FORAGING ECOLOGY OF LIVESTOCK ON THE TIBETAN CHANGTANG - A COMPARISON OF 3 ADJACENT GRAZING AREAS

In a high-altitude (above 4860 m), semiarid grazing ecosystem in western Tibet, only 28% of summer grazing areas supported aboveground net primary productivity greater than 10 g m-2. The frequencies of vegetation types selected for grazing were unique for each herd type (yak, small ruminant) in each area. Herd type and grazing area were both significant factors in daily horizontal and vertical displacement of herds from encampments. Yak were versatile foragers, consuming coarse forages when encountered, but adept at grazing small (< 3 cm high) sedges. Whereas browse was a minor component of yak diets (4.1%), it was heavily selected by goats (31.2%) and sheep (17.9%). All ruminant livestock readily consumed spinulose forbs when encountered. Statistical differences in dietary quality were attributed to grazing area effects. The overriding determinant of grazing behavior appeared to be the vegetational characteristics of the area within which livestock producers held grazing rights. Differentiating factors in these areas were (1) the availability of "low risk" vegetation (forage demonstrating predictable emergence and growth) in riparian areas watered by glacier-fed springs, versus more unpredictable nonriparian forage sources fed by monsoonal rainfall; and (2) the topographic situation of forage-yielding vegetation types.