Age-stem diameter relationships of big sagebrush and their management implications

Abstract

Decisions to control big sagebrush (Artemisia tridentata Nutt.) on North American rangelands are traditionally based on morphological characteristics (e.g., cover) rather than more ecologically based community successional criteria. Big sagebrush standage is a critical component for evaluating successional status, but has been difficult to obtain under field conditions. Assessing big sagebrush plant age based on stem diameter would provide resource managers with an efficient field tool to make management decisions based on ecological principles. For each of 3 sub-species of big sagebrush, between 75–80 stem cross-sections were collected within each of 9 stands situated at 3 regionally dispersed locations across Wyoming. Maximum basal stem diameters were measured and plant age determined from annual growth-ring assessments. Regression analysis (95% confidence interval) relating maximum basal stem diameter to plant age produced coefficients of determination (r2) of 0.70, 0.64, and 0.61 for Wyoming big sagebrush (A. tridentata ssp. wyomingensis Beetle and Young) in each of 3 regional locations; 0.53, 0.69, and 0.64 for mountain big sagebrush (A. tridentata ssp. vaseyana [Rydb.] Beetle); and 0.50, 0.62, and 0.44 for basin big sagebrush (A. tridentata ssp. tridentata). Combined regional data for each subspecies produced r2 values of 0.54 for Wyoming big sagebrush, 0.52 for mountain big sagebrush, and 0.50 for basin big sagebrush. Results indicate that maximum basal stem diameter can be used to assess the age of big sagebrush subspecies, thus, providing land managers with an ecologically based alternative method for justifying big sagebrush management decisions.