Impacts of fire and timber harvesting on long-term vegetation change in an Australian mixed-species forest

How similar are the impacts of fire and timber harvesting in forests already undergoing long-term vegetation change? Callitris forests and woodlands in eastern Australia have experienced vegetation thickening similar to that suggested for Pinus ponderosa forests in southwestern USA, probably through a combination of grazing, reduced fire frequency and episodic recruitment in peak rainfall years. They have also long been important timber-production forests. This study quantified thickening between the 1940s and 2005 and examined the effects of fire and timber harvesting on species composition, stand density and basal area in the Pilliga Forest, the largest existing remnant of this forest type. This was done using original forestry survey data, field survey, and harvesting and fire history records. Overall, stem density increased approximately 3-fold and basal area almost 4-fold over 55-60 years. The direct effects of fire and harvesting differed, but both favored Callitris glaucophylla over Eucalyptus crebra. A large fire promoted regeneration of shade-tolerant, fire-sensitive C. glaucophylla but not light-demanding resprouter E. crebra. Density and basal area of merchantable-sized C. glaucophylla stems had increased or remained stable despite 55 years of harvesting, and were probably promoted by commercial thinning of older regrowth. Certain harvesting and silvicultural practices decreased E. crebra representation in some places. However, neither fire nor harvesting was as important as 1940s forest structure in explaining 2005 forest structure, in terms of either larger stems or regrowth. This study provides important baseline information for future management as much of this forest moves from commercial mixed-use to conservation-focused management.