Beaver and Trees
Note:
This online review is updated and revised continuously, as soon as results
of new scientific research become available. It therefore presents
state-of-the-art information on the topic it covers.
The activities of beaver (Castor
canadensis and Castor fiber) influence the distribution and
abundance of many species of trees that grow near beaver ponds. In this
review, we summarize the many ways that beaver and trees interact with
each other and affect each other.
Effects of selective
tree-felling
In forests near their ponds,
beaver cut down trees of many different species. However, they most prefer
trees of the plant family Salicaceae, i.e. Populus (aspens,
cottonwoods, poplars) and Salix (willows) (Pastor and Naiman 1992).
Selective logging of Populus and Salix by beaver changes the
relative abundance and frequency of these and other tree species in the
riparian forest near the beaver pond.
For example, Johnston and Naiman
(1990) studied the effects of 6 years of beaver tree-felling in a riparian
forest alongside a beaver pond near Duluth, Minnesota. During this period,
beaver tree-felling removed over 40% of the above ground biomass of the
forest.
Before beaver occupation, the
forest was dominated by quaking aspen (Populus tremuloides), a
favorite browse tree of the beaver. However, after 6 years of beaver
foraging, the density and biomass of quaking aspen was dramatically
diminished in the forest, while the relative density and biomass of tree
species the beaver did not like to eat and therefore did cut down
increased, such as black ash (Fraxinus nigra) and tag alder (Alnus
rugosa).
Along the Lower Chippewa River,
Wisconsin, felling of trees by beaver resulted in there being only a few
trees remaining within 10 to 15 meters of the river, and tree density
increased with distance from the riverbank to at least 30 meters (Barnes
and Dibble 1988).
Beaver selected ash (Fraxinus
spp.), bitternut hickory (Carya cordiformis) and hackberry (Celtis
occidentalis) over basswood (Tilia americana), elm (Ulmus
spp.) and prickly ash (Xanthoxylum americanum). Because beaver
avoided cutting prickly ash, dense stands of this shrub persisted within
the first 10 to 15 meters of the river where most trees had been removed
(Barnes and Dibble 1988).
Another study found that in
Allegany State Park, New York, beaver clear-cut many areas around their
ponds but left mature red maples (Acer rubrum) standing (Müller-Schwarze
et al. 1994). What was particularly interesting in this case was that
while the beaver avoided red maple (a soft wood), they readily cut down
sugar maple (Acer saccharum), a hardwood tree that takes longer to
fell.
To understand why red maples
were not cut down, an experiment was set up where beaver were given a
choice of eating logs of red maple, quaking aspen, or quaking aspen
treated with extract from the bark of red maple. The beaver chose quaking
aspen logs significantly more often than quaking aspen logs treated with
red maple extract, and this result led Müller-Schwarze et al. (1994) to
conclude that the red maple bark or red maple wood contained chemicals
that repelled beavers. However, other studies suggest that red maple is
sometimes a preferred forage tree of beaver (Donker and Fryxell 2000).
Inducible defenses of
trees influence beaver choices
Many trees cut down by beaver sprout
again from the roots or stump and begin growing once more. To
protect themselves against further cutting by the beaver, these
re-sprouting trees produce higher concentrations of protective chemicals in
their bark than they did before the beaver cut them. These chemicals
deter future beaver attacks because they are toxic to beaver.
Basey et al. (1988, 1990) studied beaver
and aspen interactions at two sites: one pond in Nevada and one pond in
nearby California. The Nevada site was newly occupied by beaver
(colonized 6 months before), while the California site had been occupied
intermittently for at least 20 years. At the time the study began,
beaver had cut down only 3% of aspens within 3 meters of shore at the
Nevada site, but 23% of aspens within 3 meters of shore at the California
site.
As expected from the fact that the California site had been
used longer and more extensively by beaver, defensive chemicals in quaking
aspen bark there were significantly higher than at the Nevada site.
The highest concentrations of these defensive chemicals at the California
site were in the youngest aspen sprouts, i.e. those of the smallest
diameter (aspens that had most recently been cut down by beaver).
As the age and diameter of aspen stems increased at the
California site, the concentration of defensive chemicals gradually
decreased. In contrast, aspens of all ages and diameters at the
newly occupied Nevada site had low quantities of defensive chemicals in
their bark.
The differences in the quantity of defensive chemicals
in the aspen trees at the California and Nevada sites led to differences
in tree selection by the beaver. In Nevada, beaver preferred to cut
down aspen with small diameter trunks, while at the California site,
beaver preferred to cut down aspen with large diameter trunks, because at
the latter site the smaller aspens had too high concentrations of
defensive chemicals in their bark.
