Comment on “Has fire suppression increased the amount of carbon stored in western U.S. forests?” by A. W. Fellows and M. L. Goulden

[1] Fellows and Goulden [2008, hereafter F-G] have recently claimed 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 over a 60 year span. However the study, restricted for unspecified reasons to part of California, has several problems that I believe collectively invalidate their principal conclusion. I detail these here in increasing order of perceived importance. [2] Beginning with the interpretation of the literature, the suppression-induced carbon sink rates of 2.3 Mg/ha/yr contested by F-G [from Houghton et al., 1999], apply only to the highly fire-prone western ponderosa pine woodlands, as stated by Houghton et al. [1999, Table 2]. Houghton et al. [1999] were also clear that their estimate was [Houghton et al., 1999, p. 575] ‘‘an upper limit for the sink strength of U.S. ecosystems.’’ Ponderosa pine forests occur generally at low tomid elevations, but F-G analyzed data only fromCalifornia wilderness areas, which on the whole occupy the higher elevations and contain large areas of other forest types, such as true fir forests and subalpine woodlands [Barbour and Minnich, 2000]. A significant fraction of their plots were from above 1830 m (6000 ft), or even 2438 m (8000 ft), beyond the primary range of ponderosa pine forests in California. The effects of fire suppression in western US forests are well known to vary strongly with environment and forest type [e.g., Agee, 1993]. [3] Both the 1930s (Vegetation Type Mapping Project (VTM)) and 1990s (Forest Inventory Analysis (FIA)) data reportedly used by F-G are potentially problematic, based on specific knowledge of their purposes and methods, and the claims made by F-G [Bouldin, 1999; Keeley, 2004; Waddell and Hiserote, 2005] (see also United States Forest Service, Forest Inventory and Analysis National Program, data and tools,http://fia.fs.fed.us/tools-data/default.asp,2009,PNW-FIA data, http://www.fs.fed.us/pnw/fia/publications/data/index. shtml#1a, 2009, andPacificSouthwestRegion,RemoteSensing LaboratoryVegetation Inventory Data, http://www.fs.fed.us/ r5/rsl/projects/inventory/designs.shtml, 2009 and VTM, http:// vtm.berkeley.edu/, 2009). Regarding the FIA data, F-G indicate in their Table 1 that they use 140 wilderness FIA plots sampled between 1990 and 1994 in northern California and/or the Sierra Nevada. The only FIA plots installed in wilderness areas in that region and at that time were in the Sierra Nevada and far northwestern California. Since almost no VTM plots were collected in the latter for the necessary comparisons, this implies that all 140 FIA plots must have been located in eastern northern California (ENC: SierraNevada, Cascades and Modoc Plateau), but this exceeds the number ( 106) actually collected there. Of these, 87 (83%) were located on the Inyo National Forest, an area dominated by cold and/or dry alpine/subalpine or woodland ecosystems above 2438 m (8000 ft) elevation. Conversely, most VTM plots in ENC were established on the much more productive west side of the Sierra Nevada (VTM, http://vtm.berkeley. edu/, 2009). It would not be at all surprising to find an apparent decrease in carbon if this was in fact the comparison made. If, on the other hand, the authors only used that minority of VTM plots from the colder Inyo NF high elevations, one could well question how this represents the effect of fire reduction on carbon accrual in California forests as a whole, much less the entire western United States. [4] There is also a definitional issue relating to the plot selection criterion. The authors implicitly defined forested areas as those having a minimum of 1 tree per plot ( 12 trees/ha for the 0.08 ha VTM plots). Given that much of the California landscape was responding to fire reduction policies over the 20th century [Skinner and Chang, 1996], including upper montane forests where the evidence of catastrophic fires in the 19th century is strong [e.g., Leiberg, 1902, pp. 37, 41, 106, and 117], the area meeting this criterion may well have increased over time. This would affect both the area of the landscape sampled, and the demographic character of the plots included in the sample, changes which must be accounted for in regionwide estimates of forest carbon dynamics. [5] Relevant empirical studies bearing on the topic are scarce, as it is not trivial to separate out fire suppression effects from confounding factors such as logging, climate change and other human impacts. However, the available studies do not support F-G’s conclusion. Although climate change [e.g., Pierce et al., 2008] is indeed likely causing increased tree mortality in old growth forests in the western United States in recent decades [van Mantgem et al., 2009] this does not mean that fire reduction over the last 100 to 150 years has not had a much greater, opposing carbon impact. For example, Leiberg [1902], in a thorough early survey, noting the strong effect of fire on tree demography throughout the northern Sierra Nevada, stated [Leiberg, 1902, p. 85] ‘‘. . .this region does not contain more than 35 percent of the timber it is capable of carrying, and the deficiency is wholly due to forest fires. . .’’, and Gruell [2001] has provided long-term photographic evidence of both forest expansion and increased density and canopy cover in many locations in the Sierra since the 19th century. GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L21403, doi:10.1029/2009GL039391, 2009 Click Here for Full Article