Haemig PD  (2012)  Beaver and trees.  ECOLOGY.INFO 19

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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