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Coast Redwoods’ Response to Disturbance Events

A Review of Presettlement and Modern Disturbance Regimes in Coast Redwood Forests: Implications for the Conservation of Old-Growth Stands

Fire is an example of a disturbance event that redwoods face.
Fire is an example of a disturbance event that redwoods face.

In 2006, Save the Redwoods League recruited eight scientists to survey scientific literature about how coast redwood forests respond to “disturbance events” such as fires, windstorms and floods. The scientists considered how redwoods fit into two broad categories of trees: those that need major disturbances to perpetuate themselves and those that don’t. The seedlings of disturbance-dependent trees germinate in open spaces, grow quickly to outcompete other vegetation and tend to form even-age stands. Species that don’t need disturbances tend to be shade tolerant, slower growing and longer lived.  They usually grow in uneven-age stands.

It turns out that the redwood has both kinds of traits in a single species. “That’s the paradox of the redwood,” said lead researcher and University of Wisconsin forestry professor Craig Lorimer. “That’s unusual.” Redwoods’ thick bark enables them to survive most fires, and their seedlings often do well on bare mineral soil in the open. But redwoods can also be fairly shade-tolerant.  In Humboldt Redwoods State Park, for instance, redwood seedlings are persisting and slowly growing in shady Rockefeller Forest, an uneven-age stand. Yet they aren’t popping up beneath stands burned in the 2003 Canoe Fire. “We just don’t know whether fires are actually necessary for redwood establishment and perpetuation or whether the species simply tolerates fire, but is not dependent on it,” Lorimer said.  

Redwoods can make the best of many other types of disturbances. Unlike most conifers, they can sprout from their stumps after logging. They may be able to recover from breakage in a windstorm in the same way (though this is not yet proven).  If a flood inundates a river terrace, their roots can re-establish themselves by sending shoots up into the new layer of sediment.  Their seeds can germinate on the trunks and limbs of a fallen tree, or even on the soil tossed up by the tree’s roots. The species may also be able to set down roots in “slope failures,” the miniature landslides that in one study made up 40 percent of the total area opened up by large and small disturbances in the redwood forest. Redwoods are so large and long-lived, that only a few trees per acre—four to eight trees reaching the canopy every century—are needed to maintain their dominance.

“When you put all those facts together, you can make a fairly compelling case for the idea that redwoods might not need fire to become established,” Lorimer says. But scientists don’t know for sure. They do know that fires were frequent in coast redwood forests before European settlement, occurring on average every 6 to 26 years, and that they are rare now.

Among the additional research needed, Lorimer suggests staging some experimental controlled burns to see how they affect competitors and seedlings (“That’s pretty basic, but few results have been reported in redwoods.”) and studying slope failures to see if redwood seedlings colonize them.

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