Effects of horse grazing on plant species richness and abundance of Iris setosa in a boreal semi-natural grassland, Japan

摘要

We investigated plant community attributes and selected soil characteristics of a Japanese boreal semi-natural grassland among day-grazed (24 h), night-grazed and ungrazed sites in Akkeshi, eastern Hokkaido. We also evaluated the effects of horse grazing on plant species richness and the population of Iris setosa, a hygrophilous target species for conservation. Species richness and iris dominance were the highest and community height was the lowest at the day-grazed site; however, plant coverage and light intensity at ground level did not differ significantly among the sites. Soil aeration and water permeability of the surface soil were the lowest at the day-grazed site. Logistic regression analyses suggested direct and/or indirect positive effects of grazing on the occurrence probability of nine species. Species richness was negatively correlated with community height, soil aeration and water permeability of the surface soil and positively correlated with bulk density and soil hardness. Iris dominance was negatively correlated with aeration and water permeability and positively correlated with surface soil bulk density and volumetric water content. Iris dominance was also positively correlated with species richness. Canonical correspondence analyses demonstrated different species composition among the sites that mainly segregated with grazing intensity, soil bulk density, surface soil aeration and community height. Our results showed that all-day horse grazing in a boreal semi-natural grassland modifies surface soil characteristics and increases plant species richness and dominance of I. setosa.