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Medusahead Ecology and Management: California Annual Grasslands to the Intermountain West

Published online by Cambridge University Press:  20 January 2017

Aleta M. Nafus  and
Kirk W. Davies
Aleta M. Nafus*
Affiliation:
Department Animal and Rangeland Science, Oregon State University, Corvallis, OR, 97331
Kirk W. Davies
Affiliation:
United States Department of Agriculture-Agricultural Research Service at the Eastern Oregon Agricultural Research Center, 67826-A Hwy 205, Burns, OR 97720
*
Corresponding author's E-mail: Aleta.Nafus@oregonstate.edu
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Abstract

The spread of medusahead across the western United States has severe implications for a wide range of ecosystem services. Medusahead invasion reduces biodiversity, wildlife habitat and forage production, and often leads to increased fire frequency and restoration costs. Medusahead is problematic in the Intermountain West and California Annual Grasslands. The last review of medusahead ecology and management was completed 20 years ago. Since the last review, there have been scientific advances in medusahead management suggesting a significant need to develop an up-to-date synthesis. Medusahead continues to pose a serious threat to rangeland ecosystems. In this synthesis, we present new information regarding the ecology of medusahead, suggest a framework for managing medusahead based on invasion level, and identify research needs to further improve management of this invasive annual grass. Success of different management practices varies between the Intermountain West and California Annual Grasslands, signifying that the best management practices are those specifically tailored with consideration of climate, soil, plant community characteristics, and management objectives. Prevention and control treatments that are useful in the Intermountain West may not be practical or effective in the California Annual Grasslands and vice-versa.

Keywords

Medusahead, Taeniatherum caput-medusae (L.) NevskiELYCMControlexotic annual grassinvasive plant preventionrestorationrevegetation
Type
Research Article
Information
Invasive Plant Science and Management , Volume 7 , Issue 2 , June 2014 , pp. 210 - 221
Copyright
Copyright © Weed Science Society of America 

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References

Literature Cited

Alpert, P, Maron, JL (2000) Carbon addition as a countermeasure against biological invasion by plants. Biol Invasions 2:33–40 Google Scholar
Arredondo, JT, Jones, TA, Johnson, DA (1998) Seedling growth of Intermountain perennial and weedy annual grasses. J Range Manage 51:584–389Google Scholar
Asay, KH, Horton, WH, Jensen, KB, Palazzo, AJ (2001) Merits of native and introduced Triticeae grasses on semiarid rangelands. Can J Plant Sci 81:45–52 Google Scholar
Berner, DK, Dubin, HJ, Smallwood, EL (2007) Slender wheatgrass is susceptible to smut caused by Ustilago phrygica from Turkey. Plant Dis 91:906–906 Google Scholar
Blank, RR, Allen, FL, Young, JA (1996) Influence of simulated burning of soil-litter from low sagebrush, squirreltail, cheatgrass, and medusahead on water-soluble anions and cations. Int J Wildland Fire 6:137–143 Google Scholar
Blank, RR, Sforza, R (2007) Plant-soil relationships of the invasive annual grass Taeniatherum caput-medusae: a reciprocal transplant experiment. Plant Soil 298:7–19 Google Scholar
Bovey, RW, Tourneau, DL, Erickson, LC (1961) The chemical composition of medusahead and downy brome. Weeds 9:307–311 Google Scholar
Boyd, CS, Davies, KW (2010) Shrub microsite influences post-fire perennial grass establishment. Rangeland Ecol Manag 63:248–252 Google Scholar
