The browsing ratio as an index of the impact of deer browsing on tree seedlings

Abstract

The possibility of maintaining forested vegetation under deer browsing depends on the success of regeneration processes. Several indices have been proposed to estimate the pressure of deer browsing based on the incidence of browsing on woody plant seedlings. The maintenance of seedling populations is critically dependent on managing deer populations under the threshold levels. For two years we monitored the survival and growth of broad-leaved seedlings and the incidence of deer browsing on these seedlings to examine the effectiveness of the browsing ratio (i.e., the ratio of browsed seedlings per year to the total number of available seedlings, regardless of browse species preferences) as an index of deer browsing pressure, along with snow depth, dwarf bamboo Sasa spp. abundance, and light availability in nineteen 100 m2 plots located in natural forests in Hokkaido, northern Japan. The abundance of deer as indicated by the number of deer observed via spotlight count surveys was associated with the abundance of Sasa spp. and the snow depth. The browsing ratio was associated with snow depth, Sasa spp. abundance, light availability, and deer abundance. This result suggests that the impact of deer on seedlings differs depending on the local environment, even under similar deer abundance. Sasa spp. abundance had a negative effect on seedling density and survival. Sasa spp. are an important food source for wintering deer and deer reduce their biomass, whereas dense Sasa cover had a protective effect on small seedlings. Thus, Sasa spp. affects seedlings in various ways. Changes in both the number and mean height growth of seedlings in each plot were explained with the browsing ratio alone. Thus, the browsing ratio directly summarizes the effect of deer on seedlings. The estimated threshold browsing ratio was 0.27 for changes in seedling density and 0.51 for height growth of seedlings.