Japan is abundant in steep and complex terrain formed by crustal movement and high natural rate of erosion. Several studies suggest that such topographic conditions strongly influence spatial pattern of vegetation structure. In spite of many researches on the relationship between vegetation pattern and topography, community assembly rules on such steep topographic gradients are not well understood. In this study, we examined that relationship between community structure represented by phylogenetic diversity index (NRI: Net Relatedness Index, Web 2000) and five topographic variables (slope inclination, slope aspect, elevation, soil depth and convexity/concavity) in a 306 ha catchment on Mts Tanzawa covered by complex and steep topography. NRI is a standardized metric of phylogenetic clustering. Positive NRI values indicate phylogenetic overdispersion, while negative values indicate phylogenetic clustering.
Occurrence of woody species and soil depth were recorded in 105 quadrats (10 m × 10 m) that were set at intersection points of latitude and longitude of five-second intervals. Other topographic variables were obtained from DEM (Digital Elevation Model; 20 m × 20 m) using GIS. We combined a super-tree from a phylogeny based on APG III (APG 2009) for angiosperms and a Bayesian phylogeny for gymnosperm with the sequence data obtained from GenBank database. NRI was calculated for each quadrat. In assessing relationships between topographic variables and NRI, standard linear regressions were used.
Results/Conclusions
NRI significantly decreased along slope aspect (north-south gradient), slope inclination, and soil depth, where north facing slopes, steep slopes, and thin soils had lower values, respectively. These results suggested the effect of environmental filtering in harsh topographic conditions. Generally, north facing steep slopes with thin soils are stressful for almost all woody species, because of poor light conditions, frequent disturbance and poor nutrients, respectively, in moisture-rich environments. Therefore, species composition might be phylogenetically restricted under harsh topographic conditions. Carpinus species frequently occurred in phylogenetically clustered communities, which suggests that the species in the genus might have adaptive traits for such harsh environments than other species. Old angiosperms (e.g. Magnolia obovata and Illicium anisatum) and conifers occurred in phylogenetically overdispersed communities occurring in benign conditions. However, we could not determine whether phylogenetic overdispersion was due to the occurrence of distantly related species or limiting similarity (i.e. competitive exclusion). We conclude that topographic condition determined community assembly in steep mountainous regions.