Assessing safety margins and hydraulic strategies in Restionaceae: evaluating xylem hydraulic traits in two Elegia species

Master Thesis


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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.