Sources of Variation in Growth, Form, and Survival in Dwarf and Normal-Stature Pitch Pines (Pinus rigida, Pinaceae) in Long-Term Transplant Experiments

Determining the relative contributions of genetic and environmental factors to phenotypic variation is critical for understanding the evolutionary ecology of plant species, but few studies have examined the sources of phenotypic differentiation between nearby populations of woody plants. We conducted reciprocal transplant experiments to examine sources of variation in growth rate, form, survival, and maturation in a globally rare dwarf population of pitch pine (Pinus rigida) and in surrounding populations of normal-stature pitch pines on Long Island, New York. Transplants were monitored over a 6-yr period. The influence of seedling origin on height, growth rate, survival, and form (single-stemmed vs. multi-stemmed growth habit) was much smaller than the effect of transplanting location. Both planting site and seed origin were important factors in determining time to reproduction; seedlings originating from dwarf populations and seedlings planted at the normal-stature site reproduced earliest. These results suggest that many of the differences between dwarf and normal-stature pitch pines may be due more to plastic responses to environmental factors than to genetic differentiation among populations. Therefore, preservation of the dwarf pine habitat is essential for preserving dwarf pine communities; the dwarf pines cannot be preserved ex situ

Variation in Recruitment and Early Demography in Pinus Rigida Following Crown Fire in the Pine Barrens of Long Island, New York

Following severe, stand-replacing fires in 1995, we quantified emergence, growth and survival in Pinus rigida seedlings in eight stands in three areas of the pine barrens of Long Island, New York, USA, and examined factors contributing to spatial and temporal variation in recruitment. We followed 6431 marked seedlings in the first cohort following fire, as well as additional cohorts in subsequent years, for 7 years to assess the effects of fire intensity, soil characteristics, intraspecific density, interspecific competition and facilitation on variation in early demography at three different scales. We found substantial variation in many of the demographic parameters measured between plots within stands (10–50 m apart), among stands within areas (600– 1500 m apart), and among forest areas (5 –30 km apart), as well as among years. Almost all adult P. rigida in most of the stands studied were killed by the 1995 fires and population recovery therefore depended upon recruitment from seed. Initial recruitment appears to be largely determined by seed limitation, which was affected by fire intensity combined with serotiny and other factors. Subsequent seedling survival and growth were determined largely by inherent differences among areas and sites in factors including cover of the shrub Quercus ilicifolia and soil texture and composition. Initial seedling density, and subsequent survival and growth, varied among plots within sites, among sites within areas, and among areas. Early survival was negatively correlated with intraspecific seedling density, but was enhanced by neighbouring Q. ilicifolia, although these shrubs inhibited later seedling growth. Early demographic variation may determine many of the differences observed among mature populations in this landscape. Variation in demographic parameters across space and time, and over a hierarchy of scales, may have critical consequences at the population, community and landscape levels. A comprehensive evaluation of the nature and extent of such demographic variation across different systems would have major implications for understanding vegetation patterns at the population, community and landscape levels