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) and minimum field water potential (Ψ) to assess hydraulic safety margins. The optical and pneumatic methods produced similar estimates of P, but differed for P and P. All three species were quite vulnerable to embolism, with P of -1.9 MPa (C. virgata), -2.3 MPa (C. congesta), and -2.4 MPa (C. parviflora). Estimates of P, Ψ and Ψ aligned with habitat moisture stress, with highest values found in the semi-riparian C. virgata. Consistent differences in P, Ψ and Ψ between species resulted in consistent hydraulic safety margins across species of 0.96 ± 0.1 MPa between Ψ and P, with onset of embolism occurring 0.43 ± 0.04 MPa after Ψ 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.