Boyd, CS, Davies, KW (2012) Spatial variability in cost and success of revegetation in a Wyoming big sagebrush community. Envir Manag 50:441–450 Google Scholar
Boyd, CS, James, JJ (2013) Variation in timing of planting influences bluebunch wheatgrass demography in an arid system. Range Ecol Manag 66:117–126 Google Scholar
Brunson, JL, Pyke, DA, Perakis, SS (2010) Yield responses of ruderal plants to sucrose in invasive-dominated sagebrush steppe of the Intermountain West. Restor Ecol 18:304–312 Google Scholar
Chambers, JC, Meyer, SE, Whittaker, A, Roundy, BA, Blank, RR (2007) What makes great basin sagebrush ecosystems invasible by Bromus tectorum? Ecol Monogr 77:117–145 Google Scholar
Clausnitzer, DW, Borman, MM, Johnson, DE (1999) Competition between Elymus elymoides and Taeniatherum caput-medusae . Weed Sci. 47:720–728 Google Scholar
Connelly, JW, Reese, KP, Fischer, RA, Wakkinen, WL (2000) Response of a sage grouse breeding population to fire in southeastern Idaho. Wildlife Soc B 28:90–96 Google Scholar
Cox, RD, Allen, EB (2008) Stability of exotic annual grasses following restoration efforts in southern California coastal sage scrub. J Appl Ecol 45:495–504 Google Scholar
Cox, RD, Anderson, VJ (2004) Increasing native diversity of cheatgrass-dominated rangeland through assisted succession. J Range Manage 57:203–210 Google Scholar
D'Antonio, CM, Vitousek, PM (1992) Biological invasions by exotic grasses, the grass/fire cycle, and global change. Ann Rev Ecol Syst 23:63–87 Google Scholar
Dahl, BE, Tisdale, EW (1975) Environmental factors related to medusahead distribution. J Range Manage 28:463–468 Google Scholar
Davies, KW (2008) Medusahead dispersal and establishment in sagebrush steppe plant communities. Rangeland Ecol Manag 61:110–115 Google Scholar
Davies, KW (2010) Revegetation of medusahead-invaded sagebrush steppe. Rangeland Ecol Manag 63:564–571 Google Scholar
Davies, KW (2011) Plant community diversity and native plant abundance decline with increasing abundance of an exotic annual grass. Oecologia 167:481–491 Google Scholar
Davies, KW, Bates, JD, Svejcar, TJ, Boyd, CS (2010a) Effects of long-term livestock grazing on fuel characteristics in rangelands: an example from the sagebrush steppe. Rangeland Ecol Manag 63:662–669 Google Scholar
Davies, KW, Boyd, CS, Beck, JL, Bates, JD, Svejcar, TJ, Gregg, MA (2011) Saving the sagebrush sea: an ecosystem conservation plan for big sagebrush plant communities. Biol Conserv 144:2573–2584 Google Scholar
Davies, KW, Johnson, DD (2008) Managing medusahead in the Intermountian West is at a critical threshold. Rangelands 30:13–15 Google Scholar
Davies, KW, Johnson, DD (2011) Are we “missing the boat” on preventing the spread of invasive plants in rangelands. Invasive Plant Sci Manage 4:166–171 Google Scholar
Davies, KW, Nafus, AM, Madsen, MD (2013) Medusahead invasion along unimproved roads, animal trails, and random transects. West N Am Naturalist 73:54–59 Google Scholar
Davies, KW, Nafus, AM, Sheley, RL (2010b) Non-native competitive perennial grass impedes the spread of an invasive annual grass. Biol Invasions 12:3187–3194 Google Scholar
Davies, KW, Sheley, RL (2007a) A conceptual framework for preventing the spatial dispersal of invasive plants. Weed Sci 55:178–184 Google Scholar
Davies, KW, Sheley, RL (2007b) Influence of neighboring vegetation height on seed dispersal: implications for invasive plant management. Weed Sci 55:626–630 Google Scholar
Davies, KW, Sheley, RL (2011) Promoting native vegetation and diversity in exotic annual grass infestations. Restor Ecol 19:159–165 Google Scholar
Davies, KW, Svejcar, TJ (2008) Comparison of medusahead-invaded and noninvaded Wyoming big sagebrush steppe in Southeastern Oregon. Rangeland Ecol Manag 61:623–629 Google Scholar
