Spotted Knapweed Eradication Project, Spruce Gulch, Colorado 2010 Progress report

Transcription

1 Spotted Knapweed Eradication Project, Spruce Gulch, Colorado 2010 Progress report T.R. Seastedt, PhD. and D.K. Knochel, Ph.D. University of Colorado, Boulder The difficult we do immediately; the impossible takes us a bit longer. ( Charles Alexandre de Calonne ( )) Prefatory note A January 2009 meeting of representatives of the Colorado Weed Advisory Board, Boulder County weed manager, US Forest Service (USFS) personnel, and research scientists from CU discussed options for eradication of spotted knapweed (Centaurea stoebe) found on Federal and private lands in Spruce Gulch in Boulder County, Colorado. The advisory board passed a motion requesting that management actions allow no flowering of this species at this site in This report details 2010 results and makes recommendations for continuation of the eradication effort. Executive Summary In 2010 over 700 person-hours of mechanical removal greatly reduced densities of spotted knapweed on both USFS and private land. An herbicide (Milestone) was used on upland areas managed by the USFS. Some flowering in the basin did occur. However, seed production of those flowers was reduced approximately 95% by biological control agents, and essentially no viable knapweed seed is present in the soils of the area. Extensive data and experimental studies of this weed in Spruce Gulch have led to major scientific findings and publication in peer reviewed journals. In particular, these studies estimate 1) seed production, 2) the size and viability of the existing seed bank, and 3) population dynamics given measurements of reproduction and plant survivorship. Monitoring results indicate an 80% decline in plant densities since 2007 on an area receiving only biological control treatments. These studies and data demonstrate that the invasive threat of this particular population of knapweed is modest, and that the 2010 eradication effort, if continued at its present intensity for at least several years, will be successful. However, continued monitoring and removal of spotted knapweed plants will be required well into the future. The mandate for the management action for spotted knapweed in Boulder County is prescribed in the Colorado statutes: Designated noxious weeds - legislative declaration.(2.5) The commissioner shall prescribe integrated management techniques to achieve specified management objectives for each listed species after consulting with the state noxious weed advisory committee. The prescribed management techniques shall be 1

2 mandatory techniques for list A species and populations of list B species designated for eradication. The commissioner shall develop management techniques pursuant to science-based methodologies, peer reviewed studies, or any other method that is based on credible research. We believe that this is being met. Historical Background of Spotted Knapweed Invasion A large portion of Spruce Gulch burned in 1988 (Figure 1). Seeds used by the USFS in the restoration effort were presumably contaminated with spotted knapweed. Some of these seeds were given to the private landowner who subsequently planted these in areas below the fire. Those seeded areas became dominated by the knapweed. Beginning in the mid 1990s the private landowner began an aggressive mechanical removal activity and locally eradicated the species to the point where it was mapped in 2008 (Figure 2). Figure 1. Spruce Gulch, a rugged, roadless, and diverse landscape in the Colorado Foothills. (Photo on USFS land, looking across private land to USFS land.) The landowner contacted the CU invasive plant research group in 2001 about the potential for using biological control insects on this weed population. Small releases of the insects Larinus minutus and Cyphocleonus achates were made in 2001 and additional releases were made in subsequent years. By the early 2000s these same insects were known to have large effects on diffuse knapweed in the Colorado Front Range (Seastedt 2

3 et. al. 2003, 2005; Seastedt and Suding 2007). However, their efficacy on spotted knapweed in Colorado (and elsewhere) was unstudied and undocumented. Figure 2. Two Colorado A-listed species, spotted knapweed and myrtle spurge, mapped in Spruce Gulch by the USFS in The private landowners obtain untreated water for their house and garden from shallow cisterns connected to the stream originating in Spruce Gulch. According, they strongly oppose any chemical use in their riparian corridors. Hence, in addition to the goal of eradicating spotted knapweed, a concurrent goal was to use no chemicals on private land and minimize the chemical use on USFS lands. Under Colorado weed law, this meant that the eradication effort on the private lands would be restricted to mechanical methods, i.e., pulling and digging. The biological control agents, while present, could not under current law be used in this eradication effort. Mechanical removal intensified throughout the decade and in 2010 numerous weed pulls were conducted in the drainage. Three volunteer pulls on National Forest lands were arranged by the USFS partnering with Volunteers for Outdoor Colorado (VOC), focusing on the upstream and downstream perimeters of the infestation. A group of 7-9 teenagers from Nederland (funded by State and County sources) made five day-long visits, and multiple visits were made by colleagues of the private landowners. These pulls initially focused primarily on the upstream and downstream perimeters of the infestation, and eventually along the entire riparian corridor (so that seeds could no longer be transported 3

4 by the stream). Plants were also removed along the main trail through most of the area (so that movement by humans or animals would also be limited; Figures 3 & 4). Subsequent mechanical removal concentrated on patches where the weed occurred in dense stands, and finally, selective removal of large flowering plants throughout the Spruce Gulch drainage was conducted. June 30, a, before removal July 8, b, after removal Figure 3a,b. Spotted knapweed is replaced by grasses following removal along the trail in Spruce Gulch. The riparian zone below the lower boundaries of the infested area was patrolled to Left Hand Canyon Drive. The USFS treated about eight acres with three herbicide applications in early summer, midsummer, and fall, at the southernmost end of the infestation. While all flowering plants were not eradicated from the entire area, a sizeable portion of the drainage was treated in 2010 (Figure 5). 4

