Antelope Bitterbrush and Mule Deer

By Charlie D. Clements, Rangeland Scientist, USDA-ARS Great Basin Rangelands Research Unit

Vast areas of foothills and mountain slopes are dominated by shrubs of the sunflower family, Asteraceae, especially sagebrush. Woody members of the Rosaceae family were never dominant in the Intermountain vegetation, but fossil records indicate that they were a significant understory in forests and woodlands. From these ancestor populations evolved a few species that play important roles in current temperate desert environments because of their nutritional value to domestic livestock and wildlife, especially antelope bitterbrush, desert bitterbrush and cliffrose. It took a long time for brush species and the browse they produce to be recognized as an important component of rangeland production.

Arthur W. Sampson, one of the founders of scientific range management, was among the first to describe and discuss native range shrubs as a component of the basic forage supply on ranges. Sampson particularly mentioned the importance of antelope bitterbrush as being high in the quality of browse and that this species is sought after by cattle, sheep, goats and numerous wildlife. Antelope bitterbrush, belonging to the genus Purshia, and often referred to as “buckbrush” has been written about more than any other native shrub in North America (Fig. 1). For nearly a century antelope bitterbrush has been synonymous with deer management on ranges where big game animals seek food in winter. The continued decline of antelope bitterbrush productivity and lack of recruitment of seedlings throughout many antelope bitterbrush stands has prompted great concern among many resource managers. Antelope bitterbrush ranges from British Columbia to Montana, south to New Mexico and west to California. The concept of shrubs being an important part of the forage resources of western rangelands received vital stimulation in 1931 when William A. Dayton, early plant ecologist with the U. S. Forest Service, published Important Western Browse Plants, in which Dayton pointed out that despite the alluded bitter taste of antelope bitterbrush, this browse species is one of the most important browse species occurring on western rangelands, and some cases the most single browse species in the locality. Even though antelope bitterbrush is more widely known for its’ nutritional fall and winter forage assets, the shrub is palatable all seasons and is preferred by all classes of domestic large animals, except horse (Fig. 2)

Starting about 1920, bitterbrush and mule deer became synonymous, and for much of the range of bitterbrush species, mule deer are the most important native large herbivore. Several regional historical accounts describe the initial populations of mule deer that Europeans encountered, the decline of these herds following settlement and excessive harvesting through hunting, the regrowth of populations as the herds exceeded the carrying capacity of their habitats. In reviewing these historical accounts it is important to note how sparse the initial populations of mule deer apparently were in many areas, how rapidly the populations increased, and how huge the populations became before they crashed. It is also surprising to discover how little was known about the diet of mule deer. In fact, in 1928 J. S. Dixon published “What do deer eat?”, Dixon was a wildlife biologist in northeastern California. Shortly thereafter, following increases in mule deer populations, following hunting regulations and predator management, significant utilization of antelope bitterbrush and mountain mahogany were reported in those areas.

An example of mule deer response to improved hunting regulations and predator control was noted in the Blue Mountains of northeastern Oregon. Wildlife researcher E. P. Cliff reported that in the early 1920s the mule deer herd of the Desolation Ranger District was estimated at 3,100 animals, by 1931 Cliff estimated that the mule deer population had increased to 19,500. Cliff attributed this annual increase of 23% in mule deer population to be a result of protection from hunting and reduced predator populations as mountain lion and coyote populations were severely hunted and trapped at this time to reduce predation on domestic sheep. Antelope bitterbrush made up and estimated 30% of the browsing habitat, and following the mule deer population explosion, the carrying capacity had been met by 1929 and significant utilization on antelope bitterbrush stands occurred with no antelope bitterbrush seedling recruitment being observed. During the hard winter of 1931-1932, considerable mortality of mule deer occurred despite managers providing hay to isolated populations. Based on a sample of carcasses counted in April 1932, as many as 10,000 mule deer died during the winter.

Wildlife biologist, C. M. Aldous reported that before 1929, mule deer were not considered abundant anywhere in the Intermountain Area, but afterwards their populations exploded. Aldous noted that the Kaibab herd in northern Arizona, the Beaver Mountain and Middle Fork herds of the Salmon River area in Idaho were examples of such mule deer population explosions. Aldous also reported that buck-only hunting laws, establishment of refuges, reduction of predator populations by Government agencies, and reduced livestock grazing on National Forests were the main reasons for these significant mule deer population increases. Aldous pointed out that the excessive grazing of rangelands had resulted in an increase in browse species, especially antelope bitterbrush. Later, Aldous conducted a study of mule deer herds in White Pine County of eastern Nevada, in which he had witnessed high winter mortality of mule deer in the 1940s.

