Description. Origin and Distribution. Phenology

Transcription

1 BIOLOGY OF YELLOW STARTHISTLE Timothy S. Prather, Regional IPM Specialist, Kearney Agricultural Center Description Yellow starthistle (Centaurea solstitia/is) is a member of the tribe Cardueae Cass. in the family Asteraceae. The tribe is characterized by involucre bracts ending in a spine, bristle or membranous appendage. The achenes are obovate, flattened and often shiny on the upper third of the achene. The pappus is bristly with circular rows of bristles, the bristles increasing in length towards the center of the achene. In Centaurea, the point of attachment of the achene to the receptacle is concave. In the genus Centaurea there are species, mostly in Eurasia with a few in Africa, N. America and S. America. All members appear to be cross-pollinators. Origin and Distribution Yellow starthistle is Eurasian in origin, and has been found in Iran, Armenia, Kurdistan, Balkan-Asia Minor and the Mediterranean region (Komarov 1934). Yellow starthistle can be found in 23 of the 48 contiguous states but predominates in the west (Maddox et al 1983). It was introduced into California in the early l 800's, possibly as a contaminate of alfalfa seed. The first herbarium specimen was collected in 1869 at Oakland California (Maddox et al. 1985). In California, 52 counties are infested with yellow starthistle (Maddox et al. 1983), including coastal, intercoastal and Central Valley locations.. Yellow starthistle can be found in a wide range of habitats in California. Yellow starthistle grows well on deep well-drained soils, but also grows on shallow rocky soils. Reported precipitation range of yellow starthistle is 10 to 40 inches (Maddox 1981). Vegetation types that support yellow starthistle include grassland steppe and oak woodland (Griffin 1977). Phenology Yellow starthistle is a winter-hardy annual that often germinates in the fall (Oct. to Dec.) after precipitation begins. It occasionally acts as a biennial. The phenology in California has been described by Maddox (1981 ). Through the winter the roots continue to grow, allowing it to absorb nutrients deep in the soil profile. In early spring there are at least 7 lobed leaves that form a rosette. Through May the plant continues to produce lobed, basal leaves, as many as 26. In May to June the plants bolt, sending up a flower stalk but flower buds are not produced yet. As temperatures rise the pubescence on the plant increases producing a characteristic blue-green color. In June and July flower buds are formed and appear as small swellings at the branch tips. Yell ow-green spines appear at the tips of bracts that envelope the flower bud and the spines grow to 0.25 to 2 inches in length. Flowering takes place June through August when yellow flowers open within the flower head. In August the leaves and stems begin to tum straw colored and the bright yellow flowers fade. Once the flowers begin to fade, seeds are formed. After the 219

2 flower head has dried and the florets fall from the head, seeds are nearly mature. When the bracts open, the seeds are mature. The seeds are dimorphic (of two types), some with a white pappus and some without a white pappus. The seeds without a pappus are located along the outside edge of the flower head. They often remain in the flower head until the head degrades during winter, while the pappus bearing seeds disperse. Dispersal occurs between July and November. By December the bracts are usually lost from the flower revealing the white cottony base of the head. The plants have a characteristic grey color during late fall and winter. Seed Ecology Many species besides yellow starthistle have two types of seeds (dimorphic). Seed dimorphism often is discussed as an adaption for dispersal (Olivieri and Berger 1985) primarily among annual species. One seed type is adapted for dispersal and has no dormancy but the nondispersing type may have dormancy. The dispersing type tends to be larger and to develop a vigorous seedling. The vigorous seedling matures into a plant that usually produces more seed than the non-dispersing type. The non-dispersing type tends to have more specific germination requirements and is more light sensitive. While the dispersing type is more vigorous, it is also more costly to produce. Differences in germination timing, as well as dispersal aids in avoiding competition between seedlings from the same parent plant. The pappus bearing seeds of yellow starthistle can disperse 5 feet by wind (Roche 1991) but many seeds disperse over long distances in fur, feathers or human activity (Callihan et al. 1989a). No innate dormancy has been found for either seed type of yellow starthistle. The only difference may lie in the fact that the non-pappus bearing seeds often remain in the head until it disintegrates (Callihan et al. 1993), potentially delaying germination. The pappus bearing seeds of yellow starthistle are larger which may enable them to grow more quickly. The seed types are produced in ratios that vary somewhat but seem to average about 3 pappus bearing for each 1 non pappus bearing seed (Roche 1965). Varying light intensity and light quality does not have differential effects between seed types but germination was reduced from 85% germination without shading to 1 % under 4 inches of grass litter and 11 % under 4 inches of yellow starthistle litter (Prather, Northam, McCaffrey and Callihan, unpublished). The seed types differ in their longevity. Pappus bearing seeds can last for 10 years, buried in soil, while the non-pappus bearing seeds last for 6 years (Callihan et al. 1993). Pitcairn (unpublished) has found a high mortality of seeds in the soil ((97%) in California so while seeds have the potential to last at least 10 years, most seeds are short-lived. Yellow starthistle can germinate over a wide range of temperatures (46 to 82 F) (Callihan et al. 1989b). Germination was best at 64 F. Seeds that were 74 months old germinated better than seeds that were 2 months old, indicating the possibility of a need for an afterripening period. When fall-germinated rosettes were destroyed in the spring, seedlings emerged in April and were able to flower and set seed. These results indicate that yellow starthistle can germinate over a wide range of temperatures as long as sufficient moisture is available (Callihan et al. 1989b). 220

