Mediterranean climate ecosystems (MTE) are characterized by summer drought, but previous research suggests that Southern California has greater yearly and monthly variation in precipitation than other MTE. The Western Cape of South Africa and Southern France receive more than five times as much summer precipitation as Southern California. We hypothesized that increased summer water availability in the Western Cape of South Africa and Southern France would increase groundwater and result in lower water stress in riparian species during the summer months. We also hypothesized that riparian species in Southern California would exhibit Ymin with a larger margin of safety relative to P50 values because of the greater variation in precipitation between winter and summer. To test this we compared dominant species in the genus Salix in riparian communities in the Angeles National Forest of Southern California (Monte Cristo Creek and the Tujunga River), Provence, France (Pansard, L’Cause and Aigue Brun) and the Western Cape of South Africa (Holsloot River). These sites differ significantly in their fluvial regimes; Monte Cristo, Pansard and lower Aigue Brun are seasonal streams, and L’Cause, upper Aigue Brun and the Holsloot River are permanent streams.
We expected that Salix mucronata, along the Holsloot River and S. cinera and S. eleagnos on the upper Aigue Brun and L’Cause would be under lower water stress and be more vulnerable to cavitation as a result of maximizing water transport, whereas S. laevigata and S. lasiolepis of Southern California, and S. eleagnos and S. alba on the lower Aigue Brun and Pansard would more resistant to cavitation and be under greater water stress. However, our results reveal that S. mucronata, S. lasiolepis and S. laevigata have significantly higher P50 values than Salix species in Southern France and P50 was not higher in permanent streams. Pre-dawn water potentials were lower in for permanent streams than for seasonal streams significantly lower for permanent streams in the Western Cape and Southern France. Midday water potentials did not differ significantly. Salix species from the Western Cape and from Southern France maintained a larger margin safety between Ymin and P50 relative to species from Southern California. Our results suggest that Salix species in Southern California maximize water transport to support growth to compete for light despite being under greater water stress.