Analysing Shrub Encroachment in the Southern Kalahari: A Grid-Based Modelling Approach
Analysing Shrub Encroachment in the Southern Kalahari: A Grid-Based Modelling Approach
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DOI：
10.2307/2405265
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发表时间：
1997-12
影响因子：
5.7
通讯作者：
F. Jeltsch;S. Milton;W. Dean;N. Rooyen
中科院分区：
文献类型：
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作者：
F. Jeltsch;S. Milton;W. Dean;N. Rooyen
灌木侵占正在降低南部非洲干旱草原对牛的承载能力。尽管众所周知，灌木侵占是由于牛选择性过度放牧而造成的，但降雨和放牧之间的相互作用尚不清楚。降雨事件的数量和顺序都可能影响灌木和草的生长速度和竞争能力。灌木侵蚀是一个缓慢的过程，在干旱地区动物的饲养密度较低。因此，在各种降雨情况下确定避免灌木侵占的载养率的田间实验几乎不可能重复。我们使用基于网格的模拟模型来研究喀拉哈里南部在各种实际降雨情景和家畜饲养率下的灌木草动态。模拟实验解决了以下问题：模拟牛放牧是否会导致灌木丛侵占？该过程需要多长时间进行？植被变化的动态与放牧压力有关是连续的还是表现出阈值行为？模拟结果表明，这三个问题的答案都取决于降雨的数量和顺序。一旦载畜率超过长期平均年降雨量确定的阈值，模拟牛放牧就会导致所有降雨情景下的灌木侵占。干旱气候情景下灌木丛侵占的蓄积率阈值较中湿气候情景不太明显（即不同模拟实验中的灌木覆盖具有较高的变异系数）。这是因为来自草本层的竞争或雨水可能会限制灌木的建立。在相对温和的情况下，灌木侵占主要受到草类竞争的限制，超过一定阈值的草类放牧会导致灌木覆盖率几乎确定性的增加。然而，在干旱气候下，降雨量较低且随机性较高，针对给定放牧强度的灌木侵蚀率变得难以预测。模拟实验最重要的发现是，虽然牧场科学家目前推荐的放养率不太可能在20年内导致灌木侵占，但在一个世纪内导致灌木侵占的可能性很高。这些发现适用于卡拉哈里南部发现的大多数降雨情景，因此这个半干旱地区的牧场政策制定者特别感兴趣。
Shrub encroachment is reducing the carrying capacity of arid grasslands in southern Africa for cattle. Although shrub-encroachment is known to occur as a result of the selective overgrazing of grasses by cattle, the interactions between rainfall and grazing are not well understood. Both the quantity and sequence of rainfall events are likely to influence the growth rates and competitive abilities of shrubs and grasses. Shrub encroachment is a slow process and animals are stocked at low densities in arid regions. Thus, field experiments for determining stocking rates that avoid shrub encroachment under various rainfall scenarios are almost impossible to replicate. We used a grid-based simulation model to investigate the shrub-grass dynamics of the southern Kalahari under various realistic rainfall scenarios and stocking rates of domestic livestock. The simulation experiments addressed the following questions: Does simulated cattle grazing lead to shrub encroachment? Over what time scale does the process take place? Are the dynamics of vegetation-change continuous in relation to grazing pressure or do they show a threshold behaviour? Simulation results indicated that the answers to all three questions depended on the quantity and sequence of rainfall. Simulated cattle grazing led to shrub encroachment under all rainfall scenarios, once stocking rates exceeded a threshold determined by long-term mean annual rainfall. The stocking rate threshold for shrub encroachment was less distinct (i.e. shrub cover in different simulation experiments had a higher coefficient of variation) under xeric than mesic climatic scenarios. This is because either competition from the herbaceous layer or rain may limit shrub establishment. In relatively mesic scenarios, where shrub encroachment was limited mainly by grass competition, the grazing of grasses beyond a certain threshold led to an almost deterministic increase in shrub cover. However, under xeric climates, where rainfall was lower and more stochastic, the rate of shrub encroachment in response to a given intensity of grazing became less predictable. The most significant finding of the simulation experiments was that, although the stocking rates currently recommended by pasture scientists are unlikely to lead to shrub encroachment within 20 years, they have a high probability of bringing about shrub encroachment within a century. These findings applied to most of the rainfall scenarios found in the southern Kalahari and are therefore of particular interest to rangeland policy makers in this semi-arid region.