Yellow Starthistle Information
Reproduction
-Flower pollination
-Flower & seed
dev
-Seed dispersal
Germination
& dormancy
-Seed prod & types
-Seed development
-Germination
-Stress & germ
-Seasonal germ
-Seed longevity
Growth & establishment
-Seedling establish
-Roots
--growth
& dev
--water use
---competition
-Shoots
--growth &
dev
--light
--temperature
--allelopathy
Biology and Ecology
Growth and establishment (continued)
Roots (continued)
Water use
Heavy infestations of yellow starthistle in grasslands with loamy soils can use as much as 50% of annual stored soil moisture (Gerlach, unpublished data). In deep Central Valley soils of California, starthistle can significantly reduce soil moisture reserves to depths greater than six feet, and in three-foot-deep foothill soils it can extract soil moisture from fissures in the bedrock (Gerlach et al. 1998).
Competition with introduced annual grasses
Root partitioning between yellow starthistle and competing vegetation can greatly influence the invasibility of grasslands to starthistle. Since the root systems of most annual species are comparatively shallow, there is little competition for moisture between yellow starthistle and annual grasses during late spring and early summer. In addition to utilizing deep soil moisture, yellow starthistle can also survive at extremely low soil water potential (< -6.0 MPa) as compared to annual grasses (> -2.1 MPa) (Gerlach, unpublished data).
In a number of experiments Sheley and Larson (1994b, 1994c, 1995) showed that Bromus tectorum (cheatgrass or downy brome) has a relatively shallow fibrous rooting system and is better suited for water and nutrient capture in shallow soils. In comparison, yellow starthistle has an advantage over cheatgrass in deep soils (>0.5 m) where its roots can penetrate deeper into the soil. Thus, seedlings of many annual grasses avoid competition with yellow starthistle by occupying different rooting zones (Sheley and Larson 1994c). In addition, annual grasses, such as cheatgrass, mature earlier than yellow starthistle, which also allows them to avoid competition at later stages of growth. In contrast, the use of soil moisture by yellow starthistle was shown to be similar to that of perennial grasses (Borman et al. 1992). Like yellow starthistle, perennial grasses also have an extended growing season. These factors account for the increased competitive interactions between yellow starthistle and perennial species, compared to annual species.
Seasonal moisture can also influence the competition advantage between yellow starthistle and annual grasses. Under dry spring conditions, early maturing annual grasses would have an advantage over late season annuals, as they utilize the available moisture and complete their life cycle before later maturing species, such as starthistle (Larson and Sheley 1994). In contrast, under moderate or wet spring conditions, starthistle would have an advantage by continuing its growth later into the summer and fall and producing more seed.
Thus, in grassland systems, the greatest advantage for yellow starthistle would occur in areas 1) dominated by annual grasses, 2) with deep soil, and 3) in years with moderate to wet spring rainfall (Sheley and Larson 1992). Under these conditions, yellow starthistle would mature later, have increased seed production, and have little competition for deep soil moisture. In annual grasslands, the least competitive situation for yellow starthistle would be areas with shallow soils and low spring rainfall.
Yellow starthistle density can also affect competition with annual grasses for soil moisture. At low densities, yellow starthistle grows faster and roots grow to greater soil depths than cheatgrass (Sheley and Larson 1997). Yellow starthistle plants at low density acquire moisture primarily from greater depths in the soil profile and do not significantly utilize moisture in the upper soil zone (DiTomaso and Kyser unpublished data). In contrast, at high yellow starthistle densities, soil moisture was depleted from all depths in the soil profile throughout the duration of the season. Thus, high densities of yellow starthistle can produce growth conditions that simulate drought in grassland ecosystems. This can severely hamper restoration efforts, where surface soil moisture is critical to the establishment and survival of seeded perennial grasses and desirable annual forbs.
Competition with native species
The characteristics that enable yellow starthistle to invade grasslands can threaten native species and ecosystems processes. Native species such as blue oak (Quercus douglasii) and purple needlegrass (Nassella pulchra) depend on summer soil moisture reserves for growth and survival (Gerlach et al. 1998). Yellow starthistle, however, uses deep soil moisture reserves earlier than blue oak or purple needlegrass. Thus, when starthistle infestations are high, native species can experience drought conditions even in years with normal rainfall (Gerlach et al. 1998).
Heavy yellow starthistle infestations can also remove large amounts of stored soil moisture through plant transpiration (Gerlach et al. 1998). Most soils in California grasslands store about twelve inches of rainfall for each three feet of soil depth. In most years annual grasses reduce soil moisture reserves by about four inches of stored rainfall in the top three feet of soil. By comparison, yellow starthistle can reduce soil moisture levels by eight inches of stored rainfall for each three feet of soil depth. This is about the same as that of a mature oak tree. As a result, large yellow starthistle populations are transpiring at least an additional four inches of rainfall for each three feet of soil depth during average rainfall years and about eight inches during wet years (Gerlach et al. 1998).
