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The extent to which the increasingly frequent episodes of drought-induced tree decline and mortality could alter key soil biogeochemical cycles is unclear. Understanding this connection between tree decline and mortality and soils is important because forested ecosystems serve as important long-term sinks for carbon (C) and essential nutrients (e.g., nitrogen and phosphorus). In order to fill in this knowledge gap, we conducted a study on 13 sites distributed across the Spanish Iberian Peninsula where the dominant tree species was the Mediterranean evergreen Holm oak (Quercus ilex L. subsp. ballota [Desf.] Samp), a species that has shown important drought-induced crown defoliation and mortality rates in recent decades. Our study covered different climatic, soil, land-use type (forests, dehesas, and open woodlands), and crown defoliation (healthy, affected, and dead Holm oaks) gradients that characterize this species distribution within the Spanish Iberian Peninsula. Specifically, the soil C and nutrient content (nitrogen, N; phosphorus, P; magnesium, Mg), several functional parameters (heterotrophic respiration (R H); N mineralization (i.e., N ammonification, R amm ; and N nitrification, R nit)), and relative abundances of key microbial soil functional groups (nitrifiers and ectomycorrhizal fungi (ECM)) were studied. Our results showed that aside from the potential effects associated with the climatic gradient, Holm oak decline and mortality resulted in soil stoichiometric imbalances triggered by net losses of essential oligonutrients (e.g., Mg) and the accumulation of very mobile forms of nitrogen (NO 3 − - N) and available phosphorus (Av P). Changes in the abundance of key microbial soil functional groups (nitrifiers and ECM) co-occurred with observed nitrate and available P accumulation. Therefore, we conclude that the potential vulnerability of soil C and nutrient stocks to ongoing changes in climate may strongly depend on tree vulnerability to climate change, its effect on soil-plant relationships, and how this may impact the ecology and functioning of key soil functional groups and key metabolic pathways. Image 1 • Tree decline and mortality exacerbates the effects of drought on soil functioning. • Reduction of labile carbon (C) inputs decreased heterotrophic respiration (R H). • Tree decline and mortality alters the abundance of nitrate producers and consumers. • Tree mortality promotes accumulation of mobile forms of N and P in soil. • Stoichiometric imbalances may prevent ecosystem recovery after tree mortality. [ABSTRACT FROM AUTHOR]

Water and carbon fluxes in forests are largely related to leaf gas exchange physiology varying across spatiotemporal scales and modulated by plant responses to environmental cues. We quantified the relevance of genetic and phenotypic variation of intrinsic water-use efficiency (WUEi, ratio of net photosynthesis to stomatal conductance of water) in Pinus sylvestris L. growing in the Iberian Peninsula as inferred from tree-ring carbon isotopes. Inter-population genetic variation, evaluated in a provenance trial comprising Spanish and German populations, was low and relevant only at continental scale. In contrast, phenotypic variation, evaluated in natural stands (at spatial level) and by tree-ring chronologies (at temporal inter-annual level), was important and ten- and threefold larger than the population genetic variance, respectively. These results points to preponderance of plastic responses dominating variability in WUEi for this species. Spatial phenotypic variation in WUEi correlated negatively with soil depth (r = − 0.66; p < 0.01), while temporal phenotypic variation was mainly driven by summer precipitation. At the spatial level, WUEi could be scaled-up to ecosystem-level WUE derived from remote sensing data by accounting for soil water-holding capacity (r = 0.63; p < 0.01). This outcome demonstrates a direct influence of the variation of leaf-level WUEi on ecosystem water and carbon balance differentiation. Our findings highlight the contrasting importance of genetic variation (negligible) and plastic responses in WUEi (large, with changes of up to 33% among sites) on determining carbon and water budgets at stand and ecosystem scales in a widespread conifer such as Pinus sylvestris. [ABSTRACT FROM AUTHOR]

Abstract Climate change-induced tree mortality is occurring worldwide, at increasingly larger scales and with increasing frequency. How climate change-induced tree mortality could affect the ecology and carbon (C) sink capacity of soils remains unknown. This study investigated regional-scale drought-induced tree mortality, based on events that occurred after a very dry year (2012) in the Carpathians mountain range (Romania), which caused mortality in three common conifer species: Scots pine, Black pine, and Silver fir. This resulted in hot-spots of biogenic soil CO 2 emissions (soil respiration; Rs). Four to five years after the main mortality event, Rs-related soil CO 2 emissions under dead trees were, on average, 21% (ranging from 18 to 35%) higher than CO 2 emissions under living trees. Total (Rs) and heterotrophic (R H)-related soil CO 2 emissions were strongly related to alterations in the soil environment following tree mortality (e.g. changes in quantity and quality of soil organic matter, microclimate, pH or fine root demography). Moreover, the massive mortality event of 2012 resulted in greater presence of successional vegetation (broadleaf seedlings, shrubland and grasses), which may control the environmental factors that either directly or indirectly affected biotic soil fluxes (Rs and R H). Besides the well-known direct effects of climate change on soil CO 2 emissions, the cascading effects triggered by climate change-induced tree mortality could also exert a strong indirect impact on soil CO 2 emissions. Overall, climate change-induced tree mortality alters the magnitude of environmental controls on Rs and hence determines how the ecosystem C budget responds to climate change. Graphical abstract Image 1 Highlights • Episodes of drought-induced conifer mortality are becoming more frequent in Europe. • Conifer mortality triggers secondary succession of autochthonous broadleaf species. • Tree mortality triggers complex cascading effects which ultimately affects R s. • Changes in fine root specific root length (SRL) correlates negatively with R H. [ABSTRACT FROM AUTHOR]