Shrubs Of The Great Basin: A Natural History

Bitterbrush

Bitterbrush Purshia tridentata ALMOST AS apparent a part of the Great Basin as sagebrush is bitterbrush or antelope bitterbrush. By the pragmatically inclined, however,it is revered a great deal more than sagebrush because of its palatability for livestock and wildlife. Even the seed forms a major part of the fare of small animals. James Young and Raymond Evans, of the Agricultural Research Service at the University of Nevada, have pointed out that rodents and ants collect virtu, ally the entire bitterbrush seed crop when it falls to the ground. The natural range of bitterbrush extends from western Montana and Brit, ish Columbia south to the arid portions of California and New Mexico, in, cluding the Rocky Mountains and, of course, the Great Basin. Although good estimates of its present extent in the eleven western states are un, available, bitterbrush undoubtedly still covers tens of millions of acres and remains one of our most ubiquitous shrubs, next to the big sagebrush. Bit, terbrush can be found in an enormous variety of habitats-ranging from arid flats, provided they are not saline or as dry as the shadscale association, to alpine zones well above timberline. In favorable locations, it will get to be nearly 3 meters tall, while above timberline it hugs the ground and on wind, swept slopes may be only 15 centimeters high. Dwight Billings, who published a number of studies on the plant ecology of the Great Basin, considered the bitterbrush to be a codominant, along with rabbitbrush, littleleafhorsebrush, and green ephedra, in what he termed the sagebrush,grass zone. As the name implies, grasses are also an important element in this zone, especially various species of wild rye, wheatgrass, squirreltail, bluegrass, and needlegrass. This zone, which occurs above the shadscale zone, occupies more area in the Great Basin than any other vege, tation zone. Bitterbrush BITTERBRUSH Much of what we know concerning the ecology ofbitterbrush is the result of the studies of Eamor C. Nord during the 1950S, when he was on the staff of the Pacific Southwest Forest and Range Experiment Station. Among other things, Nord attempted to study the shrub's ecological life history and the effects of various environmental factors on such things as its growth, development, longevity, and relationship to other plants. He picked twenty~ nine locations in a variety of habitats in California for analysis. One question which Nord was able to answer involved whether or not the various forms of bitterbrush are genetically determined-that is, are they ecotypes? He grew seedlings from erect, intermediate, and semipros~ trate plants under uniform environmental conditions. Interestingly enough, seedlings from semiprostrate parents developed into erect plants 26 percent of the time, but erect parents produced only erect progeny. From this it ap~ pears that form in the bitterbrush is to a major extent determined by hered~ ity. Nord found that the squat plants above 8,000 feet in the White Moun~ tains averaged 52 years old, were over 2 meters across at the crown, and were less than 30 centimeters high. One plant was 115 years old and about 25 centimeters tall but covered an area of only about .6 square meter, while one which was 57 years old covered 12.5 square meters. Bitterbrush roots penetrate to greater depths than those of sagebrush. Nord found roots 1.3 centimeters in diameter at a depth of 5 meters, in~ dicating that they probably penetrated much deeper. This characteristic would allow bitterbrush to absorb water from deeper zones within the soil, even though adjacent sagebrush plants may show water stress as a result of their shallower root system. Bitterbrush begins to leaf out in the spring about two months sooner in the southern part of its range in California than in the northern part of the state. In nature, plants are not mature enough to produce flowers and seeds until they are about ten years of age. At low elevations and to the south, seeds mature in June, while at the opposite extreme in elevation and lati~ tude they may not mature until August or September. Once mature, the seeds drop from the plant in a very few days. On favorable sites, seed pro~ duction may be as high as 500 pounds per acre. Abundant seed production appears to occur the year after large amounts of precipitation. The seeds are heavy and cannot be carried far by the wind; apparently only rodents and...