Lübbe, Torben

English

Biodiversity is by now widely known to strengthen functioning and services, like primary production and element cycling, for a broad range of ecosystem types. However, it is less clear yet if and how this relationship also applies for forest ecosystems. Moreover, trees are supposed to be highly susceptible to deficient water supply. In this context, it seems to be most interesting whether mixed species forests might be better buffered against the climate change-induced threat of increasing drought events than monocultural stands. 
Under consideration of these questions a tree diversity experiment with potted sapling assemblages of five temperate broad-leaved species was conducted which consisted of various species combinations (n=16) differing in species richness (1, 3 and 5 species). Furthermore, a drought trial was implemented to investigate tree water consumption (stand-level), productivity (stand- and tree level) and physiological properties (tree-level) under ample and limited soil water supply. 
In case of stand-level transpiration, a significant net diversity effect was observed for the moist (increase by in average ~11%) as well as for the dry treatment (~8%). In moist soil, the increase was most reasonably realized by a selection effect (enhanced water use of the high performer species T. cordata and F. excelsior in mixed tree assemblages). Similarly, stand-level productivity was significantly enhanced by tree species diversity in the moist (increase in mean relative growth rates: ~9%), but not in the dry treatment (~5%). In contrast to tree water use, higher plant growth in species mixtures was indicated to be mainly caused by complementarity effects, even though considerable competitive asymmetries in favor of fast growing species were observed. The reduced or lacking diversity effects in the dry treatment contrast with the idea of more intense favorable interactions and increasing resistance to abiotic stress for more diverse communities. In fact, the limited soil volume of the potted tree assemblages might have restricted the potential for complementary use of deficient soil water. 
The findings about tree mixture effects on tree community functioning were also mostly apparent at tree-individual level. While T. cordata clearly enhanced its growth ability in more diverse tree assemblages, F. excelsior showed also increased performance in some water-use related physiological traits (stomatal- and sapwood hydraulic conductance). In dry soil, all species realized some plastic modifications in either the status of leaf hydration (F. excelsior) or in stem hydraulic architecture (C. betulus, F. sylvatica). Furthermore, C. betulus but also A. pseudoplatanus achieved some improvement in xylem hydraulic properties when exposed to a heterospecific tree neighborhood. Thus, we found some evidence for the beneficial impact of species mixture on tree functional performance under unfavorable site conditions.
In summary, the thesis results widely coincide with most recent findings from forest diversity research, assuming the general occurrence but inferior importance of tree diversity effects on ecosystem functioning, while tree species identity is obviously a more relevant driver. Soil desiccation clearly hampered tree individual performances, but the question if and how tree species diversity fosters community stability in a drier climate remains unsolved and still deserves more attention

Georg-August-University Göttingen

Effects of tree species diversity and soil drought on productivity, water consumption and hydraulic functioning of five temperate broad-leaved tree species

https://ediss.uni-goettingen.de/bitstream/handle/11858/00-1735-0000-0028-87A2-6/Dissertation_L%c3%bcbbe_final_SUB_Ediss.pdf?sequence=1

Effects of tree species diversity and soil drought on productivity, water consumption and hydraulic functioning of five temperate broad-leaved tree species

Abstract

Similar works

Full text

Available Versions

Georg-August-University Göttingen