U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Snake River Field Station, Boise, ID 83706, USA.
Conserv Biol. 2012 Oct;26(5):862-72. doi: 10.1111/j.1523-1739.2012.01903.x. Epub 2012 Aug 13.
Riparian and quaking aspen (Populus tremuloides) woodlands are centers of avian abundance and diversity in the western United States, but they have been affected adversely by land use practices, particularly livestock grazing. In 1990, cattle were removed from a 112,500-ha national wildlife refuge in southeastern Oregon. Thereafter, we monitored changes in vegetation and bird abundance in years 1-3 (phase 1) and 10-12 (phase 2) in 17 riparian and 9 snow-pocket aspen plots. On each 1.5-ha plot, we sampled vegetation in 6 transects. Three times during each breeding season, observers recorded all birds 50 m to each side of the plot's 150-m centerline for 25 minutes. We analyzed data with multivariate analysis of variance and paired t tests with p values adjusted for multiple comparisons. In both periods, riparian and snow-pocket aspen produced extensive regeneration of new shoots (stems/ha and 7079 stems/ha, respectively). By phase 2, a 64% increase in medium-diameter trees in riparian stands indicated successful recruitment into the overstory, but this pattern was not seen in snow-pocket stands, where the density of trees was over 2 times greater. By phase 2 in riparian and snow-pocket stands, native forb cover had increased by 68% and 57%, respectively, mesic shrub cover had increased by 29% and 58%, and sagebrush cover had decreased by 24% and 31%. Total avian abundance increased by 33% and 39% in riparian and snow-pocket aspen, respectively, ground or understory nesters increased by 133% and 67% and overstory nesters increased by 34% and 33%. Similarly, ground or understory foragers increased by 25% and 32%, aerial foragers by 55% and 57%, and overstory foragers by 66% and 43%. We interpreted the substantial regeneration of aspen shoots, increased densities of riparian forbs and shrubs, and increased avian abundances as a multitrophic-level response to the total removal of livestock and as substantial movement toward recovery of biological integrity.
河岸和颤杨（Populus tremuloides）林地是美国西部鸟类丰富度和多样性的中心，但它们受到土地利用实践的不利影响，特别是牲畜放牧。1990 年，俄勒冈州东南部一个 112500 公顷的国家野生动物保护区的牛被移除。此后，我们在 1-3 年（第 1 阶段）和 10-12 年（第 2 阶段）监测了 17 个河岸和 9 个雪袋白杨样地中植被和鸟类丰富度的变化。在每个 1.5 公顷的样地中，我们在 6 个样带中对植被进行了采样。在每个繁殖季节的三次，观察者记录了样地 150 米中心线每侧 50 米内的所有鸟类，持续 25 分钟。我们使用多元方差分析和配对 t 检验分析数据，并对多个比较进行了调整。在两个时期，河岸和雪袋白杨都产生了大量的新梢再生（每公顷茎/株和 7079 株/公顷）。到第 2 阶段，河岸林分中中径树木增加了 64%，表明成功地进入了林冠层，但在雪袋林分中没有出现这种情况，那里的树木密度超过了 2 倍。到第 2 阶段，河岸和雪袋林分中的本地草本植物盖度分别增加了 68%和 57%，中生灌木盖度分别增加了 29%和 58%，而鼠尾草盖度分别减少了 24%和 31%。河岸和雪袋白杨的鸟类总丰富度分别增加了 33%和 39%，地面或林下层筑巢者增加了 133%和 67%，林冠层筑巢者增加了 34%和 33%。同样，地面或林下层觅食者增加了 25%和 32%，空中觅食者增加了 55%和 57%，林冠层觅食者增加了 66%和 43%。我们将白杨新梢的大量再生、河岸草本植物和灌木密度的增加以及鸟类丰富度的增加解释为对牲畜全部移除的多营养层响应，并朝着生物完整性恢复的方向取得了实质性进展。
