Division of Biology, Kansas State University, Manhattan, Kansas, 66506, USA.
O'Connor Rangeland Science, 970 South Lusk Street, Boise, Idaho, 83706, USA.
Ecology. 2020 Feb;101(2):e02935. doi: 10.1002/ecy.2935. Epub 2019 Dec 26.
North American grasslands have experienced increased relative abundance of shrubs and trees over the last 150 yr. Alterations in herbivore composition, abundance, and grazing pressure along with changes in fire frequency are drivers that can regulate the transition from grassland to shrubland or woodland (a process known as woody encroachment). Historically, North American grasslands had a suite of large herbivores that grazed and/or browsed (i.e., bison, elk, pronghorn, deer), as well as frequent and intense fires. In the tallgrass prairie, many large native ungulates were extirpated by the 1860s, corresponding with increased homesteading (which led to decreased fire frequencies and intensities). Changes in the frequency and intensity of these two drivers (browsing and fire) have coincided with woody encroachment in tallgrass prairie. Within tallgrass prairie, woody encroachment can be categorized in to two groups: non-resprouting species that can be killed with fire and resprouting species that cannot be killed with fire. Resprouting species require additional active management strategies to decrease abundance and eventually be removed from the ecosystem. In this study, we investigated plant cover, ramet density, and physiological effects of continuous simulated browsing and prescribed fire on Cornus drummondii C.A. Mey, a resprouting clonal native shrub species. Browsing reduced C. drummondii canopy cover and increased grass cover. We also observed decreased ramet density, which allowed for more infilling of grasses. Photosynthetic rates between browsed and unbrowsed control shrubs did not increase in 2015 or 2016. In 2017, photosynthetic rates for browsed shrubs were higher in the unburned site than the unbrowsed control shrubs at the end of the growing season. Additionally, after the prescribed fire, browsed shrubs had ~90% decreased cover, ~50% reduced ramet density, and grass cover increased by ~80%. In the roots of browsed shrubs after the prescribed fire, nonstructural carbohydrates (NSC) experienced a twofold reduction in glucose and a threefold reduction in both sucrose and starch. The combined effects of browsing and fire show strong potential as a successful management tool to decrease the abundance of clonal-resprouting woody plants in mesic grasslands and illustrate the potential significance of browsers as a key driver in this ecosystem.
在过去的 150 年里，北美草原的灌木和树木相对丰度增加。食草动物组成、数量和放牧压力的改变，以及火灾频率的变化，是调节从草原向灌丛或林地转变（称为木质侵入）的驱动因素。历史上，北美草原有一系列大型食草动物，它们放牧和/或啃食（如野牛、麋鹿、叉角羚、鹿），并且经常发生剧烈的火灾。在高草草原，许多大型本地有蹄类动物在 19 世纪 60 年代被消灭，与之对应的是宅基地的增加（这导致火灾频率和强度降低）。这两个驱动因素（啃食和火灾）的频率和强度的变化与高草草原的木质侵入同时发生。在高草草原内，木质侵入可以分为两类：不能用火杀死的非萌生物种和不能用火杀死的萌生物种。萌生物种需要额外的积极管理策略来减少数量，最终从生态系统中移除。在这项研究中，我们调查了连续模拟啃食和规定火烧对一种萌生克隆本地灌木 Cornus drummondii C.A. Mey 的植物覆盖、嫩枝密度和生理效应。啃食降低了 C. drummondii 的冠层覆盖度，并增加了草的覆盖度。我们还观察到嫩枝密度的下降，这使得草的生长更加密集。在 2015 年和 2016 年，啃食和未啃食对照灌木的光合作用速率都没有增加。在 2017 年，在生长季节结束时，未被火烧的未被啃食对照灌木的光合作用速率高于未被啃食对照灌木。此外，在规定火烧之后，啃食灌木的盖度降低了约 90%，嫩枝密度降低了约 50%，草的覆盖度增加了约 80%。在规定火烧后的啃食灌木的根部，非结构性碳水化合物（NSC）中的葡萄糖含量减少了一倍，蔗糖和淀粉含量减少了三倍。啃食和火烧的综合作用显示出作为一种成功的管理工具，有很大潜力减少湿润草地中生有克隆的萌生木本植物的丰度，并说明食草动物作为该生态系统的关键驱动因素的潜在重要性。
