Adjustment of hydraulic, anatomical and leaf traits in Fagus sylvatica (L.) towards drier conditions. A study in mature stands.

Abstract

Hydraulic traits are closely linked to the ability of tree species to survive droughts and were therefore used to predict the disposition of trees to suffer from hydraulic failure in the face of climate change. While most studies focus on inter-specific differences in functional traits, less is known about the variability within tree species as well as within populations and about the factors influencing this variability. The species investigated in the presented thesis, European beech (F. sylvatica) is the most abundant tree species of Central Europe’s natural forest vegetation and an important timber species for many countries. Despite its wide distribution range and the tolerance of a broad variety of climatic and edaphic growth conditions, European beech is known to be relatively drought sensitive. However, a high capacity to adapt to changing climatic conditions was suggested for this species due to a high intra-specific variability in hydraulic traits. To assess the species’ adaptive and acclimative potential to drier conditions and to support the identification of potentially drought-hardier provenances, this study provides a large dataset on xylem hydraulic, anatomical and leaf traits of upper-canopy branches collected from 300 beech trees growing on 30 different sites in the lowlands of northern Germany between the North Sea coast and the Polish border. A comprehensive approach to describe the water availability across the gradient was used by including the climatic water balance (CWB) or 60-year mean annual precipitation (MAP, in case of leaf traits), as well as the plant-available water capacity of the soils (AWC) and competition between neighbouring trees (CI) in the analyses. Furthermore, the effect of branch age on xylem hydraulic and anatomical traits and the effect of tree height on anatomy were considered. As sampling was conducted in the summers of 2018 and 2019, data on leaf traits were analysed for the two years separately and results were compared between the two years. The portion of trait variation attributed to the above-mentioned variables as well as the degree of between- and within- population variability were quantified. The effect of water availability on xylem traits was rather low. Embolism resistance and vessel density were significantly related to AWC, while vessel diameter, lumen-to-sapwood area ratio and potential conductivity decreased significantly with decreasing CWB. Xylem-specific hydraulic conductivity was not affected by water availability. In contrast to the xylem traits, leaf traits showed considerable plasticity in response to climatic and edaphic aridity, with differing effects of water availability between the sampling years 2018 and 2019. For the 2018 data set, an increase in mean leaf size (LS) and a decrease in the sapwood-to-leaf area ratio (Huber value, HV) towards drier conditions was observed, indicating that water supply to the sun canopy foliage is not improved upon precipitation reduction. However, this relationship was not pronounced in the 2019 data set, instead, specific leaf area (SLA) and water potential at turgor loss point (Ptlp) were adjusted towards drier sites. Furthermore, carbon isotope signature and Ptlp were modified in response to short-term CWB. While xylem traits were not related to tree height, branch age, which varied considerably by 20 years in branches of comparable diameter, had a major influence on most of the xylem traits. Embolism resistance and vessel diameter significantly decreased, whereas vessel density increased with increasing branch age. Vessel diameter was not significantly related to embolism resistance, but vessel grouping index and the lumen-to-sapwood area ratio showed a weak but significant relation. Within-population variability was larger than between-population variability for all investigated traits except for SLA and Ptlp of the 2019 data set. Our results suggest that (1) anatomical modifications in response to water availability are rather a growth consequence of water limitation than an active acclimative response in order to increase hydraulic safety, and that (2) these modifications in terminal branches are independent of tree height. The rather small influence of water availability on embolism resistance and the high portion of unexplained variability suggest a minor acclimative capacity in this trait and indicate that embolism resistance might not be a promising trait for the identification of drought-resistant provenances. Overall, leaf traits appear to be under stronger environmental control than xylem traits. The increase in LS and decrease in HV with decreasing precipitation enhances the drought stress exposure of sun foliage at drier sites. Yet, under water shortage, osmotic adjustment to local water availability might be an important acclimative response in European beech.