Potential density and tree survival: an analysis based on South African spacing studies

Abstract

Because of the high cost of maintaining a series of unthinned, densely stocked stands over long periods of time, suitable data about potential forest density and tree survival for different planting espacements are difficult to find. Direct assessment of the potential density (which is always preferable to speculation, however ingenious the estimation may be) requires densely stocked unmanaged field studies that are remeasured regularly during long observation periods. An example of such a study is the Correlated Curve Trend (CCT) series of spacing studies established by O'Connor in South Africa. This contribution presents results for unthinned Pinus patula stands using an unusually large data set, specifically: (1) the potential density using the relationship between the quadratic mean diameter and trees per hectare, which does not confirm Reineke's constant of −1.605; (2) the relationship between average spacing and average tree diameter (known as Nilson's sparsity), which is found to be non-linear, thus contradicting previous assumptions; and (3) the development of the ratio basal area/trees per hectare, which appears to remain unchanged during the life of a planted forest, irrespective of the planting espacement. Finally, we present a tree survival analysis, based on the Weibull distribution function, for the Nelshoogte replicated CCT study, which has been observed for almost 40 years after planting and provides information about tree survival in response to planting espacements ranging from 494 to 2 965 trees per hectare.