Competition as a predisposing factor of crown dieback in a secondary forest of Betula maximowicziana in Hokkaido, northern Japan

We analyzed the probability that Betula maximowicziana Regel (monarch birch) would suffer crown dieback (crown-dieback probability) and the basal area growth rate (GB), which was found to be a predisposing stress factor making birch trees susceptible to crown dieback. First, we analyzed the relationship between the probability that birch trees would suffer from crown dieback in 1999 and GB from a period prior to the occurrence of crown dieback (1985–1987), using a data set of repeated measurements on 217 trees. Logistic regression analysis revealed that monarch birch had a larger crown-dieback probability when GB was low in the preceding period. Hence, there were predisposing stress factors that reduced GB and continued to affect trees for at least a decade. Next, we analyzed GB in the same period in relation to symmetrical and asymmetrical competition between trees and found that GB was reduced by symmetrical competition, suggesting that this was one of the predisposing factors for crown dieback. Based on these results, we used selected models for crown-dieback probability and GB to calculate crown-dieback probabilities for individuals with different initial basal areas and experiencing different intensities of symmetrical competition. The predicted crown-dieback probability decreased with decreasing symmetrical competition between trees. We discuss a possible process of crown dieback to death for monarch birch and the use of thinning as a method to reduce the risk of crown dieback.