The Basis of Browsing Tolerance in Shrubs of the Intermountain West: Growth Rates and Meristematic Potential

Relative growth rates (RGRs) and meristematic potential of big sagebrush (Artemisia tridentata, in some experiments ssp. tridentata Nutt. and in other experiments ssp. vaseyana [Rydb] Beetls), bitterbrush (Purshia tridentata [Pursh] DC), serviceberry (Amelanchier alnifolia Nutt.), birchleaf mountain mahogany (Cercocarpus montanus Raf.), curlleaf mountain mahogany (C. ledifolius Nutt.) and big tooth maple (Acer grandidentatum Nutt.) were quantified under both glasshouse and field conditions. The relationships between the RGRs, meristematic potential and regrowth capacity of the shrub species were then determined. Sagebrush exhibited a significantly higher RGR than the other species at both high and low nutrient levels under glasshouse conditions. Four of the other shrub species had similar RGRs. The shrub species also exhibited different patterns of biomass and nutrient allocation. Sagebrush allocated most of its resources to leaf production, and in contrast, the other species tended to allocate similar amounts of resources to both leaf and root production. In the glasshouse experiment, neither RGRs nor patterns of biomass and nutrient allocation were related to growth form. Under field conditions, curlleaf and birchleaf mountain mahogany exhibited similar RGRs while both serviceberry and maple had similar but significantly lower growth rates. Except for sagebrush, which died following simulated browsing, four other shrub species exactly compensated for lost tissues. The mean RGRs of those shrub species were correlated significantly with compensatory growth capacity. Loss of 90% of buds on previous year\u27s growth did not cause a meristematic constraint to regrowth production. Instead, it stimulated regrowth production. The shrub species we evaluated also exhibited differences in meristematic potential. Curlleaf and birchleaf mountain mahogany had a greater number of buds and long shoots per plant but a lower percentage of long shoots at the basal position on twigs. In contrast, serviceberry and maple had fewer buds and long shoots per plant but a higher percentage of long shoots at the basal position on twigs. The meristematic potential of the shrub species correlated with regrowth capacity