How widespread is woody plant encroachment in temperate Australia? Changes in woody vegetation cover in lowland woodland and coastal ecosystems in Victoria from 1989 to 2005

Abstract

Aim Encroachment or densification by woody plants affects natural ecosystems around the world. Many studies have reported encroachment in temperate Australia, particularly in coastal ecosystems and grassy woodlands. However, the degree to which published studies reflect broad-scale changes is unknown because most studies intentionally sampled areas with conspicuous densification. We aimed to estimate changes in woody vegetation cover within lowland grassy woodland and coastal ecosystems in Victoria from 1989 to 2005 to determine whether published reports of recent encroachment are representative of broad-scale ecosystem changes.Location All lowland grassy woodland and coastal ecosystems (c. 6.11 · 10 5 ha)in Victoria, Australia. Four major ecosystems were analysed: Plains woodlands, Herb-rich woodlands, Riverine woodlands and Coastal vegetation.Methods Changes in woody vegetation cover from 1989 to 2005 were assessed based on state-wide vegetation maps and Landsat analyses of woody vegetation cover conducted by the Australian Greenhouse Office's National Carbon Accounting System. The results show changes in woody cover within mapped patches of native vegetation, rather than changes in the extent of woody vegetation resulting from clearing and revegetation.Results When pooled across all ecosystems, woody vegetation increased by 18,730 ha from 1989 to 2005. Woody cover within Riverine woodlands and within Plains woodlands each increased by >7000 ha. At the patch scale, the mean percentage cover of woody vegetation in each polygon increased by >5% in all four ecosystems: Riverine woodlands (+9.2% on average), Herb-rich woodlands (+7.6%), Plains woodlands (+6.7%) and Coastal vegetation (+5.9%). Regression models relating degree of encroachment to geographic and climatic variables were extremely weak (r 2 £ 0.026), indicating that most variation occurred at local scales rather than across broad geographic gradients.Main conclusions At the scale of observation, woody vegetation cover increased in all lowland woodland and coastal ecosystems over the 16-year period. Thus, published examples of encroachment in selected coastal and woodland patches do appear to reflect widespread increases in woody vegetation cover in these ecosystems. This densification appears to be associated with changes in land management rather than with post-fire vegetation recovery and is likely to be ongoing and long-lasting, with substantial implications for biodiversity conservation and ecosystem services.

Evidence of increase in woody vegetation in a river corridor, Northwest England, 1984–2007

“…Likewise, vegetation changes have been studied both in riverine and other environments, many studies using satellite imagery (Bertoldi, Drake, & Gurnell, 2011) but many, especially on decadal timescales, have used aerial photographs, as here (Dixon & Johnson, 1999;Garófano-Gómez et al, 2013;Gonzalez, Gonzalez-Sanchis, Cabezas, Comin, & Muller, 2010;Kondolf, Piegay, & Landon, 2007;Lunt, Winsemius, McDonald, Morgan, & Dehaan, 2010;Yang & Tien, 2010). Increases in woody riparian area have been found but many of the cases are associated with change in river regime (Dixon & Johnson, 1999;Johnson, 1994;Perona, Molnar, Savina, & Burlando, 2009), especially due to dam control (Dixon, Johnson, Scott, Bowen, & Rabbe, 2012;Gonzalez et al, 2010;Merritt & Cooper, 2000).…”

A framework for testing the influence of Aboriginal burning on grassy ecosystems in lowland, mesic south–eastern Australia

“…Woodgate and Black 1988). Thus, woodyplant encroachment or canopy densification is only possible over a small percentage of the landscape (Lunt et al 2010). Patterns of change can be readily interpreted by comparing the historic grasslands with historic aerial photographs (~1940s) and contemporary imagery.…”

“…Thus, in areas where fire has been excluded or greatly contained over the past two centuries, there have been reports of woody-plant encroachment because of prolific woody-species seed production and the competitive suppression of grasses under intense stock grazing (Bragg and Hulbert 1976;Lunt 1998;Briggs et al 2002Briggs et al , 2005Hoch et al 2002;Franco and Morgan 2007;Lunt et al 2010Lunt et al , 2012.…”

“…The regeneration of the dominant trees in the lowland plains of south-eastern Australia is strongly episodic, with dense and widespread regeneration cohorts being triggered by major floods delivering ample summer soil moisture and reduced grass competition as a result of preceding drought (Lunt et al 2010). Given these seedlings establish under favourable climate-edaphic conditions that promote an erect, single-stemmed habit with little lignotuber development (Carr et al 1984), they tend to be more vulnerable to fire mortality until flame height is exceeded (perhaps within 3-5 years).…”

“…Global models of fire-dependent C 4 grassy ecosystems show greatest extent in the tropics and southern hemisphere, including much of Australia's eastern seaboard, which has the climate potential to support woodlands and forest, and is regulated by 'bottom-up' and 'top-down' controls on structure and composition (Jones 1999;Bond et al 2003Bond et al , 2005Waldram et al 2008;Leonard et al 2010;Lunt et al 2012;Fensham et al 2015). Thus, altering the 'top-down' controls, such as fire regime, drives a response in the structure and composition of the grassy sward, affecting grassland-woodland boundaries, and the extent of woody cover (south-eastern Australian examples: Fensham and Kirkpatrick 1992;Foreman 1996;Hadden 1998;Lunt 1998;Lunt and Morgan 1999;Franco and Morgan 2007;Price and Morgan 2009;Lunt et al 2010Lunt et al , 2012Williams et al 2015;Kirkpatrick et al 2016).…”