In certain boreal forest landscapes, the low flammability of species that quickly establish after fire - like aspen - provides a degree of resistance to forest loss as the climate warms and becomes more conducive to fire.
A mathematical model has been developed to help understand why certain landscapes are vulnerable to losing their forests and the species that rely on them, while others are more resilient.
Scientists from the Smithsonian Institute say they can determine the risk that a fire will strike again by examining the type of vegetation that grows after a forest fire.
The team developed a quantitative model based on mathematic formulas to help identify which forests are most vulnerable in the hope that these areas can be protected before it is too late.
Forests sequester carbon by absorbing carbon dioxide during photosynthesis. Some of this carbon dioxide then forms plant stems, branches, roots and leaves.
As plants and trees burn during a forest fire, a large amount of the stored carbon is released as carbon dioxide. If forests quickly recover, they may rapidly re-accumulate carbon.
However, if burning converts a forest to shrubland — and the shorter, shrub-dominated vegetation is maintained by repeated fires — the total carbon stored across the landscape may be substantially decreased.
Because carbon dioxide is a heat-trapping gas, this process further aggravates climate change creating what scientists refer to as a positive feedback loop.
Kristina Anderson-Teixeira, from the Smithsonian Conservation Ecology Centre, said that in landscapes where the vegetation that establishes after a fire is highly flammable “small increases in fire activity could result in sudden, dramatic and difficult to reverse losses of forests".
As the climate warms it may become more challenging for trees to regenerate after fire. Some types of vegetation - including shrubs and grasses - are very fire-prone and increase the chance of permanent forest loss.
But not all forests are the same. Factors including vegetation type and rainfall impact how forests will burn and recover.
The scientists applied their model to forests across the globe - including those in North America, South America and New Zealand. They found forested landscapes burn and recover in very different ways, and some are more vulnerable to being permanently lost.
Alan Tepley, also from the Smithsonian Conservation Ecology Centre, said: “In certain boreal forest landscapes, the low flammability of species that quickly establish after fire, like aspen, provides a degree of resistance to forest loss as the climate warms and becomes more conducive to fire.
“However, in many southern beech forests of the southern hemisphere, fire can convert a cool, moist forest to a highly flammable shrubland that is easily perpetuated by repeated fires.”
Beech forests also have a slow rate of recovery after fire, which makes them particularly vulnerable to conversion from moist forest to flammable shrubland in response to relatively small increases in fire frequency.
This is an example of what Anderson-Teixeira refers to as a “tipping point", meaning they could more easily convert from forested to non-forested landscapes.
The team hopes its quantitative model will help scientists identify which forests are closest to these tipping points and act accordingly to conserve these vulnerable, unique environments.
Catherine Harte is a contributing editor to The Ecologist. This story is based on a news release from the Smithsonian Institute. The study is published in the Journal of Ecology.