Tree deaths have doubled across the western US
Tree death rates have more than doubled over the last few decades in old-growth forests of the western United States, and the most probable cause of the worrisome trend is regional warming, according to a U.S. Geological Survey-led (USGS) study published in Science on January 23. The study found that 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.
Regardless of the cause, higher tree death rates ultimately could lead to substantial changes in western forests, said Phil van Mantgem, a USGS scientist and co-leader of the research team. Such changes, the team noted, can have cascading effects, such as by changing forest suitability 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.
"The same way that in any group of people a small number will die each year, in any forest a small number of trees die each year," said van Mantgem. "But our long-term monitoring shows that tree mortality has been climbing, while the establishment of replacement trees has not."
The result is that forests have begun to lose trees faster than they're gaining them, said van Mantgem.
The study's authors 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 the West rose by more than 1° F over the last few decades," said van Mantgem. "While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought."
The lengthening summer drought could be stressing trees, leading to higher death rates, he said. Warmer temperatures also might favor insects and diseases that attack trees. Some recent outbreaks of tree-killing bark beetles in the West have already been linked to warming temperatures.
"Tree death rates are like interest on a bank account – the effects compound over time," said Nate Stephenson, also with the U.S. Geological Survey and research team co-leader. "A doubling of death rates eventually could reduce average tree age in a forest by half, thus reducing average tree size."
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. "That may be our biggest concern," said Stephenson. "Is the trend we're seeing a prelude to bigger, more abrupt changes to our forests?"
Source: United States Geological Survey
Articles on the same topic
- NAU research ties tree mortality trends to climate warmingFri, 23 Jan 2009, 16:57:12 UTC
- Tree death rate in Pacific Northwest doubled in 17 yearsThu, 22 Jan 2009, 19:43:52 UTC
- Warmer climate causing huge increase in tree mortality across the WestThu, 22 Jan 2009, 19:43:48 UTC
- New study links western tree mortality to warming temperatures, water stressThu, 22 Jan 2009, 19:43:33 UTC
Other sources
- Global Warming: Tree Deaths Have Doubled Across The Western U.S.from Science DailySat, 24 Jan 2009, 23:21:19 UTC
- Study: Tree death-global warming tie foundfrom UPIFri, 23 Jan 2009, 23:42:15 UTC
- Tree death rate in Pacific Northwest doubled in 17 yearsfrom Biology News NetFri, 23 Jan 2009, 18:49:17 UTC
- Study: Western forests dying at increasing ratefrom AP ScienceFri, 23 Jan 2009, 15:28:36 UTC
- Global warming: Rate of tree deaths in western US rising due to climate change, study warnsfrom The Guardian - ScienceFri, 23 Jan 2009, 11:21:14 UTC
- Global warming already killing western treesfrom Science BlogFri, 23 Jan 2009, 3:21:08 UTC
- Tree death rate in Pacific Northwest doubled in 17 yearsfrom Biology News NetFri, 23 Jan 2009, 1:28:10 UTC
- Climate shift 'killing US trees'from BBC News: Science & NatureFri, 23 Jan 2009, 1:14:03 UTC
- Out on a Limb: Global Warming May Be Killing Old-Growth Forestsfrom Scientific AmericanFri, 23 Jan 2009, 0:56:14 UTC
- Environment Blamed in Western Tree Deathsfrom NY Times ScienceFri, 23 Jan 2009, 0:49:03 UTC
- Climate shift 'killing US trees'from BBC News: Science & NatureFri, 23 Jan 2009, 0:28:05 UTC
- Everyday tree deaths have doubledfrom Sciencenews.orgThu, 22 Jan 2009, 22:14:10 UTC
- Global warming already killing western treesfrom Science BlogThu, 22 Jan 2009, 21:42:13 UTC
- U.S. Old-Growth Forests Withering With Warmingfrom National GeographicThu, 22 Jan 2009, 21:42:04 UTC
- Tree deaths have doubled across the western USfrom PhysorgThu, 22 Jan 2009, 19:35:15 UTC
- Warming behind big increase in tree mortality in B.C., western U.S.: studyfrom CBC: Technology & ScienceThu, 22 Jan 2009, 19:21:08 UTC
- Tree Deaths Double in Western U.S. Forestsfrom Live ScienceThu, 22 Jan 2009, 19:21:05 UTC
- Warming behind big increase in tree mortality in B.C., western U.S.: studyfrom CBC: Technology & ScienceThu, 22 Jan 2009, 17:28:23 UTC