Secondary succession is driven by both biotic and abiotic factors, and studies of their relative importance have played a pivotal role in both ecosystem management and ecological theory. In Eastern North America, it has long been recognized that succession proceeds at a faster rate at lower latitudes, with unclear contributions of a warmer climate, less fertile southern soils, or different species pools of old-field herbaceous communities and pioneer trees. We factorially manipulated soil fertility and herbaceous composition across a latitudinal gradient in six locations from Syracuse, NY to Tallahassee, FL. Soil fertility was manipulated in four sand dilutions of fertile topsoil, and herbaceous communities consisted of (‘southern’) bunchgrass and (‘northern’) forb treatments plus a no-herb control. After two years of herbaceous growth, we sowed an identical seed mixture of northern and southern pioneer tree species (Juniperus virginiana, Liquidambar styraciflua, Pinus taeda, Prunus serotina) into each plot. Subsequent germination, survival, and growth of all seedlings was monitored for two growing seasons before harvest, and modeled in response to concomitant measurements of light, soil moisture, nutrients, and climatic factors, including the effects of the herbaceous community on these variables.
Contrary to our hypothesis of the role of soil fertility as a strong mediator of pioneer tree establishment in old fields, the largest driver of tree recruitment was site temperature. Two years after sowing, total seedling mass for all four pioneer species was several times greater in the southern locations, with only Juniperus virginiana recruiting well in northern locations. Furthermore, although herbaceous communities strongly limited tree recruitment through shading, herbaceous biomass showed a steady decline with latitude from north to south and was less responsive to soil fertility and herbaceous composition. Soil moisture and nitrogen availability were secondary drivers of seedling growth and influenced by an array of climatic and biotic factors. Although limited to the initial phase of secondary succession associated with tree recruitment in old fields, our results suggest that growing season duration is among the most important constraints on tree seedling growth when in competition with herbaceous communities. Further extensions of the growing season from climate change are likely to significantly accelerate the rate at which fields become forests, with important management implications.