Brookshire Jack E, Kauffman Boone J, Lytjen Danna, Otting Nick
Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR, 97331-3803, USA.
Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061-0406, USA.
Oecologia. 2002 Aug;132(4):559-566. doi: 10.1007/s00442-002-1007-4. Epub 2002 Aug 1.
We examined the effects of wild ungulates (deer and elk) and domestic sheep browsing on the growth, structure, and reproductive effort of two common willow species, Salix boothii and S. geyeriana, in a montane northeast Oregon riparian zone. With the use of exclosures, large herbivore effects on willows were studied in an area browsed by native mammals only and an adjacent area in which domestic sheep also lightly grazed during summer months. Growth variables were repeatedly measured on individual plants over a 5-year period to understand physiognomic and flowering responses of native willows to different levels of browsing pressure. At the beginning of the study, all willows were intensely browsed but were significantly taller in the area browsed only by native mammals than in the area also grazed by sheep (69 versus 51 cm, respectively). Willows inside exclosures responded with pronounced increases in height, crown area, and basal stem diameters while the stature of browsed plants outside exclosures stayed constant or declined. In the area browsed by both sheep and wild herbivores, the size of browsed plants remained at pre-treatment levels (<60 cm in height) for the duration of the study. There was no significant difference in growth rates of enclosed willows, indicating that current herbivory was the primary cause of growth retardation in the study area. Foliar area was strongly correlated with basal stem numbers for enclosed plants but much less so for browsed plants. Willows inside exclosures had more than twice as much foliar area per stem. Stem diameters were a positive function of crown area: stem-number ratios, suggesting lower photosynthetic potential was correlated with diminished radial growth among browsed plants. No flowering was observed until 2 years after exclusion when plants inside all exclosures and browsed willows in the wild ungulate area responded with a large pulse in flowering. Browsed plants in the sheep + wild ungulate area did not flower. The number of catkins produced per plant was significantly associated with willow height and plants <70 cm in height did not flower, thus suggesting a size threshold for reproduction in these species. Our results suggest that even relatively light levels of domestic livestock grazing, when coupled with intense wild ungulate browsing, can strongly affect plant structure and limit reproduction of riparian willows.
我们研究了野生有蹄类动物（鹿和麋鹿）以及家养绵羊啃食对俄勒冈州东北部山地河岸带两种常见柳树——博氏柳（Salix boothii）和盖氏柳（S. geyeriana）的生长、结构和繁殖投入的影响。通过使用围栏，在一个仅被本地哺乳动物啃食的区域以及一个在夏季家养绵羊也有轻度啃食的相邻区域，研究了大型食草动物对柳树的影响。在5年时间里，对单株植物的生长变量进行了反复测量，以了解本地柳树对不同啃食压力水平的外貌和开花反应。在研究开始时，所有柳树都被大量啃食，但仅被本地哺乳动物啃食的区域的柳树明显比也被绵羊啃食的区域的柳树更高（分别为69厘米和51厘米）。围栏内的柳树高度、树冠面积和基部茎直径显著增加，而围栏外被啃食植物的高度保持不变或下降。在绵羊和野生食草动物都啃食的区域，被啃食植物的大小在研究期间一直保持在处理前的水平（高度<60厘米）。围栏内柳树的生长速率没有显著差异，这表明当前的食草作用是研究区域内生长迟缓的主要原因。对于围栏内的植物，叶面积与基部茎数量密切相关，但对于被啃食的植物则相关性小得多。围栏内的柳树每根茎的叶面积是其两倍多。茎直径是树冠面积与茎数量比的正函数，这表明较低的光合潜力与被啃食植物径向生长的减弱相关。直到排除啃食两年后才观察到开花，此时所有围栏内的植物以及野生有蹄类动物区域内被啃食的柳树都出现了大量开花。绵羊 + 野生有蹄类动物区域内被啃食的植物没有开花。每株植物产生的柔荑花序数量与柳树高度显著相关，高度<70厘米 的植物不开花，因此表明这些物种存在繁殖的大小阈值。我们的结果表明，即使是相对较轻程度的家畜放牧，加上野生有蹄类动物的大量啃食，也会强烈影响植物结构并限制河岸柳树的繁殖。
