Variation in demographic structure and properties such as establishment, growth, and survival contribute to differences in population density, size structure, growth rate, and resilience to disturbance. In this study our goal was to understand how intra-specific variation in life-history characteristics of Pinus rigida (Pitch Pine) in the Long Island Pine Barrens (LIPB) contributes to differences in population structure at a landscape level. The LIPB is an costal Atlantic ecosystem dominated, or co-dominated, by P. rigida. It includes both normal stature P. rigida forest stands, where the mature tree canopy ranges from 8 m to 15 m tall, and the globally rare Dwarf Pine Plains, where the mature tree canopy ranges from less than 1 m to 3 m tall, and individuals have a twisted, sometimes multi-stemmed growth form. The precise cause, or causes, of the dwarf growth form remains unknown, though both abiotic conditions, such as soil characteristics, and intrinsic biotic factors, such as genetic differences between normal and dwarf stature plants, have been implicated. A long-term demographic monitoring survey was initiated in the LIPB region, including normal, dwarf, and intermediate P. rigida stands, following a series of major stand-replacing fires in 1995. More than 6000 individuals were monitored over the course of 13 years, allowing us to estimate population level survival, growth, fecundity, and prevalence of serotiny values for normal, dwarf, and intermediate stature populations.
Results/Conclusions
We found prevalence of serotiny to be significantly higher for dwarf populations (χ2 = 105.63, df = 2, P << 0.01), resulting in a substantially larger above ground seedbank in these populations. We parameterized population specific stage-based matrix models with the demographic parameter estimates, and found that differences in the above ground seedbank, which only becomes available post-fire, dramatically influences population responses to a fire. Dwarf populations have much higher seedling mortality in the years immediately following a stand-replacing fire, but because of the larger number of seeds, maintain larger, more dense populations of P. rigida. Further, based on simulation results, population stability appears to be partially dependent on shorter fire-return times at dwarf populations. This work opens up further avenues of investigation into the interactions between biotic and abiotic factors that may influence the prevalence of dwarf stature P. rigida, including such factors as intra-specific density dependence effects, and ultimately may help inform management of the rare Dwarf Pine Plains ecosystem.