Response of central Siberian Scots pine to soil water deficit and long‐term trends in atmospheric CO₂concentration

Twenty tree ring 13 C / 12 C ratio chronologies from Pinus sylvestris (Scots pine) trees were determined from five locations sampled along the Yenisei River, spaced over a total distance of ∼1000 km between the cities of Turuhansk (66°N) and Krasnoyarsk (56°N). The transect covered the major part of the natural distribution of Scots pine in the region with median growing season temperatures and precipitation varying from 12.2°C and 218 mm to 14.0°C and 278 mm for Turuhansk and Krasnoyarsk, respectively. A key focus of the study was to investigate the effects of variations in temperature, precipitation, and atmospheric CO 2 concentration on long‐ and short‐term variation in photosynthetic 13 C discrimination during photosynthesis and the marginal cost of tree water use, as reflected in the differences in the historical records of the 13 C / 12 C ratio in wood cellulose compared to that of the atmosphere (Δ 13 C c ). In 17 of the 20 samples, trees Δ 13 C c has declined during the last 150 years, particularly so during the second half of the twentieth century. Using a model of stomatal behaviour combined with a process‐based photosynthesis model, we deduce that this trend indicates a long‐term decrease in canopy stomatal conductance, probably in response to increasing atmospheric CO 2 concentrations. This response being observed for most trees along the transect is suggestive of widespread decreases in Δ 13 C c and increased water use efficiency for Scots pine in central Siberia over the last century. Overlying short‐term variations in Δ 13 C c were also accounted for by the model and were related to variations in growing season soil water deficit and atmospheric humidity.