Tree deaths outpace replacements

Reston, United States, January 23: Exacerbation of environmental changes in the western United States and Canada is playing havoc with the greenery. According to a recent study, trees in the west are deteriorating at a more than double the rate of their deaths 30 years ago.

The rising temperatures and draughts have thinned down the green cover, directly affecting the absorption of carbon dioxide from the atmosphere and eventually resulting in global warming, which in turn destroys the forests again. It’s a vicious circle.

With thin layers of trees, the forests become more vulnerable to wildfires and pests and, consequently, weaken the planet’s protective cover.

Researchers from different universities and agencies in the U.S. and Canada analyzed the tree census data since 1955 and found that the death of trees increased in 87 percent of the 76 forest plots they studied. The loss of trees is now out-pacing their replacement, they said.

"Data from unmanaged old forests in the western US showed that background mortality rates have increased rapidly in recent decades," the scientists wrote in the report, to be published in Friday’s issue of the journal Science. "Regional warming and consequent increases in water deficits are likely contributors to the increase in tree mortality rates."

The Pacific Northwest (including southwestern British Columbia), California and inland Western states were found to be the worst affected by higher temperatures. The average temperature in these regions had gone up more than one degree Fahrenheit from the mid-1970s to 2006.

"While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought," said study author, Phillip van Mantgem of the U.S. Geological Survey.

While pests and air pollution cannot be ruled out as the offenders, the most dominant factor was high temperature.

The researchers reported that the forest deterioration affected a range of species, different sized trees, and all elevations.

"The same way that in any group of people, a small number will die each year; in any forest, a small number of trees will die," said Mantgem. "But our long-term monitoring shows that tree mortality has been climbing, while the establishment of replacement trees has not."

Steve Pyne, an environmental historian at the University of Texas who has studied fires in forests, believes that the problem’s exacerbation further depends upon what grows in place of the vegetation that was being lost.

“Part of the trick here is we don’t know,” Pyne said. “It’s like the financial meltdown. It’s the uncertainty. What’s going to replace it?”