Woody cover in African savannas: the role of resources, fire and herbivory
Woody cover in African savannas: the role of resources, fire and herbivory
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DOI：
10.1111/j.1466-8238.2007.00360.x
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发表时间：
2008-03-01
影响因子：
6.4
通讯作者：
Hanan, Niall
中科院分区：
文献类型：
--
作者：
Sankaran, Mahesh;Ratnam, Jayashree;Hanan, Niall
目的确定不同驱动变量之间的函数关系及其相对重要性（平均年降水量，土壤性质，火灾和草食动物）在调节木本植物覆盖在非洲稀树草原广泛的环境梯度。位置稀树草原的东部，西部和南部非洲。方法木本覆盖的依赖平均年降水量（MAP），土壤性质（质地，氮矿化潜力和总磷），火灾制度，和草食动物（食草动物，浏览器+混合饲养者，大象生物量）确定了161个热带稀树草原网站在非洲使用随机梯度提升，回归树分析技术的改进。结果所有变量都是重要的预测因子，在我们的数据集中解释了71%的方差。然而，他们的相对重要性，作为监管机构的木本覆盖变化。MAP是最重要的预测因子，其次是火灾重现期，土壤特性和草食动物制度。木本覆盖表现出很强的正相关性MAP之间的200和700毫米，但没有依赖于MAP高于此阈值时，其他预测因子的影响占。火灾使树木覆盖率降低到马歇尔确定的水平以下。林地覆盖度与土壤全磷呈复杂的非线性关系，与粘粒含量呈负相关。有一个很强的负依赖性的木本覆盖土壤氮（N）的有效性，这表明增加N-沉积可能会导致转向草原向更多的草地状态。大象，混合饲养者和浏览器的木本覆盖有负面影响。食草动物，另一方面，抑郁的木本覆盖在低生物量，但有利于木本植被时，他们的生物量超过了一定threshold.Main结论我们的研究结果表明复杂的对比关系之间的木本覆盖，降雨，土壤性质和干扰制度在稀树草原，并建议，未来的环境变化，如改变降水制度，N-富集和CO2水平升高可能有相反的，以及潜在的相互作用，对热带稀树草原的树木和草的平衡产生影响。
Aim To determine the functional relationships between, and the relative importance of, different driver variables (mean annual precipitation, soil properties, fire and herbivory) in regulating woody plant cover across broad environmental gradients in African savannas.Location Savanna grasslands of East, West and Southern Africa.Methods The dependence of woody cover on mean annual precipitation (MAP), soil properties (texture, nitrogen mineralization potential and total phosphorus), fire regimes, and herbivory (grazer, browser + mixed feeder, and elephant biomass) was determined for 161 savanna sites across Africa using stochastic gradient boosting, a refinement of the regression tree analysis technique.Results All variables were significant predictors of woody cover, collectively explaining 71% of the variance in our data set. However, their relative importance as regulators of woody cover varied. MAP was the most important predictor, followed by fire return periods, soil characteristics and herbivory regimes. Woody cover showed a strong positive dependence on MAP between 200 and 700 mm, but no dependence on MAP above this threshold when the effects of other predictors were accounted for. Fires served to reduce woody cover below rainfall-determined levels. Woody cover showed a complex, non-linear relationship with total soil phosphorus, and was negatively correlated with clay content. There was a strong negative dependence of woody cover on soil nitrogen (N) availability, suggesting that increased N-deposition may cause shifts in savannas towards more grassy states. Elephants, mixed feeders and browsers had negative effects on woody cover. Grazers, on the other hand, depressed woody cover at low biomass, but favoured woody vegetation when their biomass exceeded a certain threshold.Main conclusions Our results indicate complex and contrasting relationships between woody cover, rainfall, soil properties and disturbance regimes in savannas, and suggest that future environmental changes such as altered precipitation regimes, N-enrichment and elevated levels of CO2 are likely to have opposing, and potentially interacting, influences on the tree-grass balance in savannas.