Davies, KW, Svejcar, TJ, Bates, JD (2009) Interaction of historical and nonhistorical disturbances maintains native plant communities. Ecol Appl 19:1536–1545 Google Scholar
DiTomaso, JM, Kyser, GB, George, MR, Doran, MP, Laca, EA (2008) Control of medusahead (Taeniatherum caput-medusae) using timely sheep grazing. Invasive Plant Sci Manage 1:241–247 Google Scholar
Duncan, CA, Jachetta, JA (2005) Introduction. Pages 1–7 in Duncan, CA, Clark, JK, eds. Invasive plants of Range and Wildlands and their environmental, economic and societal impacts. Lawrence, KS Weed Sci. Society of America Google Scholar
Duncan, CA, Jachetta, JJ, Brown, ML, Carrithers, VF, Clark, JK, DiTomaso, JM, Lym, RG, McDaniel, KC, Renz, MJ, Rice, PM (2004) Assessing the economic, environmental, and societal losses from invasive plants on rangeland and wildlands. Weed Technol 18:1411–1416 Google Scholar
Eiswerth, ME, Krauter, K, Swanson, SR, Zielinski, M (2009) Post-fire seeding on Wyoming big sagebrush ecological sites: regression analyses of seeded nonnative and native species densities. J Environ Manage 90:1320–1325 Google Scholar
Evans, RA, Young, JA (1970) Plant liter and establishment of alien annual weed species in rangeland communities. Weed Sci 18:697–703 Google Scholar
Fansler, VA, Mangold, JM (2011) Restoring native plants to crested wheatgrass stands. Restor Ecol 19:16–23 Google Scholar
George, MR, Knight, RS, Sands, PB, Demment, MW (1989) Intensive grazing increases beef production. Calif Agr 43:16–19 Google Scholar
Grey, WE, Quimby, PC, Mathre, DE, Young, JA (1995) Potential for biological control of downy brome (Bromus tectorum) and medusahead (Taeniatherum caput-medusae) with crown and root-rot fungi. Weed Technol 9:362–365 Google Scholar
Griggs, FT (2000) Vina Plains Preserve: eighteen years of adaptive management. Fremontia 27:48–51 Google Scholar
Harris, GA, Wilson, AM (1970) Competition for moisture among seedlings of annual and perennial grasses as influenced by root elongation at low temperatures. Ecology 51:530–534 Google Scholar
Harrison, S, Inouye, BD, Safford, HD (2003) Ecological heterogeneity in the effects of grazing and fire on grassland diversity. Conserv Biol 17:837–845 Google Scholar
Harwood, L (1960) Programs to control medusahead. Pages 45–49 in Proceedings of the California Section. Fresno, CA Society for Range Mangement Google Scholar
HilleRisLambers, J, Yelenik, SG, Colman, BP, Levine, JM (2010) California annual grass invaders: the drivers or passengers of change? J Ecol 98:1147–1156 Google Scholar
Hironaka, M (1961) The relative rate of root development of cheatgrass and medusahead. J Range Manage 14:263–267 Google Scholar
Hironaka, M, Sindelar, BW (1973) Reproductive success of squirreltail in medusahead infested ranges. J Range Manage 26:219–221 Google Scholar
Hironaka, M, Sindelar, BW (1975) Growth characteristics of squirreltail seedlings in competition with medusahead. J Range Manage 28:283–285 Google Scholar
Hironaka, M, Tisdale, EW (1963) Secondary succession in annual vegetation in southern Idaho. Ecology 44:810–812 Google Scholar
Hirsch, MC, Monaco, TA, Call, CA, Ransom, CV (2012) Comparison of herbicides for reducing annual grass emergence in two Great Basin soils. Rangeland Ecol Manag 65:66–75 Google Scholar
Hulet, A, Roundy, BA, Jessop, B (2010) Crested wheatgrass control and native plant establishment in Utah. Rangeland Ecol Manag 63:450–460 Google Scholar
Hull, AC, Klomp, GJ (1967) Thickening and spread of crested wheatgrass stands on southern Idaho ranges. J Range Manage 20:22–227 Google Scholar
Hulme, PE (2006) Beyond control: wider implications for the management of biological invasions. J Appl Ecol 43:835–847 Google Scholar
