- The ACT contains the largest remaining remnants of Box-Gum Grassy Woodland in good condition.
- Dieback in Blakely's Red Gum (Eucalyptus blakelyi), a dominant tree in Box-Gum Woodlands, is a major concern across the ACT.
- The Environment, Planning and Sustainable Development Directorate (EPSDD) engaged researchers Dr Alie Cowood and Dr Jasmyn Lynch at the University of Canberra to undertake a project to better understand the causes of red gum dieback in the ACT. The project utilised multi-criteria analysis and modelling to correlate dieback trends with specific data parameters, such as soils, geology, rainfall, hydrology and temperature over time, to assess possible causes of dieback.
- The project's climate analysis showed that compounding effects of an increasing temperature and variable precipitation over time is a key factor causing stress in trees.
- Blakely's Red Gum is more vulnerable to dieback than other species in Box-Gum Woodlands. While Blakely's Red Gum experienced a decline in condition over the period 2004-2017, another canopy tree (Yellow Box) increased in condition over the same period and appears more resilient to future projected climates.
- Non-climatic factors were also analysed but their association with dieback was inconclusive. For example, Psylloidea species (also known as lerps) showed no significant association with tree condition; however, limited data were available on Psylloidea in the ACT. There were too few data on mistletoe occurrence for analysis. Some threatened bird species appear to significantly overlap in distribution with habitat clusters of differing mean accumulated condition or overall change in condition. However, the current analyses did not identify if the significant differences in condition were a result of positive/negative effects of the species abundance levels or if the species selected habitat with trees of particular canopy condition.
- It is recommended that further research, monitoring and trials be conducted as part of Box-Gum Woodlands climate change adaptation, including: plantings to improve landscape connectivity; provenance trials; assisted dispersal to areas of climate refugia and distribution expansion; further data collection on cohort and biotic factors (tree age and regeneration, seed production and recruitment, understorey assemblage and condition, Psylloidea and herbivore abundance); monitoring of soil moisture and groundwater; and regular field-based observations of ecosystem condition and impacts from grazing, fire and other disturbance.
Blakely's Red Gum dieback
Blakely's Red Gum is experiencing dieback (a reduction in condition) in the ACT. It is a dominant canopy tree of the ACT-listed endangered Yellow Box–Red Gum Grassy Woodland ecological community, and forms part of the critically endangered White Box-Yellow Box-Blakely's Red Gum Grassy Woodland and Derived Native Grassland under the Australian Environment Protection and Biodiversity Conservation Act 1999. The ecological community is characterised by a species-rich understorey of native tussock grasses, herbs and scattered shrubs, and the dominance (or prior dominance) of Yellow Box (Eucalyptus melliodora) or Blakely's Red Gum trees.
The community is distributed along an arc across the western slopes and tablelands of the Great Dividing Range, from Southern Queensland through New South Wales to central Victoria. However, much of this area has been heavily cleared. Less than 5% of the original extent of the community remains. The remnants are highly fragmented and occurs in small isolated patches within a cleared, modified environment.
The Australian Government's National Recovery Plan for the ecological community states that all areas of Box-Gum Grassy Woodland should be considered critical to its survival. The ACT contains the largest remaining remnants in good condition, reflecting significantly lower levels of stock grazing and pasture improvement than has occurred elsewhere in the range of the ecological community.
What's causing dieback in the ACT?
The ACT Government has invested in research from the University of Canberra to determine the causes behind dieback in Blakely's Red Gum.
Change in condition, as a measure of dieback, of Blakely's Red Gum between 2004 and 2017 was influenced by a range of variables:
- habitat, for example soil characteristics, elevation and water table height
- climate, for example seasonal precipitation
- cohort, for example tree canopy density.
Declining condition of Blakely's Red Gum is associated with elevated maximum temperatures during the warmest month of the year and high rainfall in the wettest month. Poor condition is also correlated with fewer fires since 1900 and with increasing distance between trees. However it is important to note that the health of trees in the ecological community varies from year to year, and the relationship between condition and variables also changes considerably.
Figure 1: Occurrence of Blakely's Red Gum (Eucalyptus blakelyi) in the ACT.
Tree canopy condition
The tree canopies of the ecological community were mapped using SPOT-5 and SPOT-7 satellite imagery. Each tree canopy was assigned a condition score ranging from 1 to 5. A score of 1 represents a very poor condition tree, and a score of 5 represents a healthy tree in very good condition. Condition scores were derived for observations in 2004, 2009, 2013, 2016 and 2017. To corroborate this, random stratified sampling of more than 500 trees within the Yellow Box-Red Gum Grassy Woodland ecological community was carried out by ecologists from the ACT Government - Conservation Research unit. They collected information about condition, and fire frequency was assigned from spatial data. The field data were used to calibrate the remote sensing-derived condition mapping.
Field data collected included:
- stem count, diameter (age)
- leaf discolouration
- crown density
- dead branches
- epicormic growth
- dead tree class
Figure 2: Temporal condition mapping of Blakely's Red Gum (Eucalyptus blakelyi) over years 2004, 2009, 2013, 2016, and 2017.
This analysis confirmed that Blakely's Red Gum was experiencing a dieback event in 2016 and 2017. Condition of the ecological community varied over the years and across the ACT landscape. Condition of individual tree species indicates different spatial and temporal responses - Blakely's Red Gum experienced a decline in condition, while Yellow Box increased in condition. Condition in the north and east of the ACT was consistently lower from 2004 to 2017, while condition of trees in the south and west of the ACT was consistently higher but with greater fluctuations.
