Long-term effects of clear-cutting and selective cutting on soil methane fluxes in a temperate spruce forest in southern Germany

Abstract

Based on multi-year measurements of CH₄ exchange in sub-daily resolution we show that clear-cutting of a forest in Southern Germany increased soil temperature and moisture and decreased CH₄ uptake. CH₄ uptake in the first year after clear-cutting (−4.5 ± 0.2 μg C m⁻² h⁻¹) was three times lower than during the pre-harvest period (−14.2 ± 1.3 μg C m⁻² h⁻¹). In contrast, selective cutting did not significantly reduce CH₄ uptake. Annual mean uptake rates were −1.18 kg C ha⁻¹ yr⁻¹ (spruce control), −1.16 kg C ha⁻¹ yr⁻¹ (selective cut site) and −0.44 kg C ha⁻¹ yr⁻¹ (clear-cut site), respectively. Substantial seasonal and inter-annual variations in CH₄ fluxes were observed as a result of significant variability of weather conditions, demonstrating the need for long-term measurements. Our findings imply that a stepwise selective cutting instead of clear-cutting may contribute to mitigating global warming by maintaining a high CH₄ uptake capacity of the soil.

Introduction

Methane (CH₄) is known to be the third most important greenhouse gas after water vapor and CO₂ and currently contributes approximately 20% to global warming (IPCC, 2007). Its concentration in the atmosphere has more than doubled to ca. 1.8 ppmv during the past 200 years, mostly as a result of fossil fuel exploration, expansion of paddy rice cultivation, large-scale animal husbandry of ruminants, biomass burning and landfills (Crutzen, 1991, Le Mer and Roger, 2001). Aerated terrestrial soils act as a global sink for atmospheric CH₄ (10–44 Tg yr⁻¹), accounting for up to 10% of the global atmospheric CH₄ sink (Dutaur and Verchot, 2007). Therefore, slight alterations of the soil CH₄ sink as a result of land management or environmental changes may significantly alter the global methane budget.

Temperate forest upland soils are generally net sinks of CH₄ (Butterbach-Bahl and Papen, 2002, Borken et al., 2003). Consumption of atmospheric CH₄ by soils is microbially mediated and consequently sensitive to various environmental factors and disturbance by management (Ojima et al., 1993, Tate et al., 2007). In view of the huge spatial and temporal variability of environmental factors and the limited number and restricted temporal coverage of long-term field measurements, the annual CH₄ sink of temperate forest soils is still afflicted with a high degree of uncertainty, ranging from 0.01 to 0.64 g CH₄ m⁻² yr⁻¹ (Smith et al., 2000, Dutaur and Verchot, 2007).

Forest selective cutting and clear-cutting are the management practices most frequently used for silvicultural rotation and timber harvesting worldwide (Zerva and Mencuccini, 2005, Dannenmann et al., 2007). There is a growing number of studies pointing out that forest clear-cutting might cause a reduction in soil CH₄ consumption (Kagotani et al., 2001, Huttunen et al., 2003, Saari et al., 2004), or even switch of soils from being net CH₄ sinks to being net sources (Castro et al., 2000, Zerva and Mencuccini, 2005) by altering the biotic and abiotic factors (e.g. soil temperature, soil water content, root activity, soil nitrogen dynamics). However, very little is known about the effects of forest selective cutting on soil-atmospheric exchange of CH₄ and the existing results are not consistent. Sullivan et al. (2008) reported that methane uptake was not affected by selective cutting in ponderosa pine forest soils within one year of management. A study by Dannenmann et al. (2007) showed that forest selective logging led to a distinct reduction in CH₄ uptake in the first year after tree felling, but no significant effects of selective cutting on the magnitude of CH₄ exchange could be observed at another site 4–6 years after felling. However, an increased net CH₄ consumption after selective logging has also been reported by Bradford et al. (2000), who carried out a laboratory incubation study by collecting intact soil cores from Grizedale Forest, UK. Furthermore, most studies on the effect of forest management on soil CH₄ fluxes were performed over a relatively short period (≤2 years) or with low measurement frequencies (monthly or weekly), but the long-term effects on soil CH₄ consumption still remain unclear.

Therefore, the main objectives of this study were (1) to establish a long-term dataset (10 yrs) of continuous sub-daily soil CH₄ fluxes from a temperate forest, (2) to evaluate the long-term effects of clear-cutting and selective cutting on CH₄ exchange, (3) to assess the impact of environmental factors and soil parameters on CH₄ exchange at three sites with different management activities, and (4) to quantify annual CH₄ budgets.