Old growth forest on the western United States is dying back at an increasing rate, accoring to a study published this week in Science magazine. The study, led by USGS biologist Phil van Mantgem, found that background mortality rates — those not directly attributable to some catastrophic effect, such as fire — have risen rapidly in recent decades.
The study, which used census data from 67 long-term plots located in ancient forests (average age 450 years, some sites in excess of 1000 years), found that mortality rates had increased at the vast majority of sites, whereas recruitment rates — replacement of dead trees with new ones — had neither increased or decreased over the same period.
The dieback couldn’t be pinned down to any local effects, such as pollution, local competition or fire prevention measures. Instead, say the study’s authors, regional warming and the ensuing water problems it brings — faster snowmelt leading to longer summer drought — are putting the strain on the trees.
The observed patterns of change differ markedly from those seen in the Tropics, where diebacks have been paralleled by an increase in the growth of new trees. Mean annual temperatures in that part of the US have increased by around 0.4 degrees C per decade, a trend set to continue and indeed hasten until we take decisive action over greenhouse gas emissions.
The findings highlight the complexities the world’s forests present as a global carbon stock. As climate change bites, drought-stressed trees will become more vulnerable to insect outbreaks, which have the potential to trigger large scale diebacks. Such a scenario would lead to a feedback, where carbon released by decaying trees would increase atmospheric CO2, making global warming worse. If ever there was an ecological timebomb ticking away, this is it.
Source: van Mantgem PJ et al. (2009) Widespread increase of tree mortality rates in the Western United States. Science 323: 521–524 doi: 10.1126/science.1165000
Phil van Mantgem, lead author of the paper, was also interviewed by Science as part of the magazine’s weekly podcast. Download the file and stick it on your mp3 player to enjoy later!