Water relations of several woody riparian species across four different summer fluvial regimes in southeastern France

Background/Question/Methods

Species typical of riparian habitats in Mediterranean type ecosystems (MTE) must be able to grow rapidly during periods of high water availability (winter and spring), but must also tolerate the summer drought that characterizes MTE.  The combined effect of summer drought and winter floods makes successful establishment and persistence difficult, especially in Southern California and the Mediterranean basin, where precipitation is the most variable relative to other MTE (Cowling et al, 2005.)  This study focused on comparing drought tolerance of adult individuals of five species distributed among four rivers with different summer flow regimes in Southern France in an effort to address the question of how fluvial regime (permanent surface flows versus seasonal surface flows)and plasticity in drought tolerance might influence species distribution.  We established  two study sites on the Aigue Brun river near Loumarin , the upper (surface water presnt) and lower (surface water absent), and study sites on the L’Cause (surface water present) near Aix en Provence and the Pansard River (surface water absent) near La Londe-les-Maures in Southeastern France.  We measured  leaf specific mass (LSM) and pre-dawn and midday water potentials (ψ_maxand ψ_min)  and elevation above the thalwag for individuals of Salix eleagnos, S. cinera S. alba,  Fraxinus angustifolia, Populus alba, Alnus glutinosa, and Oleander nerium.   

Results/Conclusions

Salix. eleagnos, occurring at L’Cause and Lower Aigue Brun sites did not differ in ψ_min , -2.41 MPa and 2.28 MPa respectively, but higher leaf specific mass was associated with lower ψ_min.  Salix cinera, occurring at Aigue Brun upper and L’Cause showed a similar pattern for ψ_min  (-2.69 MPa and -2.25 MPa respectively), but LSM was higher for L’Cause, 0.015 g cm^-2 and 0.0075 g cm^-2 respectively.  Fraxinus angustifolia, which occurred at all sites had lower LSM and ψ_min at Aigue Brun upper 0.0045 g cm^-2 and –2.5 MPa compared to the L’Cause and Aigue Brun lower sites.  Pre-dawn water potentials did not reflect differences in surface flow;  F. angustifolia had the highest ψ_max, -1.2 MPa, at Pansard.  Our results suggest that summer surface water availability alone is not a predictor of species response to water stress, LSM and ψ _mindo not change for all species in the same way under different surface water availability, and the lack of correlation between LSM and Ymax is may be a response to the position of individuals above the thalwag reflecting the importance of flood disturbance to the recruitment niche.