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Belowground niche separation and competition in tree species mixtures

abgeschlossen 11/2011

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The effects of biodiversity on ecosystem functioning have emerged as a central issue in ecology in last decade and have made substantial progress on the aboveground interaction and coexistence of the different tree species, but the belowground interaction in tree species mixtures, especially for more than 3 species, has received little attention, although the root competition for water and nutrients is a very important factor determining the composition, structure and dynamics in mixed forest. There are, therefore, many uncertainties underlying the coexistence of tree species mixtures.
Here, we investigated the belowground interactions between different tree species in a long-term biodiversity experiments with temperate tree species to test the hypotheses that (i) overall soil exploitation ability of fine roots (total fine root biomass, production, length and surface area) increases with tree species richness, and that (ii) below-ground interspecific competition is size-symmetric. In addition, I explored the potential of using near infrared reflectance spectroscopy (NIRS) to distinguish and quantify the tree species composition in fine-root mixtures. The study was conducted in one of the three BIOTREE sites in Kaltenborn, Germany, where four tree species, comprising two deciduous species, European beech (Fagus sylvatica L.) and Sessile oak (Quercus petraea Liebl.), and two conifers, Norway spruce (Picea abies (L.) Karst.) and Douglas fir (Pseudotsuga menziesii (Mirb.) were planted following a simplex design. We applied a fully balanced experimental design with replacement series to investigate the fine-root dynamics in the transition zone of four saplings, which consisted of a series of tree species combinations differing in tree species richness, ranging from 1, 2, 3 to 4 species. To relate below-ground interactions to those above ground, all the saplings surrounding the fine-root sampling locations were recorded.
 
The initial overall tree fine-root biomass in the undisturbed soil was quite low in this 6-year-old temperate forest, but the live fine root mass produced in ingrowth cores after two growing seasons reached up to 378 g m-2, a level similar to those found in mature temperate forests. Fine-root production in ingrowth cores increased slightly with tree species diversity, and four-species combinations produced on average 94.8% more fine-root biomass than monocultures during the first growing season. During the second growing season, fine-root mortality increased with tree species diversity, indicating an increased fine-root turnover in species-rich communities. Furthermore, P. abies allocated more fine roots to the upper soil layer (0-15 cm), whereas P. menziesii produced more fine roots in the deeper layer (15-30 cm) in species-rich neighbourhoods than in species-poor neighbourhoods. The species-specific response of fine-root growth to mixing suggests that vertical stratification of root systems may develop early in tree mixtures.
 
In contrast to the fine-root production, overall fine-root length and surface area did not differ significantly among the different tree species richness levels. The growth rates of tree saplings were in proportion to their initial sizes with respect to basal area increments. In ingrowth cores, however, P. menziesii and P. abies had disproportionally higher fine-root growth rates in mixed stands than in monocultures, whereas the reverse was true for F. sylvatica and Q. petraea. The competitive ability index, which was significantly different between different species, demonstrated that belowground competition was size-asymmetric in this sapling stand. The nutrient enrichment in ingrowth cores did not affect proliferation rates and morphology of fine roots significantly.
 

In addition, our results showed near-infrared reflectance spectroscopy could predict the species composition of mixed samples of woody fine roots with a promising precision and accuracy regardless of species composition and concentration of each component based on artificial mixtures. This approach could serve as a complementary method to quantify species composition in root mixtures and facilitate investigation on belowground interactions in mixed forest communities.

Projektleitung:Prof. Dr. Jürgen Bauhus
Projektbearbeitung:Pifeng Lei
Finanzierung:DAAD Scholarship
Laufzeit:Sep 2007 - Nov 2011

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