The influence of air water vapor pressure deficit (VPD) on CO₂ assimilation rate (A) within a standing beech (Fagus crenata) tree crown on Mt. Fuji in Japan was examined by integrating the actual VPD during a growing season and the measurements of A and transpiration rate (E) under different VPD conditions in gas-exchange chamber by detached method. From 21 April to 31 October 1997, the maximum of VPD in sun and shade crown (20 m and 12 m height above the forest floor) was 32.8 Pa kPa^-1 and 24.2 Pa kPa^-1, respectively. The number of days in which the daily maximum of VPD exceeded 10 Pa kPa^-1 was 101 and 80 in sun and shade crown, respectively. The frequency of the measurements at half hour intervals in day time in which VPD was over 10 Pa kPa^-1 was approximately 10% in both sun and shadecrown. When the VPD in gas-exchange chamber changed from 3 to 15 Pa kPa^-1, the difference between VPD and leaf-to-air water vapor pressure deficit (ΔW) was less than 1 Pa kPa^-1 for sun and shade leaves. The A decreased to below 90% level of maximum with increasing of ΔW over 9 Pa kPa^-1 for sun and shade leaves. The stomatal conductance, intercellular CO₂ concentration, and mesophyll conductance also decreased with increasing of ΔW over 6 Pa kPa^-1. The E increased with increasing of ΔW, thus the water use efficiency of CO₂ assimilation (A/E) decreased. We conclude that (1) VPD is an important factor that decreses the A and A/E and increases E within a beech tree crown, (2) the decreasing of A is accompanied by the decreases of stomatal conductance, intercellular CO₂ concentration, and mesophyll conductance.

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Ludwigia stipulacea is one of the endangered plants in Japan. To make clear the ecological characteristics of L. stipulacea, we studied its population in an urban river in east Kanagawa Prefecture. The seasonal change of its biomass showed three periods in the lifecycle of L. stipulacea; a period of erect shoot growth (May-August), a period of level shoot growth (September-October), a period of decline (November-January). The flowers were observed from May to beginning of December, and the flowering peak was in early summer. Distributions of L. stipulacea were interspersed in about 6 km area of Kasio River. The results surveyed 44 vegetation sites in 1998, the total communities size was about 1, 500 m². Many sites of sandbar were dominated by L. stipulacea. It is characterizing the landscape of this urban river, that its communities are spreading leafs on the surface of the water. From the structure of species, we discussed the succession of L. stipulacea community. It was separated two types, early and late communities on succession, by the presence of Andropogon virginicus, Persicaria thunbergii. And it is considered that L. stipulacea community will be decline by invasion tall herbaceous species.

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In order to evaluate the surface stability of a slope covered with Sasa nipponica, we conducted field surveys of the surface failures in places on the wild slope that were considered to be caused by an earthquake. We also conducted laboratory tensile tests of the rhizome, and laboratory soil tests. Since the field surveys of the slope showed that the surface failures have an integrative pattern, we set up an analytical model of the stability of the slope against surface failures, and calculated the stability of the slope by using the observed intensities of the quake applied to the slope, and the survey and laboratory test results. As a result, the safety factors of the slope expressed as a function of the total rhizome tension per lateral sectional unit width of slope surface layer were about 1.0, indicating that the surface failures were caused by the earthquake in places on the slope. Assuming that the physical constants used in the calculation are all reasonable, the safety factor in a static condition exceeded 1.26 everywhere on the slope. The equation of this analytical model indicates that an increase of the safety factor of the slope through the total rhizome tension per lateral sectional unit width of the slope verifies the positive effect of the rhizome of the Sasa nipponica on the reinforcement of the slope against the surface failures, and that, in contrast, the safety factors of the slope without dwarf bamboo depend mainly on the strength parameters of the soil in the sliding layer. Further, as the surface failure pattern changed from that of a semicircular to one that is longer longitudinally along the slope and the area of the surface failure became smaller, the safety factor of the slope became greater.

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In Tottori sand dune, Tottori Prefecture, we investigated seed bank species composition of five vegetation types (bare area, Carex kobomugi community, Vitex rotundzfolia community, Robinia pseudoacacia community, Pinus thunbergii community), which are different in species composition and environment. On the basis of these results, we showed the relationship between the surface vegetation and seed bank species composition.
The number of buried viable seed of tree species was few in each vegetation type, so it is considered that the differences in seed bank size among communities were caused by differences in the number of seeds of herbaceous species. Buried viable seeds of annuals and biennials existed on fixed dunes and semi-fixed dunes. Buried viable seeds of sand dune plants were found on moving sands and semi-fixed dunes and those of perennial herbs, except for sand dune plants, mainly existed on fixed dunes. Buried viable seeds of tree species were found on fixed dunes.
As the seed sources of seed banks, a number of buried viable seeds were considered to be supplied within the community in Carex kobornugi community, Vitex rotundifolia community, Robinia pseudoacacia community, and Pinus thunbergii community. All buried viable seeds of bare areas were supplied from surrounding areas.

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A degree of grass roots influencing wind erosion is examined using small-scale wind tunnel test in this research. Two grass varieties were drilled in six different densities, then intermittently eroded by 3 kinds of wind velocities for 10 min. Erosion intensity of sands were evaluated by means of the surface degradation or loss in weight.
The results of this experiment are summarized as follows:
(1) Compared with sand without roots, wind erosion declined from the early period in grassy sands.
(2) The increase of the drilling rows, namely, distance between rows, is smaller and results in smaller surface degradation and loss in weight.
(3) Grass with richer fibrous roots is more effective in preventing sands from wind erosion than grass with poorer.
(4) The effect of grass to preventing erosion is stronger under a higher wind velocity than under a lower wind velocity.
(5) Through multiple regression analysis using data from the two grass variety test, the effectiveness of root system characteristics to prevent wind erosion has been generalized in this order:
Number or total length of roots in the unit surface>distance between drilled rows>weight of roots per unit volume of sand.

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