Fire-Free Forests Store Less Carbon

Note the US Forest Service official’s quote at the end: smells like something’s afoot…

ASW

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“The findings run contrary to expectation. It was
thought that more trees meant more carbon being
drawn from the atmosphere. ‘If you suppress fires
and lots of little trees show up, then you ought
to store more carbon,’ says ecologist Richard
Houghton of the Woods Hole Research Center in
Falmouth, Massachusetts.”
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Nature
14 May 2008   doi:10.1038/news.2008.818

News

Forest-fire management ‘raises carbon emissions’

California study suggests fire-free forests store less carbon.

Quenching forest fires leads to more carbon in
the air, says new research carried out in
Californian forests. The discovery suggests that
forests spared from fire may release more of the
greenhouse gas into the air than they absorb.

Decades of suppressing natural fires has
increased the number of surviving trees in
California’s forests. But this growth has been at
the expense of larger trees, which are less
resilient to drought and other stresses than
smaller, younger trees, resulting in a decline in
the total amount of carbon stored in these
forests.

Between the 1930s and the 1990s, thickening
wilderness (sic) forests have seen such a drop in
biomass that they now store one-third less carbon
than they used to, report Aaron Fellows and
Michael Goulden at the University of California,
Irvine. Their results will be published in
Geophysical Research Letters (1).

The findings run contrary to expectation. It was
thought that more trees meant more carbon being
drawn from the atmosphere. “If you suppress fires
and lots of little trees show up, then you ought
to store more carbon,” says ecologist Richard
Houghton of the Woods Hole Research Center in
Falmouth, Massachusetts.

This forest thickening was thought to be one
reason why climate researchers see more carbon
dioxide absorption in the northern mid-latitudes
than they can account for. But thickening in
California seems to turn forests into emitters of
carbon by reducing the total amount of biomass.

Small stands

Extensive historical data on tree density is
relatively rare, but Goulden and Fellows found an
inventory of California forests compiled in the
1930s. They compared the data with forest surveys
done in the 1990s in similar areas.

Overall, the number of trees had increased, with
mid-altitude conifer forests showing the greatest
growth. During the 60-year interval, the density
of those trees increased by 34%. But the total
amount of tree vegetation, and thus the amount of
carbon stored, actually decreased by 26%.

“The reason for that is that not all trees are
the same,” says Goulden. “For every big tree you
lose, you actually need 50 small trees to offset
that amount of carbon.”

In cases of drought, smaller trees quickly mop up
scarce water, which leaves larger trees
vulnerable. The authors hypothesize this
competition is the reason for the decline in
large trees.

Before human intervention, forest fires in
California burned low to the ground and were more
likely to burn young saplings and ground
vegetation, thinning out the understory. The
large, mature trees were resistant to these fires.

Source or sink

From the 1930s to the 1990s, the Californian
forests surveyed released an estimated 0.7 tonnes
of carbon per hectare per year. A healthy,
growing hectare of forest will absorb two to
three tonnes of carbon a year, says Goulden.

The average amount of carbon emitted by the
forests is small when compared with the 1.6
million tonnes of carbon released in the United
States by the burning of fossil fuels in 2003
alone. Earth’s land and oceans absorb some of
this carbon, and atmospheric measurements
indicate that North America is absorbing more
carbon dioxide than researchers can account for.

But the results suggest that forest thickening
might not account for this ‘missing carbon sink’.

“What everyone has been assuming to be one of the
terrestrial carbon sinks seems not to be,” says
Houghton. “This is the first time that I know of
when someone’s actually had measurements early
enough to really evaluate where carbon was stored
and where it was lost.”

It is hard to say what has happened to the
forests since the mid-1990s, but the thickening
may have continued, Goulden says.

Forests are now too thick to allow ground fires
to take their course. A fire would probably burn
both small and large trees. The forests “will
always have to be managed,” says Sue Exline, a
spokesperson for the Sierra National Forest in
California. “Because of the influence of society,
we can’t bring the forest back and have it take
care of itself.”

*
References:

1. Fellows, A. & Goulden, M. Geophys.
Res. Lett. doi:10.1029/2008GL033965 (2008).

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