Size is one of the most important traits in determining the ultimate success of each member of a plant population (Werner 1975;Harper 1977;Solbrig 1981;Bazzaz 1984;Silvertown & Lovett-Doust 1994). From an evolutionary point of view, however, reproductive output is a more accurate estimate of the relative contribution of each individual to the next generation. Reproductive output of a plant can be seen as the product of plant size and its reproductive allocation (RV) without considering seed quality. When reproductive allocation is constant over a wide range of plant sizes, individual size difference faithfully reflects the relative differences in seed output between individuals. However, when there is a negative correlation between plant size and RV, the size advantage of larger plants should be discounted by their smaller RV. 3. An allometric (log-log) regression between seed mass and vegetative mass fit the data better than a linear regression. The slopes of allometric regressions for each nutrient treatment were lower than 1·0. Therefore, there was a negative relationship between plant size and reproductive allocation. However, the slopes calculated over different nutrient treatments were greater than 1·0 (i.e. positive relationship between plant size and reproductive allocation). We suggest that these two reproductive allometries indicate two different biological relationships: the former represents a physiological trade-off between resource acquisition and reproductive allocation within a plant and the latter represents an allometric response to soil nutrients. 4. Genetic identity showed intrinsic effects on size dependency of reproductive allocation owing to trade-off and not as a result of the allometric response to soil nutrients. 5. Nutrient availability changed the slopes of the allometric regressions. An increase in nutrient availability alleviated the negative relationship between plant size and reproductive allocation. 6. The slopes of the allometric regressions within nutrient treatments were significantly higher in competition treatments than in competition-free treatments, although the slopes of the regressions over different nutrient levels did not differ between the two treatments. These results suggest that the effects of competition on reproductive allocation are owing to indirect effects of size difference on the allometric relationship rather than intrinsic effects.

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The results demonstrated clearly that polyploidy increases leaf size mainly by increasing the cell elongation rate, but not the duration of the period of elongation, and thus increases final cell size.

Decomposition of litter is greatly influenced not only by its chemical composition but also by activities of soil decomposers. By using leaf litter from 15 plant species collected from semi-natural and improved grasslands, we examined (1) how interspecific differences in the chemical composition of litter influence the abundance and composition of soil bacterial and fungal communities and (2) how such changes in microbial communities are related to the processes of decomposition. The litter from each species was incubated in soil of a standard composition for 60 days under controlled conditions. After incubation, the structure of bacterial and fungal communities in the soil was examined using phospholipid fatty-acid analysis and denaturing gradient gel electrophoresis. Species from improved grasslands had significantly higher rates of nitrogen mineralization and decomposition than those from semi-natural grasslands because the former were richer in nitrogen. Litter from improved grasslands was also richer in Gram-positive bacteria, whereas that from semi-natural grasslands was richer in actinomycetes and fungi. Nitrogen content of litter also influenced the composition of the fungal community. Changes in the composition of both bacterial and fungal communities were closely related to the rate of litter decomposition. These results suggest that plant species greatly influence litter decomposition not only through influencing the quality of substrate but also through changing the composition of soil microbial communities.

Although plants and soil biota have mutual functional links as producers and decomposers in terrestrial ecosystems, less is known about structural linkages between above‐ground and below‐ground communities. For this study, we examined whether plant species richness and soil microbial richness are related to each other, and how their richness is related to soil fertility and herbivore activity at 24 plots of the Shiriyazaki seminatural grassland in northern Japan. Prokaryote (bacterial) richness and eukaryote richness were evaluated by the band number of denaturing gradient gel electrophoresis (DGGE) using 16S and 18S ribosomal DNA fragments. Plant species richness was negatively related to soil nitrogen availability and phosphorus content. Prokaryote richness showed a negative correlation with soil phosphorus contents and no significant correlation with vegetation structure (height, richness and functional type composition). Eukaryote (mainly fungi) richness showed significant positive correlations with plant species richness, community height and forb species composition. These results suggest that bacterial richness is influenced by soil properties, whereas eukaryote richness is associated closely with vegetation structure.