| dc.description.abstracteng | Mountain forests play a major role for biodiversity conservation, carbon storage and hydrological cycling. Therefore, understanding different components of mountain forest ecosystems is essential to predict potential responses to global climate change. To study such responses, the investigation of elevational gradients is an important tool as they comprise a wide range of climatic variables on a small spatial scale. While changes in aboveground biodiversity along elevational gradients have been studied intensively, information on belowground systems is scarce. Besides studies focusing on litter decomposition, there is few comprehensive data available on how food web functioning changes across elevations. The soil food web is a major component of nutrient cycling within forest systems and thereby is linked to mineralization rates and tree growth. In this thesis I investigated changes in microbial resources, trophic changes in microarthropods in general and trophic changes within the functional Collembola communities along an elevational gradient (800 – 1700 m asl) of primary forests on Changbai Mountain, China. We established seven sites spaced by elevational steps of 150 m along an elevational gradient at the northern slope of Changbai Mountain. The northern slope is characterized by primary forests dominated by Korean pine with decreasing admixtures of deciduous tree species (<1100 m asl) and dark-bark spruce (>1100 m asl). The sites were sampled in early September 2019, before litter fall began. The general scope of this thesis is to understand how trophic interactions among different components of the soil food web change with elevation.
In Chapter 2 we focused on the characterization of microbial community composition and functioning across elevations as microbes are the major agents of decomposition in forest systems and form an important food resource for animal decomposers. Microbial communities in litter and soil were investigated using phospholipid fatty acid (PLFA) profiles, and microbial biomass and activity were measured using substrate-induced respiration. Our results suggest that microbial communities on Changbai Mountain are heavily influenced by elevation. Microbial communities showed strong turnover in the litter layer, most pronounced in the transition zone between the two major forest types (1100 m – 1250 m asl). While only temperature seasonality significantly structured PLFA profiles in the litter layer, temperature and precipitation seasonality, resource availability, soil moisture, Ca concentrations and pH structured microbial communities in soil. Indicators of microbial stress and microbial biomass were higher in the litter layer at higher elevations, both correlated negatively with Ca concentrations indicating increased nutritional stress in high microbial biomass communities at sites with low Ca availability.
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In Chapter 3 we report trophic positions of Collembola and Oribatida at group level, measured by bulk and amino acid stable isotopes, to increase with increasing elevation. Further, trophic positions of Collembola and Oribatida correlated with microbial stress ratios and microbial biomass. The energy channels, measured by 13C values of amino acids and neutral lipid fatty acids (NLFA), on the other hand, did not change across elevations suggesting fungi to be the major resources of both Collembola and Oribatida along the whole gradient.
In Chapter 4 we targeted trophic changes of Collembola communities across elevations at species level. Collembola species contributing 80% to the Collembola abundance per elevation were categorized into life forms and their individual body lengths and bulk stable isotopes were measured. Abundance weighted mean and minimum Δ15N values as well as Δ13C values increased with elevation, while the range of Δ15N values decreased. Isotopic uniqueness, a measure of isolated trophic niches, increased with elevation. Δ15N values of Collembola species occurring across all elevations increased with elevation, with this being most pronounced in the cosmopolitan species Folsomia octoculata. Shifts in mean Δ15N with elevation depended on the life form of Collembola and were most pronounced in litter-dwelling, hemiedaphic species. We further found changes in Δ15N with body size to depend on Collembola life form, with Δ15N increasing with body size in hemiedaphic and euedaphic species, but not in epedaphic species.
The results reported in Chapter 3 indicate trophic shifts of Collembola and Oribatida communities towards functioning more as secondary decomposers with increasing elevation. The results further indicate that these shifts are likely determined by increasing microbial stress ratios and microbial biomass with elevation, which represent the availability of microbial resources. In Chapter 4 we indeed found species functioning as primary decomposers to decrease with increasing elevation as indicated by increasing minimum Δ15N values and decreasing range of Δ15N values with elevation. These shifts in (especially) Collembola trophic niches with increasing elevation are likely linked to increasing accessibility of microbial resources with decreasing quality of litter, as reported in Chapter 2. Collembola traits, represented by Collembola life forms and body size, likely determine the capability of Collembola to access these alternative microbial resources along the elevational gradient as suggested by the results presented in Chapter 4. This thesis is the first to report detailed data on trophic changes in microarthropods along elevational gradients and therefore contributes significantly to the understanding of food web functioning in mountain forest ecosystems.
Like in mountainous ecosystems across the world, annual mean temperatures on Changbai Mountain increased and annual precipitation decreased over the last decades. According to the strong dependence on local litter factors, the trophic positions of microarthropods at higher elevations may decrease in the future as tree species from lower elevations may expand to higher elevations. There is an urgent need for comparable studies along other
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elevational gradients including different seasons to verify implications of our study and to come up with more general statements. This thesis represents a starting point of such studies. | de |