Dynamics of early successional tree species recruitment in old fields across a latitudinal gradient in the Eastern US

Background/Question/Methods The presence of a strong latitudinal gradient in the rate of woody succession in old-field ecosystems merits an understanding of how growth and survival rates of early successional woody seedlings vary with latitude.  Latitudinal variation in seedling performance independent of species identity would suggest that latitudinal variation in rates of succession might be driven by underlying climatic or edaphic factors, while significant variation in the response of species to latitude would suggest that differences in species pools might also contribute to variation in successional rates.  We estimated seedling performance (density after first growing season incorporating both germination and summer survival, and growth rates over the first growing season) for 16 pioneer species of the Eastern U.S..  We conducted these observations across a network of five old-field sites along a latitudinal gradient ranging from Syracuse (NY) to Tallahassee (FL).  We used two traits (seed mass, and location of the site relative to the species’ range) to predict species-specific demographic parameters across sites.

Results/Conclusions

For both density and growth, we found significant effects of site, species, and site by species interactions (p<0.001 for all terms), with no clear latitudinal trend.  Density was lowest and growth rates highest at the most southern site and the second-most northern site – a pattern that may be related to competition with herbaceous species in sites of higher fertility.  Across different species, there were a range of relationships between survival and growth, but there were more strong negative relationships (potentially indicative of greater competition) than strong positive relationships (indicative of particular conditions being ideal for both high growth and density).  Variation in density was strongly predicted by the location of sites relative to the species’ ranges (p=0.009), with sites south of the range limit showing decreased germination and summer survival.  Growth was significantly predicted by seed mass (p<0.001), with larger seeded species tending to have higher growth rates.  These results suggest that differences in the rate of woody succession across a latitudinal gradient are driven less by climate than by a combination of soil conditions and differences among regional species pools in the representation of certain traits such as seed size.