Cheatgrass (Bromus tectorum) is an exotic annual grass that has the potential to drastically disrupt multiple ecosystem services and processes and is considered an ecosystem “transformer.” While cheatgrass invasion is often noted in sagebrush ecosystems, cheatgrass can invade and be problematic in lower elevation ponderosa pine forests of the western US. In 2015, we measured forest understory vegetation response as part of a long-term study to understand both frequency (5-yr and 15-yr) and season of burning (spring, fall or no burning), and explore the interaction of cattle grazing and 5-yr seasonal reburning. We address three questions: 1) What is the effect of burn season and frequency on cheatgrass cover? 2) Does the exclusion of cattle alter cheatgrass response to 5-yr seasonal burning? 3) How is the extent of cheatgrass facilitated or constrained by disturbance regime, other abiotic factors, biotic resistance, and propagule pressure? Our first two questions we tackled using univariate statistics, and for our third question we developed regression trees to explore 2002 variables that might be important for explaining cheatgrass cover in 2015.
In 2015, after four 5-yr and two 15-yr interval burns, cheatgrass cover increased significantly in the two fall burn treatments compared to the control, regardless of burn frequency. Indeed, the first fall burn in 1997 appears to have “set the stage” for the patterns observed. This result is consistent with other findings that cheatgrass cover increases with time since fire. However, the greatest resource fluctuations would be associated with the most frequent reburning, yet 5-yr interval reburning (four burns) did not increase cheatgrass cover compared to 15-yr interval reburning (two burns) in the fall. Cheatgrass cover also increased in the 5-yr spring burn treatment. Interestingly, 13 years of grazing exclusion did not significantly alter cheatgrass cover compared to grazed areas. Regression tree results suggest that higher propagule pressure (frequency) on plots in 2002 facilitated higher cheatgrass cover in 2015, but biotic resistance in the form of native perennial forb cover, constrained extent but only when initial cheatgrass frequency was low. While cheatgrass cover in 2002 was limited (<1.2%), small source populations can have dramatic effects over time. Thirteen years later cheatgrass cover averaged 7% but ranged up to 82% in some areas. Our findings regarding repeat burning, grazing and cheatgrass invasion are useful to consider for future restoration and conservation efforts in burned ponderosa pine forests.