Climatic determinants of the northern range limit of Fagus crenata forests in Japan

We sought to identify the factors that determine the current northern range limit of Fagus crenata (Fagaceae) forests in Japan. Four climatic factors (warmth index, minimum temperature of the coldest month, summer precipitation and winter precipitation) can potentially explain the distribution. Five classification tree models were developed: one based on all four factors, the others based on the four permutations of three of the four climatic factors (i.e. each excluded one of the four factors). The performance and the distributions of F. crenata forests predicted by these models were then compared, with particular reference to the distributions in Hokkaido. The distribution predicted by the model developed without summer precipitation extended continuously northwards along the Sea of Japan coast beyond the current northern range limits, but the distributions predicted by the other three models showed gaps around the current limit. When the warmth index, minimum temperature of the coldest month or winter precipitation were excluded the contribution to the resulting models of the other three variables increased, thereby compensating for the exclusion. However, the exclusion of summer precipitation was not compensated by increases in the contributions of the other variables in the model developed without this factor. These results suggest that summer precipitation is the most influential factor explaining the absence of F. crenata forests in the areas immediately to the north-east of the current northern range limit.