[M-IS12] Mountain Science

[MIS12-08] Topographic conditions constraining the distribution of subalpine coniferous forests on the snowy mountains in the eastern Japan

Shiho Maniwa¹, *Atsushi Ikeda², Natsuki Sasaki³ (1.Kokusai Kogyo, 2.University of Tsukuba, 3.Meiji University)

Keywords:Abies mariesii, mountains with heavy snowfall, pseudo-alpine zone, GIS, Maxent

Abies mariesii composes subalpine coniferous forests on the mountains in the Japan Sea side of eastern Japan. However, some mountains lack such forests, the subalpine elevation is mostly covered with grass, dwarf bamboo or shrubs instead. This area called a pseudo-alpine zone was firstly thought to develop because heavy snowfall prevents coniferous trees growing. In later years, climate changes since the Last Glacial Period have been considered to control the replacement of vegetation differently on each location. However, no study has simultaneously analyzed the climate and topography through the whole distribution area of Abies mariesii. Thus, this study discussed the common topography favorable for Abies mariesii on many mountains from the central to northeastern Japan. Then, the topographic parameters were inputted into a machine learning model predicting potential distribution of species to test separation of pseudo-alpine zone from subalpine forests. Digital vegetation maps and 10-meter grid digital elevation models in the subalpine zones on 28 mountains representative of the whole region were used for the following analyses. The relationship between vegetations and three topographic parameters (elevation, slope aspects, slope angles) was examined for each mountain. The land coverage by Abies mariesii of the 28 mountains was also compared to discuss the topo-climate restrictions on the species such as vertical spreads under subalpine climates, dominant slope angles and the annual maximums of snow thickness. Abies mariesii found on 24 mountains is concentrated on gentle slopes. The medians of slope angles were often below 20° on the mountains dominated by Abies mariesii, while they were above 30° on the mountains with the pseudo-alpine zone. Maximum entropy modeling (Maxent), a software for predicting the potential distribution of species, was used to discuss the contribution of warm index (WI), rainfall, annual maximums of snow thickness, slope angles, slope curvatures and slope aspects to the distribution of Abies mariesii. These explanatory variables were prepared over the entire eastern Japan with a 250-meter grid resolution. When six explanatory variables were inputted to the Maxent at the same time, the contribution of WI (86%) and annual maximums of snow thickness (14%) only exceeded 10%. In contrast, the calculation only within the subalpine zones indicated importance of the annual maximums of snow thickness (66% contribution), slope angles (20%). The preference for gentle slopes of Abies mariesii does not contradict the previous two hypotheses. One is that Abies mariesii cannot colonize in areas with high snow pressure, and the other is that it was able to spread only on gently sloping mountains with a lot of wetlands during the postglacial period. Although the modeled probability of presence of Abies mariesii in many mountains without it decreased when the slope angle was added in the explanatory variables, high modeled probability had remained for some mountains having a pseudo-alpine zone. It suggests that the current climatic and topographic conditions are insufficient to fully explain the formation of pseudo-alpine zones.