Transition of Competitive Superiority from a Primary Pasture Canopy to the Succeeding Canopies : I. Seed size and emergence earliness as initial factors causing competition

Abstract

The transiton of competitive superiority in a primary pasture canopy to the succeeding canopies after defoliation is still questionable. For example, can competitive advantages in a primary canopy be maintained being enlarged or diminished, or reversed in the secondary canopy after the first harvesting? It is important in the viewpoint of grassland management to solve this problem, because competition usually causes autogenous shift of the community structure. Factors causing primary competition must be made clear in the beginning of searching the answer. In the present report, seed size was firstly taken up as an initial causal factor of seedling competition. Heavy and light seeds of Lolium multiflorum LAM. (cv. Mammoth) were used in the following experiments. In Expt.1, the sizes of primordial leaf and root were microscopically measured. Heavy seeds were found to contain larger primordial leaf and root than light seeds (Table 1). In Expt.2, the period of endosperm exaution and early growth of seedlings were examined under a dark an 20℃ condition. Heavy seeds showed better growth of seedling and longer heterotrophic period than light seeds (Figs.2 and 3, and Table 2). In Expt.3, heavy and light seeds were separately sown at 0 and 2cm depths under natural light condition. Seedlings emerged from heavy seeds were heavier in weight and larger in root numbers both 3 and 5 weeks after sowing, and earlier in emergence 3 weeks after sowing than those from light seeds, though the difference of emergence was almostly disappeared by 5 weeks (Tables 3-5, and Fig.4). These results suggested that the seedlings from heavy seeds might take advantage in sharing light and nutrients in comparison with those from light seeds in their mixture because not only of better growth but of earlier emergence. In Expt.4, heavy and light seeds were sown separately and in a mixture in October. Light-seeded plants were significantly suppressed due to competition 3 and 5 weeks after sowing, but heavy-seeded plants did not indicate any positive effect of competition which usually might be expected (Tables 6-7). In Expt.5, seeds were sown twice in May 9 and 16 under high and low densities so that early- and late-seeded plants could be arranged in each separate and mixed plots. Effect of competition were clearly found particularly under the low density June 20 in such a way that the early grower showed positive competitive-values and the late grower negative ones (Tables 8 and 9). The foremost effect of competition was observed both in Expt.4 and 5 when leaf area index reached approximately to one (Table 10). From these results seed size is considered to be a major autogenous factor causing primary competition in a sown grassland community. Earliness of emergence, which is usually determined by seed dormance, seed size and/or depth of sowing, also seems to be similar.