Staver A Carla, Bond William J, Stock William D, Van Rensburg Sue J, Waldram Matthew S
Botany Department, University of Cape Town, Private Bag X1, Rondebosch 7701, South Africa.
Ecol Appl. 2009 Oct;19(7):1909-19. doi: 10.1890/08-1907.1.
Disturbances from fire and herbivory strongly affect savanna vegetation dynamics. In some savannas, fire especially may be instrumental in preserving the coexistence of trees and grasses. The role of herbivory by large mammals is less clear; herbivory has been shown variously to promote and to suppress tree establishment. Here we ask how interactions between herbivory and fire act to shape savanna vegetation dynamics via their effects on tree populations in Hluhluwe iMfolozi Park in KwaZulu Natal, South Africa, a savanna with a full complement of native large mammals. We examined the effects of herbivore exclusion on tree growth, mortality, and seedling establishment from 2000 to 2007 at 10 sites located in areas of low and high herbivore pressure throughout the park. Results were analyzed statistically and using Leslie matrix models of population dynamics. Herbivory and fire acted primarily to suppress sapling growth rather than on sapling mortality or seedling establishment. This indicates that browsing, like fire, suppresses tree density by imposing a demographic bottleneck on the maturation of saplings to adults. Model results suggest that, while browsing and fire each alone impacted growth, a combination of browsing and fire had much greater effects on tree density. Only fire and browsing together were able to prevent increases in tree density. These results suggest that, while soil resources, including nutrients and moisture, are probably instrumental in determining tree growth rates, disturbances from fire and herbivory may be instrumental in limiting tree cover and facilitating the coexistence of trees and grasses in savannas.
火灾和食草作用引起的干扰对热带稀树草原植被动态有着强烈影响。在一些热带稀树草原，火灾尤其可能有助于维持树木和草本植物的共存。大型哺乳动物的食草作用的角色则不太明确；食草作用已被证明在促进和抑制树木生长方面表现各异。在此，我们探讨在南非夸祖鲁 - 纳塔尔省的Hluhluwe iMfolozi公园（一个拥有完整本地大型哺乳动物群落的热带稀树草原）中，食草作用与火灾之间的相互作用如何通过对树木种群的影响来塑造热带稀树草原植被动态。我们在2000年至2007年期间，在公园内食草动物压力低和高的区域的10个地点，研究了食草动物排除对树木生长、死亡率和幼苗建立的影响。对结果进行了统计分析，并使用了种群动态的莱斯利矩阵模型。食草作用和火灾主要抑制幼树生长，而非幼树死亡率或幼苗建立。这表明，与火灾一样，啃食通过对幼树成熟为成年树施加人口统计学瓶颈来抑制树木密度。模型结果表明，虽然啃食和火灾各自单独影响生长，但啃食和火灾的组合对树木密度的影响要大得多。只有火灾和啃食共同作用才能阻止树木密度增加。这些结果表明，虽然包括养分和水分在内的土壤资源可能有助于确定树木生长速度，但火灾和食草作用引起的干扰可能有助于限制树木覆盖，并促进热带稀树草原中树木和草本植物的共存。
