Community structure and guild patterns of soil decomposers in pure and mixed forests of European beech, Norway spruce and Douglas fir

Abstract

Anthropogenic global warming is creating new challenges for forest management in many parts of the world. One vital aspect is the choice of tree species and whether to cultivate monospecific or mixed timber plantations. Because Central Germany applies close-to-nature forest management, each piece of land needs to fulfil multiple services, including timber production and biodiversity conservation. However, scientific understanding of the effect of forest type on ecosystem functioning, especially decomposer communities, is still limited. To better understand the linkages between forest types and decomposer communities, I investigated decomposer communities in pure and mixed forests of native European beech, range-expanding Norway spruce, and non-native Douglas fir across a range of environmental conditions. Based on a review of previous work (Chapter 1). I developed the overarching hypothesis that, compared to native European beech, Douglas fir detrimentally affects the community structure of decomposers. Further, available data suggest that mixed forests may mitigate the adverse effects of pure coniferous forests. To test these hypotheses, I first investigated the structure and functioning of microbial communities using microbial respiration and phospholipid-derived fatty acid analyses (Chapter 2). The response of microbial community structure and functional indicators depends strongly on soil nutrient concentrations in the study site. Douglas fir and Norway spruce adversely affected soil microbial communities and compromised their functioning, particularly in unfavorable environments. These findings, published in Lu and Scheu 2021, call for caution when deciding whether to plant pure Douglas fir under less-favorable site conditions and overall contribute to a context-wise understanding of tree–soil interactions. Building on the concept of microbial communities as basal resources connecting trees and soil animals, I next investigated collembolans and oribatid mites in association with biotic and abiotic environmental variables (Chapter 3). Species composition of Oribatida, but not of Collembola, sensitively responded to forest type, differing most between Douglas-fir and European-beech forests. Although microarthropod richness and diversity did not differ among forest types, the abundance of both euedaphic Collembola and predatory Oribatida were lower in Douglas fir than in European beech, presumably due to lower provisioning of root-associated resources in Douglas-fir forests. The results suggest that non-native Douglas fir generally does not affect the diversity of soil microarthropods, but the limitation of root-derived resources may restrict the population development of some microarthropods in Douglas-fir forests. To further understand the intraspecific variation in food resources of oribatid mites, stable isotope ratios of 15N/14N and 13C/12C were quantified for 40 Oribatida species that occur in both litter and soil (Chapter 4). Across five forest types, Oribatida species were found to occupy virtually identical trophic niches irrespective of the soil depth at which they were recovered. Such low intraspecific variability may facilitate Oribatida niche differentiation and species coexistence. These findings are an important contribution to the understanding of the trophic ecology of oribatid mites in temperate forest ecosystems. Although basal resources of Oribatida vary between coniferous and deciduous forests, basal resources and trophic positions of Oribatida species in mixed forests are similar to those in European beech, supporting the use of mixed forests in mitigating adverse impacts of coniferous trees. Taken together, my results suggest that tree identity is an important driver for microbial and microarthropod communities. In mixed forests, microbial and microarthropod responses are intermediate compared to respective pure stands, suggesting that tree species are singular, that is, loss or addition of tree species causes detectable changes. Furthermore, the microbial response also depends on site conditions and mixture types, reflecting different responses of the tree species to environmental conditions. This also supports the idea that mixed forests provide better insurance against the changing climate (Chapter 5). Overall, mixed forests help to maintain soil microbial and microarthropod communities close to the state of native European-beech forests and mitigate the adverse impacts of coniferous forests. As a whole, this dissertation contributes to a better understanding of the structure and resource utilization of soil decomposer communities and serves as a stepping stone for the next phase of the research training group.