Higher photosynthetic capacity from higher latitude: foliar characteristics and gas exchange of southern, central and northern populations of Populus angustifolia.

Narrowleaf cottonwood (Populus angustifolia James) is an obligate riparian poplar that is a foundation species in river valleys along the Rocky Mountains, spanning 16° of latitude from southern Arizona, USA to southern Alberta, Canada. Its current distribution is fragmented, and genetic variation shows regional population structure consistent with the effects of geographic barriers and past climate. It is thus very well-suited for investigating ecophysiological adaptation associated with latitude. In other section Tacamahaca poplar species, genotypes from higher latitudes show evidence of short-season adaptation with foliar traits that contribute to higher photosynthetic capacity. We tested for similar adaptation in three populations of narrowleaf cottonwoods: from Arizona (south), Alberta (north) and Utah, near the centre of the latitudinal distribution. We propagated 20 genotypes from each population in a common garden in Alberta, and measured foliar and physiological traits after 3 years. Leaves of genotypes from the northern population had higher leaf mass per area (LMA), increased nitrogen (N) content and higher carotenoid and chlorophyll content, and these were associated with higher light-saturated net photosynthesis (Asat). In leaves of all populations the majority of stomata were abaxial, with the proportion of abaxial stomata highest in the southern population. Stomatal conductance (gs) and transpiration rates were higher in the northern population but water-use efficiency (Asat/gs) and leaf carbon isotope composition (δ(13)C) did not differ across the populations. These results (i) establish links between Asat and gs, N, chlorophyll and LMA among populations within this species, (ii) are consistent with the discrimination of populations from prior investigation of genetic variation and (iii) support the concept of latitudinal adaptation, whereby deciduous trees from higher latitudes display higher photosynthetic capacity, possibly compensating for a shorter and cooler growth season and reduced insolation.