It has been widely asserted that a high grazing pressure has led to a reduction in vegetation production at decadal time scales, implying land degradation, in African drylands, and in the Sahel in particular. We test this hypothesis by analyzing spatiotemporal patterns of vegetation production in the northwestern Ferlo in Senegal. Normalized difference vegetation index (NDVI) patterns, here used to represent vegetation production, were determined on the basis of Earth observation MODIS and PROBA‐V data. Furthermore, woody cover was assessed by very high spatial resolution (VHR) imagery. Because livestock is concentrated around deep wells in the dry season for watering, while for management they concentrate daily around pastoral family settlements or camps all year round, we studied the dependence of NDVI on distance from deep wells and camps. Locations of wells and camps were determined by visual inspection of VHR images. We found that 'inverse grazing gradients,' defined as decreasing NDVI/production with increasing distance, dominated both around wells and camps. Further, we found neither decrease nor increase in woody cover with distance to wells. Both positive and negative trends in wet season NDVI over the period 2000–2016 were identified within the study area, yet temporal trends were predominantly positive in proximity to deep wells. Around pastoral camps, positive NDVI trends were generally elevated relative to the regional average. The results question the validity of claims that high grazing pressure causes land degradation at the landscape scale, yet they show that grazing does lead to substantial spatial redistribution of vegetation production.

中文翻译：

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放牧会导致土地退化吗？来自塞内加尔北部的 Sandy Ferlo 的证据

人们普遍认为，高放牧压力导致了十年时间尺度的植被生产减少，这意味着非洲旱地，特别是萨赫勒地区的土地退化。我们通过分析塞内加尔 Ferlo 西北部植被生产的时空模式来检验这一假设。归一化差异植被指数 (NDVI) 模式，这里用于表示植被生产，是根据地球观测 MODIS 和 PROBA-V 数据确定的。此外，通过非常高的空间分辨率 (VHR) 图像评估木质覆盖。由于牲畜在旱季集中在深井附近浇水，而在管理上，它们全年每天都集中在牧民家庭聚居地或营地周围，我们研究了 NDVI 对距深井和营地距离的依赖性。井和营地的位置是通过目视检查 VHR 图像来确定的。我们发现“逆放牧梯度”（定义为 NDVI/产量随着距离的增加而降低）在井和营地周围都占主导地位。此外，我们发现木本覆盖率既没有随着距井的距离而减少也没有增加。在研究区域内确定了 2000-2016 年期间雨季 NDVI 的正负趋势，但在深井附近的时间趋势主要是正的。在牧区周围，相对于区域平均水平，NDVI 正趋势普遍升高。结果质疑高放牧压力导致景观尺度土地退化的说法的有效性，但他们表明放牧确实导致了植被生产的大量空间重新分布。