Productivity and nutrient relations of trees in deciduous forests differing in tree species diversity

Effects of biodiversity on ecosystem functioning in forests ecosystems are of increasing interest. There is a vital debate about the influence of species numbers and plant functional traits for different ecosystem functions such as primary productivity, nutrient cycling and carbon storage. Only few studies on biodiversity effects were conducted in natural forest, despite their ecological and socioeconomic importance. Here we studied 12 forest stands in the Hainich National Park (Thuringia, Central Germany). The general objectives of this comparative study were (i) to quantify aboveground tree biomass and nutrient content, (ii) to assess total stand and species-specific productivity, and (iii) to examine decomposition rates and nutrient release patterns. The focus was to find and understand relations between tree species diversity and ecosystem functions such as productivity, decomposition and nutrient release. Studied forest stands grew on similar soils (physical properties) and bedrock and differentiated by a gradient of increasing tree species from pure beech forest stands (Fagus sylvatica), to medium-diverse forests built by beech, ash (Fraxinus excelsior), and lime (Tilia cordata and T. platyphyllos), and to highly-diverse stands dominated by beech, ash, lime, maple (Acer pseudoplatanus and A. platanoides), and hornbeam (Carpinus betulus). Stem wood increment in 2006 and 2007 was measured using permanent measurement tapes. Leaf and fruits biomass from 2005 to 2007 were collected with litter samplers and a litterbag experiment was conducted over 22 months to obtain litter, lignin and nutrient release rates.Total above-ground biomass decreased significantly with tree species diversity from 480 to 200 Mg ha-1. We found distinct differences between the main tree species of our study regarding total and seasonal production of biomass e.g. Fraxinus exhibited the highest wood production, Fagus had higher basal area-related stem wood production than basal area-related leaf mass production and seasonal growth dynamics of Tilia apparently reacted most sensitive to actual climatic conditions. Total above-ground production did not differ significantly between the different forest stands, but exhibited a decreasing tendency with decreasing beech abundance (9.0, 8.5 and 7.1 Mg ha-1). Leaf biomass was constant for all forest stands and investigated years ranging from 3.1 to 3.9 Mg ha-1. Leaf litter and lignin decomposition rates were higher in the mixed species forest stands than in pure beech stands. Correspondingly, rates of nutrient release from litter mixtures were significantly higher in the highly diverse stands. Litter decomposition of Fagus was enhanced in the mixed species stands. Among individual tree species, Fagus leaf litter exhibited slowest decomposition rates (decomposition rate constant k=0.5), whereas Fraxinus leaf litter decomposed fastest (k=2). The decomposition rate constants k were closely correlated with the thickness of the organic litter layer, soil pH, soil fauna abundance and initial C:N, Ca and N values of leaf litter. We found the highest nutrient foliar concentrations in Tilia (N, P, K) and Fraxinus (Ca, K, Mg). Beech foliage and leaf litter had the lowest nutrient concentration compared to the other deciduous tree species. Basal area-related nutrient stocks were comparable in the storage compartments stem wood and branches. Species-specific differences are high within leaf litter and fruits for all nutrients. Higher basal area-related total nutrient storage of K, Mg, Ca, N, and P in mixed species stands compared to monospecific stands was found.We found no evidence of complementary resource use associated with above-ground biomass production. Higher decomposition and nutrient release rates indicated a faster nutrient cycling in the mixed species stands. However, basal area-related productivity of the monospecific stands was not higher in the mixed than in the pure stands. Instead, mixed species stands revealed higher nutrients content in all tree compartments of the mixed species stands. The results suggest that at sites that allow production of broadleaf tree species with nutrient-rich, easily decomposable foliage the establishment and promotion of these species is an important silvicultural tool to counteract natural or anthropogenic soil acidification and to maintain soil productivity. In general, all ecosystem functions were strongly dependent on the characteristic physiological, morphological and architectural traits, rather than on tree species number per se.