B4: Extent and condition of native vegetation

Introduction

For information on this indicator see Background: Biodiversity.

It was not possible to determine changes in the extent of native vegetation over the reporting period (2019–20 to 2022–23). Nor was it possible to provide an overall assessment of vegetation condition. This follows the 2015 and 2019 ACT State of the Environment reports which noted the lack of comprehensive documented information on the condition of vegetation and the amount of vegetation loss. The cumulative impact of approved vegetation clearing has 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 on the impacts of bushfire on native vegetation (see Condition of native vegetation section).

Extent

The ACT’s current native vegetation losses are from ongoing land use changes (physical removal of vegetation for land development) and from environmental pressures such as climate change, bushfires and dieback (see Condition of native vegetation section).

The majority of the ACT’s land use change is from urban development which is an increasing pressure on native vegetation and ecosystem health (see 5. Canberra’s urban boundary). Especially important is the cumulative impacts of urban development on native vegetation extent. Removal of native vegetation from one land development area may not seem consequential but total losses from many land developments across the ACT and over time will be significant. Extensive and ongoing native vegetation losses can lead to thresholds being crossed for the health of vegetation communities and ecosystem health. This may lead to the collapse of vegetation communities and the fauna they support.

The loss of mature native trees in the ACT between 2015 and 2020 was assessed as part of the Loss of Mature Native Trees Key Threatening Process Action Plan. Main findings include:

• 14,455 mature native trees were lost across urban Canberra (a 6.2% decrease from 2015)
• greenfield suburbs accounted for the largest percentage of tree loss per suburb, including Wright (42% decrease in mature native trees), Throsby (35%), Taylor (31%), Whitlam (23%), Coombs (22%) and Denman Prospect (13%)
• 1,728 mature native trees were lost across nature reserves (excluding the Namadgi and Tidbinbilla National Parks), less than a 1% decrease from 2015, and
• 4,846 mature native trees were lost in rural areas, a 1% decrease from 2015. Tree loss in the southern rural districts were mainly caused by the 2020 bushfires in the region.

Although data on the loss of mature native trees doesn’t provide an overall assessment of native vegetation loss in the ACT, the results do show that the pressures on native vegetation are significant. Particularly worrying is the net loss of mature native trees which are critical for many fauna species.

Revegetation in the ACT

The ACT Government undertakes revegetation in cooperation with various organisations and programs including the National Landcare Program, the Australian Government’s million Trees Project, Greening Australia and the Australian Government’s Clean Energy Future Biodiversity Fund. The works 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 2019 and 2023, revegetation included:

• 28 000 trees and shrubs planted in the Lower Cotter Catchment to restore habitat, connectivity and enhance the resilience of the former pine plantations
• 2,940 trees and shrubs planted in the Stony Creek Nature Reserve, and
• 3,500 trees and shrubs planted in the Murrumbidgee River Corridor, largely riparian canopy species, with some understorey planting of shrubs and ground layer vegetation to enhance habitat complexity.

In addition, 40 hectares of revegetation was undertaken on rural lands in 2022.

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

Condition

Chronic degradation of vegetation 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 leading to an increased occurrence of tree mortality from dieback (see Climate change). This chronic degradation, combined with changes to ecologically appropriate natural fire regimes, means that much the ACT’s native vegetation is under great pressure.

Impacts of fire

During the reporting period (2019–20 to 2022–23), the most significant impact on the condition of native vegetation was the 2020 bushfires. The fires burnt nearly 90,000 hectares, including 80% of Namadgi National Park (see Fire). Around 75,000 hectares burnt in 2020 were also burnt by the 2003 bushfires. This means that large areas of the ACT’s native vegetation experienced severe and widespread burning twice in just 17 years. This will affect vegetation condition for decades to come.

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. However, changes to the fire regime (fire frequency and intensity) can have significant impacts on the composition of vegetation communities and the ecosystems they support.

The impacts of the Orroral Valley bushfire on the ACT’s native vegetation is also discussed in 4. Bushfires in the ACT.

