Patterns of Giant Sequoia Regeneration in Groves Exposed to Wildfire and Retention Harvest in the Southern Sierra Nevada
Giant sequoias can live for thousands of years, but they sometimes have difficulty getting started. Unlike coast redwoods, giant sequoias rarely sprout from their bases. Their reproductive future lies in their tiny (0.2-inch-long) seeds, which need just the right combination of soil, sun and moisture to survive.
Historically, fires have provided the openings that produce these conditions. But only three wildfires hit sequoia groves in California’s Sierra Nevada between 1959 and 2009. So people can walk for miles in Sierra sequoias and find only a few scattered sequoia seedlings and saplings in unburned areas.
Part of the solution is probably to let more wildfires burn, under the right conditions. But little data has been collected to back that hunch. With partial funding from Save the Redwoods League in 2010, a US Forest Service team compared sequoia regeneration on burned and unburned areas, as well as on lands where “retention harvest” took place a quarter century ago. The latter involved removing all trees except the largest sequoias and cleaning up the slash (debris) with a prescribed burn.
Examining four groves in the southern Sierra, the researchers found that giant sequoias were successfully regenerating after moderate– and high-severity wildfires—but not after low-severity ones. Those intense fires gave the biggest boost, but retention harvest was beneficial, too, if followed by burning the slash.
“Good regeneration was strongly associated with canopy gaps,” said Marc D. Meyer, the Forest Service ecologist who led the study. He also found that healthy seedlings were more likely to be in the center of the gap than on its edges, and gaps with less forest litter on the ground were more conducive to regeneration than those that were thickly covered.
“Both wildfire and prescribed fire (preceded by harvest or not) can serve to promote giant sequoia regeneration,” the study concludes, “providing that fire intensity is sufficient to create canopy gaps, increase understory light and remove surface litter.”
There’s a caveat, however. Managing to maximize sequoia regeneration could open up too much of the canopy, hurting species that prefer ancient forests, such as Pacific fishers, California spotted owls and northern goshawks. The ideal is small patches of intense fires amid a matrix of other habitats—”a measured approach,” Meyer said.