Survival and growth of woody plant seedlings in the understorey of floodplain forests in South Carolina

Abstract

1 Survival and height growth of permanently tagged understorey seedlings in four river floodplain forests in South Carolina were monitored for 8 years. Regression models were used to determine if a common survival pattern emerged and if the pattern varied according to species, spatial 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 survival rates for 1-3 years after establishment, or brief periods (1-2 years) with much lower survival. Although the underlying survival function was best described by a negative power curve, a log-logistic model also fit the data and provided more versatility for fitting individual populations. 3 Significant differences in general survival pattern were found among species, sites (forests), years of establishment (cohorts), and all two-way interactions of species, site and cohort. Species differences were large. Spikes in mortality for individual species and cohorts were synchronized within sites and to a lesser degree among sites. In some sites, weak relationships were noted between mortality and peak river discharge during summer (negatively correlated) and winter (positively correlated). 4 Species differed strongly in their initial height, but effects of cohort and site on height were also significant as were all two-way interactions of species, site and cohort. During the 8 years of this study, very few seedlings grew taller than 30 cm. For most species, taller seedlings had greater per capita survival. Height growth, however, was not consistently related to survival. Since mean size of survivors changed little with time, age may be a better variable than size to use in models of survival. 5 Simulation models could probably be developed using a log-logistic function that includes (in order of importance): seedling age, species, intensity of winter floods (some sites) and occurrence of summer droughts (some sites).