Spring-Applied Treatments Offer Another Window of Opportunity for Revegetation of Annual Grass−Invaded Rangelands

Invasive annual grasses have degraded tens of millions of hectares of the sagebrush ecosystem of western North America. Restoration of perennial vegetation in annual grass−invaded rangelands is a management priority to decrease fire risk, increase livestock forage quality, and improve wildlife habitat. Annual grasses are traditionally controlled in the fall with preemergent herbicides, such as imazapic, and treated areas are often seeded with perennial bunchgrasses 1 yr later to avoid nontarget herbicide damage to revegetation species. However, there is a limited window of time in the fall to accomplish annual grass control treatments. Spring-applied control treatments may be another option compared with only fall control treatments, but they have received little attention. We imposed spring-applied annual grass control treatments followed by fall seeding of a perennial bunchgrasses and then measured vegetation response for the next 3 yr in cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae)−invaded communities. Spring treatments that included imazapic application (at a low rate), followed by fall seeding of perennial bunchgrasses, successfully controlled annual grasses and substantially increased perennial bunchgrass cover and density. Spring burning and glyphosate herbicide application, without imazapic, were not successful in promoting substantial increases in perennial bunchgrass cover. Spring burning before imazapic application was the most successful treatment for rehabilitation seeding. By the third yr after seeding, perennial bunchgrass cover was 17% in the spring burn−imazapic treatment, greater than what is generally found in intact Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis)-bunchgrass communities in this region. The results of this study provide strong evidence that spring-applied control treatments including imazapic can be part of successful revegetation efforts, thereby decreasing some of the logistical challenges associated with revegetation of annual grass−invaded sagebrush rangelands.