Geographical pattern in photosynthetic light response of Pinus sylvestris in Europe

1. The geographical aspects in photosynthetic light response and stomatal conductance in the shoots of Pinus sylvestris were studied together with structural properties of shoots and needles. Seven stands within the natural distribution area of P. sylvestris in Europe were chosen. CO₂ exchange, irradiance and stomatal conductance ( g_s ) for water vapour were measured and the maximum photosynthetic rate ( P_m ) was determined from the CO₂ exchange measurements.
2. There was a clear pattern in the average values of P_m along the latitudinal gradient. Highest values of P_m were found in the middle parts of the distribution area and they decreased towards both ends of the transect. The highest value was almost twice as high as the lowest one.
3. The between-site variation explained 70% of the total variation in the maximum photosynthetic rate. P_m was not clearly correlated with any single climatic variable or nitrogen concentration in the needles.
4. P_m was closely coupled with stomatal conductance ( r ²=0·74). The differences in P_m and g_s between the sites is likely to reflect adaptation and acclimation to different climates.