Browsing by Author "Saunders, Alta"

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    Investigating the photosynthetic and hydraulic trade-off during drought recovery in eucalypts
    (Stellenbosch : Stellenbosch University, 2022-04) Saunders, Alta; Drew, David M.
    ENGLISH ABSTRACT: Stomatal regulation plays a vital role in maintaining the water status of a plant by minimising water loss, however, decreases in stomatal conductance can lead to reductions in carbon uptake. The stomata balance a trade-off between water loss and carbon gain. The hydraulic system and stomatal conductance are closely linked but play opposing roles within a plant. The hydraulic system ensures that there is a sufficient water supply to leaves, while stomatal conductance regulates the loss of water from the leaves. During periods of drought, reductions in hydraulic conductance due to embolism formation can be seen, however stomatal regulation can help reduce embolism formation or prevent runaway cavitation during drought. Understanding how plant hydraulics and stomatal regulation influence production rates is becoming more important to model stomatal responses in a changing climate, especially for Eucalyptus species which is often grown in drought prone environments. Plants use a wide range of strategies to reduce or mitigate the negative impact of embolism formation, with this study focusing on the strategies utilised by commercially significant Eucalyptus hybrids. Two Eucalyptus hybrids, E. grandis X camaldulensis (GC) and E. urophylla X grandis (UG), where subjected to a drought-recovery treatment where they were periodically droughted. During the experimental period the stomatal responses, together with transpiration rates, photosynthetic capacity and biomass allocation was measured. Hydraulic measurements together with CT-scan imaging was also utilised to determine whether these plants can recover lost hydraulic pathways post-drought, and what the underlying mechanism for this might be. During the study two distinct hydraulic strategies were observed. GC was more resistant to embolism formation compared to UG, however GC showed lower levels of hydraulic recovery after rewatering. The drought responses could also be split into a resilient v. a resistant response, with the more resilient hybrid, UG, maintaining stomatal conductance throughout drought periods running the risk of hydraulic failure, however with the ability to recover lost hydraulic pathways through refilling post- drought. This is in comparison with the resistant strategy seen in GC, where water loss during drought was minimised, however this also reduced carbon uptake and production. From this study the stomatal responses could also be accurately modelled using a gain-risk model that assumes stomata optimise the trade-off between water loss and carbon gain. It was however clear, that the strategy used by plants need to be considered when using a gain-risk model, with the gain-risk model performing better for the Eucalyptus hybrid that utilised a resistant strategy, since plants that uses a resilient strategy will maintain stomatal conductance during drought, regardless of the hydraulic risk. The use of alternative models was also investigated during this study, with Machine Learning models being able to accurately predict stomatal responses on a global scale. Major increases in tree mortality are predicted due to changes in climate. To properly predict these changes, accurate models of plant responses to water limitations and other environmental changes are therefore becoming increasingly important for effective forest management. Understanding how hydraulic traits and stomatal regulation covary, can help model drought-induced tree mortality in a changing climate.