Reply to Loehle, C. Comment on “Hanson, C.T. Cumulative Severity of Thinned and Unthinned Forests in a Large California Wildfire. Land 2022, 11, 373”

Abstract

In Hanson (2022), I found that commercially thinned forests had significantly higher overall tree mortality, or cumulative severity, from the combination of pre-fire commercial thinning and the Antelope fire of 2021 in northern California. A comment on Hanson (2022) by Loehle (2025) does not dispute my central finding about cumulative severity but suggests that the findings might be limited to the Antelope fire, that differences in accuracy might exist between tree mortality estimates for thinning versus fire, and argues that there are other benefits from commercial thinning, such as financial gains for logging companies and reduced rate of fire spread in low-density forests. In this reply, I address these and other criticisms and explain why they do not affect my results or conclusions.

1. Introduction

In Hanson (2022) [1], I reported that cumulative severity in the 2021 Antelope fire in northern California was significantly higher in commercially thinned forests compared to unthinned forests, due to the combined tree mortality from both thinning and fire. A comment by Loehle (2025) [2] argues that (a) the spatial scale of the commercial thinning in the Antelope fire is inadequate to draw a broader conclusion about cumulative severity; (b) an error in Table 1 of Hanson (2022) [1] slightly altered my conclusions; (c) thinning may have provided some protection for remaining trees following thinning; (d) differences in accuracy might exist between tree mortality estimates from thinning and tree mortality estimates due to the Antelope fire; (e) unthinned forests may have higher delayed post-fire mortality in later years; (f) reducing forest density has other benefits, including financial gains for the logging industry and reduced rate of wildfire spread; and (g) even if commercial thinning increases cumulative severity, Hanson (2022) [1] did not document any benefit of forests with higher basal area, or negative impacts of higher overall tree mortality and loss due to commercial thinning. I briefly address each of these arguments below.

2. Analysis and Discussion

First, with regard to the suggestion that the spatial scale of the commercial thinning units in the Antelope fire of 2021 is insufficient to draw broader landscape-scale conclusions about cumulative severity from commercial thinning, I specifically noted this myself in Hanson (2022) [1], which is why I published it as a Communication, and stated the following: “More research is needed, in other large forest fires, to determine whether the finding, that commercial thinning killed more trees than it prevented from being killed, is common elsewhere.” Since Hanson (2022) was published, I co-authored a study of another large 2021 fire in northern California, the Caldor fire, and found that cumulative severity was significantly higher in commercially thinned forests (both in thin/burn and thin/no-burn areas) than in unthinned forests [3].

Second, Loehle (2025) [2] is correct that there was a typographical error for one of the 54 locations in Table 1 of Hanson (2022) [1], but incorrect in suggesting that this altered my results or conclusions. For location 19-3-in (thin/no-burn), the numerical entry in the column for percent basal area mortality from thinning should have read 43%, just as it did for the other commercial thinning location, 19-1-in, within commercial thinning Unit 19. This had no effect whatsoever on my results or conclusions because the Antelope fire resulted in 100% basal area mortality in location 19-3-in, thus cumulative severity was 100% from the fire alone there. Even if fire-related mortality had been less than 100% at location 19-3-in, which it was not, 43% basal area mortality from commercial thinning at that location would have increased cumulative severity, and would have slightly strengthened my results, not the opposite.

Third, with regard to the argument that commercial thinning may have provided more protection for the remaining trees after thinning, Loehle (2025) [2] acknowledged that there was no statistically significant difference in basal area mortality from fire alone between thinned and unthinned forests, and that was after excluding the pair of locations with the typographical error (19-3-in and 19-3-out). Therefore, the suggestion that commercial thinning may have led to greater protection for remaining trees is rejected by Loehle’s own analysis. Moreover, if pair 19-3 was included, the thinned location (19-3-in) had 100% basal area mortality from fire and the unthinned location (19-3-out) had only 11% basal area mortality from fire [1]. Loehle (2025) [2] asserts that, based on my data, fire-caused tree mortality is 20% higher in unthinned forests relative to thinned forests. This is factually inaccurate. As I reported, fire-related mortality alone (not including thinning) was 34.7% in thinned forests versus 38.6% in unthinned forests—a difference of less than 4 percent.

Fourth, no citations to any source or evidence of any type is offered by Loehle (2025) [2] to support the assumption about differences in accuracy between tree mortality estimates from thinning and tree mortality estimates from the Antelope fire. Thus, the criticism has no foundation.

Fifth, similar to the fourth point, no citations to any source or evidence of any type is offered by Loehle (2025) [2] to support the assumption that unthinned forests will have higher delayed post-fire mortality in later years. Thus, this criticism also has no foundation.

Sixth, as an ecologist, the extent to which logging companies or agencies financially benefit from logging is beyond the scope of my scientific research, so I will not comment on that point. The suggestion that commercial thinning provides benefits by slowing the rate of wildfire spread is unsupported by Loehle (2025) [2], and no citations to sources or evidence are offered. This assumption is contradicted by Stephens et al. (2021) [4], which reported higher rate of wildfire spread in commercially thinned forests relative to denser unthinned forests.

Last, with regard to the assertion that Hanson (2022) [1] did not document any benefits of forests with higher basal area, or any negative effects of higher overall tree mortality and lower basal area, that is incorrect. I provided in Hanson (2022) [1] citations establishing that commercial thinning substantially increases carbon emissions per hectare relative to wildfire alone [5,6,7], a finding that has since been supported by additional research [8]. I also provided in Hanson (2022) [1] citations establishing that commercial thinning causes significant adverse impacts to numerous imperiled wildlife species [9,10,11,12,13,14].