Browsing by Author "Tonkin, Christopher"

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    Leaf-level physiological response and carbon allocation in two eucalypt hybrids with contrasting water use strategies under drought
    (Stellenbosch : Stellenbosch University, 2023-03) Tonkin, Christopher; Midgley, Guy F.; Drew, David; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Eucalypts are among the fastest growing trees on earth, with high production rates. However, the high species diversity of eucalypts has allowed this taxon to be successful across a broad climatic range, and allowed the forestry industry to match species with diverse sites around the world that may demand resilience to adverse conditions in addition to productivity. The creation of hybrids has further facilitated the selection of traits which can optimize productivity and resilience under adverse conditions. Stomatal regulation is thought to underpin this optimization, but a coherent predictive theory remains elusive, with a gain:risk framing now shaping new advances. Solutions to this challenge could help position eucalypts as a vital element of nature-based solutions to anthropogenic climatic change, addressing mitigation needs while considering optimal resource use, especially of water. This study explored drought responses of two eucalypt hybrids (Eucalyptus grandis x longirostrata (E.gl) and Corymbia henryi x torelliana (C.ht)) with contrasting water use strategies, to identify how key leaf-level functional traits (stomatal and light harvesting behaviour) influence the trade-off between growth rate and drought tolerance traits. Experimental plants were grown from rooted cuttings of each clone, and subjected to a 12-week drought period. Repeated harvesting quantified growth effect using biomass, stem height and stem diameter measures. Photosynthetic response analysis determined the differential effect of drought on the photosynthetic apparatus of the two varietals. Stomatal conductance and soil water potential were measured repeatedly while varietals were subjected to soil dry-down, and used to parameterize an optimization model to estimate hydraulic parameters for both varietals. The data showed distinct water-use strategies in the two varietals, resulting in a differential fit between responses predicted by a gain:risk approach in the two varietals. Model predictions more closely matched measurements in E.gl, which showed a more isohydric response to drought (i.e. more rapid stomatal closure) and less closely in C.ht which showed a more anisohydric response (i.e. lower but less drought- responsive stomatal conductance). Both varietals showed no damage to photosynthetic apparatus under drought conditions, revealing the resilience of light harvesting in both water use strategies. This suggests that stomatal optimization modelling approaches need to better incorporate longer term adaptive responses that affect the gain:risk trade-off. Varietal differences in growth measures reflected these leaf level functional traits, with C.ht showing a significantly higher relative increase in biomass than E.gl by the end of the experiment, regardless of water treatment. Prioritisation of stem diameter growth in C.ht may support xylem development that is more tolerant of cavitation, while the E.gl varietal prioritised apical stem growth which would likely support greater productivity under sustained ideal water supply conditions, but this accumulative effect would require more time to emerge than permitted by this experiment.