Tolerable fire intervals

Tolerable fire intervals (TFI) assess the likely ecological response of native vegetation communities to fire and are based on the requirements for sensitive plant species and key habitat elements. Assessments of TFI are based on:

• Minimum TFI: the minimum period of time between fires to avoid a loss of plant biodiversity. This period is based on the time it takes fire-sensitive plant species to grow to reproductive maturity and produce adequate viable seed banks.
• Maximum TFI: the maximum period of time between fires to avoid a loss of plant biodiversity. This period is based on the likely reduced viability of shorter lived plant species which require fire to germinate seeds.

It should be noted that the TFI status is only a potential issue should a large, high-severity fire occur in areas that are below minimum TFI. There is no requirement that ecosystems be within a specific TFI, only that fire outside of these thresholds be limited. To promote maximum biodiversity, a range of TFI status is required to provide different habitat resources.

In 2023, only 18% of the total area of native vegetation assessed was found to be within the required TFI to maintain vegetation communities; 62% was below minimum TFI (fire interval too short to maintain vegetation in its optimal state), 14% above the maximum TFI (fire interval too long to maintain vegetation in its optimal state) and 6% was classed as long unburnt (Figure 11 and Figure 12).

Because over 60% 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 12 native vegetation communities assessed, only three had 50% or more of their assessable area within the preferred TFI range (Figure 13). These are the Inland Riverine Forests (100% within the preferred TFI range), Upper Riverina Dry Sclerophyll Forests (71%) and the Southern Tableland Grassy Woodlands (52%).

Five native vegetation communities had none of their extent within the preferred TFI range and also had 87% or greater of their extent below the preferred TFI. These are Montane Wet Sclerophyll Forests (100% below the preferred TFI), Southern Tableland Wet Sclerophyll Forests (100%), Alpine Heaths (100%), Subalpine Woodlands (96%) and Eastern Riverine Forests (87%). These five vegetation types were already below minimum TFI as a result of the 2003 ACT bushfires when the 2020 fires occurred.

Temperate Montane Grasslands had 59% of the assessed area above maximum TFI. Other communities with high proportions of above maximum TFI include the Upper Riverina Dry Sclerophyll Forests (28%) and Southern Tableland Grassy Woodlands (45%).

The extent of Long Unburnt (no recorded fires since 1900) vegetation in the ACT uplands has been significantly reduced after the succession of the 2003 and 2020 fires. The only upland vegetation type to still have more than 10% of extent Long Unburnt is Temperate Montane Grassland (27%). All other upland communities had either no Long Unburnt remaining, or Long Unburnt was less than 5% of the communities extent in the ACT.

The TFI assessment also showed the following trends:

• Extensive areas of the ACT uplands are below minimum TFI because of bushfires. These upland ecosystems typically have longer minimum TFIs and were still in an early recovery stage post the 2003 fires when they were extensively burnt again in 2020, just 17 years later.
• 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 the 2003 and 2020 bushfires 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 and grassy woodlands in which little burning has occurred in recent decades. These vegetation communities have a short minimum TFI and require more frequent burns to remain within their preferred TFI range. Ecological burns are now being utilised more frequently in the ACT to address these areas which may benefit from the re-introduction of fire to improve ecological condition.
• Long unburnt vegetation occurs in areas of the Namadgi National Park that have had no recorded fire. These areas support vegetation communities that were extensively impacted by the 2003 and 2020 bushfires throughout the rest of park. Roughly half the previously long unburnt areas within Namadgi National Park were burnt in the 2020 fires; the long unburnt ecosystems which remain are restricted to the very southwestern portion on the NSW border. These areas represent rare post-fire age classes for some vegetation communities. Exclusion of fire is an important conservation priority for long unburnt areas protecting mature vegetation communities while those burnt by the 2003 and 2020 bushfires recover.

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.

In 2023, the Juvenile growth stage was the most common, accounting for 35% of the total area of native vegetation communities (Figure 14 and Figure 15). The Adolescent growth stage accounted for 27% of assessed native vegetation, with Mature at 18% and the Senescent growth stage at 20%. While all growth stages are represented across the ACT, 62% of vegetation is dominated by the younger Juvenile and Adolescent growth stages. This is a result of the large bushfires in 2003 and 2020.