5 4a. Forested buffer Upper end of Spotted knapweed Infestation May b. USFS, VOC, CU pull, May 23, c. Site after second pull, July 12, 2010 Figure 4 a,b,c. A site treated in both 2009 and

6 Figure 5. Estimate of mechanical treatment on knapweed during The southernmost extension of the population was treated with Milestone. Water quality concerns kept herbicides away from the stream. New Facts about Spotted Knapweed in the Colorado Front Range During the interval, the CU Invasive Plant lab learned substantial amounts of information about the basic ecology of spotted knapweed in the Colorado Front Range, and the impacts that biological control agents can have on this species. This work has been summarized in one book chapter (Knochel and Seastedt 2009), and five peerreviewed journal articles (Seastedt et al. 2007; Knochel and Seastedt 2010; Knochel et al. 2010a, 2010b; Wooley et al., in press). These publications have been included as an electronic appendix to this report. In addition, additional research articles will be forthcoming that detail facts about specific habitat responses of knapweed and biological control agents. This project has become one of the few long-term ongoing ecological assessments of spotted knapweed in North America. These articles provide the local data and most recent literature from elsewhere that support the following fact: Spotted knapweed in the presence of multiple biological control agents and in the presence of competing vegetation no longer functions as an aggressive, invasive species. The plant not only has limited potential to spread, but declines in abundance in existing infestations. 6

7 Further, an independent measurement of knapweed seed abundance in the soils of heavily infested areas in 2010 (central portion of upper meadow and north and south facing slopes) was conducted. Tetrazolium viability tests of seed collected from these soils by the Colorado State University Seed Laboratory indicate a general lack of viable seed from these areas (CSU Seed Laboratory, Fort Collins; see appendix). Thus, the plant is largely supporting its numbers based upon past successes (i.e., monopolizing on disturbances from two fires, heavy grazing in the riparian zone and meadows, and high seed reproduction prior to establishment of biological control agents during the period ). In areas not mechanically treated, seed production estimated from two longterm transects ranged from seeds per sq. meter. With reduced seed production, new plant growth and survivorship is insufficient to maintain plant densities (Figure 6). The monitoring plots support visual evidence that the population is in decline. While these facts do not negate the logical goal to eradicate this weed from Spruce Gulch, it does alter the urgency at which this goal must be accomplished. Management actions in 2010 primarily relied on mechanical removal, but plants missed by this removal activity were still impacted by the cumulative effects of the biological control insects (Figure 6). Further, the effects of the biocontrols were likely enhanced by the large-scale mechanical removal. Since at least one of the critically important insects, Larinus minutus, was unharmed by the death of numerous knapweed plants, these insects subsequently concentrated their efforts on the remaining plants. Hence, the attack rate by at least one species was increased by mechanical management activities. The second biological agent, the root feeder Cyphocleonus achates, was unaffected and continued to develop on plants not mechanically removed. Summary: Colorado weed law dictates that spotted knapweed be eradicated in Boulder County. This remains the goal of this project, even if the plant no longer imposes the environmental threat to natural areas ascribed to it over a decade ago. We also believe that the law should be updated with current knowledge that recognizes that biological control agents can provide assistance in eradication efforts such as this one. The biological control agents reduce the invasive threat and give landowners time to use costeffective and environmentally friendly mechanical removal methods, at least for infestations of the size described here. Given the success achieved in 2010, we do not think that there is a compelling need to invoke either the economic costs or the potential environmental damage issues associated with chemical treatment of this weed species in what is a remote area with high conservation values. We recommend based upon the best, most current science information available that the eradication plan used in 2010 be continued into the future. This approach provided valuable information to researchers and managers; no eradication efforts for spotted knapweed of this size in Colorado (or elsewhere) have been published. The challenge of restoration on a site with multiple noxious weeds argues for an integrated long-term approach. Regardless, while there are a variety of methods available to kill weeds, to eradicate a large population of a perennial species in extremely complex terrain requires tenacity and a long-term commitment that is largely independent of specific management tools. 7

