Morphological Variation in the Seed of Gray Birch (Betula populifolia): The Effects of Soil-Metal Contamination
by Frank J. Gallagher1, Ildiko Pechmann2, Bernard Isaacson3, and Jason Grabosky1
1 Urban Forestry Program, Department of Ecology, Evolution, and Natural Resources, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901
2 Department of Biological Sciences, Rutgers University, 113 University Avenue, Newark, NJ 07102
3 Department of Forest and Wildlife Ecology, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI 53706.
Few species of trees are known to colonize disturbed sites with the higher than normal concentrations of soil metals typical of urban brownfields. The patchy distribution and heterogeneous makeup of such soil contamination produces strong abiotic filters limiting recruitment from regional species pools. In the northeastern United States, one of the most successful species in these situations is gray birch (Betula populifolia). Our previous work indicates that gray birch dominates the study site within Liberty State Park, a known brownfield, and that its distribution could be positively correlated with increasing soil-metal loads. Our current study questions whether gray birch seeds collected from areas of various soil-metal loads have differing wing-loading rates, and thus varying dispersal effectiveness. In addition, we investigate whether seed viability is affected by soil-metal load. Such differences could rapidly alter gene flow within isolated urban assemblages. Our results indicate that gray birch seeds exhibit a broad range of wing-loading rates, caused by differences in the surface area and mass of the seeds. Wing-loading rates (seed weight/seed surface area) exhibited no significant relationship with soil-metal load, indicating that potential for seed distribution from areas of high soil-metal load has not been impaired. However, germination success exhibited a negative correlation with increasing soil-metal load—in fact, seeds from areas of high soil-metal loads generally failed to germinate. We conclude that metal tolerance in gray birch is probably the result of genetic variation maintained within the metapopulation.
Keywords: seed dispersal, rate of descent, metaliferous soil, assemblage structure, wing load