Changes in soil bacterial community triggered by drought‐induced gap succession preceded changes in soil C stocks and quality

Jorge Curiel Yuste,Stefania Mattana,Pau Nolis,Josep Barba,Francisco Lloret,Antonio José Fernandez‐Gonzalez,Jordi Martinez‐Vilalta,Manuel Fernandez‐Lopez

doi:10.1002/ece3.409

Abstract

The aim of this study was to understand how drought-induced tree mortality and subsequent secondary succession would affect soil bacterial taxonomic composition as well as soil organic matter (SOM) quantity and quality in a mixed Mediterranean forest where the Scots pine (Pinus sylvestris) population, affected by climatic drought-induced die-off, is being replaced by Holm-oaks (HO; Quercus ilex). We apply a high throughput DNA pyrosequencing technique and 13C solid-state Nuclear Magnetic Resonance (CP-MAS 13C NMR) to soils within areas of influence (defined as an surface with 2-m radius around the trunk) of different trees: healthy and affected (defoliated) pines, pines that died a decade ago and healthy HOs. Soil respiration was also measured in the same spots during a spring campaign using a static close-chamber method (soda lime). A decade after death, and before aerial colonization by the more competitive HOs have even taken place, we could not find changes in soil C pools (quantity and/or quality) associated with tree mortality and secondary succession. Unlike C pools, bacterial diversity and community structure were strongly determined by tree mortality. Convergence between the most abundant taxa of soil bacterial communities under dead pines and colonizer trees (HOs) further suggests that physical gap colonization was occurring below-ground before above-ground colonization was taken place. Significantly higher soil respiration rates under dead trees, together with higher bacterial diversity and anomalously high representation of bacteria commonly associated with copiotrophic environments (r-strategic bacteria) further gives indications of how drought-induced tree mortality and secondary succession were influencing the structure of microbial communities and the metabolic activity of soils.

Highlights

The increase in drought-induced mortality and forest decline that has been recorded around the globe during the last decades suggests that some of the world’s forest ecosystems are already responding to climate change (Van Mantgem et al 2009; Allen et al 2010; Martınez-Vilalta et al 2011)
We did not find any significant differences in soil organic matter (SOM) composition among the four different area of influence as observed in the 13C CP nuclear magnetic resonance (NMR) spectra, including the three different indexes under study (Aliphatic C, recalcitrancy, and aromaticity; Table 1 and Table S1)
We did not find any significant differences among microbiomes in soil temperature, but soil moisture was statistically higher under Defoliated pine (DFP) than under the rest of canopy types (Table 2)

Summary

Introduction

The increase in drought-induced mortality and forest decline that has been recorded around the globe during the last decades suggests that some of the world’s forest ecosystems are already responding to climate change (Van Mantgem et al 2009; Allen et al 2010; Martınez-Vilalta et al 2011) This die-off is characterized by rapid defoliation and progressive increase in the mortality of overstory trees (Breda et al 2006). Such widespread mortality events have the capacity to transform regional landscapes on a subdecadal timescale, with significant implications for stand structure and dynamics (Royer et al 2011; McDowell 2011).

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