GROWTH OF SAGEBRUSH AND BITTERBRUSH FOLLOWING SIMULATED WINTER BROWSING - MECHANISMS OF TOLERANCE

We investigated the herbivory tolerance characteristics of shrubs in a comparative study of two Great Basin shrub species with different abilities to tolerate browsing. Simulated winter browsing treatments were applied to the less tolerant species, mountain big sagebrush (Artemisia tridentata ssp. vaseyana (Rydb.) Beetle), and to the more tolerant species, bitterbrush (Purshia tridentata (Pursh) D.C.), growing in natural stands. These treatments ranged in severity from terminal bud removal to removal of 100% of current annual growth, and were applied to (1) individual branches on shrubs and (2) to entire shrubs. We measured growth responses on entire treated branches or on typical branches when treatment was to the entire shrub. We recorded the fate (long shoot, short shoot, mortality, and in Artemisia only, flowering stem) of all nodes on each measured branch and the length, node production, and estimated biomass of all new long shoots. We also estimated the proportion of resources removed by each simulated browsing treatment. Purshia compensated for the loss of tissue to a greater extent than Artemisia. In comparison to unclipped branches, Artemisia and Purshia both increased the frequency of new long shoots produced in response to simulated browsing. However, Artemisia tended toward reduced long shoot biomass with increased browsing, while Purshia was either insensitive to the severity of browsing treatment or, in one case, increased long shoot biomass after treatment. The contrasting responses were particularly evident in the most severe treatment, which resulted in death of all Artemisia branches but in biomass production equal to controls in Purshia. Additionally, node production declined in Artemisia but in Purshia was equal to or exceeded node production in control unclipped branches. In this study, neither aboveground resource availability nor bud availability fully explained differences in the abilities of Artemisia and Purshia to tolerate browsing. In addition, the results from this and related studies suggest that inherent relative growth rate was not correlated with herbivory tolerance in these species. Developmental plasticity and the resultant altered resource allocation patterns following winter browsing appear to distinguish Artemisia and Purshia in their ability to tolerate browsing.