Did historical tree removal promote woody plant encroachment in Australian woodlands?
AbstractQuestion: Woody plants have increased in density inmany ecosystems, but thefactors promoting encroachment are often debated. Since European colonization,Callitris glaucophylla has recruited abundantly in many Eucalyptus¿Callitriswoodlands in eastern Australia following changes to disturbance regimes analogousto changes in many other ecosystems globally. We used a dynamic standmodel to disentangle effects of disturbances on Callitris encroachment andasked, to what extent was Callitris encroachment enhanced by historicalremoval of pre-settlement trees and subsequent thinning of regrowth?Location: Woodlands dominated by Eucalyptus sp. and C. glaucophylla, whichoriginally occupied ca. 100 000 km2 in eastern Australia.Methods: We used a process-driven stand simulation model to simulate treedemography, with growth, survival and recruitment mediated by annual rainfall,competition and disturbance. Following parameter calibration and modeltesting, we orthogonally manipulated historical removal of pre-settlement treesand thinning of regrowth to identify how both processes may have influencedstructural changes over 120 yr of European settlement.Results: Removal of pre-settlement trees had little effect onmodelled encroachment,as trees increased to comparable densities whether pre-settlement treeswere retained or removed. In unthinned scenarios, Callitris regeneration formed`locked¿ stands of high density but low basal area. Thinning promoted growth ofretained Callitris, increased total stand basal area, and together with directremoval of large pre-settlement Eucalyptus and thinning of Eucalyptus regrowth,transformed stands fromEucalyptus to Callitris dominance.Conclusions: Removal of pre-settlement trees does not appear to have been anecessary precursor to modelled encroachment in Eucalyptus¿Callitris woodlandsin eastern Australia, perhaps because initial tree cover was low and Callitris canregenerate beneath isolated trees.Manual thinning was required to effect majorstructural change because Callitris self-thins extremely slowly; thinning will beless important in other ecosystems that self-thinmore rapidly. The impact of historicaltree removal on encroachment is likely to vary according to initial treecover, with greatest impacts in dense ecosystems with high tree cover. Theseresults highlight the value of simulation models for disentangling the effects ofmultiple disturbances on tree encroachment and other ecosystemdynamics.