Experiments on Mechanisms of Tree Establishment in Old‐Field Succession: Seedling Emergence

Abstract

Loblolly pine (Pinus taeda) is the initial successful woody invader of abandoned old fields in the Piedmont region of North Carolina, but hardwood tree species also invade old fields and develop an understory community beneath young pine stands. The composition of this early hardwood community will be regulated by factors affecting the relative establishment success of these species in old—field habitats. Mechanisms influencing differential seedling emergence of such early successional tree species were studied in multifactor field experiments. Seeds of six wind—dispersed species (Acer rubrum, Fraxinus americana, Liquidambar styraciflua, Liriodendron tulipifera, Ulmus alata, and Pinus taeda) were sown into old fields at equal densities, and their relative germination (emergence) in two successive years was evaluated with respect to: (1) presence of old—field vegetation (tested with a weeding treatment), (2) vertebrate seed predation (tested with exclosures), (3) variation in life history traits (seed size and germination phenology), and (4) variation in physical factors (spring drought). Emergence of one species (Liriodendron) was limited by intrinsically low seed viability. In 1984, a wet spring, emergence of Pinus exceeded that of all hardwood species, and the smaller—seeded Liquidambar and Ulmus had higher emergence than the larger—seeded Acer and Fraxinus. Seed predators reduced emergence proportionately more in larger—seeded species than in smaller—seeded species. Herbaceous cover facilitated emergence of larger—seeded species by providing a moister germination microenvironment, and perhaps also by reducing visibility of seeds to predators. In 1985 an early spring drought reduced emergence of fall—dispersed species (Pinus, Liquidambar, and Fraxinus), whose earlier germination phenology coincided with the drought. Later—germinating, spring—dispersed species (Ulmus and Acer) avoided this drought period and displayed similar germination responses in both years. The prominence of drought effects in 1985 reduced the apparent impact of other experimental factors upon emergence rates. The emergence stage is an important filter on relative establishment success of tree species colonizing old—field habitats, and multiple "mechanisms" interact in complex ways to influence emergence rates.