Extent and condition of native vegetation

Background

• Background
• Extent of native vegetation
• Revegetation in the ACT
• Condition of native vegetation
• Case studies

Native vegetation is integral to ecosystem services such as the protection of biodiversity, protection of water quality and soil health, and sequestration of carbon. Declines in the extent and quality of native vegetation have profound implications for the health of the ecosystem. 

The loss of vegetation is considered to be the main threat to biodiversity in Australia. 

Historic land clearing for agriculture and urban development has produced a legacy of fragmented native vegetation in some areas of the ACT. The diversity and resilience of ecological communities relate directly to their spatial configuration, patch size, contiguity, condition and connectivity. Fragmented landscapes prevent the movement of species, limiting opportunities for mating and dispersal, and potentially creating genetic isolation. Research suggests that most animals of southern Australian woodlands and forests will not usually cross a canopy gap of more than 100 metres, and will not travel more than 1.1 kilometres from a patch of at least 10 hectares of suitable living habitat.Barrett T. and J. Love, 2012, Fine Scale Modelling of Fauna Habitat and Connectivity Values in the ACT Region, prepared for Conservation Planning and Research, EPSDD, ACT Government, Canberra

Although large-scale clearing is not an issue in the ACT, native vegetation remains under continuing pressure from urban expansion. Native vegetation in conservation areas has not been extensively cleared and is more intact than native vegetation on private land and those lands separated by urban developments. In these largely intact landscapes, vegetation communities are more likely to be resilient to natural disturbances such as fire and drought. In fragmented landscapes, native vegetation remnants are more vulnerable to natural disturbance as well as pressures arising from agriculture and residential activities. This results in the decline of vegetation, or being at risk of decline, in extent, quality and regenerative capacity. Declining vegetation quality is now a key driver of vegetation loss in the ACT. Fragmentation can also exacerbate the impacts of land use change and climate change by restricting opportunities for fauna to migrate or adapt.

Condition and trends

It was not possible to determine changes in the extent of native vegetation over the reporting period (2015–16 to 2018–19) for inclusion in this report. Nor was it possible to provide an overall assessment of vegetation condition.

The 2015 ACT State of the Environment report noted that comprehensive documented information on the condition of vegetation and the amount of clearing undertaken was lacking.Office of the Commissioner for Sustainability and the Environment, 2015, ACT State of the Environment Report 2015, ACT Government, Canberra The cumulative impact of approved clearing of vegetation had also not been documented or assessed.

These issues limit the ability to report on vegetation extent and condition changes in the ACT. However, aspects of vegetation condition assessments have been undertaken in the ACT including studies of dieback, riparian connectivity, tolerable fire intervals and growth stage, and the condition of grassland and woodland endangered ecological communities (see section on Condition of native vegetation).

Extent of native vegetation

Native vegetation losses are estimated to be small and mainly due to land use change from urban development. Urban development is likely to be an increasing pressure on native vegetation and may have major impacts on vegetation and ecosystem health, as well as the ecosystem services they provide. It is important that there is consideration of the cumulative impacts of small modifications to habitat, because these can lead to thresholds being crossed unknowingly and unintentionally for at least some aspects of vegetation and ecosystem health.

Most of the ACT’s vegetation loss has been from historic clearing and ecosystem modification for agriculture and urban development. Although there has not been a comprehensive assessment of the native vegetation extent in the ACT before European settlement, there are some examples of significant losses. For example, before European settlement, Natural Temperate Grasslands were thought to cover over 25,000 hectares or 11% of the ACT area,Gellie, N. J. H., 2005, ‘Native Vegetation of the Southern Forests: South-east Highlands, Australian Alps, South-west Slopes, and SE Corner Bioregions’, Cunninghamia, 9(2): 219–54. but today they only cover around 1,100 hectares, less than 1% of the ACT (Figure B13).

For Lowland Box Gum Woodlands, the pre-European settlement distribution was thought to be over 47,000 hectares or 20% of the ACT area,Gellie, N. J. H., 2005, ‘Native Vegetation of the Southern Forests: South-east Highlands, Australian Alps, South-west Slopes, and SE Corner Bioregions’, Cunninghamia, 9(2): 219–54. but these woodlands now only cover some 11,500 hectares, around 5% of the ACT (Figure B14). Most of the native vegetation changes are thought to be on lowlands due to the abundance of grass and absence of dense trees for agriculture, and later for urban development. It is estimated that there has been little change in the distribution of upland vegetation types.

