Species-specific fine root biomass, morphology and dynamics of six co-occurring deciduous tree species in the Hainich National Park and a conifer tree species at the alpine treeline

This thesis is subdivided into two main research areas. The first two studies were conducted in a mature mixed temperate broad leaved forest with regard to species identity effects on fine root traits, while the third study of the thesis addresses the influence of soil temperature on fine roots of Pinus cembra at the alpine treeline. In the species-rich broad leaved forest within the Hainich National Park, we assessed the role of taxonomic position and mycorrhiza type (EM and AM) on fine root biomass, fine root morphology (on root order level) and fine root dynamics of six coexisting tree species (Fagus sylvatica L., Tilia cordata Mill., Carpinus betulus L., Fraxinus excelsior L., Acer pseudoplatanus L. Acer platanoides L.). We detected similar trends of a decrease of specific root area and specific root length from the first (the root tips) to the fourth root order in all six species. Nevertheless, the root order traits differed strongly between the species, especially for the variables root tissue density and root nitrogen concentration. The highest root nitrogen contents were found in the root tips and decreased with root order. Comparing the species, F. excelsior contained the highest nitrogen content in all root orders. Fine root productivity differed strongly between the six species with the highest production in C. betulus, F. sylvatica, and F. excelsior (~ 150-170 g m-2 yr-1). Most differences in fine root production and turnover between the species were found in the deeper soil layers compared to the upper soil layer at 0-10 cm. Root turnover varied up to fivefold among the species, with lowest values in Acer pseudoplatanus and highest values in its congener A. platanoides. Even these two congeneric species differed strongly in their branching traits in the same stand, suggesting that they use differing belowground foraging strategies (e.g. more root tips per biomass in A. pseudoplatanus vs. a higher root turnover in A. platanoides). In general, species identity was found to be the most important determining factor for fine root morphology and biomass as well as for fine root productivity and turnover rates, whereas the influence of mycorrhiza type was only of secondary importance. Species differences in fine root traits were more pronounced for the respective root orders than in comparison for the whole root branches. At the alpine treeline, soil temperature is assumed to be the key factor influencing root biomass, production and morphology of fine roots. So far, most studies have been conducted on temperature influences on fine roots of tree saplings and juvenile trees in ex-situ experiments, and there is very little data on temperature dependent fine root traits of mature trees at the treeline. In this study we investigated fine root mass distribution, fine root morphology and fine root production and turnover around solitary Pinus cembra trees at the alpine treeline in the Central Eastern Alps in Austria. Those fine root data were linked to soil temperatures around the trees, measured at the time with maximal temperature deviations between shade and sun. The main objective was to determine whether soil temperature, lowered by the shade of the crown, impairs fine root growth around mature Pine trees. In contrast to previous findings regarding fine root reactions to low temperatures, we found higher fine root masses (living and dead) in cooler areas around the tree stems during a sunny summer day. Additionally in cooler, shaded soil there was a higher fine root production and turnover, which leads us to the assumption that trees invest more carbon into fine roots of cooler soil areas to compensate fine root loss and maintain optimal resource acquisition, as nutrient accessibility is lower in cold soil areas. In general, our findings suggest that fine root traits and their dependency of soil temperatures might not be comparable between tree saplings and mature trees