Nardoo Hills, the Bush Heritage Australia reserve in central Victoria, near Wedderburn, has areas of eucalypts that are stressed or have died through the influence of intense drought and/or heat stress, most likely linked to climate change. The affected area now covers over 100 hectares of the reserve. Most at risk are Grey Box (Eucalyptus microcarpa) and to some extent Yellow Box (E. melliodora). The dieback is affecting both older and younger trees; some having died outright while others have experienced the death of the main trunks and then survived with coppiced shoots or epicormic regrowth.
Traditional revegetation programs use locally sourced seed, based on the expectation that they're the most suited to the local environment. For long-lived species like eucalypts this approach is now considered high risk due to the changing climate (Breed et al., 2013). Bush Heritage seeks to develop better climate adaptation options for its reserves. To achieve this, we've started a large field trial that aims to:
- Compare the performance, from seedling to maturity, of four climate-adjusted provenances of each eucalypt species against the local provenance
- Increase the genetic diversity of two key eucalypt species and their capacity to adapt to a drying and warmer climate
- Inform Bush Heritage management and the local community of future revegetation options through tracking the planted seedlings and subsequent generations of trees
- Encourage future collaborative research to extract maximum knowledge from this trial.
Selecting seed/provenances from ‘climate analogue’ regions
We followed the suggestions of researchers at Macquarie University and CSIRO to bring together genetically diverse and climatically robust populations of grey box and yellow box using the ‘Climate-adjusted provenancing strategy’.
To do this, we initially used the Bureau of Meteorology’s Climate Analogues tool to pinpoint regions that are currently experiencing a climate that's similar to that expected at Nardoo Hills (central Victoria) in 30-70 years’ time. Such a long-term outlook is needed as the trees planted should persist for at least 70 years and need at least that time to develop into suitable habitat trees for a range of animals.
Using the Climate Analogues tool and the Australasian Virtual Herbarium (and Atlas of Living Australia), we identified a number of ‘harsh’ climate analogue regions that also had records of reasonable populations of grey box and/or yellow box; as an example Quorn SA, Griffith NSW and Condobolin NSW (see regional analogues Table below). We also chose provenances from less extreme climate analogues (e.g. Deniliquin NSW and Wagga NSW) and the local provenance of each species from the Wedderburn/St Arnaud area of Victoria.
|The regional analogues & their modelled attributes*
||Analogue climate severity rating
||Max temp (oC) vs Wedderburn
||Annual rainfall vs Wedderburn
|Quorn: Warr, RCP 4.5, Max consensus, 2090 (+2.0°C, -4%rainfall)
||1 (hotter, much drier)
|Griffith: Warr, RCP 4.5, Max consensus, 2090 (+2.0°C, -4%rainfall)
||1 (hotter, much drier)
|Condobolin: Warr, RCP 8.5, Least hot & wettest 2090 +2.8°C, +11% rainfall
||2 (much hotter, wetter)
|Deniliquin: Warr, RCP 4.5, 2050, Max consensus, +1.2°C, +1% rainfall
||3 (hotter, similar rainfall)
|Narrandera: Warr, RCP 4.5, Max consensus, 2090, +2.0°C, -4%rainfall
||1 (hotter, much drier)
|Condobolin: Warr, RCP 8.5, 2090, Least hot & wettest, +2.8°C, +11% rainfall
||2 (much hotter, wetter)
|Deniliquin: Warr, RCP 4.5, 2050, +1.2°C, +1% rainfall
||3 (hotter, similar rainfall)
|Wagga: Bend, RCP 4.5, Max consensus, 2050, +1.0°C, +1% rainfall
||3 (warmer, similar rainfall)
Sourcing seed from climate analogue regions
For those analogue regions that have established communities of grey box or yellow box, professional seed collectors or community members with a deep knowledge of their local eucalypt flora surveyed the regions to locate and collect seed from 10 well-spaced grey box or yellow box ‘mother trees’.
