Influence of Environmental Factors on Photosynthesis of Three Coniferous Species

The effect of five ecological factors (incident radiation, air and soil temperature, vapour pressure deficit (D), and soil moisture) on the net CO₂ photosynthesis rate was examined for three boreal tree species – Siberian larch (Larix sibirica), Sibirian spruce (Picea obovata), and Scots pine (Pinus sylvestris) from the Baikal lake region, by means of a coupled photosynthesis–stomatal conductance model. The model was parametrized on the data obtained from 5–year long field measurements made on sun needles of three 14–19 year-old trees of each species. Pine had the greatest photosynthesis rate while spruce had the lowest one. There were 9 free model parameters in total fitted on the whole data set that gave the model efficiency ME = 0.46–0.66. For single years and three intra–year time intervals, parameter fitting was made for three key parameters only: maximum Rubisco activity, V_cmax25, maximum stomatal conductance, g^max_sw, and quantum efficiency of photosynthesis, φ; this procedure raised fitting quality to ME = 0.61–0.72.

Photosynthesis rate varies due to current weather conditions and also due to long-term changes in the plant itself, that is in the parameters that determine the photosynthesis rate. The change of parameter values with time probably takes place due to between– and within–year weather change. Under actual weather conditions in Pre-Baikalia, larch realises 49%, spruce – 30%, аnd pine – 36% of their maximum photosynthesis rate ability. Soil moisture significantly affected V_cmax25 for larch and spruce, while for larch it was vapour pressure deficit (D) that most affected the value of V_cmax25. The coefficient of stomatal limitation of photosynthesis, L_s, offered by Sharkey, showed that stomatal regulation is maximal for pine, and minimal for larch.