Selective
tree-felling and exotic plants
Along rivers in
eastern
Montana, felling of cottonwood by beaver increased the growth rates of two
non-native pest trees seldom used by beaver: Russian olive (Elaeagnus
angustifolia) and saltcedar (Tamarisk spp.) (Lesica and Miles
2004). Therefore, wildlife managers that intend to reintroduce beaver into an area
should first eliminate such invasive exotic plants.
Selective tree-felling in hybrid tree stands
The Fremont cottonwood (Populus fremonti) and the narrowleaf cottonwood (P. angustifolia) occur sympatrically
in some areas of western North America. There, they interbreed,
producing offspring called "F1 hybrids." These F1 hybrids often backcross with pure
angustifolia but not with pure fremonti. Continued mating
of pure angustifolia with hybrids produces a series of
different backcrosses that results in a hybrid swarm of cottonwoods (Keim
1989).
Along the Weber River of Utah, the ranges of these two cottonwoods overlap
in a 13 kilometer zone where a typical hybrid swarm is produced (Bailey et
al. 2004). There, cottonwoods are the dominant tree species but the
genetics of individual cottonwoods trees are diverse, ranging from pure
fremonti to pure angustifolia, with most individuals falling in
between.
Angustifolia is better protected against beaver
herbivory because its bark has condensed tannins, a defensive chemical that is
toxic to beaver. Hybrid cottonwoods have less concentrations of
condensed tannins than angustifolia, and fremonti has none.
In the backcrossed hybrids, the concentration of condensed tannins is
greater than in the F1 hybrids, and increases as the percentage of
angustifolia genes increases.
Bailey and his research group
studied which cottonwoods were cut down most frequently by beaver and
found that fremonti was most preferred by beaver, followed by the
F1 hybrids. Backcrossed hybrids and pure angustifolia were
cut down less, however within this group those hybrids with higher
proportions of fremonti genes were most down more frequently.
Effects of beaver on forest succession
Beaver can reverse or
hasten the succession of plant communites. When beaver selectively
cut down certain tree species and create sunlit gaps in the forest,
species of sun-loving, shade-intolerant plants often regenerate there,
converting a mid-successional stand to an early successional stand (Gill
1972; Pastor and Naiman 1992).
However, if the forest is dominated
with early successional trees like aspens and willows that the beaver
likes, but has an understory of seedling late-successional trees
that the beaver does not like, such as
fir and spruce, tree-felling by beaver hastens succession by removing the
trees that hinder the growth of the understory tree seedlings (Naiman et
al. 1988; Johnston and Naiman 1990; Pastor and Naiman 1992).
Selective tree-felling in different habitats
Gallant et al. (2004) compared
beaver foraging in two types of habitats: high-quality (with a
greater proportion of deciduous trees), and low quality (with a lesser
proportion of deciduous trees). These researchers found that "with
increasing distance from the pond, beavers in high-quality habitats
selected fewer, but larger, trees and were more species selective. This
selectivity was diminished in habitats of lower quality." These
results are similar to those predicted by a model known as central place
foraging theory (Orians and Pearson 1979; Schoener 1979).
Flooding of
economically valuable timber by beaver
When streams are dammed by beaver, ponds form that flood and cover
the roots of trees which formerly stood along the stream bank. These
flooded trees die because the standing water prevents their roots from
getting air.
In some places, such as in the southeastern United States,
beaver cause extensive damage to valuable timber by flooding bottomland forests>,
felling adult trees and eating tree seedlings (Bhat et al. 1993; Härkönen 1999;Conner at al. 2000).
In such cases, the landowner can suffer significant economic losses. For information
about controlling harmful beaver, see comments in our review:
Ecology of the Beaver.
Falling
trees sometimes kill Beaver
Beaver are sometimes found dead under
fallen trees (Hitchcock 1954; Scotter and Scotter 1989). These
apparently rare, accidental deaths may be the result of beaver misjudging where trees
they are cutting will fall, or of one beaver cutting down a tree that
kills another beaver (Scotter and Scotter 1989). Still another
possibility is that a beaver was killed by a natural treefall of a
partly cut tree.
Conclusions
Although beavers are not large animals, they often have greater impact on riparian
forests than larger herbivores such as moose (Alces alces).
Johnston and Naiman (1990) give three reasons why: (1) beaver are
the only animals besides humans that can cut down mature trees, (2) beaver
concentrate their tree felling and foraging in the relatively narrow band
of forest surrounding their ponds, and (3) beaver remove far more
vegetation than they consume, because they use wood to build dams and
lodges as well as for food. Each year, in northern regions, an
average beaver family is reported to cut "at least a metric ton of wood
within approximately 100 meters of their pond" (Naiman et al. 1988, see
also McGinley and Whitham 1985).
