|Title||Mechanisms of plant succession in coastal Lake Michigan sand dunes|
|Year of Publication||1995|
|Degree||Doctor of Philosophy|
|Number of Pages||252 pp.|
|University||University of Minnesota|
|City||Saint Paul, MN|
Primary plant succession and forest ecosystem development were investigated in a comprehensive study of a beach-ridge chronosequence bordering northern Lake Michigan. A chronology of ridge formation, developed with AMS radiocarbon dates of plant macrofossils, showed that approximately 108 beach ridges formed over the past ~3500 years, averaging one ridge formed every 33 years. This high rate of ridge formation resulted in clear patterns of primary plant succession and of successional change in aboveground biomass, primary production, species diversity, and soil properties. Observational evidence indicates that plant survival, reproduction, and growth at the site might be reduced by physical stress related to sand burial, low availability of soil nitrogen, or colonization constraints in early succession and by competition for light, competition for soil resources other than nitrogen, and recruitment limitation in late succession. In addition to two classic tradeoff hypotheses of succession, three other tradeoff hypotheses were formulated and tested in field experiments. The five tradeoff hypotheses are: (1) physical stress vs. competition, (2) competition for soil nitrogen vs. competition for light or for soil resources other than nitrogen, (3) colonization constraints vs. competition, (4) growth rate vs. competition, and (5) herbivory vs. competition tradeoffs. The results of seedling-transplant and seed-addition experiments indicate that colonization vs. competition is the prevalent tradeoff faced by plants during succession and that physical stress related to sand burial vs. competition is an important tradeoff in early succession. In addition to studies of plant succession, I reconstructed a history of lake-level and regional climatic variability based on differences in dune morphology. The shore-parallel beach ridges were constructed during episodes of falling and low lake level associated with regional drought, and parabolic dunes devloped during episodes of unusually high lake level when wave erosion destablized established beach ridges. Surprisingly, parabolic dunes developed only during the 100-480 year time intervals of most frequent ridge formation and drought. This association suggests that these time intervals experienced increased variability in regional water balances, with unusually moist decades alternating with unusually dry decades, and with the time intervals of less frequent ridge formation lacking evidence of high lake level experienced less variability.