Purshia: The Wild And Bitter Roses

9 Insects and Plant Diseases

Ruminant Nutrition 149 is necessary to understand these terms and their abbreviations in order to interpret discussions of the subject. Total digestible nutrients, or tdn, is the sum of all the digestible organic compounds (proteins, sugar, cellulose, etc.), with the digestible crude fat component being multiplied by the heat factor 2.25.20 The tdn requirement of an animal is expressed as kilograms per animal per day or as a percentage of the diet. Digestible energy (de) is calculated by subtracting the gross calories in the feed from the calories in the feces. The de requirement of an animal is expressed as megacalories (c) per animal per day or as megacalories per kilogram of dry matter. The energy needs of mule deer vary according to the animal’s weight and activity . Larger animals require more tdn per day for a given activity than do smaller animals. A lactating female requires more tdn per day than a nonlactating female of similar weight. On a constant weight basis, lactation is followed in descending order of energy required by fattening, growth, gestation, and maintenance. In contrast to the generally held opinion that antelope bitterbrush is an excellent or even essential winter forage for mule deer, Bruce Welch suggested that it is more important in late fall. For evidence that deer do not prefer antelope bitterbrush as a midwinter browse he cited Leach’s work in northeastern California, Paul Tueller’s in Nevada, and the unpublished work conducted by D. E. Medin in Colorado.21 Welch and Andrus reported heavy winter use of big sagebrush and wild rose (Rosa spp.) and limited use of antelope bitterbrush.22 Leach considered the switch from antelope bitterbrush to sagebrush to be independent of the severity of the winter. We have discussed Leach’s study with field biologists who were working with the Lassen-Washoe mule deer herd at the time and have something to add to that statement. There were severe winter deaths in these mule deer herds for several winters during the late 1940s and 1950s, and the stomachs of winter killed deer often contained nothing but big sagebrush herbage. Based on this knowledge, generations of mule deer managers have accepted the notion that the animals eat big sagebrush when they are starving and then die. At the time Leach conducted his study, however, most of the antelope bitterbrush was severely overutilized and simply was not available to the deer, which had no choice but to switch to sagebrush. Tueller likewise reported that the switch from antelope bitterbrush to sagebrush on Nevada ranges was usually attributed to overutilization of the antelope bitterbrush. Leach, who is now retired, remains convinced that mule deer use antelope bitterbrush in the fall and again in the spring when the new leaves appear, but during midwinter voluntarily switch to sagebrush. He bases his conclusions on analyses of more than 3,000 deer stomachs during his career. When he originally developed this opinion, he was sharply criticized by W. P. Dassman and August Hormay, both strong proponents of the concept of key species in deer 150 Purshia management.23 For the mule deer herds of northeastern California, they believed the key species to be antelope bitterbrush. Leach describes this initial criticism as being on a professional level only, but when he reported that in mild winters a significant portion of the mule deer’s diet was foliage of the alien cheatgrass, it was more than his fellow professionals could stomach. Galen Burrell, who studied the Entiat mule deer herd in eastern Washington, reported the all too familiar story of reduction in antelope bitterbrush habitat due to wildfires, decadence, and lack of regeneration.24 Burrell’s study site was an antelope bitterbrush/bluebunch wheatgrass area adjacent to ponderosa pine/antelope bitterbrush woodlands. He based his conclusions regarding the winter diet of the mule deer on microscopic analysis of fecal samples.25 The bulk (87–93 percent, depending on site and year) of the winter diet was made up of antelope bitterbrush, buckwheat (Eriogonum spp.), arrowleaf balsamroot (Balsamorhiza sagittata), and lupine (Lupinus spp.). Burrell also compared the composition of the diet in three areas of critical winter habitat where the density of antelope bitterbrush varied. On one site the antelope bitterbrush had been killed by wildfires. On a second site the stand had been partially burned, and the third site was unburned. The first year of the study, antelope bitterbrush (when available) constituted 70 percent of the deer’s diet until...