More info for the terms: competition, density, monoecious, pappus, phase, presence
Yellow starthistle is a facultative winter annual (sometimes biennial, from a tap root) that relies on abundant seed production for population persistence and spread [80]. The reproductive biology [80,143] and life history [131] of yellow starthistle have been explored.
Breeding system: Yellow starthistle is monoecious, pollinator-dependent and facultatively xenogamous [52,80,143]. Most yellow starthistle plants are self-incompatible [80].
Pollination: European honeybees are an important pollinator of yellow starthistle and may be responsible for up to 57% of seed set [8,80]. Bumblebees are also important floral visitors, and several other insects contribute to the fertilization of yellow starthistle ovules [8,52].
Seed production: The number of flowerheads and seeds produced by yellow starthistle plants varies with soil moisture and intensity of competition. Plants can produce 1 to 1,000 flowerheads per plant and 30 to 80 seeds per flowerhead [11,74,79]. Large plants can produce nearly 75,000 seeds, and Roche [117] calculated a potential 148,928 seeds produced by a nursery plant. The number of seeds per unit area produced by an infestation of yellow starthistle has been measured by several authors, with reports ranging from 14 to 100 million seeds per acre (35-250 million seeds per hectare) [23,33,74]. The number of seeds produced on an annual basis is dependent on the size of the infestation and the amount of spring precipitation [131,133]. Sheley and Larson [131] measured seed output in yellow starthistle at 1,940 and 470 seeds per square foot (21,600/m² and 5,200/m²) under moist and dry spring conditions, respectively. The percentage of plumed seeds ranges from about 70% to over 90% [11,74,79,117]. In heavy infestations, yellow starthistle populations produce far more seeds than are necessary to reinfest the area year after year [30].
Seed dispersal: Distribution of yellow starthistle seedlings is predominantly in and near the previous year's yellow starthistle debris, suggesting a relatively slow rate of spread [117]. Plumeless seeds usually remain in seedheads until fall and winter (between November and February [133]) and fall to the soil just below the parent plant as seedheads deteriorate [20]. The size of the pappus on plumed seeds is small relative to seed size and wind moves seeds only short distances, roughly equal to the height of the plant [1,114]. Roche [114] recorded 92% of seeds falling within 2 feet (0.6 m) of the parent plant, and 48% within 1 foot (0.3 m). A combination of gusty wind and dry conditions maximizes wind dispersal. Roche also measured maximum wind dispersal at about 16 feet (<5 m) over bare ground with wind gusts of 25 miles per hour (40 km/hr). While not an effective long-distance dispersal mechanism, wind dispersal can serve to increase the area of an infestation by persistently advancing the perimeter. Yellow starthistle seed can also be transported over short to medium distances by animals and humans. The pappus bristles are covered with stiff, microscopic barbs that readily adhere to clothing and hair [20,32]. Long-distance dispersal of yellow starthistle seed is often directly related to human activities and occurs by movement of livestock, vehicles, equipment, and contaminated hay and crop seed [32,35]. Birds such as ring-necked pheasants, California quail, house finches, and American goldfinches feed heavily on yellow starthistle seeds [114,117]. While some (e.g., ring-necked pheasants) may be responsible for long distance dispersal of yellow starthistle seed, most seeds consumed by birds are lost to the regeneration pool [114].
Seed losses have been recorded by researchers as a discrepancy between total seed production and number of seeds dispersed. In Washington, plumed seed was dispersed in August with a 30% loss between crop production and dispersal, and plumeless seed was released in December with a 65% loss [74]. Sheley and Larson [130] observed a 41% loss of seed production at the time of dispersal. Further losses are observed between dispersal and seedling establishment. Roche [114] found 290 seedlings per square foot (3,230/m²) and 1.8 seeds per square foot (20/m²) in May, in an area where about 2,630 seeds per square foot (29,200/m²) were dispersed the previous fall. He speculated that the remaining seeds may have been consumed or moved by birds, rodents, insects or whirlwinds.
Seed banking: Yellow starthistle seed may remain viable in the soil for as long as 10 years [23]. In heavily infested areas, the soil seed bank of yellow starthistle approaches 13% of annual seed production and consists primarily of plumeless seeds [131,133]. However, evidence for seed banking in yellow starthistle varies. Induced dormancy, seed type, depth of burial, site fire history, and other site conditions may be responsible for observed differences in seed longevity.
On a bunchgrass site in California where yellow starthistle was present, no yellow starthistle seeds germinated from soil samples taken from 0 to 5-inch (0-13 cm) depths in October [83]. On a bunchgrass range site in Washington state that was dominated by cheatgrass and yellow starthistle (density of 16 to 21 adults per square foot (180-236/m²)), the number of yellow starthistle seeds found in soil samples ranged from about 252 to 378 per square foot (2,800-4,200/m²) [74,131]. On a California grassland site, seed density measurements in the soil under yellow starthistle in the fall varied from year to year in untreated, control plots from 309 to 911 per square foot (3,438-10,127/m²) [33].
After-ripening may prevent premature germination of yellow starthistle in dry habitats. Many yellow starthistle seeds undergo secondary dormancy after exposure to high temperatures and low moisture within 1 month of dispersal. These seeds do not then germinate even under adequate light and moisture conditions [30]. Joley and others [64] observed some evidence of after-ripening in both buried and dry-stored seed, although results were inconsistent. Benefield and others [11] report 87.6% to 95.2% viability and 84% to 87.6% germination rates at the dispersal stage at 2 California sites. Thus, nearly all viable seed was able to germinate suggesting that yellow starthistle at these locations may not have an innate or induced dormancy mechanism or an ecologically significant after-ripening period.
