The effect of mechanical canopy reduction on big sagebrush plant communities

Abstract

A major conservation challenge in the western United States is implementing management treatments that reduce fire risk, control for invasive species, and maintain herbaceous understories in big sagebrush ecosystems. Studies have found that mechanical treatment of big sagebrush can reduce fire risk and promote herbaceous understories, but a consensus on the long-term impacts of big sagebrush reduction remains unclear. We used a time series (20 years) of treated sites to understand the short- and long-term response of herbaceous plants and shrubs to mastication treatment in big sagebrush plant communities of south-central Colorado. We found that mastication to a height of 15 cm significantly reduced big sagebrush cover and increased perennial grass cover in the short term. The significant increase in perennial grass cover on recently treated (1–2 years) sites was largely attributed to C₃ rather than C₄ perennial bunchgrasses. Recently treated sites had greater annual plant cover and density than untreated sites. However, on sites treated more than 2 years ago, there was no significant difference between perennial grass and annual plant cover or density. Perennial forb cover and density was not affected by treatments. Initially reduced by nearly 80%, big sagebrush cover returned at a rapid and constant rate over time and returned to a statistically indistinguishable cover from the untreated sites within 8–10 years while height recovered slowly. Our results underscore the resilience of big sagebrush to partial canopy removal and emphasize the long-term dynamics following treatment.