Relation of mortality to DBH and available area in naturally germinated Pinus densiflora populations

Abstract To elucidate whether small diameter at breast height correlates with tree death in an overcrowded tree population, we analyzed self-thinning occurring over the course of 37 years in naturally established Pinus densiflora populations in Sug-adaira, central Japan. As trees grew, their diameters at breast height increased and the number of trees consequently decreased. Spatial distribution, which was initially clumped, changed accordingly, first becoming random and finally uniform. We calculated the “available area” of individual trees to elucidate the contribution of this parameter to tree mor -tality. Small diameter at breast height was strongly correlated with tree death, with a slight correlation observed between tree death and small available area of individual trees. Key words: available area, diameter at breast height, Pinus densiflora INTRODUCTION Spatial distribution of individual trees in an overcrowd -ed population is initially random, and then becomes uni-form as trees grow (Kenkel 1988). Because plants lack the ability to migrate, this uniform spatial distribution arises as a consequence of tree death. The dead trees are smaller individuals dying as a result of interactions with other trees (Silvertown and Charlesworth 2001, Gurevitch et al. 2006). Interactions between animals are easily observed. For example, competitive interactions associated with terri-torial defense are readily evidenced by aggressive behav-ior towards other animals. Because plants cannot move on their own, their interactions are less obvious; how-ever, a dataset of measured diameter at breast heights (DBHs) and/or heights can be generated to estimate the interaction. In this study, we used such a dataset to detect competitive interaction manifested by lower-than-usual growth rates and/or deaths of trees. Because lower growth rates and/or death may arise from factors such as disease in addition to disturbance, however, plant population mortality is difficult to predict.Mithen et al. (1984) have reported that individuals of the herb