DORMANCY AND GERMINATION OF COMMON RAGWEED SEEDS IN THE FIELD

Common ragweed (Ambrosia artemisiifolia) seeds were stored under natural environmental conditions by placing them at three soil levels (surface, 5 cm, and 15 cm) in the field on November 1, 1972. Germination tests at 4-week intervals indicated that dormancy was broken by the end of January. Germination was initially greater at high temperatures, but this difference decreased with increasing time in the field. Secondary dormancy was evident in surface seeds by March 21 but not until April 18 at 5 cm and June 13 at 15 cm. Germination in the field was greatest at the surface but was observed at all soil levels by March 21. Seedling survival was 68 % at the surface and 0% at 5 and 15 cm on June 13. Maximum and minimum soil temperatures were recorded at each soil level during the experiment and were correlated with the results. Greater germination and survival at the surface supports the evidence for ragweed's dependence on soil disturbance for germination, and the induction of secondary dormancy explains why ragweed does not constitute a dominant part of the vegetation when disturbance occurs after the soil warms to a critical point in the summer. SEED DORMANCY and germination are very important in determining the success of a species as an invader after soil disturbance. Estimates of the number of seeds (Brenchley and Warington, 1930) and viability of seeds (Crocker, 1938; Toole and Brown, 1946; Darlington and Steinbauer, 1961) in arable soil confirms the maintenance of large populations of seeds for long periods of time. After soil disturbance and subsequent exposure to light, these seeds may give rise to weedy species found in cultivated fields or weedy primary invaders of secondary succession (Saur and Struick, 1964; Wesson and Wareing, 1967, 1969). Germination of the exposed seed after disturbance depends on the existing environmental conditions and the dormancy state of the seed. Seeds of many weedy species under field conditions exhibit cyclic changes in dormancy which follow a seasonal pattern (Courtney, 1968; Taylorson, 1970, 1972; Stoller and Wax, 1974). Secondary dormancy is usually induced during early summer and is broken during the winter, resulting in nondormant seeds in the spring, and if germination doesn't occur the cycle is repeated. Laboratory germination studies have provided some information regarding the success of common ragweed (Ambrosia artemisiifolia) as a primary invader after soil disturbance (Bazzaz, 1968, 1970; Willemsen and Rice, 1972; Willem1 Received for publication 16 July 1974. Supported by a grant from The Research Council, Rutgers University. I am grateful to Mr. Ralph H. Hofmann, officer in charge of the USDA, Cereal Division Seed Branch, Federal Seed Laboratory in New Brunswick, New Jersey, for his cooperation in the use of this agency's seed germination facilities. sen, in press). However, observations on seed germination and dormancy under field conditions would be complementary to these laboratory investigations and would provide a better understanding of this species' natural behavior. MATERIALS AND METHODS-Ragweed plants bearing mature seeds were collected on October 11, 1972, from a one-year abandoned field on the William L. Hutcheson Memorial Forest property on the Piedmont of New Jersey. Fruits were removed from the plants after 4 days of air drying at 20-25 C. The seed was winnowed and stored in the dark in an air-conditioned laboratory (1824 C) until burial on November 1, 1972. For burial, 500 seeds were placed in individual 10 x 10 cm fiberglass screen envelopes (mesh size 6/cm). These envelopes were then placed on the soil surface and 5 cm and 15 cm below soil surface. Every four weeks four envelopes were collected from each soil level and cleaned under running tap water. The number of seeds which had germinated and the number of seedlings present were counted. The remaining seeds were placed on germination paper in sandwich boxes (11 x 11 x 2.5 cm) and 12 ml distilled water was added. Small quantities of water were subsequently added if drying became evident. Percent germination (as indicated by root emergence) was determined in continuous darkness and light (8 hr photoperiod) at 5/15, 10/20, 15/25, and 20/ 30 C (? 1 C). The higher temperature of each alternating temperature regime was present during the 8-hr light period. The same alternating temperature regimes were maintained in constant darkness. Ihe seeds placed in constant dark were