Effects of admixing alternative tree species to beech forests for climate change adaption on herb layer plant diversity

Abstract: Background:
The admixture of alternative tree species in monodominant beech forests is a key strategy for the silvicultural adaptation of Central European forests to climate change. The interior of pure beech forests is characterised by certain environmental conditions, such as being relatively shady, cool and humid, which are likely to change when the composition of the canopy changes. Since Central European beech forests harbour a globally unique flora, which partly depends on these environmental conditions at the forest floor, it can be assumed that the local plant communities will also be affected by the addition of alternative tree species to the canopy. To date, no studies on the effects of the admixture of alternative tree species in beech forests on local biodiversity have systematically considered the proportion of admixture of a particular tree species.
Overall aim:
The overall aim of this thesis was to assess the effects of gradual admixtures of five different alternative tree species or tree species mixtures on the diversity and composition of the forest floor vegetation of Central European beech forests. The tested tree species (from here on target species) were Douglas fir, silver fir, sessile oak, red oak and mixtures of various native broadleaves.
Research objectives:
The main research objectives, which were addressed accordingly in the individual chapters of this thesis, were:
...to analyse the effects of target species on herb layer plant species diversity and community composition along a continuous gradient of increasing canopy fractions (chapter 3)
...to compare non-native Douglas fir and native silver fir for their impact on plant species diversity, community composition and the relative abundances of plant strategy types and forest specialists (chapter 4 and 5)
...to investigate the effects of gradual admixtures of target species on herb layer functional diversity and composition (chapter 6)
Methods:
We sampled understorey vegetation in 567 forest plots across 189 mature forest stands in south-west Germany, covering a continuous gradient of mixtures of European beech and each target species that ranged from pure beech forests over beech-dominated mixed forests, codominant mixed forests and mixed forests dominated by the target species to pure target species forests. To assess the environmental conditions on the forest floor, the total canopy cover, stand density and basal area, topsoil pH and topsoil C/N ratio were recorded in each sample plot. Hypotheses were tested with generalised linear mixed-effects models.
Results and discussion:
Effects of target tree species on understory vegetation depended both on the identity of the admixed tree species and the proportion at which it was added to the beech forest. We found significantly increased plant diversity after admixtures of Douglas fir, silver fir, sessile oak and mixed broadleaves. Moreover, we found significant changes in plant community composition in all tested tree species admixtures. Admixtures of sessile oak and silver fir to acidic beech forests were the only cases, where no plant species of the initial beech forest community was found to be negatively affected. These shifts in the ground vegetation could mainly be attributed to changes in soil acidity, soil nutrient content and light availability at the forest floor.
Effects of non-native Douglas fir on understorey plant communities were more severe than those of native silver fir. Douglas fir admixtures led to shifts in herb layer plant communities towards more generalist, nitrophilous and disturbance-indicating species and additionally decreased the abundance of several beech forest specialists, as its admixture was associated with decreased soil acidity and increased nutrient availability. Admixing silver fir even increased soil acidity and nutrient-limitation of already acidic beech forests, which led to increased occurrences of acidophilic and stress-tolerating forest specialists, but without decreases of any initial beech forest species. This could be explained by the historic coexistence of European beech and silver fir in mountainous regions and the resulting overlap in ecological niches, which allow beech forest specialists to survive at even highest proportions of silver fir.
Admixture of target tree species to beech forests also affected functional diversity and shifted functional composition of beech forest herb layer plant communities. Douglas fir and silver fir both increased functional diversity whereas it was slightly decreased by admixtures of mixed broadleaves. Higher average seed masses after admixtures of silver fir and sessile oak indicated further resource limitation, whereas increased average specific leaf area after Douglas fir admixture and decreased leaf dry matter content after Douglas fir and red oak admixture pointed towards higher resource availability.
Conclusion:
From a nature conservation perspective, the impact on the ground vegetation of monodominant beech forests was most favourable after admixtures of silver fir and sessile oak. Douglas fir, red oak but also mixtures on native broadleaves significantly changed the character of Central European beech forests. Their extended use should therefore be considered with caution wherever forest management aims at the conservation of Central European beech forest habitats and the corresponding biocoenoses. Future research is needed to investigate the effects of beech forest enrichment with alternative tree species on organism groups other than ground dwelling plants, and to additionally study effects of such management interventions on β- and γ-diversity

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2024

Classification and Topics

• Keyword: Waldökosystem ; Klimaänderung ; Buchenwald ; Artenreichtum ; Waldbau ; Buche ; Biodiversität ; Vegetation ; Waldbau

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2025
• Creator: Bärmann, Lukas
• Contributor: Hauck, Markus ; Blumenstein, Kathrin ; Professur für Angewandte Vegetationsökologie ; Albert-Ludwigs-Universität Freiburg. Fakultät für Umwelt und Natürliche Ressourcen

Further information

• DOI: 10.6094/UNIFR/262017
• URN: urn:nbn:de:bsz:25-freidok-2620179
• Rights: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 15.08.2025, 7:35 AM CEST