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Wildlife, Animals, and Plants |
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BOTANICAL AND ECOLOGICAL CHARACTERISTICS
GENERAL BOTANICAL CHARACTERISTICS:Medusahead is a nonnative, cool-season annual grass [45]. Plant height ranges from 8 to 20 inches (20-50 cm), depending on the site. Plants produce tillers, but very few leaves [72,76]. Medusahead has a distinctive flowerhead. The inflorescence contains 2 to 3 spikelets per node, and each spikelet contains 1 seed. Plants produce an average of 7.1 seeds per spike [72,76]. Medusahead has 2 types of awns: both are flat, but the longer of the 2 contains barbs that point upward [72]. Plants in dense stands usually produce 1 spike; in open areas the number of spikes per plant typically increases to 3 to 5. An exceptional plant in Idaho produced 133 spikes. Medusahead-dominated stands usually have more than 100 plants/ft2. Densities of 1,500 to 2,000 plants/ft2 have been found on a valley bottom in southern Idaho [90,97].
RAUNKIAER [82] LIFE FORM:Therophyte
REGENERATION PROCESSES:Medusahead is entirely dependent upon seed production for regeneration. It is an extremely capable seeder because of its large annual production of viable seed, and because its seed maintains viability in litter and soil for at least 1 year [90]. Medusahead maintains a short-lived seedbank [9,58,90]. Plants produce up to 6,000 seeds/ft2 of soil, propagating dense stands in succeeding years [69]. Medusahead is principally self fertile. Most of the pollen grains are dispersed within the floret and only a moderate number of pollen grains are produced in each of the short anthers [39]. Some cross-pollination is effected by wind [17]. Animals, wind, and water disperse the seed, and spread is rapid [40]. A long, rough awn aids in animal dispersal of seed, and medusahead often 1st establishes along domestic sheep and cow trails [76]. Seeds are dispersed primarily from the coats and intestinal tracts of grazing animals [40,69,90,100]. Germinable seeds have been recovered in fecal material 4 to 9 days after ingestion by rabbits and domestic sheep, respectively [90]. Stiff barbs pointing in 1 direction enable the seeds to work into the duff and top layers of the soil. Seeds may germinate in fall, winter, or spring; fall germination is most common [6,90,107]. Seedlings from all seasons produce seeds by early summer [107]. Seeds germinating in the top layer of litter without soil contact may die during the 1st dry spell [57,69]. Medusahead usually germinates faster than its competitors. Germination has been observed 8 to 10 hours after moistening at 50 degrees Fahrenheit (10 oC) [45]. Germination rates are often over 90% [90]. Awn removal increases the percentage of germination [78]. A Nevada study found that medusahead seedling emergence and growth is favored by soil movement and pitting of the soil surface because these conditions maintain favorable soil temperatures and moisture levels [33]. Favorable microsites for germination and establishment of medusahead are created when plant litter covers the soil surface. In Nevada, emergence of medusahead germinants under litter was 47 times greater than emergence of germinants on bare ground by the end of March. By the end of the growing season, medusahead yield was 4 times greater under litter than on bare soil [32]. Moderate temperatures may encourage medusahead growth and yield. Maximum dry matter production of medusahead was achieved at a moderate day/night temperature regime of 75/52 degrees Fahrenheit (24/11 oC). Its yield was reduced by 75% with a high day/night temperature regime of 90/61 degrees Fahrenheit (32/16 oC), and reduced by 50% with a low day/night temperature regime of 61/41degrees Fahrenheit (16/5 oC) [28].
