Yoshio Hada

Effects of light on direct and indirect defences against herbivores of young plants of Mallotus japonicus demonstrate a trade-off between two indirect defence traits.

BACKGROUND AND AIMS Although most studies on plant defence strategies have focused on a particular defence trait, some plant species develop multiple defence traits. To clarify the effects of light on the development of multiple defence traits, the production of direct and indirect defence traits of young plants of Mallotus japonicus were examined experimentally under different light conditions. METHODS The young plants were cultivated under three light conditions in the experimental field for 3 months from May to July. Numbers of ants and pearl bodies on leaves in July were examined. After cultivation, the plants were collected and the developments of trichomes and pellucid dots, and extrafloral nectaries (EFNs) on the leaves were examined. On plants without nectar-collecting insects, the size of EFNs and the volume of extrafloral nectar secreted from the EFNs were examined. KEY RESULTS Densities of trichomes and pellucid dots did not differ significantly among the plants under the different light conditions, suggesting that the chemical and physical defences function under both high and low light availability. The number of EFNs on the leaves did not differ significantly among the plants under the different light conditions, but there appeared to be a trade-off between the size of EFNs and the number of pearl bodies; the largest EFNs and the smallest number of pearl bodies were found under high light availability. EFN size was significantly correlated with the volume of extrafloral nectar secreted for 24 h. The number of ants on the plants was smaller under low light availability than under high and moderate light availability. CONCLUSIONS These results suggest that direct defence traits function regardless of light conditions, but light conditions affected the development of indirect defence traits.

Changes in the stand structure of a pine forest after rapid growth of Quercus serrata Thunb.

Abstract We studied the species composition and stand structure of secondary pine forests affected by pine wilt disease, caused by epidemic attacks of the nematode Bursaphelenchus xylophilus, in western Japan. Based on the species composition, the vegetation was divided into four types: (1) Cladonia rangiferina, (2) Wikstroemia sikokiana, (3) Quercus glauca, and (4) Gleichenia japonica. The sequence of types from (1) to (3) represents a successional series in accordance with topographical position. The relative basal areas of Rhododendron reticulatum and Juniperus rigida in the C. rangiferina and W. sikokiana types were larger than in the Q. glauca type. The relative basal areas of Q. serrata, Clethra barbinervis, and Eurya japonica in the C. rangiferina and W. sikokiana types were less than in the Q. glauca type. The mortality of light-demanding trees was higher in the Q. glauca type than in the C. rangiferina and W. sikokiana types. In stands that were severely affected by pine wilt disease, light was reduced and soil became moist after rapid growth of Q. serrata and other species which are not affected by the disease. The numbers of species and components of evergreen broad-leaved natural forests were decreased, because pine forests in the study area had been cut frequently and the land had degraded. Pine wilt disease has shifted the dominance of pine forests from Pinus densiflora to deciduous oaks such as Q. serrata and physical conditions seemed to provide a safe site for shade-tolerant plant species, which are components of evergreen broad-leaved forests. Moreover, the seed dispersal of evergreen oaks appears to be an important factor in the succession to evergreen broad-leaved forest in degraded areas.

Different combinations of multiple defence traits in an extrafloral nectary‐bearing plant growing under various habitat conditions

Summary The effects of multiple defence traits on herbivory under different abiotic conditions have not been reported in the literature. To clarify the relations between plant defence traits and abiotic factors in the field, we investigated the plasticity of multiple defence traits in young Mallotus japonicus growing in three habitat conditions: open sites, forest edges and treefall gaps. The plants used multiple defence traits differently in three habitats. At open sites, trichomes, pellucid dots and extrafloral nectaries (EFNs) were expressed better than at the treefall gaps, but food bodies (lipid-rich particles) were fewer. Fewer ants were observed on the plants at open sites than on plants in other habitats. Leaves collected from the open sites were consumed less by the generalist herbivore, Spodoptera litura, than leaves collected from the other habitats. The effect of ants on herbivore exclusion was not readily apparent at open sites. Light intensity at the open sites was high. Soil water contents were low. Tolerance was greater in the forest edge condition, where light intensity and soil water contents were intermediate. Synthesis. The defence tactics of EFN-bearing plants in the field must be determined considering the complex effects of different abiotic factors. Each plant can change its combination of defence traits in response to abiotic habitat conditions.

Variations in direct and indirect defenses against herbivores on young plants of Mallotus japonicus in relation to soil moisture conditions

Some plant species develop multiple defense traits. To test the hypothesis that plants with both direct and indirect defense traits mainly develop the direct and indirect defense traits under the low and high soil moisture conditions, respectively, the development of multiple defense traits on the young plants of Mallotus japonicus (Thunb.) Muell. Arg., and the number of ants on the plants were experimentally examined under different soil moisture conditions. Under the low soil moisture condition, the plant growth declined, and the trichomes and pellucid dots developed well. The pearl bodies developed on the plants under the high soil moisture condition. The volume of extrafloral nectar secreted and the number of ants on the plants significantly increased under the high soil moisture condition. These results clearly show that the young plants of M. japonicus firmly develop sound direct defense traits under the low soil moisture condition, and they develop indirect defense traits that are less reliable but have relatively low costs under the high soil moisture condition.

