Daisuke Kabeya

Biotic homogenization and differentiation of soil faunal communities in the production forest landscape: taxonomic and functional perspectives

Biotic homogenization has been reported worldwide. Although simplification of communities across space is often significant at larger scales, it could also occur at the local scale by changing biotic interactions. This study aimed to elucidate local community processes driving biotic homogenization of soil faunal communities, and the possibility of biotic re-differentiation. We recorded species of oribatid mites in litter and soil layers along a gradient of forest conversion from monoculture larch plantation to primary forests in central Japan. We collected data for functional traits of the recorded species to quantify functional diversity. Then we quantified their taxonomic/functional turnover. Litter diversity was reduced in the larch-dominated stands, leading to habitat homogenization. Consequently, litter communities were biologically homogenized and differentiated in the plantations and in the natural forest, respectively. Turnover of functional traits for litter communities was lower and higher than expected by chance in the plantations and in the natural stand, respectively. This result suggests that the dominant assembly process shifts from limiting similarity to habitat filtering along the forest restoration gradient. However, support for such niche-based explanations was not observed for communities in the soil layer. In the monocultures, functional diversity expected from a given regional species pool significantly decreased for litter communities but not for those in the soil layer. Such discrepancy between communities in different layers suggests that communities more exposed to anthropogenic stresses are more vulnerable to the loss of their functional roles. Our study explains possible community processes behind the observed patterns of biological organization, which can be potentially useful in guiding approaches for restoring biodiversity.

Concordance and discordance between taxonomic and functional homogenization: responses of soil mite assemblages to forest conversion.

The compositional characteristics of ecological assemblages are often simplified; this process is termed “biotic homogenization.” This process of biological reorganization occurs not only taxonomically but also functionally. Testing both aspects of homogenization is essential if ecosystem functioning supported by a diverse mosaic of functional traits in the landscape is concerned. Here, we aimed to infer the underlying processes of taxonomic/functional homogenization at the local scale, which is a scale that is meaningful for this research question. We recorded species of litter-dwelling oribatid mites along a gradient of forest conversion from a natural forest to a monoculture larch plantation in Japan (in total 11 stands), and collected data on the functional traits of the recorded species to quantify functional diversity. We calculated the taxonomic and functional β-diversity, an index of biotic homogenization. We found that both the taxonomic and functional β-diversity decreased with larch dominance (stand homogenization). After further deconstructing β-diversity into the components of turnover and nestedness, which reflect different processes of community organization, a significant decrease in the response to larch dominance was observed only for the functional turnover. As a result, there was a steeper decline in the functional β-diversity than the taxonomic β-diversity. This discordance between the taxonomic and functional response suggests that species replacement occurs between species that are functionally redundant under environmental homogenization, ultimately leading to the stronger homogenization of functional diversity. The insights gained from community organization of oribatid mites suggest that the functional characteristics of local assemblages, which support the functionality of ecosystems, are of more concern in human-dominated forest landscapes.

Influence of reproduction on nitrogen uptake and allocation to new organs in Fagus crenata

The contributions of the internal nitrogen (N) cycle and N uptake from soil to growth in mature trees remain poorly understood, especially during reproduction. In order to elucidate how reproduction affects N uptake, allocation and remobilization, we applied pulse 15N labelling to three fruiting (F) and three non-fruiting (NF) Fagus crenata Blume trees after the leaves were fully unfurled. Three-year-old branches were sampled from upper crowns at about 2 week intervals until leaf fall. 15N content per organ dry mass (15Nexcess) and N concentration in all new shoot organs were determined. Fruiting led to greater 15Nexcess uptake from the soil during the first month following application. Cupules absorbed the highest fraction of 15Nexcess initially and nuts contained about half the 15Nexcess at the end of the growing season. Biomass of reproductive organs represented up to 70% of new shoot growth in F trees. This fruit burden led to 34% and 38% reduction in biomass and 15Nexcess, respectively, in mature leaves compared with NF trees. Moreover, the increment of 15Nexcess in new shoots of F relative to NF trees was lower than the increment of biomass between the two. These results indicate that N is a limiting resource during masting in F. crenata. 15Nexcess incorporated into nuts started to increase dramatically once 15Nexcess in leaves, branches and cupules hit seasonal maxima. Similar seasonal biomass growth patterns were also found in these organs, indicating that sink strength drives uptake and allocation of 15Nexcess between new shoot compartments. These results, together with translocation of 15Nexcess from cupules and senescing leaves to nuts (contributing to fruit ripening), suggest that a finely tuned growth phenology alleviated N limitation. Thus, fruiting did not influence the N concentration in leaves or branches. These reproduction-related variations in N uptake and allocation among new shoot compartments have implications for N dynamics in the plant-soil system.