Climate change could cause the forests of the world’s oldest national park, Yellowstone, in the Eastern Rocky Mountains, to shift to a gras and shrub ecosystem, US scientists warn. It could happen in four decades, and will likely not be a gradual process.

In recent weeks we’ve paid a lot of attention to possible feedback mechanism through which climate change may affect the world’s forests, and through which forest changes could in turn affect the carbon cycle.

Good news

Some of the news is good. We learned over the period of 1990-2007 forests were (still) a net carbon sink, compensating the extra emissions of deforestation [although that would not have been the case without the helping hand of afforestation].

A few studies have even suggested negative correlations: warming could improve nutrient availability of high latitude forests soils, thus increasing biomass production. These same northern forests could also benefit from an earlier onset of spring.

Not so good news

Other studies are more pessimistic. The same warming that could create a (temporary) growth spurt could also increase biomass oxidation processes, which release CO2. Moreover, warming of the forest soils could also increase anaerobic production of other greenhouse gases.

Problem is not all forest-climate feedbacks operate through linear biochemical processes. Some happen suddenly, driven by seasonal extremes, like the 2010 Amazon drought, or by climate-related plagues, like the pine beetles that cause megatonnes of CO2 emissions from the forests of Alberta and British Columbia.

And forest fires

Another important factor could be an increase in wildfires, a new publication in PNAS suggests. This could at least be the case in Yellowstone National Park – continental high latitude pine (and aspen) forests that is, much like the boreal forests of Canada, Scandinavia and Siberia.

Typical to the Greater Yellowstone Ecosystem are large, but infrequent wildfires. Because these are infrequent there is sufficient time for the forest biome to restore. Historically Yellowstone’s ‘fire rotation’ – the time it takes to (cumulatively) burn a park-sized area – lies at 100-300 years.

Over the period of 1972–1999 the wildfires could be correlated with monthly climate data. (One of the largest and most devastating Yellowstone forest fires occurred in 1988, during one of the worst droughts in US history.) From this scientists of the University of California [Merced campus] could create different forest fire climate models.

Forest fires increase 3-10 times

These do what we may fear to do ourselves: extrapolating. Over the rest of this century – under continued warming trends – all models indicate ‘substantial fire increases’.

Years without large forest fires were ‘common’ and will become ‘rare,’ the scientists conclude. By 2050 the fire rotation will have increased to less than 30 years – and for large pine trees that’s too much to recover from:

“The predicted new fire regime would transform the flora, fauna, and ecosystem processes in this landscape and may indicate similar changes for other subalpine forests.”

It seems like we’ve collected another balancing piece of the forest climate puzzle. Making the pieces fit is an entirely different challenge though. Forests confuse.

© Rolf Schuttenhelm | www.bitsofscience.org