As part of his research, Aldous conducted utilization studies on key winter browse species and determined the utilization of antelope bitterbrush average 51% of the current annual growth, while the closely related shrub cliffrose averaged 38% utilization. This study was one of the first to report crude protein, crude fat, and fiber content of antelope bitterbrush. Following this research, Aldous reported that there were more deer than the habitat could support and suggested an increase in buck harvest and a very regulated harvest of does was necessary as to not cause harm to the habitat.

In eastern Oregon, wildlife researcher, O. T. Edwards reported similar results of mule deer population increases and winter mortality on the Murders Creek ranges of the Malheur National Forest. Edwards reported that antelope bitterbrush comprised roughly 33% of the mule deer winter diet, and that antelope bitterbrush was being severely damaged due to excessive utilization (Fig. 3). To further investigate range utilization questions, mule deer exclosures were constructed which also excluded cattle and sheep. It was not immediate, but after 4 years antelope bitterbrush shrubs inside the exclosures were showing good signs of vigor as they flowered and produced seed. Antelope bitterbrush outside of the deer exclosures did not flower and produce seed.

Renown wildlife ecologist, A. Starker Leopold presented another reason for the growth and crash of mule deer populations. Leopold voiced his opinion that the mule deer population increases were directly related to human activity. Leopold related how 19th century mountain men like John Work and his party of Hudson’s Bay Company trappers depended on eating their own horses for food while crossing the Pitt River Valley of northeastern California in 1832 because they could not find game to kill. Yet this same area became famous in the mid-20th century for its huge mule deer populations and stands of antelope bitterbrush. Leopold concluded that most mule deer ranges were created when pristine natural resources were logged, burned or grazed and that mule deer habitat was created at an exorbitant cost in natural resources.

Mule deer production in the first half of the 20th century was at the expense of nutrients accumulated for centuries in soils, old growth forests and rangelands. The key to maximum densities of mule deer was disturbance that resulted in the dominance of woody secondary succession species such as antelope bitterbrush. Not only did the shrubs increase in density on sites formerly dominated by trees, he also insisted that these shrubs became dominant on sites formerly dominated by perennial bunchgrasses, “On the east slope of the Sierra… such forage species as sagebrush and bitterbrush have invaded foothills formerly stocked with bunchgrasses.”

Leopold concluded that similar conversions had occurred in much of the Great Basin. Leopold also cautioned that fire and grazing were a two-edged sword in regards to mule deer habitat and that even though these disturbances had created suitable mule deer habitat, but that these same disturbances also had the potential to destroy habitat they were instrumental in creating. stands to reduce over-utization, improve recruitment of new plants and restoration of stands will be essential in improving mule deer populations throughout the Intermountain west. managing

In 1952, renown pioneer wildlife ecologists, W. M. Longhurst, A. S. Leopold and R. E. Dasmann published their findings mule deer management titled, “A survey of California deer herds: Their ranges and Management Problems”. They reported in the section concerning Great Basin mule deer ranges that antelope bitterbrush was unquestionably the most important browse species on mule deer habitats. In contrast to the rest of California, where deer ranges were largely created by fire and logging, mule deer habitats in the Great Basin were created by excessive livestock grazing. This excessive grazing induced browse species that made the great expansion of mule deer populations possible. Logging and wildfire in the Sierra Nevada had increased summer habitat for mule deer, but excessive grazing of sagebrush/bunchgrass rangelands in the trans-Sierra foothills had created essential winter ranges for mule deer.

The authors noted, “…continued over-grazing of the invading browse plants by both livestock and mule deer has tended to reduce the density of more desirable species, like antelope bitterbrush, permitting a disproportionate increase in less palatable shrubs like big sagebrush and rabbitbrush. Thus, many if not most browse ranges in the Great Basin region carry fewer mule deer than they might with more conservative use. Nevertheless, it is well to remember that livestock played a part in creating these ranges, even though too many livestock, especially in combination with too many mule deer, subsequently may destroy the best elements of the brush stands”.

It should also be noted that these authors also considered the serious threat of invasive annual weeds, especially cheatgrass which significantly increases the chance of ignition and the rate and spread of wildfire, to mule deer habitats.

The significant loss of antelope bitterbrush on many mule deer ranges due wildfires is well noted. This in combination with aging antelope bitterbrush stands and lack of recruitment of new seedlings to sustain the antelope bitterbrush population continues to be of great concern as mule deer populations struggle to have adequate nutrition on transitional and winter ranges throughout the Great Basin.

Active restoration of antelope bitterbrush plants will need to be conducted to improve stand age, stand vigor and future recruitment of antelope bitterbrush (Fig. 4) needed to sustain antelope bitterbrush stands, improve much needed nutrition provided by this species and ultimately provide critical habitat for mule deer and other wildlife species dependent on the nutritional value of rangelands (Fig. 5).