Response of central Siberian Scots pine to soil water deficit and long-term trends in atmospheric CO₂ concentration

[1] Twenty tree ring C-13/C-12 ratio chronologies from Pinus sylvestris (Scots pine) trees were determined from five locations sampled along the Yenisei River, spaced over a total distance of similar to1000 km between the cities of Turuhansk (66degreesN) and Krasnoyarsk (56degreesN). 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 12.2degreesC and 218 mm to 14.0degreesC 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 CO2 concentration on long-and short-term variation in photosynthetic C-13 discrimination during photosynthesis and the marginal cost of tree water use, as reflected in the differences in the historical records of the C-13/C-12 ratio in wood cellulose compared to that of the atmosphere (Delta(13)C(c)). In 17 of the 20 samples, trees Delta(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 CO2 concentrations. This response being observed for most trees along the transect is suggestive of widespread decreases in Delta(13)C(c) and increased water use efficiency for Scots pine in central Siberia over the last century. Overlying short-term variations in Delta(13)C(c) were also accounted for by the model and were related to variations in growing season soil water deficit and atmospheric humidity