Herbivores can exert a controlling effect on the reproduction and growth of shrubs, thereby counter-acting the climate-driven encroachment of shrubs in the Arctic and the potential consequences. This control is particularly evident in the case of abundant herbivores, such as reindeer (Rangifer tarandus tarandus), whose grazing patterns are affected by management. Here, we tested how different reindeer grazing practices on the border between Finland and Norway impact the occurrence of willow (Salix spp.) dominated patches, their above-ground biomass (AGB) and the ability of willows to form dense thickets. We used a combination of multispectral and RGB imagery obtained from unoccupied aerial vehicles field data and an ensemble of machine-learning models, which allowed us to model the occurrence of plant community types (Overall accuracy = 0.80), AGB fractions (maximum R² = 0.90) and topsoil moisture (maximum R² = 0.89). With this combination of approaches, we show that willows are kept in a browsing-trap under spring and early summer grazing by reindeer, growing mostly small and scattered in the landscape. In contrast, willows under the winter grazing regime formed dense stands, particularly within riparian areas. We confirm this pattern using a random forest willow habitat distribution model based on topographical parameters. The model shows that willow biomass correlated with parameters of optimal habitat quality only in the winter grazing regime and did not respond to the same parameters under spring and summer grazing of reindeer.

中文翻译：

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驯鹿对亚北极湿地灌木化的控制：基于无人飞行器图像的空间分析

食草动物可以对灌木的繁殖和生长发挥控制作用，从而抵消气候驱动的北极灌木侵蚀及其潜在后果。这种控制在食草动物丰富的情况下尤其明显，例如驯鹿（Rangifer tarandus tarandus），其放牧模式受到管理的影响。在这里，我们测试了芬兰和挪威边境不同的驯鹿放牧方式如何影响以柳树（Salix spp.）为主的斑块的出现、其地上生物量（AGB）以及柳树形成茂密灌木丛的能力。我们结合使用从无人飞行器现场数据获得的多光谱和 RGB 图像以及机器学习模型集合，这使我们能够对植物群落类型的出现进行建模（总体精度 = 0.80）、AGB 分数（最大R ²  = 0.90）和表土湿度（最大R ²  = 0.89）。通过这种方法的结合，我们展示了在春季和初夏驯鹿放牧时，柳树被保存在捕食陷阱中，它们大多生长得较小且分散在景观中。相比之下，冬季放牧条件下的柳树形成了密集的林分，特别是在河岸地区。我们使用基于地形参数的随机森林柳树栖息地分布模型确认了这种模式。该模型表明，柳树生物量仅在冬季放牧条件下与最佳栖息地质量参数相关，而在春季和夏季驯鹿放牧下对相同参数没有响应。