Hydraulic and Anatomical Trait Plasticity in Fynbos Growth Forms Across a Climatic Gradient in the Cape Floristic Region

Arens, Matthew

Hydraulic and Anatomical Trait Plasticity in Fynbos Growth Forms Across a Climatic Gradient in the Cape Floristic Region

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

The Cape Floristic Region (CFR) is projected to experience increased frequency and intensity of drought events. Comprehensive understanding of drought tolerance and response across the fynbos vegetation remains limited. This dissertation contributes to addressing these gaps by, first, testing and refining the pneumatic method for constructing xylem vulnerability to embolism curves across three major fynbos growth forms, and second, examining variation of functional traits related to plant hydraulics and xylem anatomy, within and between species along a regional climatic gradient. The pneumatic method was implemented for Protea repens (Proteaceae), Erica monsoniana (Ericaceae) and Cannomois congesta (Restionaceae). A methodological refinement was applied that paired repeat relative water content measurements on pneumatron samples with individual water potential and relative water content relationships to improve water potential estimates. Vulnerability curves generated using the pneumatic method produced coherent metrics, especially at 50% embolised. Additionally, comparisons with published optical and centrifuge datasets showed general agreement. Protea repens and Erica monsoniana were further sampled across three sites spanning a regional climatic gradient to explore for hydraulic and anatomical trait plasticity. Within species, embolism thresholds, sapwood-specific conductivity and vessel diameters were conserved across the gradient. By contrast, osmotic and turgor traits varied among sites. Species differences were consistent, E. monsoniana exhibited more negative vulnerability thresholds and wider hydraulic safety margins than P. repens, aligning with previous understanding of these species' differences in hydraulic strategies. At regional, intraspecific scales, relationships between climate, xylem structure and embolism resistance appear marginal, implying that climatic response of functional traits at this scale primarily manifest at the leaf-level water relations.

Keywords

cape floristic region

drought

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