Dynamics of grazing lawn formation: an experimental test of the role of scale-dependent processes
Dynamics of grazing lawn formation: an experimental test of the role of scale-dependent processes
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DOI：
10.1111/j.0030-1299.2008.16651.x
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发表时间：
2008-10-01
期刊：
影响因子：
3.4
通讯作者：
Olff, Han
中科院分区：
文献类型：
--
作者：
Cromsigt, Joris P. G. M.;Olff, Han
放牧草坪是非洲稀树草原的特征，突出表现为匍匐茎耐放牧草种的密集放牧斑块。放牧草坪的发展与放牧和大型迁徙畜群增加营养投入有关。然而，我们认为，在系统中没有大规模迁移干扰，直接放牧以外，驱动草坪的发展。这种干扰，例如白蚁活动或巨型食草动物的贝登，也增加了营养输入，并保持长时间的植被。然而，实地观察表明，并非所有这些干扰都会导致放牧草坪。我们假设，最初的干扰是一个最小的阈值空间尺度，放牧强度高到足以诱导草坪形成。我们在天然的高大稀树草原上对这个想法进行了实验测试。我们修剪不同大小的地块，以模拟不同规模的初始干扰（一年内6次）和施肥的一半的地块在两年内，以模拟增加养分输入的食草动物或白蚁活动。允许放牧自然发生的食草动物，我们跟踪植被的发展超过三年。放牧使粗糙的、施肥的地块中的草堆变短，而在精细的、未施肥的地块中，草向它们的初始高度生长。此外，草坪草强烈增加覆盖在地块增加养分输入，但只有在粗尺度干扰。这些结果支持了我们的假设，即增加营养输入结合放牧确实会诱导放牧草坪的形成，但仅高于初始干扰的阈值尺度。我们的研究结果提供了一种替代机制，在系统中，缺乏大规模迁移牛群放牧草坪的发展。此外，它提供了一个新的空间维度放牧草坪发展背后的过程，因此有助于了解草食动物可能会创建和维持草地系统的空间异质性。
Grazing lawns are characteristic for African savanna grasslands, standing out as intensely grazed patches of stoloniferous grazing-tolerant grass species. Grazing lawn development has been associated with grazing and increased nutrient input by large migratory herds. However, we argue that in systems without mass migrations disturbances, other than direct grazing, drive lawn development. Such disturbances, e.g. termite activity or megaherbivore middens, also increase nutrient input and keep the bunch vegetation down for a prolonged time period. However, field observations show that not all such disturbances lead to grazing lawns. We hypothesize that the initial disturbance has to be of a minimal threshold spatial scale, for grazing intensity to be high enough to induce lawn formation. We experimentally tested this idea in natural tall savanna grassland. We mowed different-sized plots to simulate initial disturbances of different scales (six times during one year) and applied fertilizer to half of the plots during two years to simulate increased nutrient input by herbivores or termite activity. Allowing grazing by naturally occurring herbivores, we followed the vegetation development over more than three years. Grazing kept bunch grass short in coarser, fertilized plots, while grasses grew out toward their initial height in fine-scale and unfertilized plots. Moreover, lawn grasses strongly increased in cover in plots with an increased nutrient input but only after coarser scale disturbance. These results support our hypothesis that an increased nutrient input in combination with grazing indeed induces grazing lawn formation, but only above a threshold scale of the initial disturbance. Our results provide an alternative mechanism for the development of grazing lawns in systems that lack mass migrating herds. Moreover, it gives a new spatial dimension to the processes behind grazing lawn development, and hence help to understand how herbivores might create and maintain spatial heterogeneity in grassland systems.