Forty years of lowland monsoon rainforest expansion in Kakadu National Park, Northern Australia

Abstract

Monsoon rainforest is a key habitat in sustaining the natural and cultural values for which Kakadu National Park is World Heritage listed. The integrity of monsoon rainforest boundaries was thought to have been threatened by an increase in fire and feral animal disturbance over the last few decades. However, as no broad-scale assessment of rainforest boundary change had been undertaken, the rate and even direction of boundary change remained uncertain. In this study changes to the boundaries of 50 monsoon rainforest patches were assessed using temporal sequences of digitised aerial photography, with a view to understanding the relative importance of the drivers of change. Boundaries were compared for each of the years 1964, 1984, 1991 and 2004. Vegetation types were manually classified for each year with a 20 x 20 m point lattice, based primarily on the distance between tree crowns. Transition matrices, size-class distributions and fragmentation indices were calculated. Field samples of a subset of 30 rainforest patches supported the accuracy of the GIS-based mapping of rainforest boundaries. Rainforest patches increased in size between 1964 and 2004 by an average of 28.8%, with an average area increase of 4.0 ha. The expansion is likely to have been primarily driven by increases in variables such as rainfall and atmospheric CO2, but has been strongly mediated by fire regime. This project has provided land managers with an appreciation of the extent and causes of landscape- scale changes to rainforest boundaries. This will contribute to 'adaptive management' programs, especially with respect to fire management.