A micromorphological analysis was performed on the surface volcanic deposit formed by the 1990∼1995 eruption of Mount Fugen to clarify the influence of soil micromorphology and permeability on the deposit a few years after the end of the eruption. Soil micromorphology was observed in soil thin sections of the tephra deposit in two forest stands of Japanese cypress and broad-leaved trees, and the pyroclastic flow deposit. Hydraulic conductivity of these deposits was measured by experiment under saturated conditions. Depositional crust was observed in the ash layer which included small amount of the cypress litter. The conductivity of the ash layer in forest stands including litter was higher than that of the layer including a small amount of litter. The conductivity of the pyroclastic flow deposit was higher than that of the deposit overlaid with a fine-grained volcanic ash layer. These results suggest that covering the ash layer with litter or washing fine-grained ash off of the surface raises the conductivity. The result of the micromorphological analysis shows that the difference of the surface soil micromorphology between the tephra deposit and the pyroclasitic flow deposit is due to the different processes of the soil structural change of the surface deposit.

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Severe bark stripping by sika deer (Cervus nippon yesoensis) occurs during winter snow in the Tokyo University Forest in Hokkaido. The trees debarked are mainly those with small-diameter trunks. The tree species important for forest management and conservation in this Forest are Acer mono, Betula maximowicziana, Betula platyphylla var. japonica, Fraxinus mandshurica var. japonica, Kalopanax septemlobus, Maackia amurensis var. buergeri, Picea jezoensis, Prunus ssiori, Quercus mongolica var. grosseserrata, Taxus cuspidata, Ulmus davidiana var. japonica, and Ulmus laciniata. We analyzed the inner bark components of small-diameter trunks of these twelve tree species, and examined any relationship between each component and the bark preference of sika deer. M. amurensis, for which sika deer exhibited a low preference, was the only species containing alkaloids. Of the other eleven tree species, the ash and acid detergent lignin concentrations had positive and negative relationships, respectively, with the bark preferences of sika deer.

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We inoculated the pine wood nematode, Bursaphelenchus xylophilus, into Pinus thunbergii seedlings, and investigated the progress of symptoms of pine wilt disease and dynamics of B. xylophilus in the seedlings under various light conditions. Shade-treatment did not change air temperature and relative humidity. However, shade-treatment decreased the photosynthetic rate of the seedlings without nematode inoculation and advanced symptoms of pine wilt disease in nematode-inoculated seedlings. Shade treatment did not affect the dispersal of nematodes, but promoted their reproduction in seedlings. From the seedlings with discoloration of previous-year needles, more than 1,000 nematodes/g dry weight of pine segment were extracted. These results suggest that a decrease of photosynthetic rate might advance the pine wilt disease drastically from the early to the advanced stage, which might result in the promoted reproduction of nematodes in the seedlings and discoloration of needles within a short period after infection.

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We examined the relationship between dwarf-bamboo growth and small rodents on Mt. Ohdaigahara where the population density of sika deer (Cervus nippon) has been increasing and trunk debarking by the deer has become a serious problem, causing dieback of barked trees. As a result, deer-proof fences have been erected to protect the vegetation on Mt. Ohdaigahara from further damage. We established six plots with different types of dwarf-bamboo growth. Three rodent species were caught in Sherman traps: Apodemus speciosus, A. argenteus, and Eothenomys smithii. A significant negative correlation was found between the number of captured Apodemus argenteus per 100 trap-nights and dwarf-bamboo density. In areas where the deer population density has been high, the densities of Apodemus speciosus and A. argenteus inside the fence tended to be higher than those outside the fence. Eothenomys smithii was trapped inside the fence. The dwarf-bamboo (Sasa nipponica), which is the main forage of sika deer, inside the fence was higher in culm height, lower in density, and heavier in biomass than that outside the fence. The results suggest that thick dwarf-bamboo inside the fence acted as cover for rodents.

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Water repellency is recognized as an important soil property that can affect the flow and storage of water in forest soils at the slope scale, since it causes overland flow or preferential infiltration. However, the spatial distribution of water repellency on forested slopes and the factors that control it have not been investigated sufficiently. We examined the spatial pattern of soil water repellency on slopes (180×60 m in each area) in a Japanese cypress (Chamaecyparis obtusa) plantation and a nearby deciduous broad-leaved forest. The water drop penetration time (WDPT) and ethanol percentage (EP) were used as indices of repellency. In the cypress plantation, the fresh-soil WDPT (WDPT (f)), which is thought to indicate the actual water repellency, exceeded 600 sec at more than 40% of the sampling points during a dry period in summer, including those points in the middle slope. During a wet period in winter, the WDPT (f) was less than 60 sec at most of the sampling points. The air-dry WDPT (WDPT (d)), which indicates the potential water repellency, exceeded 1 h at more than 80% of the sampling points in the plantation, except on the lower slope near a stream. In contrast, potential water repellency was observed over the entire slope in the deciduous forest, although fewer than 20% of the sampling points had a WDPT (d) exceeding 1 h. Positive correlations between the total carbon content of the sampled soils and the WDPT (d) and EP were observed. Comparing soils with similar carbon contents, the soil of the cypress plantation had a greater potential water repellency than that of the deciduous forest.

