On the origins of fire scars in California redwoods

Anyone who has spent time with the redwoods has no doubt seen and even ventured inside the giant trees with fire-scarred trunks. Certain trees are so severely scarred that you wonder how they are even able to stand. Some trees contain cavernous fire-carved rooms in their base with multiple entrances and even window-like openings. I’ve marveled at these trees and looked carefully at the orientations and shapes of their scars, and in doing so have found some odd things.

Huge fire scar in giant sequoia (photo by Lee Klinger)

Having worked on fire lines in Alaska, Colorado, and California I’ve seen how fire scars are formed. A large quantity of fuel piled at the base of the tree is usually required to ignite a fire hot enough to penetrate the bark and scorch the cambium.

A characteristic burn pattern is seen on slopes where the vast majority of fire scars occur on the uphill face of the trunk (more than 90% in places)[1]. This is due to a couple of factors. First, hot air currents tend to drive ground fires upslope, especially during the day when fires burn hottest. (Keep in mind we’re talking about large ground fires, not large canopy fires which would more likely kill the tree.) The wind-driven fires tend to burn relatively quickly around the lower parts of the trunk, but eddy effects allow the fire to linger on the uphill side. Second, fallen leaves and branches tend to move downhill and accumulate on the uphill side of the trunk. Together these factors seem to account fairly well for the uphill side tendency of fire scars.

So imagine my confusion . . . 

. . . when I started looking at the orientation of fire scars on redwood trees in California. Yes, there are uphill scars, but there are also downhill scars, scars on the side, even multiple scars. Furthermore, there is the odd juxtaposition of severely fire-scarred giant redwoods right next to scarless giants.

Fire-scarred giant sequoia (photo by Lee Klinger)

I pointed this out to some friends and one of them, Tahje Lanier, decided to check it out for herself. She did a study of the scar orientation on 20 fire-scarred redwoods growing on a slope here in Big Sur. She plotted her data and compared it to the expected pattern of 90% uphill oriented scars (see below). She found that only 20% of the trees were scarred on the uphill side. Most of the scars were on the side or downhill portion of the trunk. Pretty interesting finding, thanks Tahje!

Orientation of fire scars on ancient redwoods, expected (left) vs. observed (right)

Clearly other forces are at work here that need to be considered. Woodrats are up for consideration, since I’ve seen them build their nests of sticks tucked inside the fire scars of redwoods. If and when these catch fire there is adequate fuel to generate a sizable scar. Is there some nesting site preference of woodrats that is influencing this odd fire scar pattern?

Still, one forgotten fact which is profoundly germane to the occurrence of fire-scarred redwoods is that the native people of California tended the land and trees with fire for thousands of years. Native people used their mastery of fire as a way to keep the land and forests healthy and productive. And I have little doubt that they were applying their skills for the benefit of the redwoods too.

There are countless studies showing that redwoods are a fire-adapted species. Burning around the base improves the soils and reduces competition. The National Park Service knows this, which is why they are doing prescribed burns in their redwood groves. What the native people also seemed to know is that if you set a very hot fire next to the trunk, and severely scorch and kill the cambium layer, the scar tissue that forms is thick and bulky, and it spreads out at the base. After a number of years the tissue growing next to the fire scar organizes to become an important stabilizing structure of the tree. In other words in redwoods, no fire scars = no flared bases, fire scars = flared bases. Think Eiffel Tower vs. Tower of Pisa.

Fire also improves the nutrient content of soil and tree. Just as in building strong bones, calcium is needed to build strong wood. A study by University of Missouri scientists of wood calcium content vs. fire in evergreen forests  shows that fire is linked to the calcium content of the wood (see graph below)[1].

So why are many of the biggest, oldest trees in the world severely fire scarred? Are they still with us because of, or in spite of, fire?

And, if the native people were burning to tend the redwoods, then for what purpose? Unlike oaks, redwoods are not a source of food, nor are redwood forests very abundant with game. I think I’ve found the answer simply by sitting inside an ancient hollowed out redwood and meditating on the question for a while. I encourage you to try it next time you visit a burned out redwood and share what you find.

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[1] Guyette, R.P. & Cutter, B.E. 1997. Fire history, population, and calcium cycling in the Curren River Watershed. Proceedings of the 11th Central Hardwood Forest Conference, General Technical Report NC-188. St. Paul, MN: U.S. Dept. of Agriculture, Forest Service, North Central Forest Experiment Station, pp. 354-372.

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