|Sanders, Ken - UNIVERSITY OF IDAHO|
Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 20, 2002
Publication Date: July 1, 2002
Citation: Ralphs, M.H., Sanders, K.D. 2002. Population cycles of broom snakeweed in the colorado plateau and snake river plains. Journal of Range Management. Interpretive Summary: Broom snakeweed is an aggressive native half shrub that increases following disturbance from grazing, fire and drought. The objective of this study was to monitor its populations in the Colorado Plateau and Snake River Plains and to relate its cycles to precipitation patterns. Snakeweed populations died out in drought and established and increased when winter and spring precipitation was abundant.
Technical 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 two cycles over the 13 year study period, dying out in the drought of 1989-90 and again in 1996. 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 one 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 nearly died out in 2000. Although density of mature plants did not change much, 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.