"If you attempted to go out and mess around with the forests, you'd probably end up accelerating [mortality rates]," says co-author Jerry Franklin of the University of Washington in Seattle.
"No one really knows what to do. All the sudden we are off into uncharted territory."
22 January 2009
North American tree deaths accelerate
Mortality increase correlates with climate change.
Trees in the western United States and Canada are dying more quickly than they used to, but there is no corresponding increase in "recruitment", or the number of new seedling trees. The mortality rates, which are of the order of 1%, have in many cases doubled in just a couple of decades.
This subtle trend correlates with climate change in the region, which has warmed by between 0.3 and 0.4 degrees Celsius every decade since the 1970s. The work was based on 76 long-term forest studies, and is published in the journal Science (1).
Study co-leader Phillip van Mantgem of the Western Ecological Research Center in Arcata, California explains the mortality increase in financial terms: "Think of it like a compounded interest rate; the difference between a 1% return and 2% return compounded annually over 50 years will be huge."
The team ruled out a number of competing explanations for the trend. For example, a forest that is relatively young starts out with many small saplings, and as the forest grows it also naturally thins out, until the same space is covered with many fewer, but much larger trees. For this reason, they only looked at plots where the forest was more than 200 years old - parts of the forest spared from the axe since large-scale logging began in the region in the nineteenth century. These trees are continuously dying and being born, but the forest-wide thinning process has finished.
Modern fire suppression was also ruled out by looking at the set of forests that have no history of frequent fires. They too have seen an increase in mortality.
So far, the correlation with increased temperature is just that: a correlation. "I don't think we are barking up the wrong tree with temperature," says Mantgem, "but we need to see how that plays out." The actual mechanism, he speculates, could vary between different forests.
Those in hot, arid regions may be losing trees because of drought stress. Even if rainfall is stable, higher temperatures can suck those same scanty centimeters of precipitation through the tree faster, as evaporation speeds up. In wetter regions, temperature increases may make life easier for "things that chew on trees, like bark beetles", says Mantgem.
Bark beetles have caused many massive tree die-offs in the region in recent years. And the mortality increases that Mantgem and his team captured may be symptoms of climatic stress that make the forests more liable to such catastrophes. "These changes in mortality rates are really an indication of overall system stress, and when [trees] are stressed, they can become more susceptible to things like the pine beetle," says Werner Kurz of the Canadian Forest Service in Victoria, British Columbia.
Into the unknown
What the work means for the carbon balance of the Earth is also not as obvious as it may seem. Recently, it was shown that old forests continue to suck away carbon into their third centuries and beyond (2). "With fewer live trees, you would expect that they would take up less carbon," suggests Kurz. "But old growth is very complex."
It is even less clear what action is called for in the face of the data presented by the study. "If you attempted to go out and mess around with the forests, you'd probably end up accelerating [mortality rates]," says co-author Jerry Franklin of the University of Washington in Seattle.
But if the mortality rate ramps up to higher percentages, some action could be taken, says Kurz. "Really, the options that are available are logging of the wood for biomass or long-lived wood products to keep the carbon from the atmosphere, and then replanting with species that are slightly more suited to the changing climate."
Mantgem agrees. He mentions control of any invasive species that might take over from stressed forest species, the transport of tree species to colder climes further north and more controlled burns to prepare the forests for more frequent wild fires. But he adds, "No one really knows what to do. All the sudden we are off into uncharted territory."