Trees in the western United States are dying twice as quickly as they did three decades ago and scientists think global warming is to blame.
In their surveys, ecologists found that a wide range of tree species were dying including pines, firs and hemlocks and at a variety of altitudes. The changes can have serious long-term effects including reducing biodiversity and turning western forests into a source of carbon dioxide as they die and decompose. That could lead to a runaway effect that speeds up climate change.
"The trend was pervasive across a wide variety of forest types, across all elevations, in trees of all sizes and among major species," said Phillip van Mantgem of the US Geological Survey (USGS). "At the same time, the rate of new establishment of trees didn't change."
If these trends continued, he said, forests will become sparser and store less carbon. "It introduces the possibility that western forests could be come net sources or carbon dioxide to the atmosphere, further speeding up global warming."
The forest survey, carried out by a team of scientists led by van Mantgem, is published tomorrow in the journal Science. It showed that death rates of trees overall had more than doubled since 1955. In the Pacific north-west and British Columbia, deaths had doubled in 17 years. In California, the death rate took 25 years to double.
The work is the first large-scale study of death rates in forests or temperate regions. Much of the world's population – in North America, Europe, most of China and large portions of Russia – live near temperate forests so what happens in these forests has global importance, according to Jerry Franklin, a professor of forest resources at the University of Washington and a co-author of the study.
The researchers think that warming global temperature is the most likely cause for the dramatic decline. From the 1970s to 2006, the period that includes most of the surveyors' tree data, the average annual temperature of the western US increased by 0.3C-0.4C, and increased even more at the higher elevations that are normally covered in forests.
"While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought," said van Mantgem. This longer summer drought means less water for trees and it also encourages the growth of insects and diseases that attack the plants. Recent outbreaks of tree-killing bark beetles in the western US have already been linked to warmer temperatures.
Mark Harmon, a forest ecologist at Oregon State University, said another concern from the study is that a climate feedback loop could develop from the increased death rate of trees. As temperatures rise, the smaller forests will not only absorb less CO2 but will emit more greenhouse gases ias the dead material decays. This, in turn, would lead to even higher levels of global warming.
The data for the research was gathered by several generations of scientists counting trees over more than 50 years. It included forests in Oregon, Washington, California, Arizona, Colorado, New Mexico and south-western British Columbia. All were older than 200 years, with many being established more than 500 years ago. Death rates in old forests tend to be more stable since they mostly contain very old trees.
"With many of our long-lived trees that grow very large, each year as they become larger and older, the probability of living the next year increases," said Franklin. "You might imagine that, as a tree gets larger and older, the probability of death would increase but it does not – it decreases for many of our species."
In most forests, it is the youngest trees that are most likely to die. "Often they are shaded by larger, taller trees and so they grow more slowly," said Nathan Stephenson of the USGS. "They are less resilient to changes in the environment and they also don't have as well-developed root systems so, if they run into a drought, they're more likely than a large tree to suffer."
In the latest survey, the research team found that trees of all ages were dying more quickly.
The team also ruled out factors such as overcrowding, forest fragmentation or air pollution. The main air pollutant that harms trees in the western US, for example, is ozone. "In California, where most of our forests are concentrated, ozone is fairly severe," said Stephenson. "Over the time period of the study, there was no trend in ozone and it might even have declined slightly."