Changes in forest structure over 60 years: tree densities continue to increase in the Pilliga forests, New South Wales, Australia

Abstract

Studies of long-term vegetation changes are critical for enhancing our understanding of successional dynamics in natural ecosystems. By comparing forest inventory data from the 1940s against field data from 2005, we document changes in stand structure over 60 years in forests co-dominated by Callitris glaucophylla J.Thompson & L.Johnson, Allocasuarina luehmannii (R.Baker) L.Johnson and Eucalyptus crebra F.Muell., in central Pilliga, New South Wales (NSW), Australia. Sampling was stratified across two forest types and across a 1951 wildfire boundary, to assess the effects of initial stand structure and early disturbance on stand dynamics. Stems in the size range tallied in the 1940s (>8.9 cm DBH for Callitris and >11.4 cm for Allocasuarina and Eucalyptus) of each genus increased about three-fold in density and about four-fold in basal area over 60 years, with similar trends in both forest types and fire zones. On average, there were 3638 stems ha–1 in 2010, of which 86% were small Allocasuarina and Callitris (<11.4-cm and <8.9-cm diameter at breast height, DBH, respectively). These results illustrate a continuation of forest encroachment that was initially documented in the late 1800s. However, increases in Allocasuarina have received little attention compared with Callitris recruitment. In the absence of disturbance, ongoing increases in stand stocking may be expected.