Origin of sigmoid diameter distributions
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Res. Pap. NE-718. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern Research Station. 10 p.
Diameter distributions--numbers of trees over diameter at breast height (d.b.h.)--were simulated over 20-years using six diameter-growth schedules, six mortality trends, and three initial conditions. The purpose was to determine factors responsible for the short-term development of the arithmetic rotated sigmoid form of diameter distribution characterized by a plateau, near plateau, or bump in the mid-diameter d.b.h. classes. A distinct rotated sigmoid developed when the diameter-growth schedule dropped precipitously in the mid-diameter classes; this might reflect a decadent overstory and thrifty understory. When the initial diameter distribution was distinctly sigmoid, diameter-growth schedules characterized by slow growth in the small-diameter classes tended to maintain sigmoid characteristics. A parabolic growth trend with or without parabolic mortality, also produced moderate rotated sigmoid characteristics. Under most other growth or mortality schedules, the 20-year diameter distribution was J shaped. The results reinforce the concept that diameter distributions tend to maintain or return to the J shape particularly with respect to New England northern hardwoods.
Leak, William B. 2002. Origin of sigmoid diameter distributions. Res. Pap. NE-718. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern Research Station. 10 p. https://doi.org/10.2737/NE-RP-718.