Xylem density, biomechanics and anatomical traits correlate with water stress in 17 evergreen shrub species of the Mediterranean‐type climate region of South Africa

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Abstract
Summary: Climate change in South Africa may threaten the sclerophyllous evergreen shrubs of this region. Available data suggest that they are not as tolerant of water stress as chaparral shrubs occurring in climatically similar California, USA. Seventeen species from nine angiosperm families, including both fynbos and succulent karoo species, were studied at a field site in Western Cape Province, South Africa. Minimum seasonal pressure potential (Pmin), xylem specific conductivity (Ks), stem strength against breakage (modulus of rupture, MOR), xylem density, theoretical vessel implosion resistance () and several fibre and vessel anatomical traits were measured. Species displayed great variability in Pmin, similar to the range reported for chaparral and karoo shrub species, but in contrast to previous reports for fynbos shrubs. More negative Pmin was associated with having greater xylem density, MOR and . There was no relationship between Pmin and traits associated with increased water transport efficiency. Xylem density integrates many xylem traits related to water stress tolerance, including Pmin, MOR and , as well as percentage fibre wall, parenchyma, vessel area and fibre lumen diameter. Xylem density may be an integral trait for predicting the impact of climate change on evergreen shrubs.