Effects of European colonization on indigenous ecosystems: post-settlement changes in tree stand structures in Eucalyptus–Callitris woodlands in central New South Wales, Australia
There has been considerable debate about pre-settlement stand structures in temperate woodlands in south-eastern Australia. Traditional histories assumed massive tree losses across the region, whereas a number of recent histories propose that woodlands were originally open and trees regenerated densely after settlement. To reconcile these conflicting models, we gathered quantitative data on pre-settlement stand structures in Eucalyptus–Callitris woodlands in central New South Wales Australia, including: (1) tree density, composition, basal area and canopy cover at the time of European settlement; and (2) post-settlement changes in these attributes. Location
Woodlands dominated by Eucalyptus species and Callitris glaucophylla, which originally occupied approximately 100,000 km2 in central New South Wales, Australia. Methods
We recorded all evidence of pre-settlement trees, including stumps, stags and veteran trees, from 39 relatively undisturbed 1-ha stands within 16 State Forests evenly distributed across the region. Current trees were recorded in a nested 900 m2 quadrat at each site. Allometric relationships were used to estimate girth over bark at breast height, tree basal area, and crown diameter from the girth of cut stumps. A post-settlement disturbance index was developed to assess correlations between post-settlement disturbance and attributes of pre-settlement stands. Results
The densities of all large trees (> 60 cm girth over bark at breast height) were significantly greater in current stands than at the time of European settlement (198 vs. 39 trees ha−1). Pre-settlement and current stands did not differ in basal area. However, the proportional representation of Eucalyptus and Callitris changed completely. At the time of settlement, stands were dominated by Eucalyptus (78% of basal area), whereas current stands are dominated by Callitris (74%). On average, Eucalyptus afforded 83% of crown cover at the time of settlement. Moreover, the estimated density, basal area and crown cover of Eucalyptus at the time of settlement were significantly negatively correlated with post-settlement disturbance, which suggests that these results underestimate pre-settlement Eucalyptus representation in the most disturbed stands. Main conclusions
These results incorporate elements of traditional and recent vegetation histories. Since European settlement, State Forests have been transformed from Eucalyptus to Callitris dominance as a result of the widespread clearance of pre-settlement Eucalyptus and dense post-settlement recruitment of Callitris. Tree densities did increase greatly after European settlement, but most stands were much denser at the time of settlement than recent histories suggest. The original degree of dominance by Eucalyptus was unexpected, and has been consistently underestimated in the past. This study has greatly refined our understanding of post-settlement changes in woodland stand structures, and will strengthen the foundation for management policies that incorporate historical benchmarks of landscape vegetation changes.