Woodland Fragmentation

With the exception of swamps and montane lagoons, prior to agricultural settlement, the Northern Tablelands were extensively vegetated with contiguous woodlands and forests composed of unique assemblages of flora and fauna. When travelling across the Northern Tablelands today it becomes strikingly evident that the vast majority of land is now comprised of open paddocks with few to any trees. The woodlands remaining today are composed of trees with similar age classes and occupy the steep slopes, ridgelines, or as lone standing trees amongst grazing livestock. When viewed from above using aerial imagery this contrast becomes even clearer, resembling a variegated patchwork of heavily fragmented vegetation isolated from each other by large tracts of grazing land (Figure 1). Put simply, this is the result of systematic landscape modification spanning 130 years of agricultural development. While the clearing of land across the tablelands was extensive, pastoralists tended to favour the fertile soils of plateaus, lowlands, and gently undulating topography, leaving the steep rocky and infertile ridges mostly vegetated. However, while some forests and woodlands were spared from clearing, almost all woodlands on the tablelands were exposed to regimes of sheep and cattle grazing at some time.

Why this is a problem

Fragmentation has dramatic effects on ecosystem function and affects population dynamics along with genetic and species diversity. Fragmentation occurs when previously contiguous woodland or forest is broken into smaller, greater separated patches or islands, often via intensified land use or removal of vegetation from clearing. From the time of agricultural settlement, the imposition of rapid clearing and grazing regimes has outpaced the natural cycles of disturbance these woodlands are capable of withstanding. The introduction of repeated clearing events coupled with ongoing disturbance from grazing animals has prevented woodlands from being able to recruit or recolonise the landscapes they once occupied (Figure 2). For the species of flora and fauna that call these woodlands home, landscape modification has resulted in dramatic population declines of flora and fauna, restricting remaining populations to fewer and smaller isolated woodland patches. As connectivity between woodland habitats is removed, the ability for species to naturally disperse across the landscape is impacted. The dispersal of flora and fauna across the landscape is an integral component of ecosystem function, the removal of which directly impacts the long-term persistence of many species. As patches of habitat become increasingly smaller and dispersed, both the availability and suitability of habitat becomes restricted or inaccessible, and species are forced to exist in only the most suitable locations. The effect of restricting species movement has the capacity to create genetic bottlenecks as populations have limited accessibility to breeding individuals (gene pool) with diversity in genes that is necessary to keep the gene pool resilient to perturbation. The result is a condition known as ‘inbreeding depression’, where genetic diversity within the population gene pool is gradually eroded forcing the inheritance of genes less suited to local conditions or resilient to natural levels of disturbance or disease.

A dominant feature of fragmented woodlands is their sharp transitional boundary to pasture that is maintained under frequent grazing or clearing regimes. For the original woodlands and forests on the Tablelands, clearing was performed by scalping the land rather than thinning or selectively removing vegetation, followed by the installation of grazing regimes and slashing. This resulted in the development of abrupt artificial transition zones at the woodland edge from overstory vegetation to modified pasture. In creating a sharp transitional edge, woodland and forest ecosystems become increasingly exposed to conditions that limit their capacity to buffer disturbance or extreme weather events (Figure 3). Exposure to edge effects such as wind, drought, fire, or flood has a proportionally greater impact on the vegetation community when the transition between pasture to woodland is sharp. Over time the ability of the woodland to buffer disturbance is gradually eroded resulting in rapid structural changes in woodland composition. As canopy and crown health are impacted, light resources increase within the understory that is quickly capitalised on by weeds or disturbance specialists. As weed incursion increases and native species richness declines, plant composition tends to favour those species most suited to higher frequencies of disturbance. The incursion of weeds has a direct impact on the ability of adult trees to recruit new seedlings, and for woodlands that are particularly exposed, woodland edges become populated by dead or dying trees that in turn reduce the total and functional size of the woodland. The breakdown of buffers that exist at woodland edges and transition zones has the capacity to radically shift the ecosystem processes that exist within intact vegetation communities. Functional relationships between flora and fauna are disrupted, eventually resulting in the shifting of ‘stable state’ from a grassy woodland with natural stand densities, to an open grassy woodland or dominant pasture system.

Grazing regimes are the dominant form of disturbance restricting the ability of woodlands to expand their woodland edge or increase to natural levels of stand density. For most of the remaining fragmented woodlands, shrub species and understory have been systematically removed by grazing animals and replaced with exotic pasture. Furthermore, the use of synthetic fertiliser ameliorants along with the deposition of manure favours the maintenance of pasture and severely limits or removes natural recruitment of young trees. Over time, in the absence of woodland regeneration, many remaining fragmented patches become functionally known as ‘the living dead’.

Reversing fragmentation

There are very few woodlands left on the tablelands today that have not been impacted by grazing and clearing to some extent. This is particularly true for the New-England Peppermint and Ribbon-gum – Moutain-gum – Snow-gum grassy woodland communities, the vast majority of which are now represented by small fragmented pockets of regrowth or senescing adults amongst grazing systems. Fragmentation is one of the most severe and significant problems facing these threatened woodlands, reversing or ameliorating the vortex of complications that emerge from landscape fragmentation is a challenging but important task. Pulling these woodlands out of their spiralling trajectory towards extinction requires concerted efforts from local landholders and ecosystem managers that are willing to adopt farming practices that actively promote woodland regeneration. While the task of this seems insurmountable, the tools to achieve this are well known. We provide a number of guides and resources on our website that include outline techniques on performing woodland regeneration in cost-effective ways. To reverse the trend of woodland decline we must be thinking generations ahead, for the best time to plant a tree was yesterday but the second-best time is now.

Key points

Fragmentation impacts…

• Genetic diversity within woodlands

• The ability of woodlands to withstand edge effects such as drought, wind storms, dieback, and fire

• The rate of weed incursion

• Available habitat for flora and fauna

• The movement of flora and fauna across the landscape

• The ability for woodland to naturally regenerate

References

DPE, Department of Planning & Environment (2011). NSW Woody Vegetation Extent & FPC 2011. Retrieved from https://datasets.seed.nsw.gov.au/dataset/nsw-woody-vegetation-extent-fpc-20119bb42 on 06/07/2022