Aarhus Institute of Advanced Studies, Aarhus University, Høegh-Guldbergs Gade 6B, 8000, Aarhus C, Denmark.
Department of Environmental Studies, Dartmouth College, Hanover, NH, 03755, USA.
Oecologia. 2022 May;199(1):229-242. doi: 10.1007/s00442-022-05177-w. Epub 2022 May 7.
Resolving the relative contributions of top-down versus bottom-up drivers of vegetation dynamics is a major challenge in drylands. In the coming decades, growing livestock populations and shifts in water availability will simultaneously impact many arid systems, but a lack of empirical data on plant responses to these pressures limits understanding of how plants will respond. Here, we combine ground and drone observations from an herbivore exclosure experiment to identify ungulate visitation patterns and their impacts on the cover and melon production of !nara (Acanthosicyos horridus), a large, long-lived desert plant in the hyper-arid Namib Desert. !Nara are of key ecological, social, and economic importance to Namib ecosystems and to the local Topnaar people. At our study site, we find that among native and domestic herbivores, free-ranging donkeys have the largest impact on !nara cover and melon production. !Nara cover was negatively affected by herbivores close to the desert-ephemeral river ecotone during a dry period, whereas !nara cover increased on all plants across the landscape during a wetter period, regardless of herbivore access. !Nara near the river channel and those protected from herbivores had more mature melons, particularly during the wetter period. At this site, the potential for conflict between Topnaar !nara melon harvesting and pastoral practices varies with a plant's distance from the river and prevailing abiotic conditions. This work advances monitoring approaches and adds empirical support to the understanding that top-down and bottom-up regulation of plant dynamics varies with spatiotemporal context, even within landscapes.
解决植被动态的自上而下和自下而上驱动因素的相对贡献是干旱地区的主要挑战。在未来几十年，不断增长的牲畜数量和水资源可用性的变化将同时影响许多干旱系统，但由于缺乏关于植物对这些压力的反应的经验数据，限制了对植物将如何反应的理解。在这里，我们结合了食草动物围栏实验的地面和无人机观测，以确定有蹄类动物的访问模式及其对巨柱仙人掌（Acanthosicyos horridus）覆盖和甜瓜产量的影响，巨柱仙人掌是纳米布沙漠中一种大型、长寿的沙漠植物。巨柱仙人掌对纳米布生态系统和当地托普纳人具有关键的生态、社会和经济重要性。在我们的研究地点，我们发现在本地和家养的食草动物中，自由放养的驴子对巨柱仙人掌的覆盖度和甜瓜产量的影响最大。在干旱时期，靠近沙漠-短暂河流生态交错带的食草动物会对巨柱仙人掌的覆盖度产生负面影响，而在湿润时期，无论食草动物是否可以进入，所有植物的覆盖度都会增加。靠近河道的巨柱仙人掌和受食草动物保护的巨柱仙人掌的甜瓜更成熟，尤其是在湿润时期。在这个地点，托普纳人收获巨柱仙人掌甜瓜和畜牧业之间的潜在冲突随着植物与河流的距离和流行的非生物条件而变化。这项工作推进了监测方法，并为理解植物动态的自上而下和自下而上的调节随着时空背景的变化而变化提供了经验支持，即使在景观内也是如此。
