Browsing of antelope bitterbrush (Purshia tridentata : Rosaceae) in the south Okanagan valley, British Columbia: Age preferences and seasonal differences
Pg. Krannitz et Sl. Hicks, Browsing of antelope bitterbrush (Purshia tridentata : Rosaceae) in the south Okanagan valley, British Columbia: Age preferences and seasonal differences, AM MIDL NAT, 144(1), 2000, pp. 109-122
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.