A demographic analysis of mortality caused by the pine wood nematode (Bursaphelenchus xylophilus) and pine sawyer beetles (Monochamus alternatus) in pine forests in the Seto Inland Sea-side, Japan.

Demographic characteristics were examined in 51 study sites in naturally occurring pine populations in the Seto Inland Sea-side. The pine forests consisted on average of 51.4% Japanese red pine,Pinus densiflora, 28.9% Japanese black pine,Pinus thunbergii, and 19.7% hybrids of the two species, and occurred along an elevatinal gradient from denser, younger secondary forest regrowing after disturbance, to more mature stands at higher elevations. The proportion of dead trees did not vary much among taxa (range 24-31%). Mortality was caused primarily by the introduced pine wood nematodeBursaphelenchus xylophilus, and secondarily by its vector, the Japanese pine sawyer beetleMonochamus alternatus. While the nematode attacked individuals of all three pine taxa with equal frequency (3-8% mortality), the beetle caused the greatest proportional mortality inP. thunbergii (35%), less inP. densiflora (20%), and the least in hybrids (18%). Mortality caused by both the nematode and beetle was significantly correlated with the mean tree age, height and diameter of the dead trees.The majority (70.2%) of individuals in populations on Miyajima Island that had lived 60 or more years and survived without severe damage by nematodes and beetles were hybrids, although the proportion of hybrids in the seedlings (found only in seven out of the fifty-one study sites) was quite low (1.9%). Thus it seems that hybrids could be more long-lived and better adapted than their parental species to resist both the nematode and the beetle. IfB. xylophilus andM. alternatus populations continue to increase at their current levels they may have a drastic effect on the gene pools ofP. densiflora andP. thunbergii and on the structure and composition of Japanese pine forests.