Establishment tolerance of six native sagebrush steppe species to imazapic (PLATEAU®) herbicide implications for restoration and recovery Public Deposited


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  • Scientists and land managers realize that integrated weed management (IWM) strategies are needed to attain successful and lasting improvements of weed infested landscapes. At this time no broadly reliable and environmentally safe IWM strategy has been developed to control exotic annual grasses that dominate many ecosystems of the northern Great Basin. This study determined the efficacy of several nascent control strategies at a site near Mountain Home, ID, USA with particular emphasis on the tolerance of native species to chemical control techniques applied before their emergence. In autumn 2002, prescribed burning and a single preemergent application of imazapic (PLATEAU®) herbicide were used separately and combined to control medusahead wildrye (Taeniatherum caput-medusae (L.) Nevski) and cheatgrass (Bromus tectorum L.). Seeds of six native perennial species, selected for their range of life histories and ability to provide effective competition, were planted as monocultures two-weeks after fire and herbicide applications. A monthly census determined native seedling emergence and survival from late winter through autumn 2003. In addition, end-of-season population size and reproduction were determined for both exotic annual grasses in each treatment. We hypothesized that application of imazapic would reduce and delay emergence, cause earlier mortality rates, and lower overall persistence of seeded natives because of adverse impacts to early seedling development throughout the growing season. Burning and imazapic applications combined would amplify these effects with reduced plant residue cover and increased surface evaporation. Prescribed treatments reduced densities of mature exotic annual grasses by 31.1% for burning alone, 79.1% for imazapic alone, and 92.1% for areas with burning and imazapic combined when compared to untreated controls. Few seedlings of globemallow (Sphaeralcea grossularifolia (Hook & Arn.) Rydb.) emerged from any treatment due to extreme dormancy and/or poor site adaptation. Significant responses of the five remaining native species fell into three general patterns associated with three functional/structural plant groups. Deeper-rooted perennials, big squirreltail (Elymus multisetus M.E. Jones) and Snake River wheatgrass (Elymus wawawaiensis J. Carlson & Barkworth), showed positive responses to imazapic applications. For E. multisetus, more seedlings emerged in areas treated with imazapic alone than in any other treatment (P<0.01). Rather than impacts from imazapic application, E. multisetus seedlings emerged earlier in unburned versus burned areas (P<0.01) likely due to greater moisture retention and moderated temperature extremes from the presence of surface litter. For native dicots, Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle and Young) and western yarrow (Achillea millefolium L. var. occidentalis DC.), overall emergence was reduced by an average of 20% in burned relative to unburned plots likely because of variable surface temperatures, frost heaving of the upper soil profile, and more rapid evaporation of available moisture early in the growing season. Emergence was 3.2 and 2.2 times sooner for A. tridentata (P=0.02) and A. millefolium (P=0.02) on unburned relative to burned treatments for reasons similar to those of deeper-rooted perennials. A. millefolium seedlings experienced particularly slow emergence in plots burned and treated with imazapic. Prescribed burn plots had seedlings emerge 2.1 times (P=0.04) sooner than burning with imazapic. This implies that imazapic, as well as burning, may be slowing seedling development of this species. Burned plots also exhibited seedling mortality in nine-tenths the time than unburned plots for A. tridentata and A. millefolium (P=0.03 and P=0.05). The shallow-rooted perennial, Sandberg bluegrass (Poa secunda J. Presl.), was the only seeded species to carry on a population into the fall of 2003. Untreated controls had 3.3 times more plants per m² than plots applied with imazapic alone (P=0.03) implying a degree of imazapic intolerance for this species. Although this research indicates that some native arid species are tolerant to imazapic, experiments should continue to incorporate fall preemergent applications of this herbicide to improve our understanding of native species responses and foster the development of an effective IWM strategy for arid rangelands of the Great Basin.
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