Click the following links to learn more about effects of
beaver engineering on wildlife:
birds,
frogs, salamanders,
lizards,
turtles, snakes,
invertebrates
Click this link for the introductory review: Ecology
of the Beaver.
References
Bailey JK, Schweitzer JA,
Rehill BJ, Lindroth RL, Martinsen GD, Whitham TG (2004)
Beavers as molecular geneticists: a genetic basis to the foraging of an
ecosystem engineer. Ecology 85: 603-608
Barnes WJ, Dibble E (1988) The effects of beaver in
riverbank forest succession. Canadian Journal of Botany
66:40-46
Basey JM, Jenkins SH, Busher PE (1988) Optimal
central-place foraging by beavers: tree-size selection in relation to
defensive chemicals of quaking aspen. Oecologia 76: 278-282
Basey JM, Jenkins SH, Miller GC (1990) Food
selection by beavers in relation to inducible defenses of Populus
tremuloides. Oikos 59. 57-62
Conner WH,
Inabinette LW, Brantley EF (2000) The use of tree shelters in
restoring forest species to a floodplain delta: 5-year results. Ecological
Engineering 15: S47-S56, Supplement 1
Bhat
MG, Huffaker RG, Lenhart SM (1993) Controlling forest damage
by dispersive beaver populations - centralized optimal management
strategy. Ecological Applications 3: 518-530
Donker NT, Fryxell JM (2000) Lowland boreal
forests characterization in Algonquin Provincial Park relative to beaver (Castor
canadensis) foraging and edaphic factors. Plant Ecology
148: 1-12
Gallant D, Bérubé CH, Tremblay E, Vasseur L (2004)
An extensive study of the foraging ecology of beavers (Castor
canadensis) in relation to habitat quality. Canadian Journal of
Zoology 82: 922-933
Gill D
(1972) The evolution of a discrete beaver habitat in the Mackenzie
River delta, Northwest Territories. Canadian Field-Naturalist
86: 223-239
Härkönen S (1999) Forest Damage caused by the
Canadian Beaver (Castor canadensis) in south Savo, Finland.
Silva Fennica 33: 247-259
Hitchcock HB (1954) Felled
tree kills beaver (Castor canadensis) Journal of Mammalogy
35:452
Johnston
CA, Naiman RJ (1990) Browse selection by beaver: effects on
riparian forest composition. Canadian Journal of Forest Research
20: 1036-1043
Keim P, Paige KM, Whitham TG, Lark KG (1989) Genetic analysis
of an interspecific hybrid swarm of Populus: occurence of
unidirectional introgression. Genetics 123: 557-565
Lesica P, Miles S (2004) Beavers indirectly enhance the growth
of Russian olive and tamarisk along eastern Montana rivers.
Western North American Naturalist 64: 93-100
McGinley
MA, Whitham TG (1985) Central Place foraging by beavers (Castor
canadensis): test of foraging predictions and the impact of selective
feeding on the growth form of cottonwoods (Populus fremontii).
Oecologia 66: 558-562
Müller-Schwarze
D, Schulte BA, Sun L, Müller-Schwarze A, Müller-Schwarze C
(1994) Red maple (Acer rubrum) inhibits feeding by beaver (Castor
canadensis). Journal of Chemical Ecology 20: 2021-2034
Naiman RJ,
Johnston CA, Kelley JC (1988) Alteration of North American
streams by beaver. Bioscience 38: 753-762
Orians GH, Pearson NE (1979) On the theory of central place foraging. Pp.
154-177 in Analysis of Ecological Systems. Horn DJ, Mitchell RD,
Stairs GR (editors). Ohio State University Press, Columbus
Pastor
J, Naiman RJ (1992) Selective foraging and ecosystem processes
in boreal forests. American Naturalist 139: 690-705
Scotter GW, Scotter E (1989)
Beaver, Castor canadensis, mortality caused by felled trees.
Canadian Field-Naturalist 103: 400-401
Schoener TW (1979) Generality
of the size-distance relation in models of optimal foraging.
American Naturalist 114: 903-9141
Wright
JP, Jones CG, Flecker AS (2002) An ecosystem engineer, the
beaver, increases species richness at the landscape scale. Oecologia
132: 96-101
Information
about this Review
The
author is: Dr. Paul D. Haemig (PhD in Animal Ecology)
The
proper citation is:
Haemig
PD 2012
Beaver and trees. ECOLOGY.INFO 19.
If
you are aware of any important scientific publications that were omitted
from this review, or have other suggestions for improving it, please
contact the author at his e-mail address:
director {at} ecology.info
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