Chemical and morphological response of beech saplings (Fagus sylvatica L.) to an experimental soil drought gradient

Abstract

Abstract European beech (Fagus sylvatica L.) is a common tree species in Central European forests but the fitness of this species under future climate conditions is still a matter of debate. Here, we study the response of transplanted beech saplings in terms of their chemical and morphological traits to different levels of soil drought in a mountainous, partially open, storm damaged Norway spruce forest. Throughfall was excluded by roof constructions in three consecutive growing seasons. Spatial soil drought gradients resulted from soil water consumption by heterogeneously distributed mature spruce trees. Individual drought stress dose (DSD) was assessed for each beech sapling using cumulative soil matric potential in the rooting zone. After three years of drought treatment, we analyzed biomass components, traits of leaves and fine roots of sixty beech saplings. In the third year, the classical permanent wilting point in the soil was exceeded for some beech saplings, however, all saplings survived the drought treatments. Several chemical plant traits responded to DSD sum of the last two years, while morphological traits and biomass components were mostly not or weakly correlated with DSD. Potassium concentration and associated C:K and N:K ratios in fine roots were most sensitive to drought stress. We assume that K and other nutrients were hardly available in the extremely dry forest floor and that the undersupply could not be compensated by nutrient uptake from the mineral soil. Increasing C concentration in leaves indicates an acclimation towards sclerophylly to maintain the hydraulic function of leaves. Increasing specific root length (SRL), specific root tip density (SRTD) and decreasing fine root diameter also indicate a morphological acclimation of saplings to increasing DSD. Weak correlations between morphological traits and DSD partly resulted from non-linear responses to increasing DSD. Maximum number of mycorrhizal root tips and maximum fine root biomass occurred at medium DSD. Our results suggest that the acclimation of chemical and morphological traits fortifies the resilience of beech saplings against soil drought in the years after transplantation. However, reduced uptake of soil nutrients caused by drought can only be partially compensated by redistribution within the sapling. An expansion of the rooting zone in the moist, nutrient-poor subsoil can improve the water uptake of beech saplings during drought periods, but this strategy hardly improves their nutrient supply.