James, JJ (2008a) Effect of soil nitrogen stress on the relative growth rate of annual and perennial grasses in the Intermountain West. Plant Soil 310:201–210 Google Scholar
James, JJ (2008b) Leaf nitrogen productivity as a mechanism driving the success of invasive annual grasses under low and high nitrogen supply. J Arid Environ 72:1775–1784 Google Scholar
James, JJ, Davies, KW, Sheley, RL, Aanderud, ZT (2008) Linking nitrogen partitioning and species abundance to invasion resistance in the Great Basin. Oecologia 156:637–648 Google Scholar
James, JJ, Drenovsky, RE, Monaco, TA, Rinella, MJ (2011a) Managing soil nitrogen to restore annual grass-infested plant communities: effective strategy or incomplete framework? Ecol Appl 21:490–502 Google Scholar
James, JJ, Svejcar, TJ, Rinella, MJ (2011b) Demographic processes limiting seedling recruitment in arid grassland restoration. J Appl Ecol 48:961–969 Google Scholar
James, JJ, Ziegenhagen, L, Aanderud, ZT (2010) Exploitation of nutrient-rich soil patches by invasive annual and native perennial grasses. Invasive Plant Sci Manage 3:169–177 Google Scholar
Kyser, GB, Creech, JE, Zhang, J, DiTomaso, JM (2012a) Selective control of medusahead (Taeniatherum caput-medusae) in California sagebrush scrub using low rates of glyphosate. Invasive Plant Sci Manag 5:1–8 Google Scholar
Kyser, GB, DiTomaso, JM, Doran, MP, Orloff, SB, Wilson, RG, Lancaster, DL, Lile, DF, Porath, ML (2007) Control of medusahead (Taeniatherum caput-medusae) and other annual grasses with imazapic. Weed Technol 21:66–75 Google Scholar
Kyser, GB, Doran, MP, McDougald, NK, Orloff, SB, Vargas, RN, Wilson, RG, DiTomaso, JM (2008) Site characteristics determine the success of prescribed burning for medusahead (Taeniatherum caput-medusae) control. Invasive Plant Sci Manage 1:376–384 Google Scholar
Kyser, GB, Peterson, VF, Davy, JS, DiTomaso, JM (2012b) Preemergent control of medusahead on California annual rangelands with aminopyralid. Rangeland Ecol Manag 65:418–425 Google Scholar
Kyser, GB, Wilson, RB, Zhang, J, DiTomaso, JM (2013) Herbicide-assisted restoration of Great Basin sagebrush steppe infested with medusahead and downy brome. Range Ecol Manag 66:588–596 Google Scholar
Leffler, AJ, James, JJ, Monaco, TA (2013) Temperature and functional traits influence differences in nitrogen uptake capacity between native and invasive grasses. Oecologia 171:51–60 Google Scholar
Leger, EA (2008) The adaptive value of remnant native plants in invaded communities: an example from the Great Basin. Ecol Appl 18:1226–1235 Google Scholar
Louhaichi, M, Carpinelli, MF, Richman, LM, Johnson, DE (2012) Native forb response to sulfometuron methyl on medusahead-invaded rangeland in eastern Oregon. Rangeland J 34:47–53 Google Scholar
Lusk, WC, Jones, MB, Torell, DT, McKell, CM (1961) Medusahead palatability. J Range Manage 14:248–251 Google Scholar
Mac Lauchlan, RS, Miller, HW, Hoglund, OK (1970) Lana vetch for medusahead control. J Range Manage 23:351–353 Google Scholar
Madsen, MD, Davies, KW, Mummey, D, Svejcar, TJ (2014) Improving imazapic selectivity through activated carbon seed enhancement technologies. Rangeland Ecol Manage 67:61–67 Google Scholar
Major, J, McKell, CM, Berry, LJ (1960) Improvement of medusahead infested rangeland. Davis, CA California Agriculture Experiment Station, Extension Service Leaflet 123. 6 pGoogle Scholar
Mangla, S, Sheley, RL, James, JJ (2011) Field growth comparisons of invasive alien annual and native perennial grasses in monocultures. J Arid Environ 75:206–210 Google Scholar
Maret, MP, Wilson, MV (2005) Fire and litter effects on seedling establishment in western Oregon upland prairies. Restor Ecol 13:562–568 Google Scholar
