Tuesday, February 5, 2013


Forest fires are given a really bad reputation. They destroy buildings, infrastructure, and require an enormous effort to control. Virtually every forest type has experienced fire at some point, and many types evolved with fire as the major disturbance factor, affecting the forest's successional patterns and shaping its species composition. Fires are now commonly started by foresters in settings called “controlled burns” in an attempt to reduce fuel loads within the forest, and create a healthier environment for fire dependent species.

There are two classes of fires; frequent fires, which burn at a low intensity, and occur every 1 – 5 years, and infrequent fires, which burn at a high intensity, and occur every 150+ years. The low intensity fires burn the understory, shrubs, and the top of the soil and help prevent a forest from becoming too cluttered; reducing the fuel load. High intensity fires are the result of high fuel loads. The “fuel load” of a particular forest refers to excessive understory brush, low-hanging branches, and resinous compounds within some tree species that are flammable. Without human intervention, the low intensity fires manage fuel loads, and prevent high intensity fires from happening very often. But when humans prevent low intensity fires from occurring ever 1 – 5 years within a given ecosystem, the fuel loads build up rapidly, causing high intensity fires (VERY OFTEN). This is very prominent in the western United States where fire suppression has lead to frequent high intensity fires. When a high intensity fire is lit in a forest that hasn’t had a (low intensity) fire for decades, the understory trees form a “fire ladder” which allows the flames to reach the crown of the trees, killing them. Many of the trees in fire adapted ecosystems can survive the flames, as they have evolved very thick bark.

Just a few fire adapted ecosystems in the United States
Tall grass prairie – Midwest
Chaparral – California and Southwest (refers to the various “brushlands”) MOST FLAMMABLE
Ponderosa Pine Forests – Interior West
Douglas-Fir Forests – Northwest
Loblolly and Shortleaf Pine Forests – Southeast
Jake Pine Forests – Great Lake States

So you might be thinking, how does this actually benefit the ecosystem? Well, many of these fire adapted gymnosperms have “serrotenous cones”. These cones are covered by a waxy substance that prevents the seeds from falling out, unless heated by fire. They then fall into the bed of ash left in the fires wake, and germinate in the cleansed environment. Other forms of serotiny are seeds that remain dormant in the soil until they are heated by fire, and then germinate. The post fire ecosystem is one of intrigue to ecologists. There are many species of plants, fungi, and birds that are adapted to post fire conditions, and are instrumental in recolonization. This area of research is generally understudied, and little is known about the complexity of these relationships.

Forests will continue to burn in an attempt to thin themselves (sort of natural silviculture), and humans will probably continue to put them out. This causes the economy billions of dollars each year in lost homes, insurance, and control. But the forests need to burn, and they will, one way or another. I would advise people to stop building homes in areas that burn frequently, because protection is an uphill battle.

Further Reading


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