Aaron W. Fellows

Has fire suppression increased the amount of carbon stored in western U.S. forests

Active 20th century fire suppression in western US forests, and a resulting increase in stem density, is thought to account for a significant fraction of the North American carbon sink. We compared California forest inventories from the 1930s with inventories from the 1990s to quantify changes in aboveground biomass. Stem density in mid-montane conifer forests increased by 34%, while live aboveground carbon stocks decreased by 26%. Increased stem density reflected an increase in the number of small trees and a net loss of large trees. Large trees contain a disproportionate amount of carbon, and the loss of large trees accounts for the decline in biomass between surveys. 20th century fire suppression and increasing stand density may have decreased, rather than increased, the amount of aboveground carbon in western US forests.

Rapid vegetation redistribution in Southern California during the early 2000s drought

Climate change in semi-arid, midlatitude mountain environments is expected to shift the spatial patterns of temperature, water availability, and vegetation upslope. Vegetation growing near its low-elevation range limit may prove especially vulnerable to mortality and decline. We investigated the altitudinal pattern of conifer mortality that occurred from 2002 to 2004 in Southern Californias San Jacinto Mountains. We found that conifer mortality was focused in the lower portion of the midmontane conifer range, which drove the midmontane conifer distribution upslope. We investigated past reports of conifer mortality in Southern California by searching historical newspaper accounts. We found evidence of previous episodes of conifer mortality that coincided with past droughts, and which may have caused vegetation redistribution in the past. We interpret the early 2000s mortality and associated vegetation redistribution as a response to natural decadal to centennial climate variability. Moreover, we hypothesize this response mode will dominate the early impact of global climate change on semi-arid forest, which, in turn, may complicate efforts to distinguish between ecological changes attributable to natural climate variability and those attributable to global climate change.

Reply to comment by J. Bouldin on ''Has fire suppression increased the amount of carbon stored in western U.S. forests?''

[1] We agree with some aspects of Bouldin [2009] and welcome the opportunity to clarify aspects of Fellows and Goulden [2008, hereafter F-G]. Several of Bouldin’s claims reflect his interpretation of the core message of our paper. Bouldin feels our main message was that ‘‘fire suppression, independent of logging, has brought about a decrease in live, aboveground tree carbon via increased mortality of large trees in western U.S. forests’’. Our intended message was that we found no evidence that fire suppression has led to a large increase in forest biomass, and a suggestion, along with a hypothesized mechanism, that fire suppression has led to a loss of biomass. We feel Bouldin’s interpretation is inconsistent with our paper, which repeatedly returns to the question of whether fire suppression has increased carbon stocks, and discusses the possibility of a general loss of carbon with fire suppression as a hypothesis rather than statement of fact. Our paper generated popular press, some of which may have overemphasized the possibility of carbon loss, and Bouldin may be reacting to some of these