Fire Cycles

Claralynn Nunamaker

It's not at all uncommon to hear the phrase that fire is “a natural part of the landscape.” But what does that really mean? There is a whole science devoted to fire ecology, that looks at how fire helps shape the landscape and interacts with the biological resources in that landscape. But let’s look at one particularly important concept in fire ecology, the fire cycle.

Where there is fire, there is a fire cycle. The fire cycle is the number of years, on average, that a fire historically moved through the area. It is also called the fire return interval.

Every ecosystem has a fire cycle. Even the coastal areas have fire cycles, though they are very long—perhaps 300 years or more. But in very hot, dry areas, fire cycles might be as short as every 1–7 years. In Mendocino County, fires might have come through on average every 15-40 years. The number of years depends on many variables such as the forest or vegetation type and the size of the area you are considering.

When the fire cycle is allowed to function, the result is frequent, low-intensity fires. But when the fire cycle is interrupted, the stage is set for a catastrophic fire.

Think of a few oak trees losing their leaves every year. If you burned one year's accumulation of fallen twigs and leaves, you’d have a well-behaved little fire with flame lengths of a few inches to maybe a foot. The base of the oak trees would get a little warm and might even blacken a bit.

But allow the fallen twigs and leaves to accumulate for 50 years before you decide to burn; some will have decomposed, but you’ll have a lot of fuel for that fire. Instead of a nice little fire, you’ll have a raging inferno whose flames will most likely reach into the tree itself, burning the live leaves and killing the tree.

The longer the disruption in the fire cycle, the more destructive the fire will be. Remember that it’s not if a fire will occur, it’s when.

Claralynn Nunamaker is a Registered Professional Forester and writes a column on forestry for the Mendocino County Observer. She is also the voice behind the California Forest Stewardship Helpline.