Photosynthetic differences contribute to competitive advantage of evergreen angiosperm trees over evergreen conifers in productive habitats

Here we explore the possible role of leaf-level gas exchange traits in determining growth rate differences and competitive interactions between evergreen angiosperms and conifers. We compared relationships among photosynthetic capacity (A(max)), maximum ad stomatal conductance (G(s)), leaf life span, nitrogen concentration (N) and specific leaf area (SLA), in sun leaves of 23 evergreen angiosperm and 20 conifer populations. Despite similar average leaf N(mass), conifer leaves lived longer on average (36 months) than angiosperms (25 months). At a standardized leaf N, A(mass) was higher in angiosperms (56 nmol g(-1) s(-1)) than in conifers (36 nmol g(-1) s(-1)). Stepwize regression suggested that most of this difference in photosynthetic nitrogen use efficiency could be explained by G(s) and SLA. Mean G(s) (on an area basis) of angiosperms was higher than that of conifers (152 vs 117 mmol m(2) s(-1)), but A(area)-G(s) relationships were similar for the two groups. At a given leaf N, conifers had lower SLA (projected area basis) than angiosperms. Photosynthetic differences probably contribute to the competitive advantage of angiosperm trees over conifers in productive habitats, and may be linked to the greater hydraulic capacity of vessels, enabling angiosperms to develop higher stomatal conductance and therefore sustain higher transpiration rates.