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Summary1. The ability of soil microbial communities to withstand and recover from disturbance or stress is important for the functional stability of forest ecosystems. However, the relationship between the community responses of soil microbes and variation in tree mixtures vs functional composition remains poorly understood. 2. We investigated soil biochemical properties and soil microbial resistance and resilience to drought in three 4-year-old tree monocultures (Acer saccharum Marsh, Larix laricina (Duroi) K. Koch and Pinus strobus L.) and two tree species combinations (L. laricina/A. saccharum and L. laricina/P. strobus) planted in a high-density tree field experiment located in southern Quebec, Canada. The experimentally imposed drought stress consisted of maintaining soil material for 30 days at 25% of water-holding capacity (WHC). Microbial biomass was assessed immediately after the water stress (resistance) and 15 and 30 days following drought (resilience). 3. Results showed that tree communities influenced soil chemistry, soil respirometry properties and microbial resistance and resilience. We measured significant non-additive (i.e. both synergistic and antagonistic) effects of mixing tree species in some of the soil biochemical properties measured, mostly in the L. laricina/A. saccharum mixture. However, we did not find non-additive effects of tree mixtures on microbial resistance and resilience. A structural equation modelling analysis revealed that resistance and resilience were mostly modulated by direct effects of community-weighted means (CWM) of leaf litter lignin content and mineralizable N, and by indirect links from tree density and CWM of leaf litter N content via mineralizable N. 4. This study suggests that tree species identity surpassed species mixtures as a key driver of soil microbial resistance and resilience. We showed a trade-off between microbial resistance and resilience in soil food webs, which is consistent with ecological theory. Our results indicate that differences in functional traits between tree species may rapidly be reflected in divergent soil biochemical properties and that these differences can in turn drive soil microbial resistance and resilience to drought.

1. Agricultural intensification is one of the main causes of biodiversity loss world-wide. The inclusion of semi-natural features in agricultural landscapes is suggested as a means of enhancing farm biodiversity, but this practice may have potential negative effects on yield production. Moreover, little evidence exists for effects of semi-natural features on other components of biodiversity, such as functional diversity. Yet this could provide a more comprehensive understanding of biodiversity-productivity trade-offs. 2. Here, we report the effects of semi-natural woody vegetation on taxonomic and functional diversity, and biomass production of herbaceous species at the field and farm scales by sampling 50 fields, ranging from 0 to 90% woody vegetation cover, on nine similarly managed farms in central-western Spain. 3. We found significant differences in herbaceous species richness among farms. Both taxonomic and functional b-diversity exhibited significant negative relationships with herbage production, highlighting the trade-off between biodiversity and productivity in these agroecosystems. 4. Woody vegetation cover had a significant negative relationship with biomass production and a unimodal relationship with species richness at the field scale. At high values of woody vegetation cover, species richness and functional diversity indices were decoupled, suggesting that at this extreme of the woody vegetation gradient, only herbaceous species with contrasting trait values were present. Our results showed both convergent and divergent patterns of trait values, suggesting that different assembly processes are acting concurrently along the gradient of woody vegetation. 5. Synthesis and applications. Our result indicates that management of woody vegetation may indeed increase both taxonomic and functional diversity, but this may come at the expense of key ecosystem services or other management goals, namely herbage production. Optimization of the trade-off between herbage diversity and productivity can be reached with a woody vegetation cover of c. 30% at the field scale.