Kazuhiko Terazawa

Masting behavior of Fagus crenata in northern Japan : spatial synchrony and pre-dispersal seed predation

An 11-year study of total seed and seed production in five natural Japanese beech (Fagus crenata) forests in Hokkaido, northern Japan, estimated the spatial synchrony of masting and examined whether variation in annual total seed production resulted in fluctuations in pre-dispersal seed-predator populations. We found less synchrony in total seed production than in sound seed production among sites. The coefficients of variation of sound seeds (1.3–2.1) were greater than those of total seeds (0.8–1.3) in each site. Pre-dispersal seed predation by moth larvae has a major influence on sound seed production in F. crenata. To test whether variation in annual total seed production resulted in fluctuations in predator populations, we examined the relationship between the ratio of current year total seed production to previous year total seed production, and the percentage of pre-dispersal seed predation in the current year. We found a negative correlation between insect predation and the ratio of current to previous year total seed production, suggesting that poor total seed years resulted in a decrease in predator populations. The pre-dispersal seed predators of F. crenata appeared to cope well with up to 10-fold variation in total seed production between years, but when the total seed production exceeded 20 times that of the prior year, the predators were satiated.

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.

Basal area growth and mortality of Betula maximowicziana affected by crown dieback in a secondary forest in Hokkaido, northern Japan

Since the late 1990s, decline of B. maximowicziana Regel has been observed in mature secondary forests in various parts of Hokkaido, northern Japan. To develop a method of thinning for large-timber production of B. maximowicziana, we measured basal area growth and the mortality of 217 trees during a four-year period (1999–2003) in a 90-year-old secondary forest with serious tree damage. We analyzed growth and mortality in relation to the degree of crown dieback (DC), symmetric and asymmetric competition from neighbors, and initial tree size. Individual basal area growth decreased with increasing DC, with increasing symmetric (two-sided) competition, and with decreasing initial tree size. During the four-year period, 4.1% of the observed trees died. Logistic regression analysis revealed that mortality rate increased with increasing DC and with increasing symmetric competition. These results suggest that both growth and mortality rates were affected by the same factors (i.e., DC and symmetric competition). We concluded that the resource for which individuals were competing at the study site was underground, most likely water. Modeled growth and mortality rates can be used to improve the management of damaged forests. A management plan for the damaged study site is proposed.