The Population Dynamics of a Long-Lived Conifer (Pinus palustris)

Abstract

We investigated the demography and spatial pattern of an old-growth longleaf pine population using a large plot in which all trees of at least 2 cm in dbh were mapped and tagged for individual recognition. 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. In contrast, juvenile trees were highly aggregated and were located in areas of low adult densities. Recruitment within this population thus appears to occur 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 the occurrence of summer ground fires. The absence of large gaps within the age-class distribution indicated that recruitment within the mapped plot has occurred frequently for at least the past 250 years. Neither age- nor size-class distributions, however, were stable. Instead, 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 consistent tendency for small trees to recruit into openings created by the deaths of adults. This regeneration pattern, which also occurs in other long-lived conifers, represents a spatial analogue to stochastic boundedness over time, and it may enhance the local persistence of longleaf pine populations. Efficient spatial buffering requires that open spaces not be colonized by other tree species before the establishment of longleaf recruits. We suggest that longleaf pine 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 not other tree species, can survive and reproduce. Since longleaf pine influences the fire regime, it is a key species that constitutes environmental conditions and, hence, the composition and relative abundances of other species within the habitat.