Mattise, SN, Scholten, G (1994) Mechanical control of indesirable annuals on the Boise Front, Idaho. Pages 190–193 in Proceedings: Ecology and management of Annual Rangelands. Ogden, UT USDA Forest Service Intermountain Research Station Gen. Tech. Rep. INT-GTR-313Google Scholar
McAdoo, JK, Longland, WS, Evans, RA (1989) Nongame bird community responses to sagebrush invasion of crested wheatgrass seedings. J Wildl Manage 53:494–502 Google Scholar
McKell, CM, Wilson, AM, Kay, BL (1962) Effective burning of rangelands infested with medusahead. Weeds 10:125–131 Google Scholar
Menke, JW (1992) Grazing and fire management for native perennial grass restoration in California grasslands. Fremontia 20:22–25 Google Scholar
Meyer, MD, Schiffman, PM (1999) Fire season an mulch reduction in a California grassland: a comparison of restoration strategies. Madroño 46:25–37 Google Scholar
Miller, HC (1996) Demography of medusahead on two soil types: potential for invasion into intact native communities. MS Thesis. Corvallis Oregon State Univeristy. 36 pGoogle Scholar
Monaco, TA, Johnson, DA, Norton, JM, Jones, TA, Connors, KJ, Norton, JB, Redinbaugh, MB (2003a) Contrasting responses of Intermountain West grasses to soil nitrogen. J Range Manage 56:282–290 Google Scholar
Monaco, TA, MacKown, CT, Johnson, DA, Jones, TA, Norton, JM, Norton, JB, Redinbaugh, MG (2003b) Nitrogen effects on seed germination and seedling growth. J Range Manage 56:646–653 Google Scholar
Monaco, TA, Osmond, TM, Dewey, SA (2005) Medusahead control with fall- and spring-applied herbicides on northern Utah foothills. Weed Technol 19:653–658 Google Scholar
Moody, ME, Mack, RN (1988) Controlling the spread of plant invasions — the importance of nascent foci. J Appl Ecol 25:1009–1021 Google Scholar
Murphy, AH, Lusk, WC (1961) Timing medusahead burns. California Agric 15:6–7 Google Scholar
Ott, JE, McArthur, ED, Roundy, BA (2003) Vegetation of chained and non-chained seedings after wildfire in Utah. J Range Manage 56:81–91 Google Scholar
Pellant, M, Hall, C (1994) Distribution of two exotic grasses on Intermountain rangelands: status in 1992. Pages 109–112 in Proceedings: Ecology and Management of Annual Rangelands. Ogden, UT USDA Forest Service Intermountain Research Station Gen. Tech. Rep. INT-GTR-313Google Scholar
Pierson, FB, Blackburn, WH, Van Vactor, SS (2007) Hydrologic impacts of mechanical seeding treatments on sagebrush rangelands. Rangeland Ecol Manag 60:666–674 Google Scholar
Pollak, O, Kan, T (1998) The use of prescribed fire to control invasive exotic weeds at Jepson Prairie Preserve. Pages 241–249 in Proceedings from a 1996 Conference — Ecology, Conservation, and Management of Vernal Pool Ecosystems. Sacramento, CA California Native Plant Society Google Scholar
Pyke, DA (2000) Invasive exotic plants in sagebrush ecosystems of the Intermountian West. Pages 43–54 in Proceedings: Sagebrush Steppe Ecosystems Symposium. Boise, Idaho Bureau of Land Management Publication No. BLM/ID/PT-001001+1150Google Scholar
Reiner, R, Craig, A (2011) Conservation easements in California blue oak woodlands: testing the assumption of livestock grazing as a compatible use. Nat Area J 31:408–413 Google Scholar
Reynolds, TD, Trost, CH (1981) Grazing, crested wheatgrass, and bird populations in southeastern Idaho. Northwest Sci 55:225–234 Google Scholar
Seabloom, EW, Harpole, WS, Reichman, OJ, Tilman, D (2003) Invasion, competitive dominance, and resource use by exotic and native California grassland species. P Natl Acad Sci USA 100:13384–13389 Google Scholar
Sharp, LA, Hironaka, M, Tisdale, EW (1957) Viability of medusa-head (Elymus caput-medusae L.) seed collected in Idaho. J Range Manage 10:123–126 Google Scholar
