What causes willow die-off?

Abstract

In riparian areas, willow (Salix spp.) has been identified as a key species for stream health, bank stability, vegetative filter zones and the winter survival of deer, elk, moose, beaver, hares, and ptarmigan. Consequently, the sustainability of individual willows and willow communities is fundamental to both the functioning of riparian ecosystems and the maintenance of certain wildlife populations. While we may recognize the linkage between willow survival and that of dependant wildlife species, we know little about the factors that govern reproduction, establishment, and long-term presence of willows in riparian ecosystems. Like many other woody plant species, willows are susceptible to diseases including cankers caused by fungi that enter the plant through wounds. Bark wounds can occur from browsing, hail, strong winds, frost or a host of other agents. Once infected, the plant develops a canker that begins to girdle the stem at the site of infection (Fig. 1). Information on aspen ecology from the US Forest Service indicates that while minor canker infections kill the tissue surrounding the wound, the remainder of the stem's transport tissue (xylem and phloem) will remain alive and functioning. However, if the infection grows or the plant is repeatedly infected, the canker will girdle the stem resulting in death of that stem. This event is commonly called ringing. The heavily infected stem can no longer transport nutrients past the canker to leaves or buds, and the stem soon dies and falls to the ground. The loss of numerous stems will stunt the growth of the plant and if the losses are sufficiently large, the plant may eventually die. Because quaking aspen (Populus tremuloidies) is in the same family as willows, we could use this information to predict that stem injuries may lead to canker infections in willows. Severe infections could lead to the loss of numerous willows in a riparian area. Due to the heightened awareness of the crucial role willows play in riparian ecosystems, it is important for federal, state, and private land managers to know the cause of willow die-off. Even though there are reports of willow die-offs due to declining water tables in eastern California, overuse by wildlife in Wyoming and Colorado and heavy livestock grazing throughout the West, there are very