While the loss of native vegetation remains of concern for urban development, it is unlikely to be the largest source of native vegetation change in the ACT. Chronic degradation of habitat condition, mainly in fragmented landscapes is a significant problem in the ACT. This degradation is compounded by climate change impacts such as decreasing rainfall and higher temperatures. Such degradation has led to an increased occurrence of dieback in the ACT (see section on Condition of native vegetation).

Revegetation in the ACT

The ACT Government has undertaken extensive revegetation over the reporting period (2015–16 to 2018–19). This has been undertaken in cooperation with various organisations and programs including the Australian Government’s million Trees Project, Greening Australia, and the Australian Government’s Clean Energy Future Biodiversity Fund. The works will provide vegetation corridors, riparian restoration and bank stabilisation, woodland restoration, and post-fire rehabilitation. Restoration of habitat and connectivity increases effective habitat size and access for native species, enables migration and movement to avoid temporary stressors, and aids the recovery potential and recolonisation of degraded areas. 

Between 2015 and 2019, revegetation included planting of: 

• nearly 75,000 tube stock and 20 kilograms of native seed to revegetate nearly 1,100 hectares in the Murrumbidgee River Corridor nature reserves and other areas of public lands. 
• nearly 25,000 tube stock and 180 kilograms of native seed to restore habitat in former pine plantations and other degraded areas in the Lower Cotter Catchment. 

In addition, there were revegetation activities on some 1,500 hectares of private land between 2015 and 2018, mainly through works undertaken by Greening Australia. See Community leadership in sustainability and science for more information. 

These revegetation activities will increase native vegetation extent and improve the condition of ecosystems in the future.

Condition of native vegetation

Dieback

Dieback is the gradual deterioration of health in trees, sometimes leading to tree death, and is usually caused by a combination of factors including disease and pathogens, insect attack, and additional drought and temperature stress from climate change. 

While dieback affects many species in the ACT (including Eucalyptus viminalis, E. bridgesiana and E. melliodora), recent observations have recognised a significant increase in the incidence of dieback in Blakely’s Red Gum (E. blakelyi). Dieback of Blakely’s Red Gum appears to affect any age class and is occurring across rural landscapes, urban environments and reserves within the ACT. High rates of mortality in younger trees have resulted in a lack of successful maturation across the ACT landscape. If younger trees are unable to replace the older, dying trees, the population will slowly thin out. 

A great deal of uncertainty surrounds the cause of Blakely’s Red Gum dieback in the ACT and is thought to be the result of a number of stress-inducing factors, impacts associated with climate change and reduced resilience within the landscape. The ACT Government and the University of Canberra has been undertaking research to better understand the causes and occurrence of red gum dieback in the ACT.Cowood A.L. et al., 2018, Blakely’s Red Gum Dieback in the ACT, Report to Environment, Planning and Sustainable Development Directorate, Institute for Applied Ecology, University of Canberra, Canberra.

Main findings include: 

• confirmation that Blakely’s Red Gum is currently experiencing dieback across the ACT (Figure B15). 
• Yellow Box is experiencing an increase in condition and suitability to future projected climates. 
• climate change and the compounded effects of an increasing temperature and variable precipitation over time is causing stress in trees and impacting their growth, and 
• revegetation is needed to improve landscape connectivity and also assist dispersal to areas of climate refugia and distribution expansion.

Figure B15: Temporal condition mapping of Blakely’s Red Gum (Eucalyptus blakelyi) as at 2017.

Riparian connectivity

Decline in connectivity within riparian areas is primarily due to historic clearing of vegetation for agriculture and urban development. Low river flow conditions, climate change impacts (higher temperatures, increased drought and storm events), drought and fire also impact on riparian vegetation causing loss of habitat and increased disconnection within riparian areas. 

Riparian connectivity has been assessed for the Murrumbidgee and Molonglo Rivers in the ACT. Main findings include: 

• significant areas of riparian vegetation on the Murrumbidgee and Molonglo Rivers have poor connectivity. 
• 58% of surveyed riparian vegetation on the Molonglo River was found to have high to very high connectivity, compared to 45% for the Murrumbidgee River (Figures B16 and B17). 
• the Murrumbidgee River had a higher proportion of riparian vegetation with low and poor connectivity at 36% of the area assessed, compared to 22% for the Molonglo River. 
• areas of the Murrumbidgee River and lower Molonglo River that fall outside reserved areas generally showed very low levels of connectivity. This was particularly the case for the Murrumbidgee River which had 32% of low and poor connectivity inside reserves, compared to nearly 80% outside reserves (Figure B18). 
• areas with poorest riparian connectivity include Molonglo River immediately below Scrivener Dam, Murrumbidgee River within Stony Creek Nature Reserve and the Lanyon Landscape Conservation area, and 
• several areas with extensive riparian habitat and smaller pockets that should be maintained include the Murrumbidgee River at Gigerline Nature Reserve, downstream areas of the Bullen Range Nature Reserve, Woodstock Nature Reserve (particularly adjacent to the mouth of the Molonglo River and downstream areas), and Uriarra Creek within Swamp Creek Nature Reserve. For the Molonglo River, areas within the Molonglo River Reserve should be maintained.

