Moose and Cherry Trees

Moose browse lab: Is moose browse behavior affected by wild cherry trees?


In the winter moose eat woody browse from a variety of trees and shrubs, primarily willow species, aspen, cottonwood and birch, while some species such as alder are rarely used. In Anchorage and other parts of Alaska introduced European bird cherry, Prunus padus, trees are invading moose habitat presenting moose with a potential food source or competitor with commonly used food sources.

Most of the woody plants occupying moose habitat in Alaska produce secondary chemical compounds that often function to deter moose and other herbivores from browsing the plant to death.  For example, willows contain salicin (used to develop aspirin) and cherry trees contain prunasin a cyanogenic glycoside that is converted to cyanide when eaten. Some plants store the secondary chemical compounds in higher concentrations in the shoot tips and buds and respond to damage by increasing chemical concentrations. Plants can store these chemical compounds at higher concentrations when the plant is young and most vulnerable. In some instances the secondary chemicals can be deadly. The way a plant uses secondary chemical compounds varies between species.  Moose often cope with these chemicals by varying their diets to not over consume any one particular chemical.

Three dead moose were found in Anchorage winter of 2010-2011 after consuming bird cherry trees (Woodford 2011). When an animal consumes cyanogenic plants such as cherry trees the chewing and digestive actions break the cells releasing the cyanogenic glycoside (Francisco 2000). Enzymes come in contact with the released cyanogenic glycoside and facilitate the process of breaking the chemical down and releasing hydrogen cyanide (HCN) a toxic gas (Francisco 2000). The HCN is transferred to blood cells and is redistributed throughout the body where it has a primary toxic effect on the central nervous system (CNS) (Dirikolu 2003). The redistribution effectively prevents much of the HCN from coming in contact with the CNS, and allows large animals such as moose and horses to consume small amounts of toxic HCN without death or clinical signs of poisoning (Dirikolu 2003). However, when high enough HCN concentrations are reached in an animal clinical signs such as erratic behavior, gasping for air, staggering and in some instances death will occur.

The cherry trees responsible for the recorded deaths of the moose were all planted in private landscapes (Woodford 2011). These cherry trees are invading natural forests throughout Anchorage, and in some instances appear to be the dominant vegetation (Flagstad 2010). Research on invasive cherry trees affect to salmon habitat has shown that they do not support comparable quantity or diversity of insects as other tree species (Roon 2011). Fewer insects on the tree may be from the cyanogens contained in the tree preventing insects from eating the leaves.

During the winter of 2012 Anchorage area school students quantified moose browse in plots located in nearby forests.  They found that moose indeed vary their diet within plots.  Moose seemed to prefer in order, willows, cottonwood, birch, cherry and alder (see 2012 summary). Although the only significant differences between measurements taken for each species were between willow and cherry and willow and alder.  From this information we know that where cherry trees are present moose do eat them, although they do not seem to prefer them.  What we still do not know is how does the concentration of cherry trees in a plot affect the use of browse species by moose.

Moose eat a variety of browse species in the winter, and all contain some toxic chemicals. Scientists believe that by eating a variety of species, moose are able to gain the nutrition they require while not building up high concentrations of toxic chemicals from a single browse species (Risenhoover 1989). Since the toxins act alone and not together this allows moose to avoid side effects of the toxins in available food (e.g. imagine if you could only eat jalapeno, basil, coco and coffee beans). The proposed study will explore how moose are browsing cherry and how cherry presence is affects moose browse behavior. 

Winter Forage Ecology of Moose

In Alaska and other places with substantial moose populations winter forage ecology is of interest because it helps understand natural succession of the area, and determine if the moose population is sustainable on the range.  Ecologists often focus on winter foraging ecology of moose because during the winter moose only consume woody vegetation.  While dormant in winter, woody vegetation remains available to moose. 

Moose primarily only eat the branch tips where the current annual growth (CAG) is present, and sometimes break tree leaders to reach CAG that is just out of reach.  CAG is identified on woody vegetation by following the branch tip to a point of color change and swelling.  The swelling point is where the branch began growth the previous season.  Moose gain the most nutritional benefit from branches if they bite at points no more than about 5 mm in diameter because beyond 5 mm the volume of indigestible material becomes greater than the volume of digestible material.  On occasion moose will browse beyond 5 mm in diameter and even beyond the CAG.  On some occasions they will strip bark from trees, however, moose do not gain much nutritionally from this.  Bark stripping is a sign of stress due to lack of quality browse available.

What measurements are used and what each measurement means

Forage ecologists can determine how much browse is available on the tree by counting the CAG.  The proportion removed is taken from the number of bites (bites/available = proportion removed).  Including the diameters of CAG and diameters of bites in these counts allows estimation of mass available and consumed.  Estimating mass is done by assuming the twigs are nearly conical and applying a coefficient of variation in mass to the calculation of volume. 

