When drought meets forest management: Effects on the soil microbial community of a Holm oak forest ecosystem.

The growth and survival of plants in semiarid Mediterranean forests can be improved through the benefits conferred by thinning, a forest management practice that removes trees and reduces the competition between the remaining ones. Here, we evaluate the impacts of induced drought (the exclusion of 25% of the natural rainfall for 5 years) and thinning, and their interaction, with the objective of determining whether the thinning of Holm oak (Quercus ilex L.) modulates the resistance of the soil microbial community to drought. Sequencing of 16S rRNA and ITS amplicons revealed that drought, thinning, and their interaction influenced the composition of the bacterial community, while the fungal community was exclusively affected by thinning. Thinning consisted of the removal of the aboveground parts of the Holm oak trees, which were thereafter left in forest stand. Thinning contributed to the C and N contents, with parallel increases in microbial biomass, particularly in summer. Drought increased the amounts of total organic C and total N, likely due to the reduced enzyme activities. Indeed, the composition of the bacterial community was modulated primarily by the indirect and long-term effects of drought - the accumulation of soil organic matter - rather than by the direct effect of the lower water content imposed by the drought treatments. Thinning under drought conditions did not increase soil organic C (SOC) content. However, the resistance of the soil microbial community to drought was fostered by thinning, particularly at the functional level, as indicated by the enzyme activities related to C, N and P cycles. These responses were associated to variations in the composition of the microbial communities in thinned, drought-exposed plots, in comparison to unthinned, drought-exposed plots. In conclusion, the interaction between forest management and drought influenced the soil microbial community of a Holm oak-dominated Mediterranean ecosystem.

Aboveground autotrophic respiration in a Spanish black pine forest: Comparison of scaling methods to improve component partitioning.

Total wood CO2 efflux (Rw) varies vertically within individual trees, and leaves experience large variations in foliar respiration (Rf) rates over their life spans and during daily periods. Therefore, accurate sampling approaches are required to improve aboveground autotrophic respiration (RAa) estimations in stand-scale carbon cycling studies. We scaled-up Rw (comprising stem and branch CO2 efflux; ES and EB, respectively) and Rf from biometric and flux-chamber measurements taken between 2011 and 2013 in a Spanish black pine (Pinus nigra Arn. ssp. salzmannii) forest at an unburnt (UB) site and a low burn-severity (LS) site. We measured seasonal ES at breast height (1.30m) on 9 trees at each site, which was also vertically examined on 5 of those trees. We also measured seasonal Rf in current- and previous-year needles on 3 trees at each site, and quantified Rf variations in darkness and light. Finally, we compared complex and simple scale-up methods which did or did not account for the vertical variation in Rw and the effects of leaf ageing and light inhibition on Rf, respectively. The simple methods underestimated the annual stand-level stem, branch, and total wood respiration ≈35%, 55%, and 41%, respectively, and overestimated annual stand-level whole-canopy foliage respiration ≈43% at both sites. Both methods provided similar annual stand-level RAa estimates, although the complex methods improved estimations of the relative contribution of RAa components. Thus, based on the complex methods the mean annual RAa at the stand-level was 4.53±0.25 and 4.45±0.12MgCha-1year-1 at the UB and LS sites, respectively. Our data also confirmed that the low-severity fire did not alter the RAa rates. Collectively, this study reveals that complex approaches, applicable in other forest ecosystems, enhance the accuracy of partitioning RAa sources by reducing the error in scaling-up in chamber-based measurements.

Influences of recovery from wildfire and thinning on soil respiration of a Mediterranean mixed forest.

The ecosystem recovery after wildfire and thinning practices are both key processes that have great potential to influence fluxes and storage of carbon within Mediterranean semiarid ecosystems. In this study, started 7years after a wildfire, soil respiration (SR) patterns measured from 2008 to 2010 were compared between an unmanaged-undisturbed mature forest stand (UB site) and a naturally regenerated post-wildfire stand (B site) in a Mediterranean mixed forest in Spain. The disturbed stand included a control zone (unthinned forest, BUT site) and a thinned zone (BT site). Our results indicated that SR was lower at naturally regenerated after fire sites (BUT and BT) than at unburnt one. Soil under the canopy layer of pine and oak trees exhibited higher SR rates than bare or herbaceous layer soils, regardless of the site. The effect of thinning was only manifest, with a significant increase of SR, during the 1st year after thinning practices. SR showed a clear soil temperature-dependent seasonal pattern, which was strongly modulated by soil water content (SWC), especially in summer. Site-specific polynomial regression models were defined to describe SR responses, being mainly controlled by both soil temperature (Ts) and SWC at UB site, or Ts at burnt sites. The sensitivity of SR rate to Ts variations (Q10) ranged between 0.20 and 6.89, with mean annual values varying between 0.92 and 1.35. Q10 values were higher at BT than at UB-BUT sites. The results revealed a significant, non-linear dependence, of Q10 on both Ts and SWC at UB site, and on Ts at both burnt sites. This study contributes to (i) improve the understanding of how natural recovery and management practices affect soil respiration in a Mediterranean forest during their early stages after fire disturbance and (ii) highlight the importance of Q10 values <1 which emphasizes drought stress effect on SR temperature sensitivity.

Post-wildfire effects on carbon and water vapour dynamics in a Spanish black pine forest.

Two eddy covariance systems were installed in a high-severity burned zone (BZ) and an adjacent unburned (UNB) zone to monitor water vapour and carbon dioxide fluxes for 21 months (from June 2011 to February 2013) at a Spanish black pine forest affected by a stand-replacing wildfire and located in a mountainous area of central-eastern Spain. The differences between both sites were significant especially during the growing season, affecting gross primary productivity (GPP) more than ecosystem respiration (Reco). Net ecosystem exchange (NEE) for 2012 was -3.97 and 1.80 t C ha(-1) year(-1) for the unburned and burned sites, respectively, the GPP being 64% lower for the BZ than the UNB zone. Evapotranspiration (ET) at the UNB was 18% greater than at the BZ. Difference between sites was 160 mm during the whole studied period. This study reflects the effect of one of the major disturbances that can affect Mediterranean ecosystems, showing that carbon fluxes are more dramatically concerned than water vapour fluxes.