Prolonged drought conditions in NSW added greatly to the difficulty in finding trees bearing adequate amounts of mature seed. Seed capsules were harvested using a variety of methods and stored in order to dry and extract the seed. The seed from each mother tree was packaged along with site details, including GPS coordinates.
Seed from five provenances of both grey box and yellow box were successfully collected in 2018 and 2019.
Yellow Box distribution highlighting areas (red circles) where the species has been reported in a climate analogue for Nardoo Hills (red dot).
The recorded locations of seed sourced from mother trees of Grey Box (blue) and Yellow Box (red) in NSW and Victoria. Note the Flinders Ranges (SA) Grey Box seed sourced site is not included. Nardoo Hills is represented by red square in the most northern red square in the SW corner of the map.
In designing the trial, our goal was to evaluate the 5 provenances for each of the two species in a range of varying aspects. To do this, we settled on a standard experimental design comprising18 blocks (1 ha each) placed in upper, mid and lower slope sites. The blocks are located across an undulating terrain on the north-west flank of the Nardoo Hills Reserve, and on the nearby Lawan Reserve.
Each of the 18 blocks consist of 25 plots allowing for 5 provenances to be replicated 5 times and each plot being planted with one of the five provenances. Nine blocks were planted with yellow box and nine with grey box.
Propagation: Seed cleaning and sowing
The propagation of seed from all ten provenances (100 families) will occur at Arborline Nursery in Hamilton. The process of producing tube stock commencs with seed cleaning which involves carefully removing the chaff (packing material around seed in the gum nuts) and sorting the seeds into large and smaller categories. This enabled only strong seed to be sown directly into individual cells of seed trays, optimising the survival and strength of the germinating seedlings. This is a challenging process for the Nursery because of the need to isolate and keep track of the seed and seedlings from each of the 100 families.
Site preparation for planting commenced in December 2018. The first step was to cut narrow rip lines into the soil across the trial site to capture and allow the deeper penetration of rainfall, and to facilitate subsequent planting of tube stock. Project partners GreenFleet organised contractors to undertake the ripping to a depth of about 40 cm, with each ripline spaced by 4 metres. The timing was excellent: about 90 mm of rainfall subsequently occurred through December (27% of the total 2018 rainfall).
Using the precise coordinates of the rip lines, the position of the trial blocks and plots were marked with steel stakes and recorded in a Geographic Information System (GIS).
Stage 1 planting - 2019
After several excellent falls of rain in May, June and July, the soil profile across the experimental sites had moisture to depth, particularly along the rip lines. Soil moisture conditions were almost perfect for planting in mid July.
Seedlings for 8 of the 10 provenances (9000 seedlings) were planted with each seedling individually labelled and bar coded to identify its parental line and provenance. The remaining two provenances are being planted in Stage 2 in 2020.
Each plot was planted to 25 plants from one provenance, with individual seedlings representing each of the 10 'families' (a family being seedlings all propagated from seeds collected from a single mother tree). The seedlings were planted at 4m x 4m spacings. A row of buffer trees (grey box) was planted around each block.
Planting was certainly a major, complex challenge but thanks to a large team of enthusiastic GreenFleet and Bush Heritage staff, TAFE and La Trobe University students and other generous volunteers, all seedlings were planted into their pre-determined locations within a week.
Trial assessment 2019
Following an exceptionally dry spring, the project team, supported by a strong team of volunteers, monitored all experimental plantings in early November during extremely hot, windy conditions. Plants across all blocks and plots were checked for survival, plant health, herbivory and height.
Seedling mortality, largely through desiccation, was high. Three months after the 9000 seedlings were planted across the trial in late July, 52% had survived with many appearing stressed. Plant loss was consistently high but did vary between blocks. In most sites, the presence of senesced and senescing weeds (capeweed, storks bill, Patterson’s curse, oxalis and wild oats) was obvious. The combination of exceptionally dry conditions in spring and weed competition after a wet winter put the eucalypt seedlings under stress before they had established a strong root system.
