Plant community context controls short- versus medium-term effects of pre-emergent herbicides on target and non-target species after fire

Questions Selective herbicide application is a common restoration strategy to control exotic invaders that interfere with native plant recovery after wildfire. Whether spraying with pre-emergent or bio-herbicides releases native plants from competition with exotics ("spray-and-release" strategy) and makes communities resistant to reinvasion by exotic annual grasses (e.g. cheatgrass, medusahead), without risks to non-target native plants or secondary invasion, is a major question for land managers of semiarid plant communities. Location Sagebrush steppe of southwest Idaho, USA. Methods We applied chemical herbicides (imazapic, rimsulfuron) and weed-suppressive bacteria (Pseudomonas fluorescens strains MB906 and D7) to three sagebrush steppe communities after fire. We measured plant cover prior to burning and for four years (five for exotic annual grasses) post treatment. Results Both chemical herbicides significantly reduced exotic annual grass cover in all communities in the first post-spraying year, but rimsulfuron plots were re-invaded after 1-2 years, while imazapic plots continued to resist reinvasion four and even five years post spraying, well after the herbicide should have degraded. We did not detect any increase in native perennial grass cover with either herbicide, and herbicides had both positive and negative effects on individual bunchgrass basal diameter, depending on species and plant community. Rimsulfuron was more damaging than imazapic to shallow-rooted perennial bunchgrasses. Moss and lichen cover, key components of soil integrity, increased with chemical herbicide treatments in some communities. Both herbicides increased secondary invaders (exotic forbs or grasses), which varied by plant community and herbicide. Weed-suppressive bacteria treatments had no significant effects on cover of any functional group. Conclusions While short-term effects of chemical herbicides that target exotic annual grasses were relatively consistent and predictable, longer-term effects were specific to the herbicide and plant community. The "spray-and-release" strategy may confer resistance to re-invasion by exotic annual grasses if herbicides prevent re-invasion for an extended period.