A 9 YEAR DEMOGRAPHIC-STUDY OF HELENIUM-VIRGINICUM (ASTERACEAE), A NARROW ENDEMIC SEASONAL WETLAND PLANT

I carried out a nine year demographic study of Helenium virginicum Bla ke using permanent quadrats at one sinkhole pond in the eastern United Stares to gain insight into factors controlling the distribution and abundance of this plant. I monitored recruitment and mortality of gene ts, their stage classes, and their distribution in relation to water d epth and overstory cover. The occurrence and distribution of a seed ba nk was studied as well as the survival and demography of transplants. Comparisons of soil chemistry and general patterns of distribution and abundance within sites were made between the study site and other pop ulations. The annual hydroperiod at the sinkhole pond varied from 4-16 months of continuous inundation during winter and summer with plants growing at sites with low overstory cover and at maximum water depths of from 49-59 cm. After 16 months of continuous inundation and heavy a ccumulation of floating aquatic vegetation, all H, virginicum plants h ad died at the study site. At the same time, other neighboring populat ions with inundation as long and deep did not accumulate a cover of fl oating vegetation and their populations did not decline noticeably. Th ese data suggest that H, virginicum is intolerant of shade. Following death of all H. virginicum plants at the study site, soil sampling and a large pulse of recruitment gave evidence of a seed bank which was a t least two years old. Annual recruitment and the proportion of plants in stage classes was highly variable; survival of the early stage pla nts was low. A highly variable population size suggests a high probabi lity of local extinction of the population at the study site if future investment in the seed bank should fail to occur Chemical analyses of soils from 19 of the 21 sites in which H. virginicum grows, revealed a low pH, high levels of Al and As, and low levels of B, Ca, K, Mg, an d P. Endemism of this plant may result from it being limited to refugi a where competition is reduced by a stressful soil and a variable hydr operiod, and where the variable hydroperiod leads to high rates of loc al extinction.