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

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, 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 ca. 3-fold in density and ca. 4-fold in basal area over 60 years, with similar trends in both forest types and fire zones. On average, there were 3,638 stems/ha in 2010, of which 86% were small Allocasuarina and Callitris (< 11.4 cm and < 8.9 cm 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 to Callitris recruitment. In the absence of disturbance, ongoing increases in stand stocking may be expected.