Rainfall partitioning and soil water dynamics along a tree species diversity gradient in a deciduous old-growth forest in Central Germany

Species diversity effects have been extensively investigated in grassland ecosystems, however, little is known about the influence of tree species diversity on the forest water cycle. In this dissertation several hydrological processes including soil water dynamics, rainfall partitioning and the related ion deposition with rainfall have been investigated along a tree species diversity gradient. In addition, the results were used to model the water cycle. The studies were conducted in twelve study plots in old-growth deciduous forest stands in the Hainich National Park, Germany. Monospecific plots were formed by Fagus sylvatica (beech) and mixed forest plots consisted of a variable admixture of up to eleven broad-leaved deciduous tree species such as Tilia spec. , Fraxinus excelsior, Carpinus betulus, and Acer pseudoplatanus. The first part of this thesis presents the partitioning of rainfall into throughfall, stemflow, and interception along the tree species diversity gradient. The results demonstrated that several stand characteristics influenced rainfall partitioning. The major variable explaining throughfall for different seasons was tree species diversity expressed as Shannon index. For example, in the high-rainfall summer of 2007, median throughfall per study plot was between 66% and 77% of gross precipitation and correlated positively with the Shannon index of the study plots. Stemflow contributed only 2% to 6% of gross precipitation and was negatively correlated with the Shannon index. Interception showed no correlation with this beech to mixed forest gradient. These relationships were similar in summer 2005 and autumn 2006, yet no (or only weak) correlations between throughfall and tree diversity were observed during the other study periods. Multiple linear regressions supported the assumption that combinations of several stand characteristics were important for differences in throughfall among the study plots, such as the Shannon index and the mean diameter at breast height. Influential stand characteristics varied between seasons and years due to different rainfall conditions. Differences in small-scale heterogeneity of throughfall within the study plots did not change consistently with any stand characteristic along the tree species diversity gradient. The second part of this thesis focuses on soil water dynamics along the tree species diversity gradient. Overall, seasonal patterns of soil water dynamics were similar in all study plots. During a desiccation period in summer 2006, the top soil water content was strongly correlated with tree species diversity of the twelve study plots. At the beginning of this desiccation period, soil water was extracted at higher rates in the species rich plots than in the beech-dominated plots. However, later during the desiccation period when atmospheric evaporative demand was higher, only the beech-dominated stands were able to further increase soil water extraction. On plots of high tree species diversity, soil water reserves were already low and soil water extraction reduced. Possible explanations for high water extraction rates in mixed species plots at the beginning of the desiccation period include species specific characteristics such as high maximum water use rate of some species, enhanced exploitation of soil water resources in mixed stands (complementarity effect), and additional water use of the herb layer, which increased along the tree species diversity gradient. Differences in small-scale heterogeneity of volumetric soil water content within the study plots did not change consistently with any stand characteristic along the tree species diversity gradient. The third part of this thesis presents ion deposition with rainfall and related processes, i.e. interception deposition and canopy exchange, along the tree species diversity gradient. Precipitation deposition and stand deposition (throughfall plus stemflow) of the ions Na+, Cl-, K+, Ca2+, Mg2+, PO43-, SO42-, H+, Mn2+, Al3+, Fe2+, NH4+, NO3- and Norg were measured in nine of the twelve study plots along the tree species diversity gradient. Interception deposition and canopy exchange rates were calculated with a canopy budget model. The interception deposition of all ions except for SO42- decreased with increasing tree species diversity, whereas the canopy leaching rates of K+, Ca2+, Mg2+, Fe2+, Cl- and PO43- increased with increasing tree species diversity. Stand deposition of K+, Ca2+, Mg2+ and Cl- increased and stand deposition of Mn2+ and H+ decreased along the gradient. Possible reasons are differences in ion status of the leaves, tree physiology, and soil characteristics. In conclusion, the nutrient input to the soil was higher in mixed species plots than in monospecific beech plots, whereas the acid input was highest in monospecific beech plots. In the forth part of this thesis, the observations of the first two studies were combined and used in the modelling framework Expert-N to simulate the water fluxes for a monospecific beech study plot and for two mixed study plots. Expert-N combines models for physiological and hydrological processes of the plant-soil system. Water fluxes were simulated by considering rainfall interception, evaporation, soil water flow, drainage, and root water uptake. Observations in the study plots provided reliable data for the parameterisation and the calibration of the model. Differences in rainfall interception and root water uptake among the study plots were realistically described. The applied stand-level model was thus able to simulate the water dynamics of the monospecific and mixed forest stands. The calculated water fluxes were sensitive to the species composition of the forest. The results of this dissertation demonstrate that rainfall partitioning, soil water extraction, and stand deposition of ions differed along the investigated tree species diversity gradient during certain periods. Both selection effects and complementarity effects played a role. These results provide information for the management of deciduous forests in Central Europe to meet public demands related to water resources as well as to ensure forest vitality under changing climatic conditions.