Aboveground and belowground response of European beech to drought: field studies and experiments

Abstract

Global warming is predicted to shift the seasonal distribution of precipitation in Germany to reduced rainfall in summer and enhanced precipitation in winter. The estimated rates of future climatic change are expected to outpace migration or succession rates of many plants. In Germany, the economically most important broad-leaved tree species is European beech (Fagus sylvatica L.). Although beech is a late-successional tree species with a high dominance on a broad range of soil chemical and hydrological site conditions, F. sylvatica is more drought-sensitive than other temperate broad-leaved trees. Hence, increasing summer droughts could in future impair the vitality of beech at the drought limit of its occurrence. In this study, the drought response of beech was studied on the two key interfaces for water flow in the soil-plant-atmosphere continuum (SPAC): fine roots and leaves. Major aims were to investigate (i) long-term adaptive responses of adult beech trees to low soil moisture across a precipitation gradient and (ii) the specific contribution of phenotypic plasticity and genetic variability to the drought response of beech saplings in a common garden experiment. Surprisingly, adult trees responded to a large decrease in annual rainfall with an increase of leaf area and leaf area index (LAI). The explanation for this unexpected drought response of European beech can partly be found in phenology: leaf production is temporarily uncoupled from summer droughts. Producing a high LAI despite the desiccated soil in summer supports the survival strategies of this late-successional tree species to outmatch competitors by casting deep shade. The optimal resource partitioning theory predicts enhanced root growth during drought in order to reduce water limitations in a desiccated soil, which could not be supported for European beech. Under water limitation, beech rather decreased its fine root biomass by about a third due to reduced fine root longevity and increased root turnover. Fine roots were readily shed to reduce maintenance costs, thus maximizing overall tree productivity. Aboveground and belowground organs of beech adapt to summer drought by fundamentally different strategies in terms of genetic and environmental control: the limited between-population genetic variability determines important leaf traits of European beech. Yet, an increased drought tolerance is in this species presumably primarily based on the enhanced phenotypic plasticity of the fine root system: expansive fine root growth in favourable soil conditions, reduced fine root longevity and increased turnover with drought, and the ability to provide for the water demands even with a strongly reduced fine root mass.