Susceptible conditions for debarking by deer in subalpine coniferous forests in central Japan

Iijima and Nagaike Forest Ecosystems (2015) 2:33 DOI 10.1186/s40663-015-0059-y RESEARCH Open Access Susceptible conditions for debarking by deer in subalpine coniferous forests in central Japan Hayato Iijima* and Takuo Nagaike Abstract Background: Recently, deer have expanded their distribution to higher altitude ranges including subalpine forests. However, culling deer and construction of deer fence in subalpine forests are difficult because of steep slopes and complex topography. Thus it is necessary to clarify the factors which are associated with debarking by deer for the effective protection of subalpine forests. In this study, we examined which factors are associated with debarking by sika deer (Cervus nippon) in subalpine coniferous forests. Methods: We conducted our survey in Minami-Alps National Park, central Japan. We established 24 10 m × 40 m plots and surveyed the occurrence of debarking on saplings >30 cm in height and <3 cm in diameter at breast height (DBH) and on trees >3 cm in DBH, as well as sapling density within each plot. Minimum distances to nearest grassland of plots were calculated (tentatively assuming grassland would attract deer and would cause high debarking pressure in the surrounding subalpine forests). Results: The mean percentage of debarked live saplings was higher than that of live trees. The mean percentage of debarked saplings which had already died was 81.6 %. Debarking of saplings increased with lower elevation, taller sapling size, and marginally increased near grassland. Sapling density was lower in plots with low basal area of conspecific trees near grassland and differed among species. Sapling density marginally decreased with decreasing elevation and increasing stand tree density. Debarking of trees was positively related to small DBH and low elevation, and marginally increased near grassland and differed among species. Conclusions: Our results suggest that tall saplings in subalpine forests of low elevation or near subalpine grassland were susceptible to debarking by deer and monitoring of these areas may permit the early detection of the impacts of deer in subalpine coniferous forests. Keywords: Abies, Cervus nippon, Debarking, Grassland, Picea, Sapling density, Subalpine region Background In recent years, the population densities of large ungulates, especially deer species, have increased worldwide (Stewart and Burrows 1989; Fuller and Gill 2001; Rooney 2001; Apollonio et al. 2010; Iijima et al. 2013). An increase of deer density has been shown to result in more prevalent debarking of trees (Akashi and Nakashizuka 1999; Nagaike and Hayashi 2003; Iijima and Nagaike 2015) and browsing of saplings and understory vegetation (Gill and Beardall 2001; Beguin et al. 2009; Suzuki * Correspondence: Yamanashi Forest Research Institute, Saisyoji 2290-1, Fujikawa, Yamanashi 400-0502, Japan et al. 2013) in forests. To deal with the overabundance of deer and its effect on forest ecosystems, control of nuisance deer and the construction of fences to protect vegetation have been conducted. However, such countermeasures are difficult to perform in subalpine forests because of difficult access (e.g., steep slopes, complex topography, great distances from roads). Deer seasonally migrate to higher elevations, where delayed bud flush caused by low temperature provides fresh leaves even in mid-summer and they can escape from predators and more intense hunting pressure in low-elevation areas (Mysterud et al. 2011). These factors also have caused the number of deer in many higher elevation areas to © 2015 Iijima and Nagaike. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Iijima and Nagaike Forest Ecosystems (2015) 2:33 increase (Takatsuki 2009), and debarking in subalpine forests has increased (Yokoyama et al. 2001; Takeuchi et al. 2011) in recent years. Therefore, factors correlated with debarking by deer in subalpine forests should be clarified to help effectively conserve these forests. Many studies have examined the factors influencing debarking and browsing in lower elevation forests. Debarking was shown to depend on the size of trees (e.g., Nagaike and Hayashi 2003; Koda and Fujita 2011; Borkowski and Ukalski 2012), their species (e.g., Kay 1993; Akashi and Nakashizuka 1999; Moore et al. 1999; Takeuchi et al. 2011), proportion of coniferous stands (Ligot et al. 2013), the distance from forest road (McLaren et al. 2000), and snow depth (Iijima and Nagaike 2015). In addition to these factors, spatial variation of deer impact was observed in lower elevation forests: the higher the deer density, the higher the proportion of debarked trees (Iijima and Nagaike 2015) and browsed saplings (Akashi et al. 2011) and the lower the sapling density (Beguin et al. 2009). The spatial variation of deer density across low-elevation areas was explained by the presence of attractive habitat (e.g., artificial grassland; Kamei et al. 2010; Iijima et al. 2013). However, the attractive habitat in subalpine forests, which are located on steeper slopes and have more complex topography than lower elevation areas, has not been well studied. Takeuchi et al. (2011) reported that elevation was an important factor affecting expansion of deer density into subalpine zones. In addition to elevation, the amount of Page 2 of 7 attractive habitat (e.g., subalpine grassland) may affect deer density. Deer generally prefer herbaceous plants to woody plants (Takatsuki 1986; Winnie 2012), and severe browsing of herbaceous plants in subalpine grasslands by deer has been reported (Schütz et al. 2003; Nagaike 2012; Nagaike et al. 2014). Thus, subalpine forests which were surrounded by large subalpine grasslands would be more susceptible for debarking by deer. Thus far, however, direct evaluation of the relationship between these factors and the intensity of debarking in subalpine forest is rare. The objective of this study was to clarify the conditions which are correlated with debarking by sika deer in subalpine forests in order to improve the effective management and conservation of these subalpine forests. We hypothesized that the occurrence of debarking and sapling density would increase and decrease in low altitude and abundant grassland. Methods Study area The present study was conducted in subalpine forests in Minami-Alps National Park (357.5 km2) in central Japan (Fig. 1), where sika deer (Cervus nippon) density has been increasing in recent years (Izumiyama and Mochizuki 2008; Izumiyama et al. 2009). The park is characterized by steep slopes and a concentration of many mountains within a small area (Fig. 1). S (...truncated)