DECOMPOSITION OF SLASH RESIDUES IN THINNED REGROWTH EUCALYPT FOREST IN WESTERN-AUSTRALIA

1. Stand thinning is being used increasingly by forest managers to max imize wood production from regrowth eucalypt forests in Australia. Man y of the ecological effects of this intensive method of management, in cluding changes in nutrient cycling and the susceptibility of sites to fire due to addition of slash residues to the forest floor, are not w ell understood. 2. Decomposition of slash residues was studied at one site in regrowth karri (Eucalyptus diversicolor) forest, a tall, open forest growing in south-western Australia. 3. Thinning reduced tree de nsity from 4000 to 380 stems ha(-1), reduced tree basal area from 32 t o 17 m(2) ha(-1) and removed 2000 stems ha(-1) of understorey plants. Approximately 78 t ha(-1) slash residues from the thinning operation w ere added to the existing 40 t ha(-1) of forest floor litter. 4. Rates of decomposition of 27 components of slash were determined during a 2 -year period, following thinning using the mesh bag technique. Compone nts studied consisted of 5 leaf fractions from the overstorey and main understorey species, 15 twig and stem components from the understorey , 6 twig and stem components from the overstorey and bark from the ove rstorey. 5. The rate of weight loss of each slash fraction was assesse d using single and double exponential decay models. The single exponen tial model adequately described the decay of karri stem and twig wood, and of understorey stem wood. Decay of leaf fractions, bark and small -diameter understorey wood were best described by the double exponenti al model. 6. Fractions of labile components in leaf, bark and small un derstorey wood were lost rapidly during the initial decay phase (half- lives 1-6 weeks); thereafter decomposition proceeded more linearly and was dominated by the rate of decay of the resistant fraction of each slash component. Model predictions of the fraction of each slash resid ue present as labile compounds were significantly related (r(2) = 0.94 ) to the amounts determined in laboratory extractions of the residues. 7. The decay rate of wood components was slower than for leaf fractio ns and was dependent on the size of the wood and its source. Rate of d ecay decreased with wood diameter and was more rapid for understorey w ood than overstorey wood.8. Half-lives of leaf components of slash ran ged from a few weeks up to about 18 months, whereas half-lives of wood in the slash residues were much greater and exceeded 30 years for the largest diameter overstorey wood fraction. 9. Fire control and nutrie nt conservation are critical considerations in formulating strategies for management of thinning residues in eucalypt forests. Thinning resi dues deposited on the forest floor increase the risk of fire. However, where this hazard can be tolerated for limited periods, a delay in fu el-reduction burning for up to 2 years following thinning will allow t ime for much of the leaf material to decompose. This will reduce the f ire risk and enhance conservation of the nutrient capital.