Northern Yellowstone National Park provides an example of passive restoration, as wetlands and riparian areas there lost most tall willows in the 20th century, due to intensive herbivory by elk (Cervus canadensis). Following large carnivore restoration in the late 1990s, elk numbers decreased, and some researchers reported willows growing taller with reductions in browsing, evidence of a shift toward willow recovery. Others questioned the extent and significance of these changes. To investigate how willow heights have changed in northern Yellowstone since the 1990s, and to assess the importance of browsing as a driver of willow height and canopy cover, we compared data from three time periods: 1988 to 1993 when elk densities were high and most willows very short, 2001 to 2004 when willows may have begun to recover, and 2016 to 2018. We found a strong contrast between sites along streams, compared to wet meadows (meadow sites). Willows in meadow sites did not increase in height, but willows in stream sites increased significantly, exceeding 200 cm mean height in summer by 2001–2004, and in spring by 2016, a height indicative of recovery. Where height did not increase, this was due to loss of annual growth to herbivory. Overall willow height distribution changed from mostly short to become clearly bimodal, with a new peak around 300–400 cm. Bison increased, and in some sites where bison congregate willows remained suppressed at heights below 120 cm, a condition strongly correlated with summer browsing. We also located and measured willow thickets (groups of 5 of more willows >200 cm spring height and <2 m apart) along streams in the study area. We found willow thickets in all stream reaches surveyed, a significant change from past conditions. Thickets occupied >80% of willow patches in some sites, but as little as 22% in others. Tall willow thickets are an important habitat feature and an indicator of willow recovery. Our results demonstrate that the reduction of elk herbivory over the last two decades in northern Yellowstone has allowed willows to grow taller in many places, despite a warming and drying climate, while increased herbivory by bison continues to suppress willows in some locations.

中文翻译：

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高高的柳树灌丛回到黄石北部

北黄石国家公园是被动恢复的一个例子，由于麋鹿（加拿大鹿）大量放牧，20世纪那里的湿地和河岸地区失去了最高的柳树。）。在1990年代后期恢复大型食肉动物之后，麋鹿数量减少了，一些研究人员报告说，随着浏览量的减少，柳树变得越来越高，这证明了向柳树恢复的转变。其他人则质疑这些变化的程度和重要性。为了调查1990年代以来黄石北部的柳树高度如何变化，并评估浏览作为柳树高度和树冠覆盖的驱动因素的重要性，我们比较了三个时间段的数据：1988年至1993年麋鹿密度高和大多数柳树很短的时间（2001年至2004年，当柳树可能已开始恢复），以及2016年至2018年。我们发现，与湿润的草地（草甸场所）相比，沿溪流的站点之间存在强烈的对比。草地上的柳树的高度没有增加，但是溪流中的柳树却显着增加，到2001年至2004年夏季，平均高度超过200厘米，到2016年春季，平均高度超过200厘米，这表明已恢复。在高度没有增加的地方，这是由于草食性的年生长损失。柳树的总体高度分布从大部分变短为明显的双峰分布，在300-400 cm处出现了一个新峰。野牛增加了，在一些野牛聚集的柳树在120厘米以下的高度仍被抑制的情况下，这种状况与夏季浏览密切相关。我们还沿着研究区域的溪流定位并测量了柳树灌木丛（每组5个以上的柳树，其春季高度> 200厘米且相距<2 m）。我们发现在所调查的所有溪流中柳树灌丛都比过去的状况发生了重大变化。在某些地方，灌木丛占柳树斑块的80％以上，而在另一些地方，仅占22％。高柳灌木丛是重要的栖息地特征，是柳树恢复的指标。我们的结果表明，尽管气候变暖和干燥，近二十年来黄石公园北部麋鹿的食草性减少使柳树在许多地方变得更高，而野牛增加的草食性在某些地方继续抑制了柳树。