8 Flowering stems per sq. m no data Lower meadow Upper meadow Figure 6. Stem densities of spotted knapweed following exclusion by cattle are not correlated with climate variables, but appear to be declining due to increased plant competition and reduced reproduction and survivorship. (values are means and Std errors, n= 20 plots in lower meadow, n=29 plots in upper meadow.) Note: Cover of the knapweed in the infested areas outside these transects was under 13% of total vegetative cover in June of 2010 (unpublished results). Acknowledgements: The assistance of Beverly Baker and the Boulder Ranger District, USFS, and Steve Sauer of Boulder County Parks and Open Space were crucial in organizing the volunteer effort used in this project. We thank Beverly Baker and Steve Popovich for commenting and improving earlier drafts of this report. Literature Cited Seastedt, T.R., Nathan Gregory and David Buckner Reduction of diffuse knapweed by biocontrol insects in a Colorado grassland. Weed Science 51: Seastedt, T.R., K.N. Suding and K.D. LeJeune Understanding Invasions: The rise and fall of diffuse knapweed (Centaurea diffusa) in North America. Pages In: Inderjit (Ed.) Invasive plants: ecological and agricultural aspects. Basal, Switzerland: Birkhauser-Verlag AG. Seastedt, T.R., K.N. Suding, Biotic constraints on the invasion of diffuse knapweed (Centaurea diffusa) in a Colorado grassland. Oecologia 151:

9 Appendix A. Publications using findings of Spruce Gulch Studies: (PDFs available on line at: knapweed.htm) Seastedt, T.R., D Knochel, M Garmoe, and S. Shosky Interactions and effects of multiple biological control insects on diffuse and spotted knapweed in the Front Range of Colorado. Biological Control 42: Knochel, D. and T.R. Seastedt Sustainable control of spotted knapweed (Centaurea stoebe). Pages In: Management of Invasive Weeds (Inderjit, ed.), Springer, Heidelberg, Germany. Knochel, D.G. and T. R. Seastedt Reconciling contradictory findings of herbivore impacts on the growth and reproduction of spotted knapweed (Centaurea stoebe). Ecological Applications 20: Knochel, D.G.,C. Flagg, and T.R. Seastedt Effects of plant competition, seed predation, and nutrient limitation on seedling survivorship of spotted knapweed (Centaurea stoebe) Biological Invasions. 12: Knochel, D.G., N.D. Monson, and T.R. Seastedt Additive effects of above- and belowground herbivores on the dominance of spotted knapweed (Centaurea stoebe). Oecologia 164: Wooley, S.C.,, B. Smith, C. King, T.R. Seastedt, and D. G.Knochel. In press. The lesser of two weevils: physiological responses of spotted knapweed (Centaurea stoebe) to above- and belowground herbivory by Larinus minutus and Cyphocleonus achates. Biocontrol Science and Technology. Waltermyer, E Results of Tetrazolium Tests (unpublished report to Dr. David Knochel) 9

10 Appendix B. Additional photos A view of Spruce Gulch from the northeast, looking southwest. The knapweed is found at the upper portions of the drainage. Photo by David Knochel, taken Aug. 17, 2010 The above photo shows mostly private lands, with Forest Service property found on the left (north facing) slope, along with a small portion of the Spruce Gulch riparian zone. In addition to the eradication of spotted knapweed, more than ten separate populations of myrtle spurge have been found on private property and the USFS lands seen in the photo. All myrtle spurge populations were mechanically treated in 2010, and all will be revisited and similarly treated in subsequent years. 10

11 This 2006 photo shows a site at the upper boundary of private land. In 2006 the area was desert-like as a result of overgrazing and drought. These factors set up conditions for an increase in densities of spotted knapweed. Grazing has been greatly reduced since then and vegetation recovery is evident on most areas. Recent studies on spotted knapweed all point to the fact that the plant exploits disturbances such as that shown above to obtain dominance. Intact grasslands are slow to invade and often do not allow the plant to form a monoculture. At Spruce Gulch we ve found that the insect effects are strongest in areas where these grasslands were least effected by grazing and roads. Further, forest succession is facilitating the reduction of the knapweed, particularly on north-facing areas. 11

12 Mechanical removal of one of the knapweed monoculture areas Removal of knapweed along the riparian corridor was a priority in

13 Stem densities of knapweed have Declined by ~50% in the last three years. A heavily infested site in 2007 shows the blue-green hue of the knapweed monoculture. The same site in 2010 shows increased cover by shrubs and trees but also that large patches of Monarda fistulosa (bee balm or horse-mint) are invading this patch. Elsewhere, a native buckbrush (Ceanothus fendleri), Oregon grape (Mahonia aquifolium) and a number of native and introduced grasses are also appear to be outcompeting the now-stressed knapweed. The boundaries between USFS and private lands remain uncertain. The area in the above photo was assumed to be on private land and was used for research during the interval. However, re-inspection of topographic maps with boundary markings suggests the above property may be on USFS land. 13

14 June 30, 2005 July 12, 2010 Upper meadow and saddle The enhanced greenness in the 2010 photo is due, in part, to greater nitrogen content of foliage and greater leaf area in the second photo. The restoration of forests on northfacing slopes as a result of regrowth since the 1988 fire is evident. Knapweed does not fare well beneath forest canopies. Note also the green blotches in the meadow in the second photograph. Those are native buckbrush bushes. 14

15 June 30, 2005 July 12, 2010 Notice eventual loss of early successional weedy non-natives that took hold after 2004 fire (left): mullein, toadflax, clump of cheatgrass and re-emergence of native forbs and grasses (right): needlegrass, yellow aster, sage. Native species are on the increase. A group from Volunteers for Outdoor Colorado prepare to do some damage to the knapweed in June,