Spatial dispersion of trees in an old-growth temperate hardwood forest over 60 years of succession

An analysis of spatial dispersion was conducted for individual tree species in the old-growth forest at the Davis-Purdue Research Forest in Indiana. This 20.6 ha stand has been left largely undisturbed by exogenous factors since its acquisition by Purdue in 1917. It is the only long-term study plot of its size for a temperate hardwood forest with x–y coordinates (rectangular plot) for all species above 10 cm diameter. Full censuses have been conducted spanning 60 years (1926, 1976 and 1986).

Ripley’s L(t) function revealed that most species are characterized by some form of aggregation, agreeing with a prior evaluation from 1981. Heterogeneity of spatial structure was evident between two large plots, indicating that differences in site quality and history had influenced spatial structure. Shade-intolerant species were numerically dominant and spatially aggregated in 1926 but have declined over the 60-year interval and become more random in spatial dispersion. Shade-tolerant species have increased in number and become more aggregated over time, or they exhibit little change in spatial structure. Examples of the latter include Acer saccharum and Ulmus americana, species that experienced explosive population growth. These contrasting patterns are masked by stand-level patterns that show a trend toward uniformity over the same time frame.

These data reveal that changes in dispersion accompany the demographic failure experienced by numerous tree species in Central Hardwood old-growth stands, and these changes may feed back into a negative population cycle and further impede regeneration. The simultaneous manipulation of dispersion and density should be considered as a tool for influencing forest succession and promoting regeneration of desired tree species.