Geographic pattern of genetic variation in photosynthetic capacity and growth in two hardwood species from British Columbia

Geographic patterns of intraspecific variations in traits related to photos ynthesis and biomass were examined in two separate common garden experiment s using seed collected from 26 Sitka alder (Alnus sinuata Rydb.) and 18 pap er birch (Betula papyrifera Marsh.) populations from climatically diverse l ocations in British Columbia, Canada. Exchange rates of carbon dioxide and water vapour were measured on 2-year-old seedlings to determine the maximum net instantaneous photosynthetic rate, mesophyll conductance, stomatal con ductance, and photosynthetic water use efficiency. Height, stem diameter, r oot and shoot dry mass and fall frost hardiness data were also obtained. Me an population maximum photosynthetic rate ranged from 10.35 to 14.57 mu mol CO2 m(-2) s(-1) in Sitka alder and from 14.76 to 17.55 mu mol CO2 m(-2) s( -1) in paper birch. Based on canonical correlation analyses, populations fr om locations with colder winters and shorter (but not necessarily cooler) s ummers had higher maximum photosynthetic rates implying the existence of an inverse relationship between leaf longevity and photosynthetic capacity. S ignificant canonical variates based on climatic variables derived for the s eed collection sites explained 58% and 41% of variation in the rate of phot osynthesis in Sitka alder and paper birch, respectively. Since growing seas on length is reflected in date of frost hardiness development, an intrinsic relationship was found between photosynthetic capacity and the level of fa ll frost hardiness. The correlation was particularly strong for paper birch (r=-0.77) and less strong for Sitka alder (r=-0.60). Mean population bioma ss accumulation decreased with increased climate coldness. These patterns m ay be consequential for evaluation of the impact of climate change and exte nsion of the growing season on plant communities.