Sir

Covington states in his Commentary1 that the open ponderosa pine forests of the western United States are “in widespread collapse” because fire suppression by humans has eliminated the low-intensity surface fire regime that maintained the open, park-like structure of these forests. He fears this will lead to an “unprecedented” crown fire regime that will eliminate forests.

Although a crown fire regime in ponderosa pine forests may be unprecedented, there is a wide variety of ecosystems in North America that have always had a high-intensity crown fire regime, including chaparral in southern and central California, subalpine forests in the Rocky Mountains, and the boreal forest of Canada and Alaska2, among others. Some forest managers and conservationists believe that wildfires in these crown fire regime ecosystems are more extreme in their behaviour and effects than ever before, but in most ecosystems there is little or no good evidence to support this belief3.

In the chaparral ecosystems of southern and central coastal California, for example, the crown fire regime has changed little since Europeans first settled the area, despite extensive fire suppression efforts over the past decades4. There has been an increase in the number of human-caused fires; these, however, have had an insignificant impact on the total area burnt each year. Consequently, high-intensity wildfires continue to dominate in these ecosystems and are not an artefact of fire management policy.

In boreal and subalpine forest ecosystems, it is often stated that, because of fire suppression, fire intensity will increase because fuel accumulates as forest stands age. However, while large fuels may accumulate,the fine fuels (1 cm diameter) that contribute most to fire intensity and spread reach an equilibrium well before stand maturity. Thus, we should not expect a significant change in fire behaviour owing to “unnatural fuel accumulation” in these closed-canopied forests.

Although there may be evidence to suggest a shift from low-intensity surface fires to high-intensity crown fires in the ponderosa pine forests of the southwest, it would be prudent for resource managers from other western ecosystems to demand convincing evidence before applying Covington's suggestions to their areas.