Callihan and others [23] harvested plumed and plumeless yellow starthistle seeds from a site in Idaho, and buried them in a sandy loam soil under an annual grass community in southeast Washington at depths of 1, 2, and 6 inches (2.5, 5, and 15 cm). They detected no effects of burial depth on seed longevity, but did find that viability decreased over time. Average longevity of plumed and plumeless seeds was 10 and 6 years, respectively [23,96]. Joley and others [64] buried plumed and plumeless seeds at 2 inches (5 cm) for 72 months in the field in a sandy loam soil under a fallow orchard in northeastern California. They found no effects of seed type on germination. In another study, plumed seeds were buried at several depths. After 13 months, germinable seeds increased with depth, with 0.5%, 3.9%, 63.1%, and 88.1% germinability at 0.2-, 0.4-, 1-, and 2-inch (0.5, 1, 2.5, and 5 cm) depths, respectively. The loss of germinable seeds at shallow depths is attributed primarily to germination, as indicated by split seedcoat.
One year of prescribed burning in Sonoma County, California, significantly reduced the seed bank of yellow starthistle (P<0.05) and 3 consecutive years of burning, with no further seed recruitment, further depleted the seed bank. Prior to burning, yellow starthistle seed density ranged from 309 to 911 seeds per square foot (3,438-10,127/m²). Seed density declined to 240 seeds per square foot (2,673/ m²) after 1 year of burning, 38 seeds per square foot (421/m²) after 2 years of burning, and 5 to 11 seeds per square foot (52-127/m²) after 3 consecutive years of prescribed burning. Corresponding decreases in seedling density the following springs were also observed [33]. Similarly, Joley and others [64] reported over 83% depletion in the seed bank 1 year after preventing seed rain by mowing or clipping mature plants, and after 3 years, only 3.9% of the original seeds remained. Benefield and others [11] sowed 1,000 seeds 0.4 inch (1 cm) deep in weed-free soil containers in California. They observed that after 1 year, germination and seed recovery accounted for 69% and 57% of the original 1,000 seeds sown, for plumed and plumeless seeds, respectively. Of the seeds recovered, over 80% were damaged or degraded by microbial or insect activity. Unrecovered seed was speculated to have been lost to bird or rodent predation. These results support the contention that, under California soil and climatic conditions and average field conditions where seeds are predominantly dispersed on the soil surface, yellow starthistle seeds are relatively short-lived. Microbial degradation and predation of yellow starthistle seeds probably contribute to the rapid depletion of the soil seed bank [11,30,64].
Two to 3 years of effective control can dramatically reduce yellow starthistle infestation and presence in the soil seed bank [33,64]. Yet the survival of even a small percentage of seeds can potentially lead to reinfestation of a site, even in the absence of off-site seed recruitment [23,33,64].
Germination: The mean number of viable and germinable yellow starthistle seeds increases consistently with advancing phenological stage, with some germinable seed developing as early as the late flowering stage [80] and late senescence stage (see Seasonal Development) [11]. Plumeless seeds mature earlier than plumed seeds, and germination of plumed seeds is slightly greater than plumeless seeds [35]. Dimorphic seeds may be associated with the success of yellow starthistle in exploiting variable soil moisture and temperature regimes in semiarid environments [75]. The different seed types tend to have different optimums with regard to temperature and moisture in the lab [75,117]. In the field, the germination rate of plumed seed is higher in late fall and winter, and that of plumeless seed is higher in spring [11].
Over 90% of yellow starthistle seeds are germinable 1 week after seed dispersal [11]. Yellow starthistle seeds germinate at both low and high temperatures, allowing for both fall and spring germination when moisture is adequate [20]. In the field, seeds usually germinate in late fall or early winter, when sufficient soil moisture is present [79,133], though germination can continue throughout winter and into spring in northern California [63]. Germination is closely correlated with winter and spring rainfall events [11,79], with germination occurring throughout the rainy season, and emergence highest after early-season rainfall events [11]. The extended period of germination increases the difficulty of controlling yellow starthistle populations with late winter and early spring treatments, as germination subsequent to control efforts can result in continued infestations.
Joley and others [63] found that temperature, light, seed type, collection date, dormancy and storage, and interactions of these factors all affected yellow starthistle germination in the lab. Nearly 100% germination occurs when seeds are exposed to moisture, light and constant temperatures of 50, 59, or 68 °F (10, 15, or 20 °C), or alternating temperatures of 59:41 or 68:50 °F (15:5 or 20:10 °C). Sheley and others [128,134] found that under similar conditions of moisture and temperature and no light, germination occurred rapidly, with nearly all seed germinating within 96 hours. Total germination is reduced at temperatures above 86 °F (30 °C) and below 43 °F (6 °C) [122]. Although seeds can germinate in the dark, germination is greatly reduced in dark environments [63] and appears to be stimulated by white light [92].
Seedling establishment/growth: Yellow starthistle seedlings first allocate resources to root extension and then to leaf expansion. Yellow starthistle root growth is vigorous and can extend deeper than 3.3 feet (1 m) during early seedling establishment [116].
In exposed areas, high germination can result in extremely dense seedling populations [74,145]. Near Walla Walla, Washington, yellow starthistle seedling populations reached winter (mid-January) densities approaching 2,500 plants per square foot (26,875/m²). Subsequent frost heaving reduced seedling populations by about 40%, and yellow starthistle density was further reduced by 75% during the juvenile phase (late May to mid-June) [131,133]. Similarly, Roche [117] observed a 58% reduction in yellow starthistle density from April to July. Adult density of yellow starthistle at a particular site can be closely associated with soil depth and thus, late season water storage capacity. Roche and others [116] observed a significant relationship (P< 0.001) between the number of yellow starthistle plants per unit area and total soil moisture from 19 May to 29 August in southeastern Washington.
Asexual regeneration: There is no evidence for asexual regeneration in yellow starthistle.