Evidence of altitudinal increase in photosynthetic capacity: gas exchange measurements at ambient and constant CO2 partial pressures

? Because all microclimatic variables change with elevation, it is difficult to compare plant performance and especially photosynthetic capacity at different elevations. Indeed, most previous studies investigated photosynthetic capacity of low- and high-elevation plants using constant temperature, humidity and light but varying CO₂ partial pressures (P _{CO 2}).

? Using gas exchange measurements, we compared here maximum assimilation rates (A _max) at ambient and constant-low-elevation P _{CO 2}for two temperate tree species along an altitudinal gradient (100 to 1600 m) in the Pyrénées mountains.

? Significant differences in A _max were observed between the CO₂ partial pressure treatments for elevations above 600 m, the between-treatment differences increasing with elevation up to 4 μmol m^?2 s^?1. We found an increase in A _max with increasing elevation at constant-low-elevation P _{CO 2} but not at ambient P _{CO 2} for both species. Given a 10% change in P _{CO 2}, a proportionally higher shift in maximum assimilation rate was found for both species.

? Our results showed that high elevation populations had higher photosynthetic capacity and therefore demonstrated that trees coped with extreme environmental conditions by a combination of adaptation (genetic evolution) and of acclimation. Our study also highlighted the importance of using constant CO₂ partial pressure to assess plant adaptation at different elevations.