Abstract
Myoginohana Marsh is a lowland reed marsh with high indigenous species richness, including 19 endangered species. However, the area dominated by Ischaemum aristatum var. glaucum, which once occupied most of the area of the marsh, has been declining since the late 1990s. We hypothesized that I. aristatum var. glaucum is an indicator of indigenous species richness in the marsh, due to its tussocks, which provide suitable habitat for other species. The marsh vegetation was classified into three distinct types: type C, where Carex deispalata dominates in the middle layer beneath Phragmites australis, which is dominant in the grass canopy; I^+, where I. aristatum var. glaucum dominates in the middle layer, with a patchy moss-layer on the soil surface; and type I^-, where I. aristatum var. glaucum dominates in the middle layer, without moss. The average species density within a 1-m^2 quadrat was highest in type I^+. A whole-marsh-scale survey revealed that the distribution of four endangered species was significantly associated with type I^+. A microsite-scale survey revealed that species density was significantly higher in tussock microsites than in microsites with tussocks. Type I^+ vegetation was characterized by specific environmental conditions: a lower density of P. australis in the high vegetation layer; greater light availability above the middle and low layers of vegetation; and higher frequency of winter mowing or burning than the other vegetation types. These results suggested the importance of I. aristatum var. glaucum as an indicator of plant species richness in the marsh. The ongoing loss of the area dominated by the species may cause a rapid decline in species richness and local extinctions of some endangered plants.
