Demography of Two Facultative Biennial Plant Species in an Unproductive Habitat

SUMMARY character: the 'sparse' and 'dense' sites. The number of seeds produced were also estimated on the two sites. (2) The demography of P. hieracioides differed greatly on the two sites. On the dense site, more seedlings emerged and a greater proportion established, probably due to the higher water retention than on the sparse site. Conversely, a lower proportion of plants at the dense site survived to maturity, reproduction was delayed, and there was reduced seed output by mature plants, possibly due to competition. Survival and reproduction was highest in 1976, the year with the most rainfall during the growing season. Populations of P. hieracioides declined during the study on both sites, apparently because of low survival and establishment of seedlings during 4 subsequent years of drought. Populations persisted through the dry years by reproductive delay coupled with comparatively high survival of established plants. (3) Most aspects of Echium vulgare demography appeared unaffected by site conditions; only reproduction was delayed longer on the dense site. Moreover, the rates of survival from seedling emergence to maturity were as l gh during years of drought as when rainfall was abundant. Echium vulgare plants survived better during drought than those of Picris hieracioides. Populations of Echium vulgare increased even during 4 years of drought, suggesting that the species may have only recently begun colonizing the quarry. Echium vulgare appears to tolerate better the dry, infertile quarry habitat than Picris hieracioides, indicating that there is substantial between-species variability among facultative biennials in their ability to colonize unproductive sites. (4) Unlike old-field populations of facultative biennials that typically become extinct after one or two generations due to successional changes, P. hieracioides and Echium vulgare showed no decline resulting from competitive displacement. The populations will probably continue to persist in the quarry for many years.