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Host pine trees infected with pine wilt disease show various histological responses. The pathogenic pinewood nematodes (Bursaphelenchus xylophilus) migrate through the resin canals in cortex and xylem, and epithelial cells surrounding resin canals are destroyed soon after nematode invasion. Lignin and suberin-like substances accumulate around the resin canals in the cortex. Various histological changes occur in xylem parenchyma cells, and after the symptom develops severely, the contents of xylem parenchyma cells spread into surrounding tracheids and incrust the tracheids and the bordered pits. In resistant pine species, cytological changes in xylem parenchyma cells are less induced than in susceptible pine species, and such changes are restricted in nematode-inoculated regions. When inoculated with avirulent isolates of pinewood nematode or avirulent nematode species (B. mucronatus), nematode distribution and histological changes are restricted in nematode-inoculated regions. The detailed mechanisms of symptom development of pine wilt disease still remain unknown, and further studies are needed to clarify them completely.

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Biochemical responses of pines after the infection of the pinewood nematode, Bursaphelenchus xylophilus are reviewed. Biochemical changes, which result in wilting and death of trees, progress with the nematode attack in susceptible pines, and endogenous toxic substances are produced to cause cell death and blockage of water conduction. Preformed inhibitory activity is detected in some resistant pines. Production of Inhibitins and/or phytoalexins and histological changes are also observed as the expression of resistance, and are supposed to be more important resistance factors.

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The pathogenicity of Bursaphelenchus xylophilus, a pathogen of pine wilt disease, to conifers mainly Pinus, was reviewed, and the differentiation of its pathogenicity and intraspecific variation of virulence were discussed. Seventy four species of conifers in 7 genera of Pinaceae in North America, Europe, and East Asia have been reported as host trees of the nematode. The degree of resistance differed with the pine species. Pine species in Eurasia including Japan tended to be susceptible to the disease and species in the eastern part of North America tended to be resistant. The pathogenicity of the nematode may have already differentiated in North America, but not in Japan. The degree of virulence also differed with the nematode. Although the factor that affects virulence remains unclear, virulent nematode isolates tended to migrate and increase in population in the nematode-infected tree or reproduce in fungal culture rapidly, and have high cellulase activity compared with avirulent isolates.

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The genus Bursaphelenchus, a group of mostly obligate mycophagous nematodes, has been considered a potential risk to cultivated conifers and palms and natural forest trees, because the genus contains two lethal plant pathogens, pinewood nematode (B. xylophilus) and red ring nematode (B. cocophilus). Recently, the importance of the taxonomy and identification of this genus has been re-realized due to the accidental introduction of B. xylophilus into Europe. In this context, the current systematic status of the genus Bursaphelenchus is reviewed and compared. There are about 80 morphologically and ecologically divergent species of Bursaphelenchus that are classified into several groups according to their morphology and/or molecular phylogenetic traits. The present morphological taxonomic system, which focuses on the spicule morphology (male secondary characteristics) was compared with the results of molecular phylogenetic analysis. This comparison demonstrated that the spicule morphology partly reflects the inferred molecular phylogenetic relationships of these nematodes, however convergence of spicule morphologies were observed in several morphological groups. In addition, comparisons between life history traits and molecular phylogeny suggested that the phoretic relationships between Bursaphelenchus and different insects have independently occurred several times in this genus.

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The pinewood nematode, Bursaphelenchus xylophilus, is the causative agent of pine wilt disease (PWD), and is transmitted among host trees by cerambycid beetles in the genus Monochamus. In Japan, its principal vector is M. alternatus and there is a mutualistic relationship between B. xylophilus and M. alternatus at population level. After the roles of B. xylophilus and M. alternatus in PWD were clarified, the biology of and the interaction between the two species have been energetically investigated. Especially for the last decade, the study on interaction between them has been rapidly advanced by the development of artificial pupal chambers, the analysis for factors affecting B. xylophilus transfer to and departure from M. alternatus adults, the discovery of new transmission pathways of B. xylophilus to pine trees and so on. In this review, I summarize the studies on the transmission mechanism of B. xylophilus containing the latest information and discuss the future direction of research on this topic.

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Although geographical variation was proposed for morphological, physiological and ecological traits of Monochamus alternatus, there has been no information about its genetic structure. Recent progress in molecular ecological methods, e.g. the development of microsatellite markers and phylogeographical analysis using mitochondrial DNA, enabled us to study genetic differentiation among populations. After I introduced reports of geographical variation in some traits, I reviewed population structure in M. alternatus detected by molecular markers. First, the process of population expansion was inferred from genetic structure in Japan, China and Taiwan populations revealed by mitochondrial DNA and microsatellite markers. Second, microsatellite markers showed the pine sawyers' dispersal route in the past in Akita and Iwate Prefectures, located in northeast part of Honshu, Japan. These areas are the frontier damaged area of the pine wilt disease. At the end of the article, I discussed further subjects in studies of the sawyers' geographical variation.

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The severe epidemic of pine wilt disease (PWD) in Japan is caused by the pinewood nematode, Bursaphelenchus xylophilus, an alien pest allied with the endemic vector insect, Monochamus alternatus, originally having remarkable reproductive potential and dispersal ability. In the Japanese pine forest ecosystem, strategies and methods effective for some endemic pest insects are not applicable for control of PWD. The only strategy for disease control is to keep the damage at a sub-epidemic level and this will be embodied in a series of procedures: first, designation of the pine forest to be reserved, second, suppression of the PWD damage to a sub-epidemic level, and third, maintenance of the damage at a sub-epidemic level. To suppress the damage level in an affected pine forest, we must carry out the following measures precisely: removing the surrounding source of infection, eradicating of the dead pine trees infested with M. alternatus, and conducting preventive measures against infection for the living trees. Suitable measures for the above three procedures and prevention factors on-site are discussed, and an example of the application of these measures on-site is shown.

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