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The aboveground impacts of climate change receive extensive research attention, but climate change could also alter belowground processes such as the delicate balance between free-living fungal decomposers and nutrient-scavenging mycorrhizal fungi that can inhibit decomposition through a mechanism called the Gadgil effect. We investigated how climate change-induced reductions in plant survival, photosynthesis and productivity alter soil fungal community composition in a mixed arbuscular/ectomycorrhizal (AM/EM) semiarid shrubland exposed to experimental warming (W) and/or rainfall reduction (RR). We hypothesised that increased EM host plant mortality under a warmer and drier climate might decrease ectomycorrhizal fungal (EMF) abundance, thereby favouring the proliferation and activity of fungal saprotrophs. The relative abundance of EMF sequences decreased by 57.5% under W+RR, which was accompanied by reductions in the activity of hydrolytic enzymes involved in the acquisition of organic-bound nutrients by EMF and their host plants. W+RR thereby created an enhanced potential for soil organic matter (SOM) breakdown and nitrogen mineralisation by decomposers, as revealed by 127–190% increases in dissolved organic carbon and nitrogen, respectively, and decreasing SOM content in soil. Climate aridification impacts on vegetation can cascade belowground through shifts in fungal guild structure that alter ecosystem biogeochemistry and accelerate SOM decomposition by reducing the Gadgil effect., This study was funded by the Ministerio de Economıa y Competitividad (projects CGL2010-21064 and CGL2013-48753-R co-funded by European Union FEDER funds) and Fundacion Seneca (19477/PI/14). MdMA acknowledges a mobility fellowship funded by the ‘Jose Castillejo’ programme (CAS14/00023) of the Ministerio de Educacion y Formacion Profesional

Drylands cover around 41% of Earth’s land surface and include 20% of the major centers of global plant diversity. Drylands are especially vulnerable to climate change and desertification. Maintenance of biodiversity in drylands is crucial to prevent those negative impacts. In order to do so, it is important to understand how dryland plant communities are organized and the processes that allow plant species coexistence in these severely water limited environments. Afforested plantations of Pinus halepensis Mill., that were extensively planted in semiarid areas of the Mediterranean basin during the second half of the XXth century are particularly vulnerable to climate change and drought intensification. The aim of the present PhD thesis was to investigate the water use strategies of some of the most common plant species naturally occurring in the semiarid Iberian SE, and to assess how plant-plant interactions influence the resource acquisition and physiological processes of vegetation in natural and afforested P. halepensis woodlands. We also wanted to assess how thinning, one of the most common silvicultural practices, influences the physiology of the remaining trees. Given the potential of isotopic techniques in ecophysiological studies, we wanted to evaluate their utility for investigating plant water use strategies and plant-plant interactions involving water in a semiarid ecosystem of the Iberian SE.