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  Behavioral Education for Human, Animal,
Vegetation & Ecosystem Management

Stories of Applied Animal Behavior
Created by members of a graduate Foraging Ecology Class
     at the University of Idaho and Washington State University
     under the direction of Drs. Karen Launchbaugh and Lisa Shipley

The Dark Secrets of Willow Browsing

By Brad Stumph

Research conducted in Yellowstone (YNP) and Rocky Mountain National Parks (RMNP) has documented the decline of willow communities on elk winter ranges.  Both parks have documented a decline in height, density, and crown volumes of willow communities since the 1930s.  Although moose have resided in YNP since its inception, they have just recently colonized RMNP.  Moose concentrate their foraging efforts on willow communities in all seasons of the year.  Moose may affect willow decline more dramatically in RMNP because most effective predators have been removed while YNP supports predators such as wolves and grizzly bears.  However, elk and moose employ very different social and foraging strategies regardless of predation pressure.  Therefore, they occur at different densities and utilize willow most heavily at different times of the year.   

Problems that arise from the differing responses of willow to browsing

     Willow communities respond differently to harvest by elk and moose according to the season in which they are browsed.  Given the variation in seasonal response of willow to herbivory, areas containing multiple herbivores, which use willow communities differently in time and space, may contain willow communities of highly contrasting structure and composition.  The variable nature of willow communities can be problematic for land managers assessing the health and long-term viability of willow stands because a single standard can not be applied to all willow communities.  The manager must clearly understand the limitations placed on willow communities by wildlife.  Therefore, designing riparian area management plans in our national parks requires intricate knowledge of the foraging behavior of wildlife utilizing willow stands as well as the seasonal response of willows to herbivory.

      For over-utilized willow stands, wildlife exclosure research provides managers with a standard to measure the success of willow restoration projects as well a projection of recovery time required.  Wildlife exclosures are fenced areas that protect vegetation from herbivores such as moose, elk, and deer.  Researches use exclosures to determine the effects that wildlife have on vegetation.  Wildlife managers need to understand how the herds they manage affect willow communities because willows are an important source of forage along western streams.

The value of willow to wildlife and cattle

      Throughout the western United States, willows dominate stream-side vegetation communities.  Vegetation surrounding streams occupies a transitional zone between aquatic and upland areas known as the riparian zone.  Riparian zones maintain critical ecosystem functions by mediating the cycling of nutrients and sediments, increasing the retention of water in vegetation, and providing habitat for wildlife.  Willows are an important food source for wildlife and livestock in riparian areas.  Unlike most species, especially grasses, willow provide a stable protein source throughout the summer months.  The protein content of upland grasses decreases from 15-17% in early summer to less than 5% in late summer.  In contrast, willows may only experience a 7% decrease in protein content during the course of the summer.  When protein levels of most rangeland plants begin to decrease in mid-summer, deer, elk, and livestock may shift their foraging efforts towards willow communities in order to meet nutritional requirements.

      The seasonal pattern in which animals feed on willow depends on their foraging strategy and the availability of other forage types.  For example, grazing animals such as cattle and deer may only eat willow in late summer or winter whereas moose and other browsers eat willow year-round.  Willows react to a loss of stems and leaves in vastly different ways depending on the season and level of utilization.  Willows rely on their stems and leaves for energy production and transport. When a willow’s ability to produce and transport food is reduced by wildlife and livestock, it must respond by reducing energy requirements or reproducing the lost structures.  In general, willows browsed during the summer (growing season), especially late summer when little time remains for regrowth, respond the following summer with decreased stem and leaf production; whereas willows browsed during the winter (dormant season) respond the following summer with increased stem and leaf production but decreased seed production.  Of course the magnitude of the response depends on the density of foraging animals utilizing the willow community.

Genetics: the key to willow resilience

      The variation in willow response to animal harvest reflects their many adaptations to the riparian environment.  Willows flourish in the riparian zone where water levels pulse seasonally.  Many of the adaptations that allow willows to survive in the riparian zone also enable regrowth to occur after animals remove stems and leaves.  Regrowth, or compensatory growth, counteracts the effects of animal foraging by replacing stems and leaves.  Some adaptations of willows providing a high potential for compensatory growth include deciduous leaves, rhizomatous growth, and rapid photosynthetic rates.  Because willows do not have to spend energy to maintain leaves during winter months, they save energy for growth the following summer.  Wildlife such as elk and moose eat willow stems during winter months.  Stems are more difficult to digest than leaves and thus are not consumed as quickly.  Deciduous leaves reduce a plant’s maintenance cost and the amount of energy lost to wildlife during the winter.  If willows are browsed during the winter, they have more energy reserves available for growth the following summer than they would have had if leaves were present.  Willows also conserve energy in underground structures called rhizomes.  Rhizomes are modified roots that are capable of sprouting and reproducing an entire plant, a response known as rhizomatous growth.  By storing energy inaccessible to livestock, elk, deer, or moose, rhizomes ensure a willow’s ability to rebound from the effects of foraging animals.  In addition, willows are capable of photosynthesizing energy quickly.  Because of their high photosynthetic rates, willows quickly replace lost leaves.  With so many adaptations that counteract the effects of wildlife and livestock browsing, how can willow be declining in our national parks?

What information is being attained to reduce the problem?

      Comparison of long-term trends in willow decline between the 2 parks could describe not only the herbivore-plant interaction but also the predator-plant interaction.  Initial evidence from YNP shows that wolves alter the foraging behavior of elk by pushing them out of stream bottoms where willow communities occur.  Comparative studies between YNP and RMNP could also reveal in what season and at what herd densities willow communities are most vulnerable.  As elk and moose become more abundant in national parks across the west, the demand placed on riparian vegetation will increase.  Willow communities are critical components of western riparian zones and should be managed to preserve the ecological integrity of stream systems.

Additional references

Augustine, D. J., and S. J. McNaughton. 1998. Ungulate effects on the functional species composition of plant communities: herbivore selectivity and plant tolerance. Journal of Wildlife Management 62:1165-1183.

Karrenberg, S., P. Edwards, and J. Kollman. 2002. The life history of Salicaceae living in the active zone of  floodplains. Freshwater Biology 47:733-748.

Singer, F. J., L. C. Zeigenfuss, R. G. Cates, and D. T. Barnett. 1998. Elk, multiple factors, and persistence of  willows in national parks. Wildlife Society Bulletin 26:419-428.

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Learn more about the Foraging Ecology Class by visiting http://www.cnr.uidaho.edu/range556/