Analysis of the reproductive ecology of white clover and its application to the restoration of grazed swards

本研究旨在加深对白色三叶草种子繁殖生态学的认识。一系列的实验进行了使用两个品种，Kopu（大叶）和Tahora（小叶）。主要结果如下。一个共同的环境实验表明，Kopu表现出较大的花头和更广泛的安排比Tahora每株花头。采用析因试验，研究了落叶胁迫和资源水平对品种的可塑性反应。试验结果表明，种子繁殖性状的反应程度不同，甚至每荚种子数（决定结实率）和单个种子大小由于强烈的脱叶酸胁迫而减少。结果还表明，不同品种对中度落叶胁迫的可塑性反应具有复杂的模式。资源水平对落叶胁迫的塑性反应有微妙的影响。本研究结果和前人的研究结果表明，在个体水平上，花穗多、开花期长、种子萌发不同步的基因型有利于持续存在。并进行了田间生殖生态观察。种群大小、结实率和访花频率之间存在明显的正相关关系。个体极小的种群，传粉者访花频率低，结实率低。结实率的显着下降可能会严重限制成功恢复。在有小块孤立的白色三叶草的放牧草地上，这种可能性很大。三叶草含量的自我恢复需要成虫的数量和分布格局与结实相适应。

The aims of this research are to deepen our understanding of the seed reproductive ecology of white clover. A series of experiments were conducted using two cultivars, Kopu (large-leaved) and Tahora (small-leaved). Major results are described below. A common environment experiment showed that Kopu exhibited larger flower-heads and wider arrangement of flower-heads per genet than Tahora. A factorial experiment, in which both of defoliation stress and resource level were manipulated, was conducted to examine the plastic responses of the cultivars. The experiment showed that the seed reproductive characters differed in the extent of the responses, and even seed number per pod (which determine seed set rate) and individual seed size decreased due to strong defolation stress. It also showed that the cultivars had complex patterns of the plastic responses to moderate defoliation stress. Resource level affected the plastic responses to defoliation stresses subtly. The results obtained in this research and the previous studies suggest that the genotypes which have many flower-heads and long periods of flowering at individual level and asynchronous seed germination would be advantageous in persisting. I also conducted the observation on reproductive ecology in fields. It clearly showed positive relationships among population size, seed set rate and visitation frequency of pollinators. The populations with extremely small size had low visitation frequency of pollinators, resulting in reduction in seed set rate. Marked reduction in seed set rate can impose serious limitation to successful restoration. It is highly possible in the grazed swards with small isolated patches of white clover. It would be essential to self-restoration of clover content that the abundance and distributional pattern of adults are proper for the seed set.