Soil microbial communities play an important role in forest ecosystem functioning, but how climate change will affect the community composition and consequently bacterial functions is poorly understood. We assessed the effects of reduced precipitation with the aim of simulating realistic future drought conditions for one growing season on the bacterial community and its relation to soil properties and forest management. We manipulated precipitation in beech and conifer forest plots managed at different levels of intensity in three different regions across Germany. The precipitation reduction decreased soil water content across the growing season by between 2 to 8% depending on plot and region. T-RFLP analysis and pyrosequencing of the 16S rRNA gene were used to study the total soil bacterial community and its active members after six months of precipitation reduction. The effect of reduced precipitation on the total bacterial community structure was negligible while significant effects could be observed for the active bacteria. However, the effect was secondary to the stronger influence of specific soil characteristics across the three regions and management selection of overstorey tree species and their respective understorey vegetation. The impact of reduced precipitation differed between the studied plots; however, we could not determine the particular parameters being able to modify the response of the active bacterial community among plots. We conclude that the moderate drought induced by the precipitation manipulation treatment started to affect the active but not the total bacterial community, which points to an adequate resistance of the soil microbial system over one growing season.

Abstract. Precipitation patterns across Central Europe are expected to change over the 21st century due to climate change. This may reduce water availability during the plantgrowing season and hence affect the performance and vitality of forest ecosystems. We established a novel rainfall reduction experiment on nine sites in Germany to investigate drought effects on soil-forest-understory ecosystems. A realistic, but extreme annual drought with a return period of 40 years, which corresponds to the 2.5 % percentile of the annual precipitation, was imposed. At all sites, we were able to reach the target values of rainfall reduction, while other important ecosystem variables like air temperature, humidity, and soil temperature remained unaffected due to the novel design of a flexible roof. The first year of drought showed considerable changes in the soil moisture dynamics relative to the control sites, which affected leaf stomatal conductance of understory species as well as evapotranspiration rates of the forest understory.

& Key message Understory plant communities are essential for the recruitment of trees making up future forests. Independent of plant diversity, the understory across different forest ecosystems shows considerable physiological acclimation and structural stability towards drought events, which are expected to occur more frequently in future. & Context Understory plant communities are essential for the recruitment of trees making up the future forest. It is so far poorly understood how climate change will affect understory in beech and conifer forests managed at different intensity levels. & Aims We hypothesized that drought would affect transpiration and carbon isotope discrimination but not species richness and diversity. Moreover, we assumed that forest management intensity will modify the responses to drought of the understory community. & Methods We set up roofs in forests with a gradient of management intensities (unmanaged beech-managed beech-intensively managed conifer forests) in three regions across Germany. A drought event close to the 2003 drought was imposed in two consecutive years. & Results After 2 years, the realized precipitation reduction was between 27% and 34%. The averaged water content in the top 20 cm of the soil under the roof was reduced by 2% to 8% compared with the control. In the 1st year, leaf level transpiration was reduced for different functional groups, which scaled to community transpiration modified by additional effects of drought on functional group leaf area. Acclimation effects in most functional groups were observed in the 2nd year. & Conclusion Forest understory shows high plasticity at the leaf and community level, and high structural stability to changing climate conditions with drought events.

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Abstract. Climate change is predicted to severely affect precipitation patterns across central Europe. This may reduce water availability during the plant-growing season and hence affect the performance and vitality of forest ecosystems. We established a novel rainfall reduction experiment on nine sites in Germany to investigate drought effects on soil-forest-understory-ecosystems. A realistic, but extreme annual drought with a return period of 40 years, which corresponds to the 2.5% percentile of the annual precipitation, was imposed. At all sites, we were able to reach the target values of rainfall reduction, while other important ecosystem variables like air temperature, humidity and soil temperature remained unaffected due to the novel design of a flexible roof. The first year of drought showed considerable changes in the soil moisture dynamics relative to the control sites, which affected leaf stomatal conductance of understory species as well as evapotranspiration rates of the forest understory.

Citation: Welk, A., E. Welk, M. Baudis, J. B€ ockelmann, and H. Bruelheide. 2019. Plant species' range type determines local responses to biotic interactions and land use. Ecology 100(12):Abstract. Geographic plant distribution is often assumed to be predominantly limited directly by the climatic tolerances of species. However, the role of climate is now known to be mainly an indirect one mostly mediating dispersal and establishment, species interactions, or habitat characteristics, which all are often modified by human land use. In these complex systems, negative biotic interactions are predicted to increase in relative importance toward benign climatic conditions. We tested this hypothesis experimentally by exposing plant species with different geographic distribution ranges to different climates, biotic interactions, and land use. Thereby, species predominantly distributed in regions with benign climatic conditions were expected to be better able to cope with negative biotic interactions than species from regions with environmentally stressful climatic conditions. We present results of a fully crossed twoyear transplantation field experiment replicated in 45 plots in three study regions along a precipitation gradient across Germany. We manipulated biotic interactions (presence/absence of competition and mollusk herbivory) in grasslands of different management regimes (meadows, mown pastures, pastures). The transplanted phytometers consisted of six congeneric species pairs, each representing one oceanic and one distinctly more continental range type. The oceanic range type is predominantly distributed in benign climatic conditions in Western Europe, while the more continental type is distributed in regions with more stressful climatic conditions in Eastern Europe. This experimental setting allowed us to study the impact of negative biotic interactions along an abiotic stress gradient under realistic land-use conditions. Under competition and mollusk herbivory, growth performance was more strongly reduced in continental compared to oceanic species. Range types also differed in their responses to grassland management. Differences in survival between the congeneric species were found to be region-specific and largely unaffected by biotic interactions and land use. In consequence, our results suggest that local responses to biotic interactions and land-use practices of otherwise very similar plant species can differ strongly depending on species' large-scale geographical distribution. Regionally differing responses to biotic interactions also show that local conditions can drastically change responses expected from macroecological theory.

Current climate change scenarios predict changes in precipitation patterns resulting in a longer and more intensive droughts. These changes pose a challenge for Fagus sylvatica as the most common European deciduous tree species. We asked how different genotypes respond to drought conditions and paid particular attention to how soil moisture availability can be manipulated. We established a tube technique that allowed to add water horizontally at different rooting depths, which allowed to measure water contents and to observe root growth along the full length of 1-m deep tubes. Three F. sylvatica provenances were compared in their response to different depth of water supply with respect to growth, leaf stomatal conductance as well as root and leaf traits. There were short-lasting differences between the watering regimes in soil water content and depth of soil moisture maxima. Adding water to depths of 30-60 cm resulted in reduced leaf stomatal conductance and lower survival but increased root tip growth rates compared to watering the tubes at 10-20 cm. There were differences between the three provenances for specific leaf area and stem dry mass, which however, were independent of the watering regime and did not support the idea of local adaptation to drought of populations from dry or moist origins. The findings indicate that differences between provenances might have been caused by environmental factors other than drought. Technically, we demonstrated the high potential of using the tube technique for a fine-tuned manipulation of soil moisture in the rooting space.

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