The Effect of Environmental Factors on Photosynthesis and Transpiration Rates in Pinus sylvestris L. and Picea abies L.

We compared photosynthesis and transpiration in pine and spruce trees growing in natural conditions with optimal soil moisture at the beginning of the growing season. The research was carried out at the Serebryanoborsk Forest District of the Institute of Forest Science of the Russian Academy of Sciences in the Moscow Region. The research relevance stems from the significant amount of dead spruce stands and the need for pine reforestation in these areas. There are few comparative studies of photosynthesis and transpiration in pine and spruce trees growing under similar conditions. Leaky open chambers were used to measure photosynthesis and transpiration. The rates of photosynthesis and transpiration in the needle shoots were measured based on the difference in the CO2 and H2O concentrations, respectively, between the air outside the chamber and the air exiting the chamber, the airflow velocity, and the horizontal surface area of the needles in the exposure chamber. The CO2 and H2O concentrations were measured using a LI-840 portable infrared gas analyzer (Li-Cor, USA). The air depletion by CO2 in the chamber was, on average, 2 % lower than in the ambient air. The air flow was driven by diaphragm microcompressors (Sonic-388, China). The data showed that, on a mostly sunny day, the rate of photosynthesis in pine trees was more than 88 % higher than in spruce trees. On a cloudy day, the differences were significantly lower (20 %). On a mostly sunny and overcast days, the differences in transpiration rates between the two species were smaller; 18.5 and 20 %, respectively. The relationship between CO2 exchange in pine and spruce and solar radiation and air temperature is similar; differences are observed only in terms of intensity, although in conditions of water stress, pine shows higher photosynthesis rates. The CO2 gas exchange intensity in pine trees under optimal forest growing conditions is nearly twice as high as that in spruce trees. The calculated data on photosynthesis and transpiration were close to the experimental values. The nature of the relationship between transpiration and other factors differs significantly more between pine and spruce. In pine, the differences are mainly due to solar radiation, whereas in spruce, they occur due to air temperature.