Climate change is causing devastation in America’s forests, owing to fire, insect-infestations and drought. All are related to a warming climate.
In 2013, Yosemite National Park was affected by California’s third-largest ever wildfire, which burnt over a quarter of a million acres.
In 2015, over 10m of America’s 766m acres of forest were consumed by wildfires. This was the biggest area burned since 1960, and cost the government over $2 billion.
The growth of wildfires is a worldwide problem. Siberia, Tasmania, Canada and Indonesia have seen record-breaking fires in recent years. Fire consumed over 7m acres of Russian forest in 2015-2016. The area of Canada’s forest burning each year has roughly doubled since the 1970s. In May 2016 a wildfire near Fort McMurray, in Alberta, destroyed 1.5m acres of forest and 2,400 buildings. The fire is believed to be Canada’s most expensive natural disaster.
The devastation caused by insects is ecologically as dramatic. The United States Forest Service (USFS) claimed that 26m trees had been killed by the insects and drought in California.
Such destruction, partly caused by global warming, will itself cause more warming. Many American forests are growing at high densities, and are a significant carbon sink. They absorb around 13% of the greenhouse gases that the US emits through burning fossil fuels. The USFS predicts that by 2036, American forests will become a carbon source.
Fire is an important part of the natural system. Wildfires clear away disease; remove leaf litter and create space for new growth. They prevent the build-up of woody material.
Climate change is believed to have made California’s drought 15-20% more severe; in Alaska, where the average winter temperature has risen by over 3°C in the past six decades—over twice the average for the rest of America—its impact is greater. By accelerating the melting of winter snow, for example, in Alaska and the mountains of the West—the Rockies, Cascades, Sierra Nevada—hotter temperatures have made the fire season longer. Since 1970 the average duration has increased from 50 to around 125 days.
Mismanagement is also fuelling the flames. Since 1910, the US government has suppressed wildfires. Logging has reinforced the effect, by planting dense forest cover.
In addition, drought-stressed trees are losing their ability to repel insects. Since the 1990s, 42m acres of North American pine forest are estimated to have succumbed to bark beetles. In the boreal forest spruce-beetle populations are increasing rapidly.
The reasons for the surging insect numbers are similar as for wildfires. Warmer, drier weather, including milder, shorter winters, has led to an increase in insect populations, as well as in the number of stressed trees they infest. Both problems, fire and pests, are liable to get worse as the climate warms. The USFS expects the area of forest burned to double by the middle of the century. However, it predicts that beetles will often kill more trees in a given year than fire. Even if the climate were stable, this onslaught would make it hard for the forests to regenerate.
Putting out fires
Mitigating those effects would require a massive intervention to clear dead trees and plant new ones, which is currently unthinkable. In 2015 USFS spent more than half its budget on firefighting, and by 2025, they estimate that 67% of its budget will be used for firefighting. That leaves very little for mitigation.
A warmer, drier climate should force trees uphill and to higher latitudes; the Ponderosa pine will climb from the montane to the subalpine zones, displacing or finding refuge among white firs and lodge pole pines. The effect of fire and bug-death has created opportunity for a massive experiment in tree migration and regrowth.
Whether a species can migrate may depend as much on factors such as soil type, distance from a seed source, the pace at which it reaches reproductive maturity and the vulnerability of higher-elevation vegetation to infiltration as on temperature alone. It took the Ponderosa pine 11,000 years to migrate from New Mexico to Wyoming after the most recent Ice Age: it is unlikely to keep stride with rapid warming.
The USFS suggests that nearly 60% of tree species are experiencing a contraction in their environmental range. Only 20% are making the predicted northward shift. High-elevation species, such as whitebark pine and Rocky Mountain fir, are likely to become extinct.
The losses will be partially offset by new growth: America’s forests will be vast and productive for a long time yet. As its snows melt and permafrost thaws, the Arctic is getting greener, so Alaska should grow more and bigger trees.
There is a growing acceptance in the USA of the need to thin forests, including by controlled burning. Over the past 15 years, partnerships have been formed in many cities to improve management of their forested watersheds. Some, such as in in Denver, Flagstaff and Santa Fe, include provisions for downstream water-users to help pay for forest management. In the Ashland drainage basin in Oregon foresters and environmentalists work together to survey, thin and improve the forest; the city has introduced a water tax, on households in the city, to help pay for the work.