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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 willows 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 plants 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 willows 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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