The impact of sheep grazing on shrub communities at the Afrikaner and Hereford grazing trials at Grootfontein in the Eastern Upper Karoo

In the Nama-Karoo, grazing by livestock is an important driver of Karoo shrub composition. Many different breeds of livestock are farmed in the Nama-Karoo, and the stocking rate varies greatly over the region. In this study we examine how grazing by dorper and merino sheep applied at three stocking rates affected two Karoo shrublands on a long-term grazing trial at Grootfontein in the eastern Karoo. The effects of grazing treatment on shrub communities at two sites of the Afrikaner-Hereford grazing trial differed greatly and were measurable after ten years of rest. At the sandier, grassier Afrikaner site, heavy grazing by dorper sheep had a significant effect on shrub composition, density and cover, whereas the effect of grazing treatment was less pronounced at the Hereford site. Lenient grazing by merino sheep resulted in a vastly different community compared to all other grazing treatments. Individual Karoo shrub species responded differently to grazing treatment. Overall shrub recruitment has faltered for all but three species, and populations were dominated by intermediate-sized and large individuals. Results indicate that grazing management, animal type and stocking rate are important determinants of Nama-Karoo rangeland composition and structure with evidence of a legacy effect longer than ten years.

在纳米卡鲁（Nama-Karoo）地区，牲畜放牧是调控卡鲁灌丛群落组成的重要驱动因素。该区域内养殖有多种不同的牲畜品种，且各地的放牧载畜量（stocking rate）差异显著。本研究以东卡鲁地区格罗特方泰恩（Grootfontein）的长期放牧试验为平台，探究三种载畜量下杜泊羊（Dorper）与美利奴羊（Merino）的放牧活动对两处卡鲁灌丛草地的影响。阿非利卡-赫里福德（Afrikaner-Hereford）放牧试验的两个样地中，放牧处理对灌丛群落的影响差异显著，且在休牧十年后即可被观测到。在沙质更高、草本更为繁茂的阿非利卡样地中，杜泊羊的高强度放牧对灌丛的组成、密度与盖度均产生了显著影响；而在赫里福德样地中，放牧处理的效应则相对微弱。美利奴羊的轻度放牧处理下，灌丛群落组成与其余所有放牧处理均存在显著差异。不同卡鲁灌丛物种对放牧处理的响应各不相同。除三个物种外，其余所有灌丛物种的整体更新均受阻，种群以中、大型个体为主。研究结果表明，放牧管理方式、牲畜种类与载畜量是决定纳米卡鲁牧场群落组成与结构的关键因子，且存在超过十年的遗留效应。