Severe drought impacts tree traits and associated soil microbial communities of clonal oaks.

Abstract

Background: Biotic and abiotic factors, including plant age, soil pH, soil organic matter concentration, and especially water availability, significantly influence soil microbial populations and plant characteristics. While many ecosystems are adapted to occasional droughts, climate change is increasing the frequency and severity of drought events, which negatively impacts plant productivity and survival. Long-lived, drought-sensitive tree species such as Quercus robur are particularly vulnerable to water shortages. Drought also alters soil microbial communities, reducing and reshaping microbial diversity, biomass, and activity, which can in turn disrupt key ecosystem functions. The objective of this study was to investigate the effects of natural drought conditions on soil physicochemical variables, plant traits and microbial communities of the oak clone DF159 in Central Germany. Our research focuses on two study sites, Bad Lauchstädt and Kreinitz, which differ in soil water retention capacity. Data collection spans two periods: before and after a severe drought in 2018. Oak traits and environmental data was collected from 2011 to 2023 covering two oak time series with trees planted annually between 2010 and 2019. Microbial communities were analyzed every second year between 2015 and 2021 around trees representing five different ages.

Results: We found that plant traits, including apical growth, branch elongation and number of shoot flushes, were positively correlated with precipitation and relative humidity. Although the study sites differed in oak leaf number per shoot flush and number of shoot flushes, the 2018 drought negatively impacted all measured plant traits, regardless of sites. Soil bacterial richness and diversity declined at both study sites, independent of plant age, while fungal richness specifically increased in Bad Lauchstädt, which has a higher water-holding capacity, following the drought event. Bacterial community composition was more strongly affected by drought than fungal communities, whereas the latter was more responsive to plant age than bacterial communities.

Conclusions: Given their strong functional links during drought, interactions among vegetation, microbial communities, and soil functioning may ultimately influence major ecosystem services. Bacterial communities were particularly sensitive to drought, while fungal communities exhibited greater resistance, suggesting their potential role in supporting plant survival under drought stress. These findings highlight the risk that prolonged drought may cause irreversible shifts in microbial communities, with significant implications for soil functions and plant-microbe interactions.