Fences are more effective than repellents in reducing deer browsing on planted two conifer species but their effectiveness is reduced by higher deer density, deeper snow, and steeper slope

Abstract

Population expansion of deer and the damage to planted trees caused by them is becoming a serious conflict between deer and humans. Although there are many methods for tree protection, their effectiveness varies from case to case, and the determinant of effectiveness has rarely been clarified. We investigated the following three specific questions: 1) What factors influence forest managers' choice of protection methods? 2) What factors were important in reducing tree damage by each protection method? 3) Which of the repellents and fences was more effective in reducing tree damage? In 2018, we obtained data of plantation tree damages caused by sika deer (Cervus nippon) and tree protection methods from 1424 plantations in Japan. No protection was chosen at low deer densities, and repellents and fences were chosen at middle and high densities by forest managers. In the case of repellents, the damage rank increased with increasing deer density, tree age, and maximum snow depth, but was not affected by planted tree species (Cryptomeria japonica and Chamaecyparis obtusa). In the case of fences, the damage rank increased with increasing deer density, tree age, maximum snow depth, slope, and decreasing net height. The damage rank also increased with the usage of separable digging prevention net and the net without stainless wire. When the age of planted trees is seven, repellents can suppress the tree damage under 50% only when the deer density is below 10 deer/km2 (no snow) or 6 deer/km2 (snow depth is 16 cm). Fences can suppress tree damage below 50% when deer density was below 38 deer/km2 (snow depth: 0, slope: 35°) or 25 deer/km2 (snow depth: 16, slope: 35°). Deer protection methods should be selected according to environmental conditions such as deer density, snow depth, and slope degree in the case of our treated tree species.