Extent and composition of dead wood in Australian lowland tropical rainforest with different management histories

Dead wood (coarse woody debris or CWD and dead standing trees or snags), is regarded as an important ecological component of temperate forests, yet its status in tropical forests has gone largely unreported. Its extent and composition was studied in a series of sites in the lowland rainforests of Australia's Daintree region. The aim was to compare the amounts of CWD and snags in forests with historically different intensities of management: undisturbed old-growth, selectively logged and re-growth resulting from previous total clearance. 1671 individual pieces of CWD and 173 snags were recorded and measured at 81 sampling locations. Dead wood was very patchily distributed, and occurred at considerably lower levels than in most equivalent temperate or boreal forests. Old-growth forest generally contained slightly greater mass and volume of CWD, and significantly greater volumes in the larger size-classes (> 40 cm diameter), than either logged or re-growth forest; snag volumes were so variable that no relationship with management history was detectable. Overall volumes of CWD ranged from 35.68 m(3) ha(-1) in old-growth to 20.16 m(3) ha(-1) in re-growth sites, with logged sites intermediate; for snags, volumes ranged from 27.00 m(3) ha(-1) in logged to 10.01 m(3) ha(-1) in re-growth, with old-growth intermediate. Overall dead wood size-class distribution was closely correlated with living tree stand structure across all nine study sites. CWD volume, especially that in larger size-classes, was correlated with basal area and with basal area contributed by trees in the larger size-classes. These in turn are linked to management history, reaching their highest values in old-growth sites and their lowest in re-growth. These results suggest that any management system that systematically reduces the proportion of larger trees will, over the longer term, affect the forest's ability to generate dead wood, especially in the larger size-classes. The proportion of forest-dwelling species that are dependent on dead wood is unknown, but may be a fifth or more, with many associated only with the larger size-classes. Thus managing tropical rainforests according to ecologically sustainable principles requires a commitment to maintaining stand structures that allow the continued generation of dead wood in a full range of sizes. (C) 2001 Elsevier Science B.V. All rights reserved.