© 2001 by Leo Poppoff

April 26, 2001

Basin Watch

by

Leo Poppoff

WHAT ARE THE CHANCES OF A WILDFIRE IN THE TAHOE BASIN?

Fire suppression in the Tahoe Basin is very successful. The last runaway
fire was in 1918. We're lucky to have early detection and prompt
response by the basin's fire fighting groups.

Still, successful fire suppression is why Tahoe Basin forests are loaded
with fuel, and why the likelihood of destructive wildfires is
increasing. According to the authors of Chapter 5 of the "Lake Tahoe
Watershed Assessment," there would've been three to five cycles of fire
in the basin during the past 76 years -- and, therefore, smaller
accumulations of fuel -- if fire suppression hadn't been so successful.

Patricia Manley and colleagues at the U.S. Forest Service and UC-Davis
wrote the 195-page Chapter 5 on "Biological Integrity." It's a thorough
description of plants and animals of the Tahoe Basin, and the issues
surrounding them.

One of those issues is the potential for wildfires. The authors analyzed
the basin's vegetation, microclimates and weather regimes. They prepared
maps and tables showing high and low wildfire potential areas. If you
have any interest in wildfires, this chapter is a must. We'll hit some
high spots.

For millennia, fires occurred regularly in Tahoe forests. They were
ignited by lightning and, probably, by Native Americans. The result was
that brush and small trees were burned off. Larger trees were able to
withstand the heat of those smaller fires because of their thicker bark
and higher limbs. So, before European types took over the basin, the
fire regime produced large, well-spaced trees.

In the latter half of the 1800s, during the Comstock lumbering boom,
Tahoe's forests were clear-cut. Now, most of the lower altitude forests
are second-growth trees, smaller and closer together. Young forests are
more susceptible to fire because trees are shorter, smaller and have
thinner bark. So, the authors conclude, a combination of decades of fire
suppression and smaller second-growth trees has increased the likelihood
of severe fire.

The odds of having wildfires are greater at lower altitudes, where
tree-stands are thicker, growing seasons are longer, and climates are
warmer and drier. At higher altitudes, it's cooler and wetter. Also,
there are more barren areas, so lightning strikes aren't as apt to start
big fires. According to the authors, sparse forests and barren stretches
along the western crest of the basin also protect us from wildfires
blowing over from the west slope of the Sierra.

Analysis of local weather associated with forest fires in the basin
shows that, on average, there are 10 days per year when the likelihood
of fire is severe. Extreme fires are more likely during hot, dry drought
days. During the last 92 years, 31 were drought years.

Lightning fires aren't as dangerous because they usually occur during
wet weather and at higher altitudes where fuel is scarcer. The authors
explain that most fires that burn more than 10 acres in the basin are
caused by humans -- camp fires, debris burning, smoking, arson, kids
playing with matches, etc. Of nine fires that burned more than 10 acres
in the basin since 1973, only one was caused by lightning.

From scars on older trees and stumps, estimates were made of fire
frequencies in the basin. The most frequent were in lower altitude,
drier sites with Jeffrey pine and Jeffrey pine-white fir forests. Higher
altitude red fir forests on the east side of the lake were also dry and
had frequent fires. Higher altitude red fir forests on the west side,
where there is more precipitation and heavier snowpacks, had fewer
fires. Wetter riparian areas and north-facing slopes are more fire
resistant.

So, what do we do to lessen fire danger in the Tahoe Basin? The authors
recommend two approaches. The first is increased fire prevention patrols
and education to reduce human-caused ignitions. Second, is to reduce
fuels in urban areas and in the urban-wildland interface. Most fires are
ignited in these regions, where the heaviest fuel loads and longest fire
seasons occur. Treating these areas would reduce fire risk for the whole
basin.

The emphasis should be on surface fuels because they increase the
intensity of fires and the speed at which they spread. Thinning trees to
reduce crown density may be needed. Yet, the authors point out, reducing
crown density is not effective if surface fuels aren't treated first and
effectively. The authors note that true crown fires are rare in the
Sierra.

Prescription burns are important tools for reducing fuel loads. And,
it's important to reintroduce fire into Tahoe Basin ecology. But,
prescribed burns are expensive and limited by weather conditions. Also,
they impact air and water quality. So, the authors recommend that the
highest priorities for prescription burns should be the remaining
patches of old-growth forest, because that's how they were created.