Non‐native tree (NNT) species have been transported worldwide to create or enhance services that are fundamental for human well‐being, such as timber provision, erosion control or ornamental value; yet NNTs can also produce undesired effects, such as fire proneness or pollen allergenicity. Despite the variety of effects that NNTs have on multiple ecosystem services, a global quantitative assessment of their costs and benefits is still lacking. Such information is critical for decision‐making, management and sustainable exploitation of NNTs. We present here a global assessment of NNT effects on the three main categories of ecosystem services, including regulating (RES), provisioning (PES) and cultural services (CES), and on an ecosystem disservice (EDS), i.e. pollen allergenicity. By searching the scientific literature, country forestry reports, and social media, we compiled a global data set of 1683 case studies from over 125 NNT species, covering 44 countries, all continents but Antarctica, and seven biomes. Using different meta‐analysis techniques, we found that, while NNTs increase most RES (e.g. climate regulation, soil erosion control, fertility and formation), they decrease PES (e.g. NNTs contribute less than native trees to global timber provision). Also, they have different effects on CES (e.g. increase aesthetic values but decrease scientific interest), and no effect on the EDS considered. NNT effects on each ecosystem (dis)service showed a strong context dependency, varying across NNT types, biomes and socio‐economic conditions. For instance, some RES are increased more by NNTs able to fix atmospheric nitrogen, and when the ecosystem is located in low‐latitude biomes; some CES are increased more by NNTs in less‐wealthy countries or in countries with higher gross domestic products. The effects of NNTs on several ecosystem (dis)services exhibited some synergies (e.g. among soil fertility, soil formation and climate regulation or between aesthetic values and pollen allergenicity), but also trade‐offs (e.g. between fire regulation and soil erosion control). Our analyses provide a quantitative understanding of the complex synergies, trade‐offs and context dependencies involved for the effects of NNTs that is essential for attaining a sustained provision of ecosystem services.

In Mediterranean ecosystems the effect of aboveground and belowground environmental factors on soil microbial biomass and nutrient immobilization-release cycles may be conditioned by the distinctive seasonal pattern of the Mediterraneantype climates. We studied the effects of season, canopy cover and soil depth on microbial C, N and P in soils of two Mediterranean forests using the fumigation-extraction procedure. Average microbial values recorded were 820 lg C g -1 , 115 lg N g -1 and 19 lg P g -1 , which accounted for 2.7, 4.7 and 8.8% of the total pools in the surface soil, respectively. Microbial N and P pools were about 10 times higher than the inorganic N and P fractions available for plants. Microbial C values differed between forest sites but in each site they were similar across seasons. Both microbial and inorganic N and P showed maximum values in spring and minimum values in summer, which were positively correlated with soil moisture. Significant differences in soil microbial properties among canopy cover types were observed in the surface soil but only under favourable environmental conditions (spring) and not during summer. Soil depth affected microbial contents which decreased twofold from surface to subsurface soil.Microbial nutrient ratios (C/N, C/P and N/P) varied with seasons and soil depth. Soil moisture regime, which was intimately related to seasonality, emerged as a potential key factor for microbial biomass growth in the studied forests. Our research shows that under a Mediterranean-type climate the interaction among season, vegetation type and structure and soil properties affect microbial nutrient immobilization and thus could influence the biogeochemical cycles of C, N and P in Mediterranean forest ecosystems.

We synthesize a series of independent but integrated studies on the functioning of a mixed Mediterranean oak forest to demonstrate the tree-soil interactions underpinning a positive feedback process that sustains the coexistence of two oak species. The studies focused on the foliar functional traits, plant regeneration patterns, biogeochemical cycles, soil microbial biomass and ectomycorrhizal (ECM) fungal diversity associated with the co-dominant evergreen Quercus suber and deciduous Q. canariensis in a Mediterranean forest in southern Spain. Foliar attributes differed between oak species, with Q. canariensis having higher nutrient content and lower carbon to nutrient ratios and leaf mass per area than Q. suber. These attributes reflected their distinct resource use strategies and adaptation to high and low resourceavailability environments, respectively. Leaf-fall nutrient concentrations were higher in Q. canariensis than in Q. suber and were correlated with concentrations in the fresh leaves. Leaffall nutrient concentrations influenced nutrient return, leaf-fall decay rate and the proportion of nutrients released from decomposing leaf-fall, all of which were higher for Q. canariensis than for Q. suber. This generated a differential net nutrient input into the soil that led to increased soil nutrient concentrations under the canopy of Q. canariensis as compared to Q. suber. The fraction of slowly decomposing leaf-fall that builds up soil organic matter was higher for Q. canariensis, further raising the nutrient and moisture retention of its soils. Differences between species in soil properties disappeared with increasing soil depth, which was consistent with the hypothesised leaf-fall-mediated effect. Tree-species-generated changes in soil properties had further impacts on soil organisms. Soil microbial biomass (Cmic) and nutrients (Nmic, Pmic) were higher under Q. canariensis than under Q. suber and were positively related to soil moisture content and substrate availability (particularly soil N). The composition of the ECM fungal community shifted between the two oaks in response to changes in the soil properties, particularly soil Ca and pH. Lower ECM phylogenetic diversity and higher abundance of mycorrhizal species with saprophytic abilities were related to the greater soil fertility under Q. canariensis. Overall, the two oak species generated soil conditions that aligned with their resource-use strategies and would enhance their own competitive capabilities, potentially creating a positive feedback. The two Quercus created soil spatial heterogeneity that could enable their coexistence through spatial 3 niche partitioning. This study demonstrates the critical role of aboveground-belowground interactions underpinning forest community composition.

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