Scientific publications

Gaseous CO2 exchange and shoot growth over a growing season were studied in 3‑yearold seedlings of silver birch (Betula pendula Roth) growing in 10‑litre pots with different soil temperatures. Initially, activation of growth and an increase of CO₂ exchange were observed in seedlings experiencing higher soil temperatures compared to seedlingsgrowing in colder soil. Also, an increase in stomatal conductance and a higher pigment content were detected. In the seedlings growing in cold soil, a consistent rise of CO₂ exchange and dark respiration was observed as the air temperature and soil temperature were growing. In the period before intensive growth, the total leaf area in seedlings rooted in warmer soil was more than three times higher than that of seedlings in cold soil. Until July, air temperatures were not high enough to initiate intensive growth. Therefore, a great deal of assimilates in the seedlings growing under higher soil temperature were largely being deposited, particularly in roots and bark. In the absence of powerful attracting points, assimilates were in excess, and photosynthesis was inhibited. A significant difference in CO₂ exchange and stomatal conductance between the two groups of seedlings was seen at the beginning of the growing season. Later on, as the air temperature was rising, CO₂ exchange increased insignificantly in seedlings growing in soil of higher temperature. Warming up of the soil smoothed down the difference in CO2 exchange between the two groups of seedlings. In the period of intensive growth in July, CO₂ exchange in seedlings growing naturally in the field was higher than in pot-grown seedlings. We suppose that the increase in soil temperature early in the growing season results in a rise in the concentration of sucrose in the phloem exudate, and promotes bark thickening. This process is quite pronounced in Karelian (curly) birch – an ecological form of silver birch (Betula pendula Roth var. carelica (Merclin) Hämet Ahti).