Tree survival and maximum density of planted forests – Observations from South African spacing studies

Abstract

Among the most important aspects of risk and hazard studies relating to forest ecosystems are maximum forest density and density-dependent tree survival. Long-term observations about the maximum density of unthinned Pinus patula and P. elliottii field plots based on the Correlated Curve Trend (CCT) spacing studies which were established almost 8 decades ago by O’Connor (Forest Research with Special Reference to Planting Distances and Thinning, 1935) in South Africa. Three specific approaches were introduced for analysing maximum density and tree survival, namely the ‘limiting line’, Nilson’s sparsity and tree survival with the Weibull function. The main results are: a) Maximum densities differ greatly among the two species grown on the same site and within the same species grown on different sites; it is possible to relate these differences to site index in both species. b) The relationship between the quadratic mean diameter and the minimum average spacing of surviving trees (known as Nilson’s Sparsity) appears to be surprisingly similar in both species. c) An analysis of tree survival in response to different initial planting espacements shows that the Weibull survival function parameters can be estimated if the initial planting density is known. This result is presented for each of the eight large experiments used in this study. This study contributes to a better understanding of tree survival and maximum density which are the key factors required for estimating risk and uncertainty. The risk of tree mortality is not constant, but varies with tree species, planting density, tree age and growing site. For estimating that risk, therefore, continuous long-term observation on different sites and with varying planting densities, as provided by the unthinned CCT series, are essential.