The most commonly used indicator of forest cutting in the United States is volume removed, which has been tracked by the USFS FIA in a relatively consistent manner for a long time period22. Therefore, we compared our estimate of the bole C loss with multiple studies based on FIA inventories. Since our estimate is for the eastern United States (e.g., 104 Tg C yr−1 for 2002–2010), the total removal of bole (e.g., 128 Tg C yr−1) was calculated based on the assumption that the removal in the western United States accounts for 19% of the total removal in the conterminous United States, proposed by Oswalt et al.23, which is comparable to the average of previously published estimates (115 ± 19 Tg C yr−1) (Table 2)8,12,13,14,15,16,17,24,25,26. These estimates might not be comparable in a strict sense as they represented estimates for different time periods (experienced various land use practices) using different inventories and calculation methods. The purpose of this comparison is to provide consistency and verification check on our calculation procedures. However, most of the previous estimates are based on periodic inventories and empirical models or process models; the results were highly dependent on the capability of the inventories and the models in tracking forests changes11,16,27. Apparently, the varying sampling designs and data collection methods of periodic inventories would introduce large uncertainties into detecting the nation's forest dynamics by comparing the successive inventories directly28. In addition, the accuracy of the model, if utilized, depended strongly on the model parameterization16. In contrast, we estimated the bole C loss in live biomass using the re-measured plots in annualized forest inventory data directly. The high consistency of the collected data ensured an unprecedentedly direct and integrated quantification of U.S. forest cutting and its impacts on C dynamics in this study10,20.

Table 2 Comparison of live C loss in bole (Tg C yr−1) for the conterminous United States from this study and a sample of previous estimatesa Full size table

Top-limbs, stump and belowground biomass of the removed trees together were estimated to account for 38% of the total C loss in this study. These sectors can exert substantial impacts on the C cycle since 1) the top-limbs of the removed trees are an important source of woody debris and their post-treatments have a great impact on the C cycle29; and 2) the cutting-related loss of live biomass in stump and belowground roots would increase the down deadwood in the forest ecosystem30. Unfortunately, all of the components were usually ignored or simplified in the cutting-related C accounting15,16,24,25. Therefore, it is important to consider the C dynamics of the other sectors of trees induced by forest cutting disturbances besides the bole biomass.

Partial cutting, usually ignored in large-scale C accounting4,8,21, was found to be the dominant activity in the eastern United States (Figure 2), which was broadly in agreement with earlier estimates22,23. We further revealed that partial cutting was the major cutting practices regardless of forest type, stand age and geographic location (Figures 3 and 5, Table 1). The C changes following partial cutting differ greatly from the well-known clear-cutting events31. For instance, most studies reported a decrease in the total ecosystem C stocks following the direct removal of live tree biomass via clear cutting32,33. On the contrary, partial cutting was documented to exert variable impacts on the total ecosystem C stocks34,35. Thus, our results highlight the critical role of partial cutting in regional and global C budgets.

The cutting activities occurred at different rates among forest types. Overall, softwood forests experienced more intensive cutting activities than hardwood and mixed forests (Table 1), mainly because of the high productivity of softwood that attracted large investments in practicing high-intensity forestry22. However, hardwood cutting accounted for a larger amount of total C loss relative to softwood harvesting, which was attributed mainly to the substantially large forest area (Table 1) and high merchantable biomass of timber on the landscape taken by hardwood23. That justifies a comparable amount of C loss to softwood (hardwood vs. softwood: 81 vs. 75 Tg C yr−1) even with a significantly lower C loss per unit forest area (0.79 vs.1.96 Mg C ha−1 yr−1).

Softwood was mostly cut at a much younger age than hardwood and mixed forest was in between (Figure 4a). Interestingly, the C loss density decreased substantially after a dramatic increase for softwood, but it remained nearly stable after a gradual increase for hardwood along cutting ages (Figure 4b). This feature can be attributed to both natural and economic factors. First, the frequency distributions of the forestland area across various forest types (Figure 7a) is closely linked to the C loss distributions (Figure 4a) over age gradients (with the square correlation coefficients of 0.86, 0.69 and 0.53 for hardwood, softwood and mixed forests, respectively), suggesting the pre-disturbance forest area is a major factor in determining cutting events. Second, the rapid growth of softwood ensures younger-age harvesting in softwood22, which can be seen by the differences of frequency distributions between C loss and forest area (Figures 4a, 7 a and 7 b) over age gradients. For example, the frequencies of C loss in age 20–60 for softwood were greater than the frequencies of forest area over the same age ranges (i.e., the ratio in Figure 7b was more than 1). By contrast, the large and stable C loss density in hardwood over age 60 may be due mainly to the high and stable pre-disturbance live C density in old-age hardwood37, indicating by a relative larger frequency in C loss than in hardwood forest areas over age 60.

Figure 7 The frequency distributions of forest area (a) and the ratios between the frequencies of total C loss and the forest area derived from both Figures 4a and 7a (b) over different age ranges for each forest type. Full size image

Cutting-related C loss showed a large geographical heterogeneity. In the northern portion of the eastern United States, the Northeast experienced the largest C loss, followed by Northern Lake States and Northern Prairie States (Table 1), which can be primarily explained by the availability of their pre-disturbance live biomass15 or forest area. The region with a large forest area was estimated to share a large live C loss (Table 1). The southern regions of the eastern United States, however, accounted for a substantially greater amount of C loss and had a higher C loss per unit forest area than the North (Table 1), although their pre-disturbance live C densities are less than those in the Northeast15. This can be mostly attributed to the fast growth conditions and large area allocated to forestry use in the South and forest management policies11,22. First, the southern portion of the eastern United States contributed 54% to the forest area in the eastern United States (Table 1) and over half of the area was allocated to forestry use36, which provides a strong foundation for forest cutting activities. Second, the high productivity and rapid growth conditions in the South mean a high-return investment and thus this region usually experienced high-intensity forestry22,37. Finally, public policy greatly affected the rate of forest cutting. For example, timber harvest on federal lands in the Northwest declined since the enactment of Northwest Forest Plan in 199338; consequently, harvests increased on private lands that were largely distributed in the southern portion of the eastern United States10,36.

This study estimated that the total cutting-related loss of live biomass in the eastern United States was 168 Tg C yr−1 in 2002–2010, which was equivalent to 70% of the total U.S. forest C sink (240 Tg C yr−1)5 and 11% of the national annual CO 2 emissions from fossil-fuel combustion over the same period39, emphasizing a great potential to mitigate climate change by forest management.

However, the C loss estimated in this study does not equate to the net cutting-related C emissions as some of the dead biomass is not returned immediately to the atmosphere but remains stored in a durable status such as in wood products19,40, which (if long-lived) can be considered a C sink8. In contrast, emissions associated with forest cutting from combustion, decomposition of debris, disturbed soil, the slow decay of leaves, wood and roots and harvested wood products are potentially large sources of C to the atmosphere12,16 and the source is likely to be strengthened by the reduced C accumulation rate due to the removal of leaf area (which is the physiological basis for tree productivity41). These uncertainties demonstrate the importance of a systematic quantification of the C fate in each forest sector following forest cutting.