Cheatgrass Encroachment on a Ponderosa Pine Forest Ecological Restoration Project in Northern Arizona

Abstract

Land managers frequently thin small-diameter trees and apply prescribed fire to reduce fuel loads and restore ecosystem structure, function, and process in forested areas. There is increasing concern that disturbances associated with these management practices can facilitate non-native plant invasions. Cheatgrass (Bromus tectorum), an annual grass from the Mediterranean, has invaded large areas of the interior West and has become the dominant species in many of these areas. In 2003, a ponderosa pine (Pinus ponderosa) ecological restoration site on Mount Trumbull in the Uinkaret Mountains of northern Arizona experienced a large increase in cheatgrass. Thinning and burning projects had been conducted on this site since 1996. Cheatgrass cover increased 90-fold on the thinned and burned plots between 1996 and 2003. While cheatgrass also increased on thinned plots that were not burned and the untreated control plots, the cover of cheatgrass remained low. There were two additional factors that may have influenced the cheatgrass invasion. In 2002, the region experienced the most extreme drought recorded in the past 100 years. Substantial rainfall returned to the area in September 2002, coincident with the timing of cheatgrass germination. Additionally, cattle were reintroduced to the study area in August 2002 after a four-year hiatus in grazing. We present data suggesting that the interaction of prescribed fire and small-diameter tree thinning, potentially exacerbated by cattle grazing and drought, was the primary cause of the spread of cheatgrass. Furthermore, we offer management recommendations for reducing the risk of non-native plant invasion on ecological restoration projects.