Water use physiologies of co-occurring goldenrods (Solidago juncea and S. canadensis): Implications for natural distributions.

Water use patterns and the seasonal progression of functional leaf area were determined for Solidago canadensis L. var. scabra and S. juncea Ait., two species of cooccurring goldenrods which differ in their competitive ability and distribution along soil moisture gradients. Field measurements of diurnal trends in stomatal conductances and leaf water potentials indicate little difference between the species. Laboratory gas exchange measurements of assimilation rates (13.15-13.25 micromoles CO2 m-2 sec-1), stomatal conductances (31.53-38.44 centimoles H2O m-2 sec-1), water use efficiencies (8.10-9.66 mg CO2 g H2O-1) and stomatal response to low leaf water potentials (i.e. initiation of stomatal closure at -16 to -20 bars) were also similar for the two species. Differences in their maximum functional leaf areas (421 cm2 vs 209 cm2 for S. canadensis and S. juncea, respectively, at maturity), phenologies (S. juncea flowers about one month earlier than S. canadensis) and the presence of the non-reproductive rosette habit in the dry site species (S. juncea) are probably more important in explaining the differential distributions of these two species than differences in their water use patterns.