Silvertown Jonathan, Smith Bridget
Biology Department, Open University, MK7 6AA, Milton Keynes, UK.
Oecologia. 1989 Nov;81(3):369-373. doi: 10.1007/BF00377086.
The size and number of rosettes of Cirsium vulgare were censused in a 4 ha sheep grazing trial on lowland calcareous grassland in paddocks receiving controlled winter, spring and summer grazing treatments. Spring grazing significantly increased thistle rosette numbers, and there were always fewer rosettes in size classes 250mm-300 mm under the heavier grazing treatments. The emergence of seeds of Cirsium vulgare sown by hand into each of the grazing treatments was monitored and showed a positive effect of spring grazing. When the percent emergence of sown seeds was used as a covariate in the analysis of rosette numbers in the experimental paddocks, it was found to account for 77% of the variance in thistle numbers. It was concluded that the main effect of grazing on thistle rosette numbers is an indirect one exercised through the effects of grazing on germination conditions. Seeds of Cirsium vulgare were sown into artificial gaps at another grassland site, to determine the effect of gap-size upon seedling emergence. Germination was poor, but significantly more seedlings emerged in gaps 10-20 cm diameter, than in 5 cm gaps or in control plots without a gap. A computer simulation model was used to explore the relationship between gap density and thistle population dynamics. A threshold density of gaps was found to exist, below which thistle populations went extinct, and above which the thistle population grew geometrically. The degree of aggregation of dispersed seeds did not alter the threshold gap-density for plant extinction, but did affect the rate of increase of the thistle population when the threshold gap-density was exceeded. It is concluded that strategies for controlling Cirsium vulgare populations by grazing manangement will be most effective if aimed at reducing suitable sites for establishment in spring. The success of such attempts will depend upon the soil fertility of the site, and sudden outbreaks of Cirsium vulgare infestation can be expected if a sward gradually deteriorates through over-grazing.
在一片4公顷的低地钙质草地上进行了一项绵羊放牧试验，对接受可控冬季、春季和夏季放牧处理的围场中的大翅蓟莲座叶丛的大小和数量进行了统计。春季放牧显著增加了蓟莲座叶丛的数量，在较重放牧处理下，250毫米至300毫米大小等级的莲座叶丛数量总是较少。对人工播撒到各放牧处理区域的大翅蓟种子的出苗情况进行了监测，结果表明春季放牧有积极影响。在分析试验围场中莲座叶丛数量时，将播种种子的出苗百分比用作协变量，发现其占蓟数量变异的77%。得出的结论是，放牧对蓟莲座叶丛数量的主要影响是通过放牧对发芽条件的作用而产生的间接影响。在另一片草地的人工间隙中播种了大翅蓟种子，以确定间隙大小对幼苗出苗的影响。发芽情况不佳，但直径10 - 20厘米的间隙中出苗的幼苗明显多于5厘米间隙或无间隙的对照地块。使用计算机模拟模型探索间隙密度与蓟种群动态之间的关系。发现存在一个间隙阈值密度，低于该密度蓟种群灭绝，高于该密度蓟种群呈几何级数增长。分散种子的聚集程度并未改变植物灭绝的阈值间隙密度，但在超过阈值间隙密度时会影响蓟种群的增长速度。得出的结论是，如果旨在减少春季适合植株建立的地点，通过放牧管理控制大翅蓟种群的策略将最为有效。此类尝试的成功将取决于场地的土壤肥力，如果草地因过度放牧而逐渐退化，预计大翅蓟虫害会突然爆发。
