Browsing of antelope bitterbrush (Purshia tridentata : Rosaceae) in the south Okanagan valley, British Columbia: Age preferences and seasonal differences

We compared browsing on twigs of small and large antelope bitterbrush (Purs hia tridentata) shrubs among tell sites in the south Okanagan valley, Briti sh Columbia. We tested whether there were any age preferences by browsers a nd determined whether these preferences changed between seasons and mode of browsing. Two different types of browsing were observed: leaf stripping wh ich occurred in the summer and twig clipping which occurred predominantly i n the winter. We calculated age and size relationships showing that shoot v olume and especially stem diameter were good predictors of shrub age. Among the ten sites, clipping removed 0.02 to 15.7% of a shrub's total twig leng th and stripping removed leaves from 0 to 5.2% of total twig length. Observ ations suggested that California bighorn sheep (Ovis canadensis california) stripped antelope bitterbrush leaves in late summer, mule deer (Odocoileus hemionus hemionus) clipped twigs in the winter and cattle clipped twigs in the summer, Browsers preferred to clip twigs on smaller and hence younger antelope bitterbrush shrubs. In contrast, larger and older shrubs were pref erred for leaf stripping. Since twig clipping was more prevalent than leaf stripping in antelope bitterbrush, overall preference for younger shrubs ma y lead to difficulties in seedling establishment in regions where it is hea vily used as winter forage.