Fine root dynamics in broad-leaved deciduous forest stands differing in tree species diversity

Abstract

Biodiversity effects on ecosystem functioning in forests have attracted increasing attention only recently. The vast majority of studies in forests focussed on above-ground responses to differences in tree species diversity, while systematic analyses of the effects of biodiversity on root systems are virtually missing. By investigating the tree fine root systems in 12 temperate deciduous forest stands in the Hainich National Park (Thuringia, Germany), we tested the hypotheses that (i) stand fine root biomass increases with increasing tree species diversity, (ii) that tree species mixtures result in a spatial segregation of the fine root systems of different tree species, (iii) that seasonal changes in fine root mass are more pronounced in tree species-poor than in tree species-rich stands, and that (iv) productivity and resilience of the fine root system (i.e. recovery after disturbance) increase with increasing tree species diversity. The selected 12 forest stands represent a gradient in tree species diversity on similar bedrock from almost pure beech forests to medium-diverse forests built by beech, ash, and lime, and to highly-diverse stands dominated by beech, ash, lime, maple, and hornbeam. We used a morphological key to identify the fine roots of the different tree species by colour, periderm surface structure and branching patterns. We investigated fine root bio- and necromass and their spatial distribution patterns at 24 profiles per stand and analysed species differences in fine root morphology by microscopic analysis. Furthermore, the seasonal changes in fine root bio- and necromass were recorded during a 12-month period and fine root productivity was estimated with two independent approaches (ingrowth cores, sequential coring). Fine root biomass ranged from 440 to 480 g m-2 in the monospecific to the 5-species stands, with 63-77 % being concentrated in the upper 20 cm of the soil. In all tree species present, fine root density (biomass per soil volume) decreased exponentially with soil depth at very similar rates. Moreover, indications of a spatial segregation of the root systems of different species in the species-rich stands were not found. Fine root morphology showed marked distinctions between tree species, but these root morphological differences did not lead to significant differences in fine root surface area or root tip abundance on a stand area basis. Moreover, differences in tree species composition of the stands did not alter fine root morphology of the species. According to the analysis of 360 ingrowth cores, fine root growth into the root-free soil increased with tree species diversity from 72 g m-2 yr-1 in the monospecific plots to 166 g m-2 yr-1 in the 5-species plots indicating a more rapid recovery of the root system after soil disturbance in the species-rich stands. Fine root productivity as approximated by the sequential coring data also indicated a roughly threefold increase from the monospecific to the 5-species stand. We found no indication of a more pronounced seasonality of fine root mass in species-poor as compared to species-rich stands. We conclude that "overyielding" in terms of fine root biomass does not occur in the species-rich stands, which is most likely caused by the absence of significant spatial segregation of the root systems of these late-successional species. Our study produced first evidence in support of the hypothesis that more diverse forest stands are more resilient after soil disturbance, and perhaps more productive, in their fine root system than species-poor forests. Species identification on the fine root level, as conducted here, may open new perspectives in further studies focusing on tree species effects on root dynamics.