Department of Plant Sciences, University of California, Davis, CA 95616, USA.
Oecologia. 2012 Mar;168(3):749-59. doi: 10.1007/s00442-011-2123-9. Epub 2011 Sep 21.
Spatial heterogeneity in woody cover affects biodiversity and ecosystem function, and may be particularly influential in savanna ecosystems. Browsing and interactions with herbaceous plants can create and maintain heterogeneity in woody cover, but the relative importance of these drivers remains unclear, especially when considered across multiple edaphic contexts. In African savannas, abandoned temporary livestock corrals (bomas) develop into long-term, nutrient-rich ecosystem hotspots with unique vegetation. In central Kenya, abandoned corral sites persist for decades as treeless 'glades' in a wooded matrix. Though glades are treeless, areas between adjacent glades have higher tree densities than the background savanna or areas near isolated glades. The mechanisms maintaining these distinctive woody cover patterns remain unclear. We asked whether browsing or interactions with herbaceous plants help to maintain landscape heterogeneity by differentially impacting young trees in different locations. We planted the mono-dominant tree species (Acacia drepanolobium) in four locations: inside glades, far from glades, at edges of isolated glades and at edges between adjacent glades. Within each location, we assessed the separate and combined effects of herbivore exclusion (caging) and herbaceous plant removal (clearing) on tree survival and growth. Both caging and clearing improved tree survival and growth inside glades. When herbaceous plants were removed, trees inside glades grew more than trees in other locations, suggesting that glade soils were favorable for tree growth. Different types of glade edges (isolated vs. non-isolated) did not have significantly different impacts on tree performance. This represents one of the first field-based experiments testing the separate and interactive effects of browsing, grass competition and edaphic context on savanna tree performance. Our findings suggest that, by excluding trees from otherwise favorable sites, both herbaceous plants and herbivores help to maintain functionally important landscape heterogeneity in African savannas.
木质覆盖物的空间异质性会影响生物多样性和生态系统功能，在热带稀树草原生态系统中可能尤为重要。食草动物的啃食和与草本植物的相互作用可以在木质覆盖物中创造和维持异质性，但这些驱动因素的相对重要性尚不清楚，特别是在考虑多个土壤背景的情况下。在非洲热带稀树草原，废弃的临时牲畜围栏（bomas）会发展成为长期存在的、富含养分的生态热点，具有独特的植被。在内罗毕中部，废弃的畜栏遗址在树木繁茂的基质中保留了几十年，成为无树的“林间空地”。虽然林间空地没有树木，但相邻林间空地之间的区域树木密度高于背景稀树草原或孤立林间空地附近的区域。维持这些独特木质覆盖模式的机制尚不清楚。我们想知道，食草动物的啃食或与草本植物的相互作用是否通过对不同位置的幼树产生不同的影响来帮助维持景观异质性。我们在四个位置种植了单优势种树木（金合欢树）：林间空地内、远离林间空地、孤立林间空地边缘和相邻林间空地边缘。在每个位置内，我们评估了食草动物排除（笼养）和草本植物清除（清理）对树木存活和生长的单独和综合影响。笼养和清理都提高了林间空地内树木的存活率和生长。当清除草本植物时，林间空地内的树木比其他位置的树木生长得更好，这表明林间空地的土壤有利于树木生长。不同类型的林间空地边缘（孤立的与非孤立的）对树木生长没有显著不同的影响。这是首次在野外进行的测试食草动物啃食、草本植物竞争和土壤背景对热带稀树草原树木生长的单独和交互影响的实验之一。我们的研究结果表明，通过将树木排除在其他有利的位置之外，食草动物和草本植物都有助于维持非洲热带稀树草原中具有重要功能的景观异质性。