SITE CHARACTERISTICS:Medusahead grows in areas that have relatively mild to cold temperatures in winter but are hot in summer [69,72]. It is generally found in areas that receive fall, winter, and spring moisture followed by dry summers [99]. It occurs in areas with annual precipitation of 10 to 40 inches (250-1,000 mm), with an upper limit of precipitation approximately 50 inches (1,270 mm) [69,72,90]. Infestations primarily occur in former sagebrush-grass or bunchgrass communities that receive 10 to 20 inches (250-500 mm) of precipitation [72,90]. Areas above 4,500 feet (1,370 m) elevation, and well-drained coarse soils, may be less susceptible to invasion. Medusahead often dominates disturbed areas on soils with high moisture-holding capacities and slow percolation rates [31]. Sites particularly susceptible to medusahead invasion in the more arid portions of Idaho are either those with well-developed soil profiles, particularly with high clay content either at or near the surface; or those occupying topographic positions that receive additional run-off from adjacent sites. In more mesic climates, moderately well-developed soils are as susceptible to invasion as well-developed soils. Conversely, soils with little profile development, particularly those that are well drained, remain dominated by cheatgrass in early seral stages regardless of whether they are in a more arid or mesic area [27]. In a northwestern California site where medusahead is prevalent, 60% of the vegetation is grassland or woodland/grass. The climate is mediterranean, with cool wet winters and hot dry summers. Mean annual rainfall is 39 inches (980 mm). Soil is fine sandy loam 2 to 3.3 feet (0.6-1.0 m) deep with rapid surface drainage; slope is 10% on a southeast aspect [5,6]. Foothills in southwestern Oregon where medusahead is found have silty clay loam soil, with 1,600 feet (500 m) elevation and 20-30% slope on a west aspect. On southwest aspects, medusahead occurs on 5-20% slopes at 2,000 feet (600 m) elevation. The climate has a mediterranean/maritime pattern with cool, wet winters and hot dry summers and annual precipitation of 20 inches (500 mm) [13]. Medusahead and cheatgrass are often in competition with each other, and soil and topographic factors affect their distribution [27,37]. Each can replace other herbaceous vegetation and share dominance with the other. Cheatgrass occupies a larger geographical area than medusahead, extending to drier areas of the semiarid western U.S. than does medusahead [28]. In arid conditions, medusahead is more dependent on additional moisture for survival. This may be because cheatgrass matures when soil moisture is still plentiful in May, and medusahead does not mature until 3 weeks later when moisture is more confined to depressions and clay soils [27,37]. Because medusahead matures approximately 1 month later than cheatgrass, it initially only replaces cheatgrass on soils with sufficient moisture holding capacity, such as clay textured soils, so that some soil moisture remains after cheatgrass matures. Medusahead's root system can exploit all soil moisture in the soil profile [108]. In the Columbia River Basin, medusahead is dominant on soils high in montmorillonite clay within 10 to 12 inches (25-30 cm) of the surface, and on soils low in clay but on with favorable topographic positions. Cheatgrass is dominant on weakly developed soils low in montmorillonite clay.
SUCCESSIONAL STATUS:Medusahead occurs in seral and late-successional plant communities. It has invaded vast areas formerly dominated by perennial grasses. Medusahead often colonizes portions of range previously dominated by cheatgrass [26,53]. The growth habits, life cycles, and ecological adaptations of medusahead and cheatgrass are similar, and the annuals typically grow in association until medusahead becomes dominant and eventually exclusive [15]. Southwestern Idaho stands in which medusahead was sparse were all seral. In virtually all cases studied, the sites invaded by medusahead had been occupied previously by seral species, mainly annuals, which had replaced perennial bunchgrasses depleted by overgrazing, fire, or cultivation [60,72,93]. Medusahead has potential for successionally replacing cheatgrass in the 11- inch (280 mm) and above precipitation zone in the northern Great Basin [50]. Cheatgrass usually grows in dense stands and readily ignites and carries fire. After fire strikes a cheatgrass-infested community, cheatgrass usually flourishes. However, medusahead can thrives in the wake of cheatgrass-driven fires [30]. Medusahead is a seral invader after disturbance [93]. Medusahead often grows in dense stands on disturbed sites where climax perennial grasses have been removed, often to the exclusion of other species [47]. The abundance of bluebunch wheatgrass has "significantly" decreased in the Great Basin because of the invasion of introduced annuals such as medusahead [74]. Past heavy grazing of foothills, pastures, and rangelands of southeastern Oregon has resulted in dominance by annual grasses such as medusahead and annual forbs including yellow starthistle [14].
SEASONAL DEVELOPMENT:Medusahead is a cool-season annual, and temperature is an important factor in controlling its phenology. During winter, growth is slowed markedly with low temperatures, and the plant resumes active growth when the temperature increases at the beginning of spring [24]. Medusahead germinates during autumn, late winter, or early spring [24,72]. It usually germinates in October and continues to grow through the winter in mesic climates. Leaves, stems and roots increase in number through the winter and roots can reach 40 inches (100 cm) depth by early February [47]. This allows medusahead to outcompete desirable grasses such as bluebunch wheatgrass [42,47]. Growth accelerates in the spring; by late May or early June, seeds are in the milk or early dough stage [40]. Seed are generally mature by late June to early July, a few weeks later than most annual grasses [72]. Seeds remain in spikes until dispersal in late summer or early fall [90]. Late maturity and greater availability of soil moisture late in the growing season allow medusahead to reach maturity and produce large amounts of seeds, which might enhance site occupation in subsequent generations [28]. Medusahead phenology was as follows in northern Idaho [15]:
Related categories for SPECIES: Taeniatherum caput-medusae | Medusahead |
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