USA — Timber! Every day an area of forest larger than New York City is felled to fuel our growing appetite for wood. This logging activity, along with deforestation, accounts for more than 15% of greenhouse gas emissions worldwide. But logging is far from the only threat trees face. A new study of forestry in the western US shows that bark beetle outbreaks and wildfires are killing more trees than commercial logging. Over the last few decades, bark beetles and wildfires combined have potentially released a staggering 9% of the carbon stored in trees in this region.
Wildfires and insect outbreaks have wiped out millions of hectares of forest in western North America in recent years. In particular, the mountain pine beetle has destroyed vast areas of lodgepole pine forest, including more than 16 million hectares in British Columbia alone. But what kind of impact does this have on the carbon cycle?
To answer this question, Jeffrey Hicke from the University of Idaho and colleagues used tree mortality maps and logging data to estimate the amount of carbon released by trees in the western US. Using existing spatial datasets of forest biomass, burn severity and beetle-caused tree mortality, they were able to tot up the amount of above- and below-ground carbon in killed trees across the region.
Their results, published in Environmental Research Letters (ERL), showed that between 1984 and 2010, wildfires released between 511 Tg of carbon per year in the western US. Meanwhile, between 1997 and 2010, beetle outbreaks killed enough trees to release between 224 Tg of carbon per year, with the outbreaks peaking after the year 2000. Added together, these disturbances have killed a volume of trees equivalent to 9% of the total forest carbon in the western US all in the space of three decades.
“The annual amount of carbon in trees killed by fires and beetles exceeds that by harvest in the western US in recent years, which came as a surprise to us,” said Hicke.
Anecdotal evidence indicates that these disturbances have increased in recent years. A warmer and drier climate is favourable to bark beetles and helps encourage fire. Meanwhile, forest clearing and wildfires around 1900, together with later fire suppression, have led to dense forests today and a high proportion of larger and older trees, which bark beetles prefer.
Different forest management techniques could help to reduce a forests susceptibility to fire and pests. “The reintroduction of fire and other ways to reduce fuel loads, through thinning for example, will reduce severe fires as well as susceptibility to beetle outbreaks,” said Hicke. Bark beetles prefer certain tree species, so harvesting carefully to maximise diversity in both tree species and tree sizes will also help.
For western North America at least, these natural disturbances are having a significant effect on the carbon budget, and need to be considered in any calculations. The figures will be different for other parts of the world, but it looks like carbon loss from natural disturbances cant be ignored.