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 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 Letters1.
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.
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."
Fellows, A. & Goulden, M. Geophys. Res. Lett. doi:10.1029/2008GL033965 (2008).