Historical changes in woodland tree densities

By Ian Lunt, Nigel Jones & Maree Petrow, Charles Sturt University, Dec 2003

There has been considerable debate recently about how many trees grew in the woodlands of central NSW at the time of European settlement. This debate has been triggered, among other things, by

Fig. 1. Estimated minimum number of trees / ha at the time of European settlement in 22 remnant woodlands in south-central NSW

Fig. 1. Estimated minimum number of trees / ha at the time of European settlement in 22 remnant woodlands in south-central NSW

Fig. 1. Estimated minimum number of trees / ha at the time of European settlement in 22 remnant woodlands in south-central NSW

vegetation clearance legislation and suggested approaches to salinity management. Recently, some people have argued that tree densities were originally very low (about 8 trees/ha) and that densities increased greatly soon after settlement, after regular fires ceased. Others have argued that densities were always much greater than this (about 30 trees/ha). Unfortunately, little accurate information is available.

To help resolve this issue, we have been documenting the density of trees in central NSW at the time of European settlement, by counting stumps in remnant forests scattered across the region. Stumps from pre-settlement trees can be identified by their size, cutting style, degree of decomposition and presence of fire scars. This report summarises preliminary data from 22 study sites from the Victorian border to north of Narrandera (ongoing work will extend our results to west of Dubbo).

We found that pre-settlement tree densities varied greatly between sites, from 18 to 81 trees/ha, averaging 39 trees/ha (Fig. 1). These are estimates of the minimum number of trees that existed at the time of settlement, and actual densities may have been greater still. Most stands were originally dominated by eucalypts. In contrast, dense cypress-pine dominates all sites today. Tree densities increased greatly in the late 1800s and, on average, almost 300 trees/ha currently grow at the study sites.

These results confirm that tree densities increased greatly after European settlement. However, they also illustrate that pre-settlement stands were not as open as has been suggested. Many sites supported well-stocked eucalypt woodlands prior to settlement, not open woodlands or grasslands with sparse, scattered trees.

Fig. 1. Estimated minimum number of trees / ha at the time of European settlement in 22 remnant woodlands in south-central NSW

The results also suggest that the wave of dense cypress-pine regeneration which occurred across south-central NSW in the late 1800s was triggered (at least in part) by widespread ringbarking, which had taken place at most sites. Forestry studies have shown that cypress-pine will not regenerate densely beneath well-stocked eucalypt stands and, in some sites, it seems likely that the 1800s regeneration could only have survived if the eucalypts were first killed by ringbarking.

This study provides the first accurate information on regional changes in tree densities in grassy woodlands, and will greatly enhance our understanding of the effects of European settlement on woodland landscapes.

This study was supported by a Discovery grant from the Australian Research Council.