Responses of timber species to climatic changes in Northern Germany: Studies on the radial stem growth and the fine root system of European beech, Sessile oak, Scots pine and Douglas fir along a gradient of increasing aridity
by Stella Gribbe
Date of Examination:2025-01-23
Date of issue:2025-12-17
Advisor:Dr. Dietrich Hertel
Referee:Prof. Dr. Christoph Leuschner
Referee:Prof. Dr. Dirk Hölscher
Referee:Prof. Dr. Alexander Knohl
Referee:Prof. Dr. Erwin Bergmeier
Referee:Prof. Dr. Holger Kreft
Referee:Prof. Dr. Mark Maraun
Persistent Address:
http://dx.doi.org/10.53846/goediss-11714
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Abstract
English
Climate change with rapid warming and increasing atmospheric and edaphic drought exposes trees and forests to growing stress. Most of Central Europe’s economically important tree species have suffered vitality losses, growth decline and locally increased mortality during recent hot droughts. Foresters thus require comparative assessments of the major timber species’ climate vulnerability. This thesis consists of two field studies from the disciplines of dendro- and root ecology to compare the drought response of the major timber species European beech (Fagus sylvatica L.), sessile oak (Quercus petraea (Matt.) Liebl.), Scots pine (Pinus slyvestris L.) and Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) along a precipitation gradient (834 − 570 mm yr-1). The study sites were located in mature, monospecific forest stands in the lowlands of Northern Germany on deep sandy to sandy-loamy soils. The aim of the thesis was to explore the four species’ climate vulnerability, assess their adaptive potential to reduced precipitation and to expand the knowledge about the species’ response to increasing climatic aridity. The overarching goal in combining the two studies was to facilitate cultivation decisions for policy makers and forest managers. Temperate trees preserve information on secondary growth constraints within the distinct pattern of their tree rings. To assess a tree species’ response to climatic stress, dendroecology uses wood increment cores as natural archives to analyze past growth patterns under variable climatic conditions. The dendroecological study in the thesis at hand compared long-term radial growth trends (basal area increment; BAI) and climate-growth relationships along the gradient and explored growth patterns at the stand level (growth synchrony). Two wood increment cores per tree were taken in early spring of 2021 from 20 (co-)dominant trees on each of the five sites per species, for a total of 400 sampled trees. The results show that the BAI of Douglas fir and beech had decreased in recent decades at most sites, while pine and oak growth was predominantly stable or had even increased. Yet, the growth synchrony of all species had increased at most stands over the past decades, indicating an increase of climatic stress. Increasing BAI of oak and pine at drier sites may indicate successful acclimation of these species to reduced precipitation. The conifers profited primarily from sufficient moisture in late-summer and higher temperature in February and March, the broadleaf trees from a moister and cooler previous-year summer. Winter temperature emerged as an important new driver of growth in pine, Douglas fir and oak, which may promote the growth of these species at some sites through reduced winter cold stress. While there is already extensive research addressing aboveground traits, less is known about the drought resistance and drought response of the trees’ fine root system, the site of water and nutrient uptake. The root ecological field study therefore comprised two separate sampling methods on the fine root system and its productivity with the aim to explore potential acclimatization to water scarcity and providing a comprehensive understanding of the drought response of this most vital part of the plants. A fine root inventory was carried out in autumn of 2021 after a relatively moist summer to assess fine root dynamics as well as morphological and chemical traits. An in-growth core field experiment in the subsequent year yielded data on fine root productivity and turnover rate. For each target species, fine root biomass (FRB), productivity (FRP), lifespan, and morphological traits were studied and the response to mean annual precipitation across sites was modelled. FRB increased in beech toward drier sites, decreased slightly in oak, and remained unchanged in pine and Douglas fir. FRP increased in beech and Douglas fir with drier conditions, but remained stable in oak and pine. All species exhibited increased root tissue density under drought, but only conifers showed a consistent shift to more conservative traits, reflected in longer root lifespan under reduced precipitation. The study revealed diverse belowground drought responses among the species, including altered fine root dynamics and morphological adjustments, indicating multiple resilience strategies. Root trait modifications, although minor, were complex, species-specific, and influenced by phylogeny, underscoring the need for further field studies and refined models to better understand belowground drought responses. Both studies and a literature evaluation led to the conclusion that all four species, but especially Douglas fir and beech, are increasingly negatively affected by the drier summers the study region and seem vulnerable to climate warming in the North German Lowlands. Oak and pine showed a successful drought strategy under increasingly warmer and drier conditions, when they are acclimated to drier conditions. Although oak was assessed as the most drought tolerant species, there were indications that oak’s specific drought response may cause increased vulnerability to both abiotic and biotic agents in a future with a higher drought frequency, if imbalances in the trees carbon budget arise. Thus, forest management were advised to increasingly consider other minor timber species for silvicultural decisions.
Keywords: dendrochronology; growth synchrony; growth trend; lowlands; precipitation gradient; fine root biomass; fine root production; Northern Germany; root lifespan; drought acclimatization; Fagus sylvatica; Quercus petraea; Pinus sylvestris; Pseudotsuga menziesii; basal area increment
