Browsing of Antelope Bitterbrush (Purshia Tridentata: Rosaceae) in the South Okanagan Valley, British Columbia: Age Preferences and Seasonal Differences

By Krannitz, P. G.; Hicks, S. L. | The American Midland Naturalist, July 2000 | Go to article overview

Browsing of Antelope Bitterbrush (Purshia Tridentata: Rosaceae) in the South Okanagan Valley, British Columbia: Age Preferences and Seasonal Differences


Krannitz, P. G., Hicks, S. L., The American Midland Naturalist


ABSTRACT.-We compared browsing on twigs of small and large antelope bitterbrush (Purshia tridentata) shrubs among ten sites in the south Okanagan valley, British Columbia. We tested whether there were any age preferences by browsers and determined whether these preferences changed between seasons and mode of browsing. Two different types of browsing were observed: leaf stripping which occurred in the summer and twig clipping which occurred predominantly in the winter. We calculated age and size relationships showing that shoot volume 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 length and stripping removed leaves from 0 to 5.2% of total twig length. Observations 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 preferred for leaf stripping. Since twig clipping was more prevalent than leaf stripping in antelope bitterbrush, overall preference for younger shrubs may lead to difficulties in seedling establishment in regions where it is heavily used as winter forage.

INTRODUCTION

Severe browsing of juvenile plants can prevent regeneration of preferred browse species (Huntly, 1991). Establishment of woody plants sometimes occurs in waves of cohorts of seedlings during low browser years (Pastor et al., 1988). For example, younger balsam fir juveniles were completely uprooted by moose in Newfoundland; 2-y old trees were eliminated, but 7-y old juveniles were largely left alone (Bergerud and Manuel, 1968). Similarly, over 80% of seedlings of a tropical rainforest tree sustained some herbivory in the first year (Clark and Clark, 1985). Because of this intense selective pressure, some species have evolved specialized defenses in the juvenile stage against herbivory, including physical defenses (thorns) and chemical defenses (Bryant et al., 1991; Bryant et al., 1992). Antelope bitterbrush (Purshia tyidentata) is an important woody browse species that, instead of defending itself against herbivory, appears to be resilient to intense levels of browsing (Wandera et al., 1992). The branching architecture results in the stimulation of long shoot production when more than 50% of annual production is removed (Bilbrough and Richards, 1993).

Antelope bitterbrush is considered one of the most important browse species on western ranges for a wide variety of wildlife and domestic livestock, except horses (Stubbendieck et aL, 1992). The leaves and younger twigs of antelope bitterbrush provide summer forage for cattle, mule deer (Odocoileus hemionus hemionus) and domestic sheep (Wood et aL, 1995; Griffith and Peek, 1989). The percent dietary contribution of this shrub increases throughout the summer as palatable forbs and other browse species become less available (Kufeld et aL, 1973; Austin et aL, 1984). Mule deer often use antelope bitterbrush for winter forage (Welch and Wagstaff, 1992) because it supplies more carotene, crude protein and phosphorus than dormant grass (Welch et al., 1983). Though resilient, severe browsing can result in reduced growth (Kay, 1995). Decline of antelope bitterbrush populations has been observed throughout its range (Rickard and Sauer, 1982; Updike et aL, 1990). If juvenile antelope bitterbrush are browsed more severely than established shrubs, regeneration might be limited partially because of overuse.

Whether or not younger antelope bitterbrush shrubs are subjected to more browsing than older individuals is unknown. We tested whether there were any preferences for young or old antelope bitterbrush shrubs by ungulates or domestic livestock and determined whether these preferences changed between seasons and mode of browsing. …

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