Medium-term dynamics of soil respiration in a Mediterranean mountain ecosystem: The effects of burn severity, post-fire burnt-wood management, and slope-aspect

Abstract

The interaction between burn severity and subsequent post-fire logging practices may exert a direct effect on soil respiration (SR) in recently burnt stands. This effect is associated with the modification of microclimatic conditions, soil carbon inputs, and the decay rates of woody detritus. In addition, slope aspect may determine SR rates by modifying the microclimatic conditions in post-fire environments. In this study, we assessed the changes in SR rates during the early (1.5–4 years) post-fire stages in a burnt and logged Spanish Black pine forest along a burn-severity gradient: (1) an unburnt site (UB), (2) a low burn-severity site (LS), (3) a south-facing high burn-severity site (HSS), and (4) a north-facing high burn-severity site (HSN). Monthly or fortnightly manual SR measurements (SRM) were taken at midday between 2011 and 2013. In addition, we also quantified the litter layer and fine-root biomass at each site. Multiple regression models combining abiotic (both soil temperature and water content, Ts and SWC, respectively) and biotic (tree diameter at breast height, 1.30m) were used for midday SRM modelling. For temporal and spatial scaling of SR at the stand-level, we performed 8 seasonal campaigns of automated SR measurements (SRA) along 4 linear gradients from trees or stumps to inter-tree/stump gaps with the aim of: (1) determining the main soil-surface areas (soil close to trees or stumps and/or soil away from them; SC and SA soil, respectively) and (2) correcting the modelled daily daytime and night-time SRM rates.Our results showed a significant reduction in fine-root biomass at the burnt sites, although over the 3-year study period this figure did tend to increase. The amount of litter was similar between sites suggesting that logging practices may influence the amount of post-fire detritus. We also found that the spatial variability in soil microclimate was high and was primarily influenced by post-fire canopy cover as well as the slope aspect. SR rates significantly varied between years and were higher in SA soil compared to SC soil at all the sites we studied. Higher SR rates were observed close to stumps compared to those close to trees, highlighting the large relative contribution of this decaying debris to post-fire SR rates. In the medium-term, the combined effect of fire and logging practices may lead to an increase in the annual stand-level SR at high burn-severity sites after fire. In contrast, the interaction between fire and logging practices did not alter the post-fire SR rates at the LS site during the same time period. Our results also underscored the dependence of post-fire SR rates on the slope aspects within the high-severity fire area. Finally, this study showed that our integrated spatio-temporal approach could be a useful tool for obtaining more accurate SR estimates for short-, medium- and long-term stand-scale carbon cycling studies in burnt areas.