Intraspecific variation of drought adaptation in two south-east Australian Eucalyptus species
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2026
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Melbourne Authors
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School of Agriculture, Food and Ecosystem Sciences
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PhD thesis
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© 2026 Rhys Browning
Abstract
Recent trends in temperature and rainfall suggest that south-eastern Australia is getting both warmer and drier. These trends are associated with an increase in reports and concern about potential drought-induced dieback in Eucalyptus communities. Furthermore, many of the landscapes in the south-east have already been significantly impacted by deforestation during European settlement, leading to an interest in revegetating Eucalyptus communities to create landscapes that are ‘climate-ready’. This is primarily being done through the practice of provenancing: the integration of potentially adapted genotypes (ecotypes) of a species from hot and dry regions into landscape restoration. However, understanding what constitutes drought adaptation in trees is difficult. Trees can have varying strategies in response to drought, and an assortment of functional traits that complement these strategies. Therefore, developing an understanding of both drought strategies and drought traits is integral to understanding what may constitute an adapted provenance.
This thesis explored drought strategies and adaptations in Eucalyptus melliodora A. Cunn ex Schauer and Eucalyptus microcarpa Maiden, two ‘box’ species within the section Adnataria. Both species are integral components of threatened grassy-woodland ecosystems in south-eastern Australia. The distribution of the species encompasses regions that are predicted to become warmer and drier in the coming decades. To establish whether the potential for provenancing in the drier regions of each species is possible, I investigated traits related to drought adaptation and strategies in provenances of each species. This work complements a provenance trial established by Bush Heritage Australia in Central Victoria, where four provenances each of E. microcarpa and E. melliodora were selected as ‘climate-analogues’ from regions where current rainfall and temperature patterns are predicted to match those of Central Victoria in the coming decades. The two species co-exist in south-eastern Australian woodlands, however the distribution of E. microcarpa extends into drier and warmer regions in comparison to E. melliodora. The provenances of both species are separated by large distances; however, all provenances are from within the drier ranges of their respective distributions.
In the first experiment, I studied five provenances of each species that match the Bush Heritage Australia trial, as well as an additional provenance of each species from wetter regions. One year old trees were planted in soil under a rainout shelter. After a five-month long adjustment period, irrigation was withheld for 17 weeks to emulate a chronic drought, while control trees were irrigated daily. Predawn and midday water potentials and stomatal conductance were measured weekly on five replicate trees per provenance for each species. Trees were then harvested at the end of the drought period, and biomass allocation and leaf osmotic potential was measured. Neither species developed substantial water deficits during the drought period, but both species exhibited osmotic adjustment. Predawn and midday water potentials were significantly lower and stomatal conductance was higher in droughted E. microcarpa in comparison to droughted E. melliodora, however the differences were small. Both trees reduced biomass and adjusted biomass allocation, including an increase in root:shoot ratio. The adjustment was much more pronounced in E. microcarpa, which also had significant plasticity of leaf morphology resulting in a reduction in SLA. The greater growth and reduced water stress in droughted E. melliodora is likely due to significantly greater biomass allocation to roots evident in this species, resulting in this species effectively ‘avoiding’ drought in the experiment. Provenance differences were limited and only observed in droughted E. microcarpa, where provenances from drier regions had comparatively less drought stress while also having the greatest reduction in height, although the differences were also small. No provenance differences were observed in E. melliodora in this experiment.
The second experiment involved testing the responses of five provenances of E. microcarpa and four provenances of E. melliodora to an acute drought. This was done by placing 40-month-old trees in 70 L pots on scales and measuring the weight at minute intervals. To establish a controlled drought, daily amounts of irrigation were calculated from daily evapotranspiration so that the total amount of water was reduced by approximately 500 mL per day. During this period, leaf water potential, stomatal conductance, hydraulic conductance and evapotranspiration was continually measured. The results showed contrasting drought strategies in terms of whole-tree water use between the two species. E. melliodora had a lower hydroscape area (HA), less negative minimum midday water potential in saturated soil and a significant and rapid reduction in stomatal conductance as the acute drought progressed. E. microcarpa was much more drought tolerant, evident via greater comparative hydraulic function at greater water deficits and a greater HA. Hydraulic traits also differed between the species, evident via a much greater maximum hydraulic conductance in E. melliodora. Provenance differences were limited, and again only evident in E. microcarpa. Drier provenances had marginally lower leaf specific hydraulic conductance, suggesting there may be a small difference in hydraulic safety between the provenances within this species.
The third experimental chapter involved measuring seasonal growth and water stress in provenances that had been planted in Lawan, a revegetation trial set up by Bush Heritage Australia in 2018. The reserve is near the Wychitella Nature Conservation Reserve, 75 km NW of Bendigo, Central Victoria. Seasonal water relations, overall growth and leaf hydraulic traits were measured in three winters in 2021, 2022 and 2023 and two summers in 2022 and 2024. Predawn and midday water potentials varied seasonally in both species, however, the contrasting drought strategies were again evident. E. microcarpa had greater stomatal conductance and lower predawn and midday water potentials than E. melliodora after the hot and dry summer in 2024. Leaf xylem cavitation was also significantly different between the species, with leaf xylem p50 (the water potential at which point 50% of the leaf xylem has cavitated) ranging from -4.15 MPa in E. melliodora to -5.45 MPa in E. microcarpa. What is noteworthy is that despite the contrasting drought strategies and traits, both species had the same growth outcome over a 5-year period. In this chapter, provenance differences were only observed in E. melliodora, whereby the local provenance had greater predawn water deficits, suggesting there may be some level of greater comparative stress in the local provenance in this species.
This thesis suggests that, within similar climate niches, provenances of both species do not appear to have any obvious adaptive advantage to drought. Therefore, where a distribution of either species is already on the periphery of the climate niche, provenancing may not have significant outcomes in terms of the persistence of trees if climate change results in an increase in drought events. However, within-provenance variation was significant in many traits. Therefore, the introduction of different genotypes into the landscape will likely introduce greater intraspecific variation in both species. Most notable was the foundational difference in drought strategy between the two closely related, co-existing species; in all three studies, E. microcarpa was comparatively more drought tolerant than E. melliodora, while E. melliodora had more stringent stomatal control under water limitation. However, within the distributions of both species, half of the climate niche (in terms of mean annual rainfall) is shared. Therefore, the trade-off in traits that complement the respective strategies allows both species to function in south-east Australian woodlands.
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Eucalyptus, Drought, Drought adaptation, Tree water-use, Provenance, Intraspecific variation, Water potential