3 Invasion Susceptibility Why does yellow starthistle occupy vast areas in California, especially in the Central Valley? Part of this question can be answered by determining what the native vegetation was prior to alien introductions; this is still a point of debate. Heady (1977) has stated that the valley was dominated by Stipa pu/chra, a perennial bunchgrass. Other perennial bunchgrasses included Aristida divaricata, E/ymus glaucus, Festuca idahoensis, Koeleria cristata, Muhlenbergia rigens, Stipa cernua and Stipa coronata (Heady 1977;Daubenmire 1978; Bartolome et al. 1986). Historically, large herds of grazing animals such as bison were not present in large numbers west of the Rocky Mountains, leading to evolution of grasslands dominated by these bunchgrasses instead of grasslands dominated by rhizomatous or stoloniferous grasses that tolerate grazing (Mack and Thompson 1982). Grasslands that evolved with grazing are more resistant to invasion by alien plants (Mack and Thompson 1982). When agronomic and livestock production began in the Central Valley, these grasslands were lost, replaced by annual grasslands composed predominately of alien species. While this explanation is probable in the western states, it may not have been the case in the Central Valley. Wester (1981) proposes that bunchgrasses dominated in the hills, mountains and coastal ranges but not on the valley floor. On the valley floor, the original grassland was composed of annual species. If the Central Valley is predisposed to domination by annual species, then aggressive annuals like yellow starthistle are capable of, and will continue, invading and dominating the annual communities. Competitive ability Whether once dominated by perennial or annual grasses, by the time yellow starthistle colonized in California, the grasslands were dominated by annual. Yellow starthistle has several adaptions that increase its competitive ability with annuals. Yellow starthistle roots grow through the winter, allowing it to compete with the annual grasses (Callihan et al. l 989b). There can be as many as 6000 seedlings yd-2 (Pitcairn unpublished), creating an effective shade barrier to annual grass germination and seedling development. As the season progresses, up to 95% of the seedlings die (Pitcairn unpublished) but suppression of the annual grasses has occurred. Water usually is a limiting resource on rangelands and pastures and yellow starthistle has several adaptions that make it competitive in habitats that have limited water available. Stems and leaves of yellow starthistle are covered with a light pubescence. This pubescence reflects some of the light that impinges on them, reducing the heat load and the transpiration demand. The stems are winged. The winged stems have added surface area that is vertically oriented and acts as a radiator, further reducing the heat load and subsequently reduced transpiration demand. Annual grass biomass has been reduced by as much as 85% when yellow starthistle was present (Northam and Callihan 1989). Yellow starthistle also is competitive with seedlings of perennial grasses (Prather and Callihan 1991). If yellow starthistle densities are high in a perennial grass stand then recruitment of grasses from seed becomes less likely. However, in a 29 month old stand of Oahe intermediate wheatgrass, biomass and density of yellow starthistle were reduced by 91 % (Northam and Callihan 1988). In another study, yellow starthistle biomass was 53% less in plots seeded to intermediate wheatgrass than in plots without intermediate wheatgrass, 5 years after 221

