net sources of carbon dioxide to the atmosphere, further speeding up the pace of global warming.
Our study ruled out a number of possible sources of the increasing tree deaths, including air pollution, long-term effects of fire suppression, and normal forest dynamics. In contrast, increasing regional temperature was correlated with tree deaths.
Average temperature in western North America rose by more than 1.0° C over the last few decades. While this change may sound modest, it was enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought.
Changes in length of summer drought could be stressing trees, leading to higher death rates. Warmer temperatures also might favor insects and diseases that attack trees. For example the recent outbreak of mountain pine beetle in British Columbia has already been linked to warming temperatures.
In some cases, increasing tree deaths could indicate forests vulnerable to sudden, extensive die-back, similar to forest die-back seen over the last few years in parts of the southwestern states, Colorado, and British Columbia. This is a major concern – the trend of increased death rates indicates our forests are stressed and may be susceptible to bigger, more abrupt changes.
van Mantgem et al. 2009. Widespread increase of tree mortality rates in the western United States. Science 323: 521-524.
It has been a pleasure working with the lead- and co-authors of this study:
Phillip J. van Mantgem (USGS)
Nathan L. Stephenson (USGS)
John C. Byrne (U.S. Forest Service)
Lori D. Daniels (Unversity of British Columbia)
Jerry F. Franklin (University of Washington)
Peter Z. Fulé (Northern Arizona University)
Mark E. Harmon (Oregon State University)
Andrew J. Larson (University of Washington)
Jeremy M. Smith (University of Colorado)
Alan H. Taylor (Pennsylvania State University)
Thomas T. Veblen (University of Colorado).
Links to some Canadian Media Coverage of this research…
CBC Quirks and Quarks – http://www.cbc.ca/quirks/archives/08-09/qq-2009-01-24.html
CanWest News – http://www.canada.com/topics/news/story.html?id=1207075
The Globe and Mail –