Did historical tree removal promote woody plant encroachment in Australian woodlands?

Abstract

Question: Woody plants have increased in density in many ecosystems, but the factors promoting encroachment are often debated. Since European colonization, Callitris glaucophylla has recruited abundantly in many Eucalyptus-Callitris woodlands in eastern Australia following changes to disturbance regimes analogous to changes in many other ecosystems globally. We used a dynamic stand model to disentangle effects of disturbances on Callitris encroachment and asked, to what extent was Callitris encroachment enhanced by historical removal of pre-settlement trees and subsequent thinning of regrowth? Location: Woodlands dominated by Eucalyptus sp. and C. glaucophylla, which originally occupied ca. 100 000 km 2 in eastern Australia. Methods:We used a process-driven stand simulation model to simulate tree demography, with growth, survival and recruitment mediated by annual rainfall, competition and disturbance. Following parameter calibration and model testing, we orthogonally manipulated historical removal of pre-settlement trees and thinning of regrowth to identify how both processes may have influenced structural changes over 120 yr of European settlement.Results: Removal of pre-settlement trees had little effect on modelled encroachment, as trees increased to comparable densities whether pre-settlement trees were retained or removed. In unthinned scenarios, Callitris regeneration formed 'locked' stands of high density but low basal area. Thinning promoted growth of retained Callitris, increased total stand basal area, and together with direct removal of large pre-settlement Eucalyptus and thinning of Eucalyptus regrowth, transformed stands from Eucalyptus to Callitris dominance.Conclusions: Removal of pre-settlement trees does not appear to have been a necessary precursor to modelled encroachment in Eucalyptus-Callitris woodlands in eastern Australia, perhaps because initial tree cover was low and Callitris can regenerate beneath isolated trees. Manual thinning was required to effect major structural change because Callitris self-thins extremely slowly; thinning will be less important in other ecosystems that self-thin more rapidly. The impact of historical tree removal on encroachment is likely to vary according to initial tree cover, with greatest impacts in dense ecosystems with high tree cover. These results highlight the value of simulation models for disentangling the effects of multiple disturbances on tree encroachment and other ecosystem dynamics.Recent increases in the density of woody plants (variously termed encroachment, densification or thickening) have been observed in many ecosystems around the globe. In regions settled relatively recently by Europeans (e.g. Americas and Australia), encroachment has resulted in conversion of open grassland, savanna and woodland systems to woody shrublands and forests (

Long term thinning and logging in Australian cypress pine forest: Changes in habitat attributes and response of fauna

“…Our study was primarily driven by a priori hypotheses that in the long-term, thinned and logged forest will have fewer foraging, shelter and nesting resources provided by large live and dead trees and small trees and shrubs as a consequence of active removal of these elements (Eyre, 2005;Cockle et al, 2008;Harrod et al, 2009;Politi et al, 2010;Adams and Law, 2011;Ross et al, 2012), but more habitat resources provided by coarse woody debris and leaf litter as a consequence of logging residue and fire exclusion (Grove, 2001;Eyre et al, 2010;Collins et al, 2012). We predicted indirect effects of thinning and logging on faunal groups, not only through changed availability of habitat attributes but also through the facilitation of deleterious interspecific interactions; specifically, the competitive exclusion of small birds by the despotic noisy miner Manorina melanocephala.…”

“…Cypress pine is also slow growing. It is an obligate seeder capable of episodic recruitment events and tolerant of subsisting in highly dense stands for long periods of time where growth and self-thinning is negligible, meaning silvicultural thinning of regrowth is widely used to promote growth in retained cypress trees (Thompson and Eldridge, 2005;Ross et al, 2008Ross et al, , 2012Ngugi et al, 2013). Since cypress pine forests in timber reserves are protected from fire and are both selectively logged and thinned, they are useful for studies on the structural and compositional change due to mechanical removal of stems, and faunal responses to this change, because other potentially confounding influences are minimised.…”

Demographic patterns of a widespread long‐lived tree are associated with rainfall and disturbances along rainfall gradients in SE Australia

“…A current lack of high intensity fires, prevalent before agriculture and land fragmentation (Noble ; Noble and Dargavel ) also promotes these changes (Ross et al. ). This phenomenon is similar to that found in fire‐sensitive Pinus ponderosa populations in the long‐term absence of fire in mixed Pinus ‐ Quercus forests in the western U. S. A.…”

Interactions between dense Callitris regeneration and Eucalyptus and Callitris canopy trees in semiarid woodlands

“…This encroachment coincided with widespread changes in fire and grazing regimes and canopy tree removal (Noble 1997). In many places woodland dominants, C. glaucophylla and a range of Eucalyptus species, were both removed or ringbarked at different rates, shifting dominance from Eucalyptus species to C. glaucophylla (Lunt et al 2006;Ross et al 2012). Although the dynamics of monospecific stands of C. glaucophylla have been well researched at the stand scale (Lacey 1972(Lacey , 1973FCNSW 1988), there is comparatively little research on dynamics within mixed stands of Eucalyptus species and C. glaucophylla and this is restricted to temperate climates (Clayton-Greene 1981;Clayton-Greene & Ashton 1990;Lunt et al 2011).…”

“…For example, canopies more commonly facilitate seedling establishment at lower than higher rainfall (Holmgren et al 1997). Canopy tree densities are likely to decline with declining rainfall (Specht & Specht 1999) or may be influenced by disturbances, like logging (Lunt et al 2006;Ross et al 2012). Drought may also have a greater negative effect on plant survival at lower than higher rainfall (Lacey 1972).…”