Short-term effects of thinning on soil CO2, N2O and CH4 fluxes in Mediterranean forest ecosystems

Abstract

In Mediterranean ecosystems an increasing demand for in situ trace gas exchange data is emerging to enhance the adaptation and mitigation strategies under forest degradation. Field-chamber green-house gas fluxes and site characteristics were analysed in two Mediterranean peri-urban pine forests showing degradation symptoms. We examined the effect of different thinning interventions on soil CO₂, CH₄ and N₂O fluxes, addressing the relationships with the environmental variables and C and N contents along forest floor-soil layers. Soil temperature resulted as the main driving variable for CO₂ efflux and CH₄ uptake. Soil moisture content and organic matter availability affected CO₂ emission patterns in the two sites. N₂O fluxes showed a positive correlation with soil moisture under wetter climatic conditions only. GHG fluxes showed significant correlations with C and N content of both forest floor and mineral soil, especially in the deepest layers, suggesting that it should be considered, together with environmental variables when accounting GHG fluxes in degraded forests. Short-term effects of thinning on CO₂ emissions were dependent on disturbance induced by logging operations and organic matter inputs. After thinning CH₄ uptake increased significantly under selective treatment, independently from specific site-induced effects. N₂O fluxes were characterized by low emissions in both sites and were not affected by treatments. Soil CO₂ efflux was the largest component of global warming potential (GWP) from both sites (11,553 kg ha^-1 y^-1 on average). Although it has a large global warming potential, N₂O contribution to GWP was about 131 kg CO₂eq ha^-1 y^-1. The contribution of CH₄-CO₂ equivalent to total GWP showed a clear and significant CH₄ sink behaviour under selective treatment (36 kg ha^-1 y^-1 on average). However, in the short-term both thinning approaches produced a weak effect on total GWP.

Keywords: Forest floor; Global warming potential; Green-house gas fluxes; Pine plantations restoration; Soil moisture; Soil temperature.