Assessing vulnerability to embolism and hydraulic safety margins in reed-like Restionaceae

The African Restionaceae (Poales), the dominant graminoid layer in the megadiverse Cape Floristic Region of South Africa, are distributed across a wide range of moisture availability, yet currently there is very little known about the underlying hydraulics of this group. We tested two methods for measuring culm vulnerability to embolism, the optical and pneumatic methods, in three species of Cannomois ranging in habitat from semi-riparian (Cannomois virgata) to dryland (Cannomois parviflora and C. congesta). Estimates of culm xylem vulnerability were coupled with measures of turgor loss point (Ψ_TLP) and minimum field water potential (Ψ_MD) to assess hydraulic safety margins. The optical and pneumatic methods produced similar estimates of P₅₀, but differed for P₁₂ and P₈₈. All three species were quite vulnerable to embolism, with P₅₀ of -1.9 MPa (C. virgata), -2.3 MPa (C. congesta), and -2.4 MPa (C. parviflora). Estimates of P₅₀, Ψ_TLP and Ψ_MD aligned with habitat moisture stress, with highest values found in the semi-riparian C. virgata. Consistent differences in P₅₀, Ψ_MD and Ψ_TLP between species resulted in consistent hydraulic safety margins across species of 0.96 ± 0.1 MPa between Ψ_MD and P₅₀, with onset of embolism occurring 0.43 ± 0.04 MPa after Ψ_TLP for all three species. Our study demonstrates that restio occupancy of dry environments involves more than the evolution of highly resistant xylem, suggesting that other aspects of water relations are key to understanding trait-environment relationships in this group.