Condition of grassland and woodland endangered ecological communities

The ACT Government has been monitoring the condition of Box Gum Woodland and Natural Temperate Grassland endangered ecological communities since 2009. Results to date show the following trends: 

• Although the trend in native plant species richness differs significantly between Woodlands, Natural Temperate Grasslands and Secondary Grasslands, all vegetation formations have displayed an increase in native plant species richness since monitoring commenced in 2009. 
• Woodland native species richness peaked in 2014 but has since seen a slight overall decline, possibly due to drought conditions experienced in 2018. 
• Grassland species richness has continued to improve since surveys commenced. 
• There has been an average increase of 1.4 rare species per survey plot per decade for the three vegetation formations. This suggests that Box Gum Woodland and Natural Temperate Grasslands are becoming more floristically diverse. 
• Native grass cover across all vegetation formations has been declining since 2012. This decline does not appear to be linked to drought. More analysis is required to determine if it is linked to management practices. The decline in this indicator is of concern because it has implications for fauna habitat and for weed invasion. 
• There has been a slight increase in exotic species richness across Box Gum Woodland and Natural Temperate Grassland. This trend is partly due to the decline of native grass cover that provides more opportunities for exotic species to establish themselves. There is evidence that the increase in exotic species richness has declined since a high in 2012, but it is not known if this more recent trend can be attributed to management. 

Overall the long term condition is positive with Box Gum Woodland and Natural Temperate Grassland endangered ecological communities in the ACT becoming more diverse. However, it is clear that short-term variations are having an impact on grass cover and the number of exotic species. Ongoing monitoring and development of management practices will be important to maintain these important communities.

Impacts of fire on native vegetation

NOTE: The condition and trends information presented here do not include the impacts of the 2019-20 fire season which burnt extensive areas of the ACT. These fires will have a significant impact on vegetation tolerable fire intervals and growth stage.

Bushfire is an important occurrence for many native vegetation communities in the ACT. Although fire can cause a temporary loss of vegetation, fire is necessary for the regeneration and regrowth of many plant species. The appropriate fire regime to promote native biodiversity (intensity, frequency, season, extent and type of fire) varies between native vegetation communities. Changes to ecologically appropriate natural fire regimes can have significant impacts on the composition of vegetation communities and the ecosystems they support. 

The ACT’s fire regimes have changed over time due to increased human sources of ignition, the suppression of natural fire to protect human life and assets, and prescribed burning practices for the management of fuel loads (see Fire section). In addition, periods of prolonged drought and higher temperatures increase the risk of more frequent and severe fires. Climate change is expected to further influence the occurrence of bushfires in the ACT.

Tolerable fire intervals

In 2018, only 34% of the total area of native vegetation assessed was found to be within the required TFI to maintain vegetation communities; 53% was below minimum TFI (fire interval too short to maintain vegetation in its optimal state), 7% above the maximum TFI (fire interval too long to maintain vegetation in its optimal state), and 6% was classed as long unburnt (Figures B19 and B20).

Because over 50% of the total native vegetation assessed is below TFI, large areas of the ACT will remain outside optimal TFI irrespective of the level of prescribed burning and future fire events. This potentially places species with life cycles dependent on long inter-fire intervals at increased risk.

For the 10 native vegetation communities assessed, only 4 had 50% or more of their assessable area within the preferred TFI range (Figure B21). These are the Southern Tableland Grassy Woodlands (58%), Upper Riverina Dry Sclerophyll Forests (70%), Southern Tableland Dry Sclerophyll Forests (77%), and Tableland Clay Grassy Woodlands (89%). Five native vegetation communities had none of their assessable area within TFI, and one only has 3% within TFI. These communities had fire regimes mostly below minimum TFI, the exception being the Temperate Montane Grasslands which had 60% of the assessed area above maximum TFI. Other communities with high proportions of above maximum TFI include the Southern Tableland Grassy Woodlands (39%) and Upper Riverina Dry Sclerophyll Forests (28%).

