Background/Question/Methods
The Western Cape of South Africa is characterized by a Mediterranean type climate with a pronounced summer drought.  Species in riparian habitats within this climate must cope with water limitations during the period of highest evaporative demand.  Climate change models forecast decreasing rainfall in the Western Cape of South Africa exacerbating summer water limitation in riparian habitats.  Riparian habitats in the Western Cape already face pressure from invasive species and increasing human demand for water which diverts water from plant and animal species.  This study links anatomy and water relations of a dominant riparian species, Metrosideros angustifolia in an effort to understand how decreasing water availability might affect the distributions of populations of this ubiquitous riparian species.  A previous study suggested that vulnerability to cavitation might change across different water availability regimes, but no mechanism explaining this change was identified.  Individuals from two different reaches with different summer water availability on the Eerste River in the Jonkershoek Nature Reserve in the Western Cape of South Africa were compared.  

Results/Conclusions
The high water availability site has an annual minimum discharge of about 2.5 m³/s while the lower water availability site has an annual minimum discharge of less than 1.0 m³/s.  Midday water potentials were higher in the lower water availability site, and diurnal stomatal conductances were higher at the higher water availability site. Leaf specific conductivity and specific leaf area did not differ between the two sites, but xylem density was lower in the higher water availability site.  Xylem vessel lumen diameter to wall thickness ratios from the lower water availability site were smaller than the higher water availability site.  These results suggest that individuals growing under conditions of lower water availability adjust xylem traits to minimize loss of conductivity, and also adjust water demand by reducing leaf area and reducing water loss through stomata.  Individuals growing under higher water availability maximize vessel area for hydraulic transport which may support increased stomatal conductance, although at the expense of lower water potentials.