To maximise survival of the remaining plants through their first summer, GreenFleet and Bush Heritage committed to one watering. Under normal circumstance, this procedure would be undesirable but considering the drought and importance of the trial, watering was successful in enabling the survival of enough seedlings to ensure the trial could continue as planned.
Stage 2 planting - 2020
Seed stock for the provenances of Quorn (Grey Box) and Condobolin (Yellow Box) were not available for the first planting, so were planted a year later in 2020. To facilitate future comparisons, the control provenances were supplemented at the same time with additional plants from the control seed stock. With much more favourable planting conditions, seedling survival of these plants has been 95% (much higher than the first planting).
Trial assessments 2020-2022
At the time of writing, survival, growth and health of each seedling has been monitored seven times, in spring 2019, autumn 2020, winter 2020, spring 2020, autumn 2021, spring 2021 and autumn 2022. We've had two successive seasons of exceptional rainfall and mild temperatures in 2021 and 2022 contributing to strong growth and great survivorship of the seedlings. Grey Box are averaging 93cm and Yellow Box are averaging 82cm in height.
We've meticulously collected and labelled leaf samples (2 to 3 leaves) from a little under half the plants (2350 trees) from across the entire trial (5080 trees). The leaves from randomly selected plants were placed in labelled envelopes and stored in silica gel to preserve the DNA in the leaves. These samples will allow us to study and understand the genomic differences between provenances and their families.
Introducing provenances adapted to hotter or harsher environments into cooler/milder environments (as currently experienced at Nardoo Hills during a La Nina climate phase) means that seedlings/saplings were at risk of being poorly adapted to establish in current conditions.
We certainly expected that the local provenances would do better until they're confronted by consistently hotter/dryer conditions. This year’s results may support that expectation. Local provenances of both species (Grey Box-Wedderburn, Yellow Box-St Arnaud), along with one climate-adjusted provenance (Grey Box-Fifield and Yellow Box-Narrandera), are significantly larger (in terms of height and stem diameter) than other provenances. Further, considering Grey Box-Fifield and Yellow Box-Narrandera were as tall as the local provenances, and that there's no evidence yet that other climate-adjusted provenances are struggling, the risk of serious mal-adaptation of introduced climate-adjusted provenances in this trial appears low. Survival of seedlings from climate adjusted provenances have also not been significantly lower than local provenance seedlings.
This trial is a collaboration between Bush Heritage Australia and GreenFleet. This project received scientific guidance from Prof Ary Hoffmann, The University of Melbourne; Drs Suzanne Prober, Linda Broadbent and Rebecca Jordan, CSIRO; Dr Martin Breed, University of Adelaide and Dr Peter Harrison and Prof Brad Potts (University of Tasmania). Additional and extensive technical guidance was provided by GreenFleet (Eoghan O'Connor and Jo Sasse) and Arborline (Keith Cumming). Both GreenFleet and Arborline have gone to special lengths, well beyond commercial expectations, to support the establishment of the quality requirements of this project. Financial support was generously provided by VicRoads for seed collection.
Seed collections were generously organised/undertaken by Andrew Knop (Outback Seeds); Dan Frost (Creswick Harvest); The Australian Tree Seed Centre; Nella Smith (Narrandera, NSW); Dick Green (Wagga, NSW) and Anne Brown (Wirrabara, SA). We do applaud the extra lengths to which our seed collectors went to secure appropriate quantities of seed in a very challenging season and environment. We also thank Sue Logie (Land Services Officer, Murray Seedbank); Jess Hill (Senior Local Land Services Officer, Riverina LSS), Martin Driver (Wagga NSW) and Nicki Taws (Greening Australia) for their assistance in locating either seed-bearing trees or seed collectors.
Breed, M.F., Stead, M.G., Ottewell, K.M., Gardner, M.G., Lowe, A.J., 2013.
Which provenance and where?: Seed sourcing strategies for revegetation in a changing environment.
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Hancock, N., Harris, R., Broadhurst, L., Hughes, L., 2016.
Climate-ready revegetation. A guide for natural resource managers.
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Garry McDonald, Matt Appleby and Angela Hawdon.