A preliminary study of the responsiveness to seasonal atmospheric and rainfall patterns of wash woodland species in the arid Richtersveld

Seasonal carbon and water relations were compared among seven tree or shrub wash woodland species in the winter rainfall desert of the Richtersveld Na tional Park, South Africa. Plants were generally aseasonal with respect to gas exchange, but responsive to rainfall events with respect to water relat ions and phenology. Relatively narrow annual ranges in potential evapotrans piration due to the maritime influence could explain why these plants respo nd more to fluctuations in water acquisition potential than to evaporative demand. Two species were summer-deciduous, but one of them (Ozoroa concolor ) responded to aseasonal summer rainfall by leafing out and flowering. Thes e two species had high shoot xylem water potentials when in leaf. All other species were sclerophyllous evergreens with low water potentials, particul arly the shallow-rooted shrub Zygophyllum prismatocarpum, and Boscia albitr unca which may have a different rooting pattern to the other phreatophytes. The latter species was also unique due to its high leaf nitrogen contents, photosynthetic rates and stomatal conductances, despite very low leaf wate r potentials. Leaf stable carbon isotope composition ((13)delta C) varied b etween species (-22 to -27 parts per thousand), but was lower than the mean for arid regions worldwide. The values indicated moderately high levels of water use efficiency, but a less conservative strategy in two species, inc luding Boscia albitrunca. The affinities of these species to summer rainfal l biomes, their apparent decline in the western arid regions in recent geol ogical history following aridification, and their absence southwards in the winter rainfall regions, suggest that these wash species rely on sporadic summer rainfall events to some extent. They may be at risk if predicted inc reases in temperature and changes in rainfall patterns alter their effectiv e moisture availability.