The population dynamics of a long-lived conifer (Pinus palustris)

W. J. Platt, Louisiana State University
G. W. Evans, Louisiana State University
S. L. Rathbun, Louisiana State University


In an old-growth longleaf pine population in which all trees of at least 2 cm in dbh were mapped and tagged, the population was of uneven age and size; tree size correlated positively with tree age. Large or old trees were only loosely aggregated, forming a background matrix that filled the forest. Juvenile trees were highly aggregated, located in areas of low adult densities. Recruitment thus occurs primarily within open spaces created by the deaths of large trees. Variable time lags may occur before the colonization of open spaces, however, because of temporal variation in seed production and occurrence of summer ground fires. Recruitment within the mapped plot has occurred frequently for at least the past 250 yr. Temporal variation in adult mortality and recruitment into open spaces, coupled with strong negative interactions between cohorts of different ages, appears likely to produce alternating phases of population growth and decline that are highly variable in length and magnitude. An upper bound to population size occurs when all available space is filled with trees; but no lower bound exists, and extinction probabilities may be increased at very low densities. The population is buffered from declines to very low densities, however, by the tendency for small trees to recruit into openings created by the deaths of adults. Longleaf pine possibly maintains the environment in an open state suitable for its own regeneration by transmuting a localized disturbance (lightning) into a widespread disturbance (ground fires). Fire facilitation results in an extended, but indefinite, increase in the persistence of environmental conditions in which longleaf pine, but no other tree species, can survive and reproduce. -from Authors