For the 14 native vegetation communities assessed, eight had 50% or more of their ACT extent within Juvenile and three had 50% or more of their ACT extent in Adolescent growth stages (Figure 16). Six of these communities had over 80% of their extent in Juvenile or Adolescent growth stages, including Alpine Bogs and Fens, Alpine Heaths, Alpine Herbfields, Subalpine Woodlands, Southern Tablelands Wet Sclerophyll Forests and Montane Wet Sclerophyll Forests. It should be noted that Alpine Herbfields and Alpine Bogs and Fens are in the Juvenile growth stage across the entire ACT extent of these communities.

Only three vegetation communities had greater than 50% of their ACT extent within the Mature growth stage including Montane Bogs and Fens, Upper Riverina Dry Sclerophyll Forests and Southern Tableland Grassy Woodlands. In the case of Montane Bogs and Fens this likely reflects the majority of this vegetation community escaping the 2020 Orroral Valley bushfire.

Native vegetation post-fire growth stages across the ACT mirror the pattern seen with TFI status, with extensive areas of the uplands being dominated by Juvenile and Adolescent growth stages. This is a result of the 2003 and 2020 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 the habitat features of older growth stages including established overstorey canopies, deep litter layers and tree hollows.

The growth stage assessment also showed the following trends:

• Vegetation communities which have 100% of their extent assessed as juvenile post-fire growth stages include Alpine Bogs and Fens and Alpine Herbfields.
• Most other upland ecosystems are 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, and then burnt again in the 2020 bushfires.
• 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 uplands where dry forest and grassy 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.
• Senescent / late post-fire growth stages are primarily distributed in the far south of Namadgi National Park (which has had no recorded fire since 1900, escaping both the 2003 and 2020 bushfires) and in the lowland grasslands and grassy 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 high severity bushfire until more of the landscape reaches greater post-fire maturity.

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. Causes of dieback in the ACT are not well understood.^[6]

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 Consequently, revegetation is needed to improve landscape connectivity and also assist dispersal to areas of climate refugia and distribution expansion.

More recently, extensive dieback of subalpine snow gums (Eucalyptus debeuzevillei and E. niphophila) has been identified as a major cause for concern in the Brindabellas. The dieback is caused by the larvae of a long-horned beetle and appears to be associated with ongoing drought. To date, dieback of subalpine eucalypts has received little research or management attention.^[7]

The ACT’s Loss of Mature Native Trees Key Threatening Process Action Plan notes that Eucalypt dieback is a complex and widespread phenomenon in the ACT lacking a clear policy response.

There has been no update on the occurrence of dieback in the ACT since 2017. More information can be found in the ACT State of the Environment 2019.

Connectivity

The Unnatural Fragmentation of Habitats was listed in 2019 as a key threatening process in the ACT. The listing reflects the substantial fragmentation of habitat, particularly in urban development areas and agricultural landscapes. Fragmentation of habitat and vegetation significantly increases the likelihood of extinction of flora and fauna through the loss of loss of ecological connectivity and disruption to biological processes.

Despite this, comprehensive information on the status and trends of vegetation and habitat fragmentation in the ACT was not available for this report. However, the ACT is currently assessing fragmentation to determine ecological connectivity requirements for a range of species.

It was also not possible to update riparian vegetation connectivity data for this report. The decline of connectivity for riparian vegetation was discussed in the ACT State of the Environment 2019. Main findings showed that significant areas of riparian vegetation on the Murrumbidgee and Molonglo rivers have poor connectivity. The condition of riparian vegetation is also discussed in Water – Indicator W1: Aquatic ecosystem health.

Data gaps

• There is a lack of comprehensive information on the extent and condition of native vegetation in the ACT, meaning it is not possible to assess vegetation changes over time.
• The Unnatural Fragmentation of Habitats was listed in 2019 as a key threatening process in the ACT. Despite this, comprehensive information on the status and trends of vegetation and habitat fragmentation in the ACT was not available for this report.