Relationship between the incidence of pine wilt disease and the drainage area

We examined the relationship between the incidence of pine wilt disease and moisture conditions in the stand level ofPinus densiflora Sieb. et Zucc. forests in the warm-temperate zone of the western part of Japan. For this analysis, pine trees killed by pine wilt disease were distinguished from the suppressed trees by their position in the layer of the pine forest stand. The drainage area, which is small in the upper part and large in the lower part of the slope, was adopted for representing the moisture conditions in the soil of the slope. The percentage of the pine trees killed by pine wilt disease increased as the size of the drainage area increased. This result suggested that the incidence of pine wilt disease tended to be high in areas with moist conditions. Pine trees attacked by the pinewood nematode die from extensive water deficit due to tracheid cavitations. Pine wilt disease mainly emerges in the summer when the soil water conditions become especially severe, and the radical water stress is thought to accelerate the disease. It was assumed that pine trees in the plots with the small drainage area resisted the influence of the attack of the pinewood nematode because pine trees in the plots with the small drainage area encountered long-term water stress and acquired drought tolerance. Pine trees in the plots with the large drainage area were presumed to be well established in the moist conditions and not to have acquired drought tolerance. The drought tolerance of pine trees was thought to be an important factor in resistance to pine wilt disease.

Plasticity in the expression of direct and indirect defence traits of young plants of Mallotus japonicus in relation to soil nutritional conditions

Although soil nutrients can influence the defence strategy of plants that have multiple defence traits, to date, there have been few studies to examine this. To evaluate the effect of soil nutrients on multiple plant defences, we cultivated Mallotus japonicus under three soil nutritional conditions in the field, and experimentally examined the expression of a physical defence trait (trichomes), chemical traits (pellucid dots), and biotic traits (extrafloral nectaries (EFNs) and pearl bodies) of the plants, and the number of ants visiting them. Under the low soil nutritional condition, plants strongly expressed the physical defence by trichomes and chemical defence by pellucid dots, meaning that the young plants adopted direct defences under the poor soil nutritional condition. Under the high soil nutritional condition, in contrast, the plants strongly expressed the indirect defence traits. They produced abundant EFNs and pearl bodies, and attracted many ants. These results suggest that young plants of M. japonicus use different defence modes in response to different soil nutritional conditions.

Impacts on Natural Ecosystems

Japan extends over the East Asian monsoon region, covering slightly more than 20° in latitude from 25° North to 45° North, and 3,000 km from north to south. A major characteristic of this entire region is its location in a rain-forest zone, which supports forest growth well. Located in the mid-latitudes where latitudinal gradients in temperature are steep, these forests range from subtropical evergreen broad-leaved forests to cold-temperate coniferous forests. The mountainous regions in excess of 1,000 meter elevations that form the backbone of Japan serve as migration routes as well as distributional barriers. There are also many instances in which marine straits serve as distributional boundaries, and global warming is anticipated to have a major effect in terms of forced migration of biotas and ecosystems

Aggressiveness of ants attracted to the extrafloral nectary-bearing plant, Mallotus japonicus, and temporal fluctuations in their abundance

Many plants that bear extrafloral nectaries (EFNs) attract various ant species that can exclude herbivores. The aggressiveness of the attracted ants and their temporal activity patterns are important factors that can affect the efficiency of herbivore exclusion from the plant. However, the characteristics of this mutualistic relationship between EFN‐bearing plants and ants have not been sufficiently elucidated. We investigated the aggressiveness of six ant species against the common armyworm, Spodoptera litura Fabr., and temporal fluctuations in the abundance of four aggressive ant species on an EFN‐bearing plant, Mallotus japonicus (L.f.) Müll. Arg. Workers of Crematogaster teranishii Santschi, Pheidole noda Smith, Pristomyrmex punctatus Smith and Formica japonica Motschoulsky were observed to be highly aggressive. In contrast, workers of Camponotus vitiosus Smith showed low aggressiveness. Paratrechina flavipes Smith workers did not attack the herbivore. The activity patterns of the four aggressive ant species greatly differed. Crematogaster teranishii and Ph. noda workers were constantly active throughout the day and night. In contrast, F. japonica was diurnal. Pristomyrmex punctatus was principally nocturnal. Formica japonica workers foraged solitarily, whereas workers of the other three species foraged in a group or recruited nestmates. Our results suggest that the efficacy of the indirect defense in M. japonicus depends principally on the attracted ant species.

Applied Ecology of Succession in Pine Forests of western Japan

One of the most representative vegetation types in western Japan is Pinus densiflora forest, given that most primary evergreen, broad-leaved forests have been largely destroyed by human activities. Pine trees regenerate and persist at sites where forest cutting, land clearing, and fire have occurred, because P. densiflora requires full sunlight and bare mineral soil for seed germination. For a long time, pine trees have been used for timber and fuel; shrubs for fuel; and litter, for compost. However, after the 1960s, petrochemicals such as propane gas and chemical fertilizer became widely used in Japan. Since then, pine forests have been abandoned and have gradually been replaced by broad-leaved forests. In addition, pine forests have suffered from mass dieback due to pine wilt disease, which is caused by epidemic attacks of the nematode Bursaphelenchus xylophilus (Steiner et Buhrer) Nickel, vectored by the beetle Monochamus alternatus Hope (Kiyohara and Tokushige, 1971; Morimoto and Iwasaki, 1972). Many pine forests in Japan suffer from pine wilt disease except in northern areas such as Hokkaido and Aomori Prefecture (Kishi, 1995; Rutherford et al., 1990). One-fourth of pine forests in Japan have been infested by the disease (Mamiya, 1983).