Ecologists consider browse intensity on a tree, often referred to as architecture, as another measure of moose behavior.  Moose ecologists typically categorize browse intensity on a tree as broomed, browsed and unbrowsed.  It is important to note that unlike CAG bite counting, browse intensity accounts for browsing from past and present seasons.  A broomed tree often resembles an upside down broom because of broken leaders (main stem of tree), and greater than 50% of available browse is eaten.  Broomed trees are often an indication of intense browsing for multiple years.   Browsed trees have bite marks on the branch tips, however, the structure of the tree seems unaffected and less than 50% of available browse is removed from the tree.  An unbrowsed tree has no evidence of past season or present season bite marks on the tree.  These data can help determine the history of browse behavior in a given area.

Bark stripping is not a nutritionally advantageous behavior for moose, and is usually a sign of nutritional stress due to a lack of available quality browse.  Often bark stripping is measured as presence absence.  In Anchorage severe bark stripping by moose is sometimes observed, primarily on ornamental trees.  Besides stress bark stripping might indicate a moose desire to eat that particular tree species more than others or lack of movement from the area for prolonged periods.

Browse species composition and moose browse behavior

Ecologists study the effect of moose primary forage species quantity and diversity on selectivity and browse intensity of moose using bite counts, bit diameter, and browse intensity.  Often studies focus on the relation of moose behavior to the density of common preferred browse species.  Some of these studies find that moose tend to intensely browse a preferred species when it is in lower concentration (Mansson 2007, Edenious 2002).   However, moose consumption at sites is generally higher with increased availability of desirable forage.   Moose browsing behavior with increasing intensity on a ramet when densities are low is often attributed to an effort by some large ungulates to increase diversity in their diet. Increasing diversity in a diet prevents over accumulation of toxins that are present in a specific tree species.

The proposed study seeks to determine how the concentration of bird cherry affects moose forage behavior.  Previous work has established that moose eat cherry trees in Anchorage forests although with less intensity than cottonwood and willow species.  We are now interested in testing three hypotheses about the effect availability of cherry relative to preferred browse changes moose browsing behavior:

1)     High concentrations of cherry trees relative to other species causes increased intensity of browse on preferred species because moose are seeking out preferred species especially when they are at low concentrations. 

2)     At low cherry tree concentrations moose do not browse cherry trees more intensely as they would a preferred browse species.

3)     Cherry tree concentration relative to other species is negatively related to overall consumption at a site.  In other words moose eat less in areas with more cherry trees.




Browse- (verb) the act of eating a plant.

Woody Browse- (noun) Species of trees or shrubs that moose will eat.

Herbivore- (noun) Animal that eats only plant material.

Shoot- (noun) the part of a plant that is actively growing

Bud- (noun) part of plant that will flower during the growing season.

Enzyme- (noun) Protein that binds to specific chemicals to facilitate a reaction resulting in the chemical breaking down into different compounds.

Cyanogenic- (noun) Compound that releases hydrogen cyanide, a poisonous gas, when broken down.

Glycoside- (noun) Compound that plants often use to store toxins in a manner that keeps the toxin in a harmless state.

Ramet – (noun)  An individual within a clonal patch of plants.

Works Cited:

Dirikolu, Levent, C. Hughes, D. Harkins, J. Boyles, J. Bosken, F. Lehner, A. Troppman, K. McDowell, and T. Tobin. 2003. The Toxicokinetics of Cyanide and Mandelonitrile in the Horse and Their Relevance to the Mare Reproductive Loss Syndrome. Toxicology Mechanisms and Methods. 13: 199-211.

Edenius, Lars, G. Ericsson and P. Naslund.  2002.  Selectivity by moose vs. the spatial distribution of aspen: a natural experiment.  Ecography. 25(3): 289-294. 

Flagstad, L., H. Cortés-Burns, and T.L. Roberts. 2010a. Invasive plant inventory and Bird Cherry control trials. Phase II: Bird Cherry distribution, demography and reproduction biology along the Chester and Campbell Creek trails, Anchorage, Alaska. Prepared for The Municipality of Anchorage and The Anchorage Parks Foundation. Alaska Natural Heritage Program, University of Alaska Anchorage, Anchorage, AK. 61 pp. Downloaded 2/16/2012.


Francisco, Ilza A. and M. H. Pimenta-Pinotti. 2000. Cyanogenic Glycosides in Plants. Brazilian Archives of Biology and Technology. 43(5): 487-492.

Mansson, Johan, H. Andren, A. Pehrson and R. Bergstrom. 2007.  Moose browsing and forage availability: A scale-dependent relationship?  Canadian Journal of Zoology.  85: 372-380. 

Roon, David A. 2011. “Ecological effects of invasive European bird cherry (Prunus padus) on salmonid food webs in Anchorage, Alaska streams.” M.Sc. Thesis, University of Alaska Fairbanks. Downloaded 2/16/2012.


Risenhoover, Kenneth L. 1989. Composition and Quality of Moose winter Diets in Interior Alaska. Journal of Wildlife Management. 53(3): 568-577.


Woodford, Riley, and C. Harms. 2011. Cyanide-poisoned Moose Ornamental Chokecherry Tree a Devil in Disguise. Alaska Fish and Wildlife News. March 2011. Downloaded 1/27/12.


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