Sheley, RL, Bingham, BS, Davies, KW (2012a) Rehabilitating medusahead (Taeniatherum caput-medusae) infested rangeland using a single-entry approach. Weed Sci 60:612–617 Google Scholar
Sheley, RL, Bingham, BS, Svejcar, TJ (2008) Crested wheatgrass defoliation intensity and season on medusahead invasion. Rangeland Ecol Manag 61:211–217 Google Scholar
Sheley, RL, Carpinelli, MF, Morghan, KJR (2007) Effects of imazapic on target and nontarget vegetation during revegetation. Weed Technol 21:1071–1081 Google Scholar
Sheley, RL, James, JJ (2010) Resistance of Native Plant Functional Groups to Invasion by Medusahead (Taeniatherum caput-medusae). Invasive Plant Sci Manage 3:294–300 Google Scholar
Sheley, RL, Kedzie-Webb, S, Maxwell, BD (2003) Integrated weed management on rangelands. Pages 57–68 in Sheley, RL, Petroff, JK, eds. Biology and Management of Noxious Rangeland Weeds. Corvallis, OR Oregon State University Press Google Scholar
Sheley, RL, Svejcar, TJ (2009) Response of bluebunch wheatgrass and medusahead to defoliation. Rangeland Ecol Manag 62:278–283 Google Scholar
Sheley, RL, Vasquez, EA, Chamberlain, AM, Smith, BS (2012b) Landscape-scale rehabilitation of medusahead (Taeniatherum caput-medusae) dominated sagebrush steppe. Invasive Plant Sci Manage 5:436–442 Google Scholar
Shinn, SL, Thill, DC (2004) Tolerance of several perennial grasses to imazapic. Weed Technol 18:60–65 Google Scholar
Siegwart, M, Bon, MC, Widmer, TL, Crespy, N, Sforza, R (2003) First report of Fusarium arthrosporioides on medusahead (Taeniatherum caput–medusae) and preliminary tests for host-specificity. Plant Pathol 52:416–416 Google Scholar
Simberlloff, D, Martin, L, Genovesi, P, Maris, V, Wardle, A, Aronson, J, Courchamp, F, Galil, B, Garcia-Berthou, E, Pascal, M, Pysek, P, Sousa, R, Tabacchi, E, Vila, M (2013) Impacts of biological invasions: what's what and the way forward. Trends Ecol Evol 28:58–65 Google Scholar
Smith, HA, Johnson, WS, Shonkwiler, JS, Swanson, SR (1999) The implications of variable or constant expansion rates in invasive weed infestations. Weed Sci 47:62–66 Google Scholar
Stromberg, MR, Griffin, JR (1996) Long-term patterns in coastal California grasslands in relation to cultivation, gophers, and grazing. Ecol Appl 6:1189–1211 Google Scholar
Suding, K, Stein, C (2010) Overgrazing as a trigger for invasion in California rangelands:. Abstracts of the 63rd Annual Meeting of the Society for Range Management, 2010, Denver, COGoogle Scholar
Sweet, SB, Kyser, GB, DiTomaso, JM (2008) Susceptibility of exotic annual grass seeds to fire. Invasive Plant Sci Manage 1:158–167 Google Scholar
Torell, PJ, Erickson, LC, Haas, RH (1961) The medusahead problem in Idaho. Weeds 9:124–131 Google Scholar
USDA, NRCS (2012)) The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA. http://plants.usda.gov. Accessed May 3, 2012Google Scholar
Wagner, JA, Delmas, RE, Young, JA (2001) 30 years of medusahead: return to fly blown-flat. Rangelands 23:6–9 Google Scholar
Young, JA (1992) Ecology and management of medusahead [Taeniatherum caput-medusae ssp. asperum (Simk.) Melderis]. Great Basin Nat 52:245–252 Google Scholar
Young, JA, Evans, RA, Eckert, RE (1969) Wheatgrass establishment with tillage and herbicides in a mesic medusahead community. J Range Manage 22:151–155 Google Scholar
Young, JA, Evans, RA, Robison, J (1972) Influence of repeated annual burning on a medusahead community. J Range Manage 25:372–375 Google Scholar
Young, JA, Trent, JD, Blank, RR, Palmquist, DE (1998) Nitrogen interactions with medusahead (Taeniatherum caput-medusae ssp. asperum) seedbanks. Weed Sci 46:191–195 Google Scholar
Young, K, Mangold, J (2008) Medusahead (Taeniatherum caput-medusae) outperforms squirreltail (Elymus elymoides) through interference and growth rate. Invasive Plant Sci Manage 1:73–8Google Scholar