4 seeding (Prather et al. 1988). The effect of intermediate wheatgrass was significant when both herbicide and fertilizer no longer reduced yellow starthistle biomass (5 years after application). The perennial grasses can suppress yellow starthistle when the stands of grasses are healthy, and under these circumstances, recruitment from seedlings may be possible. Yellow starthistle's dominance of annual grass communities and its suppression in perennial grass communities is important. If the Central Valley is indeed predisposed to domination by annual grasses then seeding perennial grasses may not be successful as a method to control yellow starthistle. However, if the area was dominated by perennial grasses in the past, it may be possible to establish perennial grasses for control of yellow starthistle. Literature Cited Bartolome, J. W., S. E. Klukkert, and W. J. Barry Opal phytoliths as evidence for displacement of native Californian grassland. Madrono 33 : Callihan, R.H., T. S. Prather and F. E. Northam Longevity of yellow starthistle (Centaurea solstitia/is) achenes in soil. Weed Tech. 7: Callihan, R.H., FE. Northam, J.B. Johnson, E. L. Michalson and T. S. Prather. 1989a. Yellow starthistle biology and management in pasture and rangeland. Univ. of Idaho, College of Agric. CIS 634. Callihan, R.H., T. S. Prather, and F. E. Northam. 1989b. Invasion by yellow starthistle. Proc 1989 Knapweed Symposium, Ext. Bull. 45. Montana State Univ. Bozeman, MT.pp Daubenmire, R Plant Geography. Academic Press, New York~ 338 pp. Griffin, J. R Oak woodland, in Terrestrial Vegetation of California. M. G. Barbour and J. Major, eds. John Wiley and Sons, New York 1002 pp. Heady, H.F Valley grassland, in Terrestrial Vegetation of California, M. G. Barbour and J. Major eds, John Wiley and Sons, New York 1002 pp. Komarov, V. L Flora of the U.S.S.R. Akademiya Nauk SSSR. Botanicheskii lnstitut 28: Mack, R. N. and J. N. Thompson Evolution in steppe with few, large, hooved mammals. Amer. Nat., 119: Maddox, D. M., A. Mayfield and N. H. Portiz Distribution of yellow starthistle (Centaurea solstitia/is) and Russian knapweed (Centaurea repens). Weed Sci. 33: Maddox, D. M Introduction, phenology, and density of yellow starthistle in coastal, intercoastal, and Central Valley situations in California. USDA-ARS, Agric. Res. Reults, ARR-W pp. 222

5 Northam, F. E. and R.H. Callihan Effects of eleven herbicides on a yellow starthistle community. pp 54-58, West. Soc. Weed Sci. Res. Prog. Rep. Northam, F. E. and R.H. Callihan Yellow starthistle presence in 29 month-old stands of eight grasses. pp 60-63, West. Soc. Weed Sci. Res. Prog. Rep. Olivieri, I. and A. Berger Seed dimorphism for dispersal: physiological, genetic and demographical aspects, in Genetic Differentiation an Dispersal in Plants, P. Jacquard (ed) Springer Verlag Berlin Heidelberg.pp Prather, T. S. and R.H. Callihan Interference between yellow starthistle and pubescent wheatgrass during grass establishment. J. Range Mgt. 44: Prather, T. S., R. H. Callihan and D. C. Thill Revegetating yellow starthistle infested land with intermediate wheatgrass. pp 68-69, West. Soc. Weed Sci. Res. Prog. Rep. Roche, B. F Achene dispersal in yellow starthistle (Centaurea solstitialis L.). Northwest Sci 66: Roche, B. F Ecologic studies of yellow starthistle (Centaurea solstitia/is). Ph.D. dissertation. Dep. Range Resources, Univ. Idaho, ID