The objective of this study was to use hierarchical phytosociological data to clarify the process of succession from a native willow riparian forest to one dominated by the invasive species
Robinia pseudoacacia. We applied two-way indicator species analysis (TWINSPAN) and species-composition data obtained from vegetation surveys. In community-classification approaches, the species composition of each community type is often clarified using the highest value of cover rank for each species in each vertical forest layer. However, this method cannot elucidate distributional relationships among the upper and lower layers, so it cannot provide information on succession or species shifts among communities. In this study, we analyzed species distributions in terms of individual cover in each of the tall-tree, sub-tree, shrub, and herb layers using the TWINSPAN method, which distinguishes the distributional structures of the upper and lower layers. The TWINSPAN results for our hierarchical data show that
Robinia pseudoacacia persisted beneath tall-growth willow (
Salix serissaefolia), which was dominant in the riparian zone, but that
S. serissaefolia seedlings and shrubs did not grow in the understory of the
R. pseudoacacia forest canopy. These results imply that in the absence of large-scale disturbance by river water native
S. serissaefolia forests will shift toward non-native
R. pseudoacacia forests. Our results indicate that TWINSPAN is useful for understanding forest regeneration and succession involving invasive species, in particular, even in the absence of tree-height and stem-diameter data from surveys of individual trees.
View full abstract