Population cycles of broom snakeweed in the Colorado Plateau and Snake River Plains

Abstract

Broom snakeweed (Gutierrezia sarothrae (Pursh) Britt. Rusby) is one of the most widespread range weeds in North America. The objective of this study was to monitor broom snakeweed populations in the salt-desert shrub community of the Colorado Plateau and in crested wheatgrass (Agropyron desertorum (Link) Schultes) seedings typical of the Snake River Plains and Great Basin, and determine if its population cycles are related to precipitation patterns. Foliar cover of broom snakeweed and associated plant species was measured along 7.6 or 33 m transects by the line intercept technique. Density of snakeweed age classes (seedling, juvenile, mature) was counted in 1 m2 quadrats at the beginning and end of each transect. Correlations were made between snakeweed cover and density, and seasonal precipitation. The snakeweed population at the Colorado Plateau site completed 2 cycles over the 13 year study period, dying out in the drought of 1989-90 and again in 2001. There were positive correlations between density of snakeweed classes and seasonal precipitation: seedlings with spring precipitation (r = 0.63); juveniles with winter precipitation (r = 0.69); and mature plants with previous fall precipitation (r = 0.62). Only 1 cycle occurred at the Snake River Plains site. Following the snakeweed invasion into crested wheatgrass seedings in the mid 1980's, the population was at the top of its population cycle when the study began in 1990, dropped back and fluctuated between 6-10% cover from 1992 to 1999, and died out in 2001. Although density of mature plants did not change much during the middle part of the study, the change in snakeweed cover was correlated with spring (r = 0.81) and total precipitation (r = 0.60), reflecting increase and decrease in size of plants in response to precipitation.