Reversing Woody Thickening in an Urban Grassy Woodland Remnant

ABSTRACT Woody thickening threatens the future of many open grassy ecosystems globally. The Queens Domain in Nipaluna/Hobart, Lutruwita/Tasmania, is one such grassy eucalypt woodland reserve that has experienced woody thickening since the cessation of Indigenous burning practices. This study evaluates the effects of burning, mowing, and mechanical thinning on the structure and composition of tree species in this remnant, with the aim to help develop appropriate prescriptions to reverse woody thickening while preserving conservation values. We used the Bitterlich wedge method to determine basal area (BA) (m 2 /ha) of tree species in 164 randomly located points. Linear models were created for the 2022 data, and the change between 2000 and 2022 for total BA, Allocasuarina verticillata BA, Eucalyptus viminalis BA, and all tree species BA (total BA) with the predictors mown (yes/no), thinned (4 classes), fire in last two years (yes/no), elevation, aspect, equinoxial radiation, number of fires 2013–2022, and years since last fire (before 2022). The same predictor variables were related individually to the direction of change (positive or negative) for other less common tree species, using one‐way analysis of variance or Chi‐square. Basal area for both A. verticillata and E. viminalis in 2022 was negatively affected by fire frequency. Allocasuarina verticillata and the total BA had reverse‐J size class distributions. Eucalyptus viminalis decreased dramatically with a size class distribution biased towards medium‐sized trees and benefited from both frequent fire and mowing. Conservation values of the Domain are promoted by reducing understorey trees and increasing the number and size of eucalypts. However, retaining some thickets is important for native animal shelter. Where low closed‐forest or closed‐scrub understoreys dominated by A. verticillata have formed, mechanical thinning of at least two thirds of stems, followed by fire or slashing/mowing of understorey tree regeneration at least every five years, seems the best option for maintaining grassy woodland with healthy eucalypts.