Rice fields are irrigated throughout the season from planting to harvesting. Irrigation ponds are constructed in areas where people can not get enough water from rivers. Particularly, there are numerous ponds in the Seto Inland Sea district of western Japan. As a case study of irrigation ponds, we studied water quality and aquatic plants of ponds in Hojo area, Shikoku Island, in the Seto Inland Sea district. The area's landscape is characterized by citrus orchards distributed widely on hillside slopes. Many ponds have watersheds partly or completely occupied by orchards. Ponds located in agriculturally intensive watersheds had extremely eutrophic water. Ponds located in forested watersheds had lower nutrient levels than the ponds in agricultural lands. Submerged and free-floating taxa dominated the aquatic flora. Many of them are known wet rice field weeds. Floating-leaved taxa were rare except Trapa japonica. Very eutrophic ponds were populated only with Lemnaceae species. Species-rich flora were restricted to the nutrient-poor and clear ponds located in forested watersheds. Repair work on two ponds resulted in recovery of floral diversity under a decline in phosphorus concentration. This suggests that land use changes, including citrus farming expansion, caused sensitive species to disappear. It also suggests the possibility of restoration and conservation of species-rich aquatic flora by environmental improvement where seed banks contain formerly diverse flora. Species composition differs even between ponds of a similar environment within a small geographical area. Therefore, it is desirable to conserve diverse ponds as much as possible to conserve diverse aquatic flora.

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The process of succession from a secondary coppice forest of pine and deciduous oaks to an evergreen broadleaved oak forest was investigated on a hilly site in Toyota City, Aichi Prefecture, Japan. A 100×150m plot was established in the secondary forest, and the plot was divided into 150 quadrats categorized for three topographic positions: ridge, slope, and valley. The distribution, height and age of evergreen Quercus glauca individuals were measured in each quadrat. Further, the measurment of the growth of leader shoots was conducted and dead individuals were checked from June 1999 to April 2001. There occurred numerous Q. glauca saplings in the study plot, being 683/ha in density. The height of all individuals except one was less than 5.0m, showing the maximum age of about 40. The density of Quercus saplings was lower (3.3/100m^2) on the ridge, but higher on the slope and in the valley (8.5/100m^2 and 10.3/100m^2, respectively). This low density on the ridge is correspondent with the high mortality of the saplings on the ridge compared with higher density and lower mortality of those saplings on the slope and in the valley. The height growth per year was the largest in the valley. Based on our findings, we predict that the coppice forest will be replaced by Q. glauca faster in the valley than on the slope and the replacement will likely be retarded for longer on the ridge.

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In order to obtain information on the effects of topography and other environmental variables on the regeneration of Abies homolepis Sieb. et Zucc., one of the main species inhabiting the cool temperate forests of Japan, we examined the population density and size structure of the species in 6 topographically distinct stands in the headwaters of the Kinugawa River in central Japan. The floodplain comprised 3 stands. The stand on the lowest terrace was dominated by the pioneer species, Alnus hirsuta and Toisusu urbaniana, etc. and it had a high density of A. homolepis [diameter at breast height (DBH), ≦10cm]. Although A. homolepis was the most dominant species in the stands on the middle and highest terrace, A. homolepis of DBH 20-60cm was not found in the stand on the highest terrace. This implied a depression of regeneration with progress of succession after flooding. On a steep, rocky slope, the A. homolepis population showed a continuous, inverse J-shaped size distribution, which implied continuous regeneration. In contrast, lower proportions of A. homolepis of small and intermediate sizes were found at two other sites on a mountain slope; this indicated that the species present at these sites showed less continuous regeneration. In addition, inverse correlations were found between sapling density and dwarf bamboo coverage, and sapling density and soil depth, indicating that the sapling density was greater in shallow soil and in places where dwarf bamboo coverage was less. The results of this study implied that flooding or uprooting, which creates disturbances in the soil and result in the generation of bare sites, is favorable for the establishment of A. homolepis. In conclusion, A. homolepis might regenerate vigorously where disturbances in soil are frequent, for example, a steep rocky slope with a thin layer of soil, and behave like an edaphic climax species. On the other hand, this species may behave like a long-lived seral species at sites where disturbances in soil are less frequent, for example, a floodplain or a gentle slope.

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The demography of the epiphytic orchid Sarcochilus japonicus was investigated in a Cryptomeria japonica artificial forest disturbed by a windstorm in Asago City, which is in the northern region of Hyogo Prefecture. The number of leaf sheaths and number of fruits that each individual was currently bearing or had borne in the past were recorded. The demography of S. japonicus was assessed on the basis of the survival and reproduction rates by using a transition matrix. We studied 1335 individuals; the maximum number of leaf sheaths per individual was 53; 70 individuals were currently bearing fruits; and the minimum number of leaf sheaths for individuals that were currently bearing or had borne fruits in the past was 10. The number of fruits per individual did not exceed 4, and the mean number of fruits per individual was 1.56. We estimated the survival curve and reproduction rate assuming four conditions: (1) an individual grows 2.5 new leaves per year, (2) the number of fruits and mean establishment rate remain constant, (3) the seed input from and output to the outside of the population are balanced, and (4) the seeds are not dormant for more than one year. The survival rates were estimated by assuming that the individuals in each sheath class were exponentially distributed. These rates were calculated separately for small and large individuals; individuals were classified as small or large on the basis of a cutoff point for the number of leaf sheaths so that the sum of squared residuals was minimized. This cutoff number of leaf sheaths was eight, and the survival rates thus estimated were 0.8161 (L≤8) and 0.9131 (L>8), where L represents the number of leaf sheaths. The reproduction rate was calculated by multiplying the number of fruits per individual by the mean establishment rate per fruit and dividing this product by 2.5 assumed number of leaves grown per year. The number of fruits borne by each individual with 10 or more sheaths was regressed against the number of leaf sheaths by using the generalized linear model with Poisson errors and log link. Thus, the equation for the regression model of the number of fruits per individual was F_L=exp(0.0484L-2.2043) (L≥10), where F_L is the number of fruits per individual with L number of leaf sheaths. The mean establishment rate per fruit was calculated by dividing the number of established individuals by the number of fruits. The total number of fruits was 109, and the number of established individuals was 490; thus, the number of established individuals per fruit was 4.5. The reproduction rate was expressed as R_L=1.7994exp(0.0484L-2.2043) (L≥10), where R_L is the number of fruits per individual with L number of leaf sheaths. The finite rate of natural increase was thus determined to be 1.0235. We concluded that there may be an increase in the population of S. japonicus under the study conditions.

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Stand structure and dynamics of Betula alnoides forests in the Caiyanghe Nature Reserve located in Yunnan Province, China, were studied. These were secondary forests distributed over the subtropical zone; they were established after human activities led to the destruction of evergreen broad-leaved forests. A total of 10 plots were established in stands that were aged 10-40 years and were at different stages of development i.e., from young, small-sized stands to fairly mature stands. The height and DBH (diameter at breast height) of stems≧3cm in DBH were recorded. The number of stems per hectare of B. alnoides showed a strong negative correlation with the maximum DBH of the species. Juveniles (<3cm in DBH) of B. alnoides were not found in any of the stands. DBH- and height-class distributions of the mature stands showed that regeneration of B. alnoides was discontinuous. More than 89.7% of B. alnoides stems found in the mature stands were canopy trees. On the basis of these results, it was concluded that B. alnoides is a typical shade-intolerant pioneer species that regenerates in sites with ample sunlight. In addition, the present study suggested that B. alnoides grows to a height of 25m and 40m within 20 years and 40 years, respectively.

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