The TFI assessment also showed the following trends:

• Extensive areas of the ACT uplands are below minimum TFI as a result of the 2003 Canberra bushfires which burnt some 90% of the Namadgi National Park. These upland ecosystems typically have longer minimum TFI and are still in an early recovery stage.
• In contrast, many areas of the ACT lowlands and on the lower margins of the uplands are within TFI or above maximum TFI. These areas either escaped 2003 bushfire and/or are communities that recover more quickly from fire.
• Much of the above maximum TFI in the northeast of the ACT are native grasslands which have a short minimum TFI and in which little burning has occurred in recent decades.
• Long unburnt areas are those areas in the Namadgi National Park that have had no recorded fire but support vegetation communities that were otherwise extensively impacted by the 2003 bushfire throughout the rest of park. Most of the long unburnt ecosystems in Namadgi National Park are restricted to the very southern portion on the NSW border. These areas therefore represent rare post-fire age classes for some vegetation communities and exclusion of fire in the foreseeable future is a particularly high conservation priority while other areas of these communities recover from the 2003 fires.

Growth stage

Post-fire growth stages represent the recovery of native vegetation communities after fire and the progression from early response (re-sprouting and seed germination) to the maturation of plant species and animal populations, and eventually senescence and species turnover at longer times subsequent to fire. Each growth stage is characterised by a different structural arrangement of vegetation and may be dominated by different component species. Similarly, the faunal community supported by an area will vary as the vegetation progresses through growth stages. Biodiversity values are most likely enhanced at a landscape scale by achieving a range of growth stages within each vegetation community and across the landscape.

In 2018, the adolescent growth was the most common growth stage accounting for 44% of the total area of native vegetation communities assessed (Figures B22 and B23). The next most common growth stage was mature at 38% of the total area of native vegetation assessed, with both juvenile and senescent both accounting for 9% each. While all growth stages are represented across the ACT, the landscape is dominated by early and young stages which account for over half of the assessed vegetation communities.

For the 13 native vegetation communities assessed, 8 had 50% or more of their assessable area within juvenile or adolescent growth stages (Figure B24). Five of these communities had over 80% of their assessed area in juvenile or adolescent growth stages, including Alpine Bogs and Fens, Alpine Herbfields, Subalpine Woodlands, Southern Tablelands Wet Sclerophyll Forests, and Montane Wet Sclerophyll Forests. Only 4 vegetation communities had greater than 50% of their assessed area within the mature growth stage including Montane Bogs and Fens, Southern Tableland Dry Sclerophyll Forests, Southern Tableland Grassy Woodlands, and Upper Riverina Dry Sclerophyll Forests.

Native vegetation growth stages across the ACT reflect TFI status, with extensive areas of the uplands being dominated by early and young growth stages. This is a result of the 2003 bushfires and the relatively slow recovery rate of many upland ecosystems. The dominance of early and young growth stages has significant implications for biodiversity, especially for fauna that require older growth stages.

The growth stage assessment also showed the following trends: 

• Vegetation communities dominated by early post-fire growth stages include Alpine Bogs and Fens and Alpine Herbfields, and to a lesser extent Montane Bogs and Fens, which grow at lower altitudes and were not as extensively impacted in 2003. 
• Most other upland ecosystems are also dominated by juvenile and adolescent growth stages including Subalpine Woodlands, Southern Tableland Wet Sclerophyll Forests and Montane Wet Sclerophyll Forests. 
• Early recovery stages are extensive in parts of the Namadgi National Park which have been subjected to prescribed burning since 2003, primarily as a part of the Regional Fire Management Plan, between 2013 and 2018. 
• Some lower elevation communities are also dominated by earlier growth stages including the Eastern Riverine Forests and Tableland Clay Grassy Woodlands. 
• Mature growth stages are more common on the lower elevation eastern and northern margins of the upland where dry forest and woodlands that recover more quickly from fire dominate. 
• Dry sclerophyll forest communities and Southern Tableland Grassy Woodlands have a significant proportion of their extent in mature and senescent growth stages. 
• Older growth stages are currently rare in the ACT uplands and are known to support different collections of species to ecosystems dominated by younger growth stages. The biodiversity these areas support is at risk from future bushfires because of the time required to transition from early to later growth stages and to replace slow developing habitat features such as mature trees and tree hollows. 
• Late post-fire growth stages are primarily distributed in the far south of Namadgi National Park (which escaped the 2003 bushfires and has not been burnt in recorded history) and in the lowland grasslands and woodlands around Canberra. 

In the longer term, conservation priority needs to focus on diversifying growth stages within and between ecosystems to maximise persistence of biodiversity. In the lowlands, this can be achieved through the ecological burning of late and mature growth stages for vegetation resilient to fire. However, in the uplands achieving growth stage diversity will require time and deliberate protection of the relatively rare older growth stages from prescribed fire and bushfire until more of the landscape reaches post-fire maturity.

Case studies

• Grassland restoration project
• ACT Woodland research and restoration