Habitat similarity – cluster analysis
Clustering analysis was used to group areas of similar habitat characteristics (for example soil, slope and land use) across the ecological community. Following this, another cluster analysis was run for the ecological community using habitat variables that were found to correlate with condition. The clustering analysis developed differing numbers of habitat clusters for the ecological community as a whole (10 clusters), Blakely's Red Gum (14 clusters) and Yellow Box (11 clusters) across the ACT.
Results from these analyses determined that the condition of the ecological community was influenced by variation in habitat character. However, individual variables are not primarily responsible for the spatial and temporal patterns in tree canopy condition. The condition of individual species within the ecological community also showed differing significant relationships with habitat variation, but not for all condition observations. Habitat clusters where Blakely's Red Gum show decline in condition showed improved condition for Yellow Box.
Figure 3: Identified habitat clusters.
Current climate characteristics
Further clustering analysis was used to develop four current climate clusters for the ACT. All Blakely's Red Gum and Yellow Box occur in Cluster 4. Cluster 4 covers the northern and eastern half of the ACT. This area represents the highest mean daytime temperature range, warmest temperatures for the warmest and coldest months, lowest precipitation for the wettest and driest months, largest negative water deficit value and smallest aridity index value (meaning the highest degree of dryness in the ACT).
The analysis shows that the ecological community condition was influenced by variation in current climate character. Condition of individual species within the ecological community was not able to be further assessed for the influence of current climate variation.
Figure 4: Current climate - All Blakely's Red Gum and Yellow Box are located in Cluster 4.
Historical climate characterisation: calculation of a growth stress index
The stress index (SI) is a measure of the compounded stressful effect on trees of a period of unusually wet or dry weather spanning the time when trees are actively growing. The wetter the wet period and the drier the dry period, the larger the SI values. Sudden and large changes in SI values are likely to represent more stressful conditions than lesser and gentler changes, and a change from high to low values represents a decline or absence of stressful weather.
Calculation of SI values was undertaken using the most complete long-term precipitation records from representative Bureau of Meteorology (BOM) stations around the ACT. The ACT climate has been changing across the long-term climate record (1940 to 2017). Change is trending towards an increasingly warmer climate. Change in rainfall patterns contributed to increasing consecutive years of stressful conditions during the trees' growing season.
The growing season SI values and 3 and 10 year running means are showing an increasing trend over time. Typically, the 10 year running mean has stayed positive (increasing stress) for 19 years since the 1998 - 1999 growing season, and while values are reducing they have not yet fallen below 0 (negative stress) at any of the BOM stations. The 3 year running mean shows decreased stress in recent years. Yellow Box seem to respond more quickly to increasing favourable climatic conditions (following the 3 year mean), while Red Gum responds slower (following the 10 year mean).
Near-future climate characterisation
Climate across the ACT is projected to change in the near future. Agreement across the 12 near future climate projections from NARCLiM models shows projected change to an increasingly warmer and drier climate for the northern and eastern half of the ACT, warmer climate for the central areas, wetter in the higher elevations, and cooler and wetter for the montane areas. This indicates a possibility of refugia to the south-west of the ACT for the Yellow Box – Red Gum Woodland ecological community. Agreement on the direction of temperature change and volume of precipitation between the 12 NARCliM models is low to moderate. However, the majority of models agree that the seasonality of precipitation will change.
Figure 5: Near-future climate clusters.
Biotic factors – insects and birds
Psylloidea species - also known as lerps - observations showed no significant association with tree condition, however the analysis was based on very low sample numbers. Field observations of regeneration and advanced epicormic growth indicate that Yellow Box may be showing greater resilience than Blakely's Red Gum.
The distribution of some bird species appeared to be associated with habitats of certain conditions or an overall change in condition. However, the current analysis was not able to determine if the significant differences in condition are a result of positive/negative effects of the species abundance levels, or if the species are selecting habitat with trees in of certain tree canopy condition.
- Regular field-based observations are needed.
- Refined field-based observation, data collection and further analysis can be undertaken for reserves and provenance trial locations.
- Monitoring of soil moisture, water table and groundwater should be undertaken.
- Investigate collection and translocation plantings of seed from endemic ACT sub-population provenance of Blakely's Red Gum tree canopies shown to be more resilient to dieback from this study to conserve the local genetic variability in the species.
- Plantings should be implemented to improve landscape connectivity of Box Gum Woodland and also to assist its dispersal to areas of climate refugia and distribution expansion.
Aridity index – a numerical indicator of the degree of dryness of the climate at a given location.
Dieback – a disease of shrubs or trees characterized by progressive death of twigs, branches, shoots, or roots, starting at the tips.
Ecological community - is defined under the Nature Conservation Act 2014 and equates to the definition ‘ecosystem' under the IUCN.
NARCLiM - the NSW and ACT Regional Climate Projections project provides information on project climate changes in our region. NARCliM provides detail down to the nearest 10 kilometres, the finest climate projections for the region available for the near (2030) and far (2070) future.
Refugia - areas where desirable native species are most likely to remain suitable under climate change.
Stress index - the stress index (SI) of White (1969, 1986) is a measure of the compounded stressful effect on trees of a period of unusually wet or dry weather spanning the time when trees are actively growing. The wetter the wet period and the drier the dry period, the larger the SI values.
Email GrassyWoodlands@act.gov.au to request a copy of the Blakely's Red Gum Dieback in the ACT report.