Forest soil carbon is threatened by intensive biomass harvesting

Abstract

Forests play a key role in the carbon cycle as they store huge quantities of organic carbon, most of which is stored in soils, with a smaller part being held in vegetation. While the carbon storage capacity of forests is influenced by forestry, the long-term impacts of forest managers' decisions on soil organic carbon (SOC) remain unclear. Using a meta-analysis approach, we showed that conventional biomass harvests preserved the SOC of forests, unlike intensive harvests where logging residues were harvested to produce fuelwood. Conventional harvests caused a decrease in carbon storage in the forest floor, but when the whole soil profile was taken into account, we found that this loss in the forest floor was compensated by an accumulation of SOC in deeper soil layers. Conversely, we found that intensive harvests led to SOC losses in all layers of forest soils. We assessed the potential impact of intensive harvests on the carbon budget, focusing on managed European forests. Estimated carbon losses from forest soils suggested that intensive biomass harvests could constitute an important source of carbon transfer from forests to the atmosphere (142-497 Tg-C), partly neutralizing the role of a carbon sink played by forest soils.

Figure 1. Distribution of the sites used in this meta-analysis on the effects of conventional and intensive harvests on soil C stocks.

See more details on the geographical location of the sites in Fig. S1. Map created in Python Language version 2.7 (Python Software Foundation; www.python.org), using the basemap package (https://pypi.python.org/pypi/basemap/1.0.7) of the matplotlib library (http://matplotlib.org).

Figure 2. General effects of conventional and intensive harvests on SOC stocks as a function of soil depth (individual soil layers) and in the entire soil profile (cumulated soil layers).

(A) Effects of conventional harvest (clear-cutting and thinning; means ± standard errors). (B) Effects of intensive harvest compared with stem-only harvest (means ± standard errors). (C) Combined effects of conventional and intensive harvests. Values are expressed as relative responses: (A) log(clear-cutting or thinning harvest/unharvested control) (B) log(whole-tree harvest/stem-only harvest) (C) log(whole-tree harvest/unharvested control). For the sake of clarity, comparisons between treatments and controls are also presented as the mean arithmetic difference (in italics, expressed in %). Results in (C) were obtained using the two datasets (data in A,B) and a bootstrap resampling method. For each panel, number of case studies (or sites) and number of bootstrap samples are shown in italics to the right of each bar. There were not enough data for FTMD in (B). Significant differences between relative responses and 0 are denoted by an asterisk (t test). See more results in Fig. S4.

Figure 3. Effects of conventional and intensive harvests on SOC stocks in forest floor (F) and top mineral soil (T) in relation to time elapsed since harvesting.

(A) Forest floor. (B) Top soil. Effects were assessed considering two periods (0–10 years and > 10 years since harvesting; Means ± standard errors). Values are expressed as relative responses: log(clear-cutting or thinning harvest/unharvested control) or log(whole-tree harvest/stem-only harvest). For the sake of clarity, comparisons between treatments and controls are also presented as the mean arithmetic difference (in italics, expressed in %). Number of case studies (or sites) ranged from 16 to 100. Significant differences between relative responses and value 0 are denoted by an asterisk (t test). The P values in brackets were calculated using all intensive harvest treatments (whole-tree harvest and whole-tree + forest floor harvest). Effects of conventional clear-cutting on C stocks in the forest floor are shown for more time classes in Fig. 4.

Figure 4. Effects of conventional clear-cutting harvest on SOC stocks in forest floor (F) in relation to time elapsed since harvesting.

Effects were assessed considering five periods (0–2, 2–5, 5–10, 10–20 and >20 years since harvesting; Means ± standard errors). Values are expressed as relative responses: log(conventional clear-cutting harvest/unharvested control). For the sake of clarity, comparisons between treatments and controls are also presented as the mean arithmetic difference (in %). Number of case studies (or sites) ranged from 12 to 31. Significant differences between relative responses and value 0 are denoted by an asterisk (t test). Temporal changes associated with other harvest types are shown in Supplementary Information (conventional harvest at thinning: Fig. S2; intensive harvest: Fig. S3).

Figure 5. Effects of intensive harvest on C stocks in mid soil (M) related to mean annual temperature (MAT) and effective evapotranspiration (ETR).

(A) MAT; (B) ETR. Values are expressed as relative responses [log(whole-tree harvest/stem-only harvest)]. For the sake of clarity, comparisons between treatments and controls are also presented as the mean arithmetic difference (% higher or lower). A similar trend (P < 0.1) was observed between ETR and SOC losses for the topsoil layer also (data not shown).

Figure 6. Effect of intensive harvest on C stocks in the forest floor (F), top soil (T) and mid soil (M) related to Köppen climate classes.

(A) forest floor (all sites or selected sites (time elapsed since harvesting <10 years)); (B) top soil; (C) mid soil (all sites). Means ± standard errors. Values are expressed as relative responses [log(whole-tree harvest/stem-only harvest)]. For the sake of clarity, comparisons between treatments and controls are also presented as the mean arithmetic difference (% higher or lower). Number of sites ranged from 7 to 23 (insufficient data for tropical climates). Significant differences between relative responses and value 0 are denoted by an asterisk (t test). There were also significant differences among classes (ANOVA, P = 0.040 for the mid soil, P = 0.078 for the forest floor with selected sites (0–10 years)).