Browsing by Author "Skelton, Robert"

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    A theoretical and empirical framework for measuring minimum conductance, an understudied plant drought trait
    (2025) Irlam, Huw; West, Adam; Skelton, Robert
    In light of increased drought frequency and intensity due to climate change, knowledge of how plant drought tolerance is affected is important for understanding plant vulnerability to these changes. Minimum conductance (gmin) describes the residual rate of water loss by a plant through its leaves when it has closed its stomata in order to minimise that water loss rate. This includes contributions to water loss through incompletely closed or leaky stomata, the waxy cuticle layer, or through wounding and scarring. This understudied conductance parameter is a critical piece to understanding plant dry-down time to death. However, definitions of this trait have historically been inconsistent leading to non-standardised methods for measurement, which lack an underlying theoretical basis. This obscures assessment of the existing variability and sensitivity of this important trait. In this thesis, clear definitions for minimum conductance are provided. A theory-based and biologically meaningful framework for measuring this trait over the stomatal safety margin is proposed. There is a focus on standardisation of existing methods, while increasing applicability and reproducibility on plants with a variety of morphologies. Using this framework, an assessment of minimum conductance was carried out on three characteristic and well-studied plant families of the Cape Floristic Region, namely Proteaceae, Ericaceae, and Restionaceae, to improve our understanding of drought response in these groups while demonstrating how this framework helps to improve minimum conductance measurements. Results showed a diverse response of minimum conductance values indicative of diverse drought strategies. This highlights the importance of including accurate values of minimum conductance in models predicting plant mortality under changing climate conditions.
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    Assessing safety margins and hydraulic strategies in Restionaceae: evaluating xylem hydraulic traits in two Elegia species
    (2021) Atkins, Kayla; West, Adam; Skelton, Robert
    The movement of water through a plant is responsible for acquiring nutrients, avoiding desiccation and enabling gas exchange. Therefore, access to soil water is vital in determining vegetation structure in a landscape, which suggests that plant hydraulic strategies are an important ecological driver of responses to seasonality and intensity of drought events, especially in the face of climate and habitat change. Wide-spread mortality has been observed over recent decades as a result of drought stress. Many plant species have exhibited hydraulic niche segregation as a result of a trade-offs between stomatal regulation and xylem integrity, and the associated physiological adaptations. This is concerning in the face of climate change predictions of more frequent and intense drought and flooding events because predicted habitat water availability changes may exceed the physiological tolerance ranges of many species. The hydraulic safety margin (difference between xylem vulnerability (P50; MPa) to cavitation and point at which stomata are considered closed (turgor loss point; MPa)) displays the tolerance margins based on plant physiological capacity. Restionaceae have shown extensive evidence of hydraulic niche segregation and are a key demographic of fynbos vegetation but not much is known about their hydraulic strategies. The optical vulnerability method was used to construct vulnerability curves and the turgor loss point (ΨTLP) was acquired from pressure-volume curves. E. fenestrata, a localized seep species, was more vulnerable (P12 = −0.52 MPa; P50 = −1.07 MPa; ΨTLP = −1.57 MPa; negative safety margin = −0.5 MPa) than E. tectorum, a widespread, dry habitat species (P12 = −0.99 MPa; P50 = −1.6 MPa; ΨTLP = −1.64 MPa; slightly negative safety margin = -0.04 MPa). The Cape Floristic region is predicted to receive less rainfall and become drier with ongoing climate change. We expect that this overall drying trend will have a profound impact on the Restionaceae, particularly E. fenestrata that does not have the physiological capacity to deal with severe drought stress.
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    Variation in leaf attributes and their effects on physiological processes in Leucospermum conocarpodendron L. Buek
    (2007) Skelton, Robert; Midgley, Jeremy J
    Two subspecies of Leucospermum conocarpodendron with considerable variation in leaf traits occur along the Cape Peninsula along clearly delimited geographical distributions. We attempted to quantify the difference in leaf trait dimensions between green (L. conocarpodendron ssp. viridum) and grey (L. conocarpodendron ssp. conocarpodendron) individuals. Leaves from grey individuals are shown to have higher reflectance across the entire photosynthetically active region (PAR) and greater stomatal density. Leaves from grey individuals were found to have higher specific leaf area (SLA] and a greater size boundary layer. We found no correlation between the transpiration rate and stomatal density, size of the boundary layer and hairiness. We hypothesize that the leaf traits are having an effect on rate of photosynthesis and subsequently determining growth strategy of each type. Grey individuals, because of increased reflectance from the leaves are able to persist throughout summer in hot, dry conditions, although they may suffer the cost of reduced photosynthetic rate during late winter and early summer when conditions are relatively mild. Green individuals are able to start growing earlier due to higher SLA and increased rate of photosynthesis but are not able to persist for as long as the grey individuals during mid to late summer. Finally we argue that adaptation to edaphic environment may be associated with subsequent shifts in flowering phenology.