SURVIVAL AND GROWTH OF WOODY PLANT SEEDLINGS IN THE UNDERSTOREY OF FLOODPLAIN FORESTS IN SOUTH-CAROLINA

1 Survival and height growth of permanently tagged understorey seedlin gs in four river floodplain forests in South Carolina were monitored f or 8 years. Regression models were used to determine if a common survi val pattern emerged and if the pattern varied according to species, sp atial location, time, seedling size and seedling growth. 2 For most of the populations, per capita survival was initially poor but increased steadily with age. A few populations, however, had relatively high su rvival rates for 1-3 years after establishment, or brief periods (1-2 years) with much lower survival. Although the underlying survival func tion was best described by a negative power curve, a log-logistic mode l also fit the data and provided more versatility for fitting individu al populations. 3 Significant differences in general survival pattern were found among species, sites (forests), years of establishment (coh orts), and all two-way interactions of species, site and cohort. Speci es differences were large. Spikes in mortality for individual species and cohorts were synchronized within sites and to a lesser degree amon g sites. In some sites, weak relationships were noted between mortalit y and peak river discharge during summer (negatively correlated) and w inter (positively correlated). 4 Species differed strongly in their in itial height, but effects of cohort and site on height were also signi ficant as were all two-way interactions of species, site and cohort. D uring the 8 years of this study, very few seedlings grew taller than 3 0 cm. For most species, taller seedlings had greater per capita surviv al. Height growth, however, was not consistently related to survival. Since mean size of survivors changed little with time, age may be a be tter variable than size to use in models of survival. 5 Simulation mod els could probably be developed using a log-logistic function that inc ludes (in order of importance): seedling age, species, intensity of wi nter floods (some sites) and occurrence of summer droughts (some sites ).