• Tree Establishment During Dry Spells At An Oak Savanna In Minnesota

      Ziegler, Susy Svatek; Larson, Evan R.; Rauchfuss, Julia; Elliott, Grant P.; Minnesota Dendroecology Laboratory, Department of Geography, University of Minnesota (Tree-Ring Society, 2008-06)
      Recent research has challenged the long-standing hypothesis that forests in the Upper Midwest of the United States developed during wetter periods and retreated during dry periods. We explored this debate by examining patterns of tree establishment on an oak savanna in east-central Minnesota within the context of variable moisture availability and fire suppression. We used superposed epoch analyses (SEA) to evaluate the mean moisture conditions for a 21-year window surrounding tree establishment dates. Before effective fire suppression (1809–1939), 24 of 42 trees with pith dates (62%) grew to 30-cm height during dry years (Palmer Drought Severity Index < -1), versus only 5 of 42 (12%) that established in wet years (PDSI > 1). Significantly more trees established during dry periods (negative PDSI values) than would be expected with the proportion of wet-to-dry years (x²= 10.738, df = 1, p-value = 0.001). Twenty of the complete sample of 74 trees with pith dates (27%) established during drought in the 1930s. We hypothesize that dry conditions limited plant productivity, which in turn decreased competition between grasses and tree seedlings and reduced rates of accumulation of fine fuels, enabling seedlings to grow tall enough to resist subsequent fires. We recommend SEA as a methodological approach to compare historical climate conditions with the timing of regeneration success in other regions of forest expansion.
    • When Is One Core Per Tree Suffifcient To Characterize Stand Attributes? Results Of A Pinus Ponderosa Case Study

      Woodall, C. W.; USDA Forest Service, Northern Research Station, 1992 Folwell Avenue, St. Paul, MN 55108, USA (Tree-Ring Society, 2008-06)
      Increment cores are invaluable for assessing tree attributes such as inside bark diameter, radial growth, and sapwood area. However, because trees accrue growth and sapwood unevenly around their pith, tree attributes derived from one increment core may not provide sufficient precision for forest management/research activities. To assess the variability in a tree’s inside bark radius, sapwood radius, and 10-year radial growth estimated by tree cores, two increment cores at 90 degree angles were collected from ponderosa pine (Pinus ponderosa) trees in eastern Montana (n = 2,156). Paired core measurements varied substantially with 13% mean difference for inside bark radius, 19% mean difference for sapwood radius, and 23% mean difference for estimates of radial increment. Furthermore, decreasing crown ratio, decreasing diameter, and increasing site slope were all found to increase differences in estimates derived from paired cores. Whether for management or research purposes, the number of cores that should collected per tree depend on a stand’s susceptibility to reaction wood, required measurement precision, and budgetary constraints.