The concentration of soil carbon dioxide increased and methane decreased by soil Fr at the end of dormancy. At the end of the follow-up growing season, the hydraulic conductance and impedance loss factor of roots were higher in Fr than in NoFr, but there were no differences in above- and belowground biomasses.
At the beginning of the growing season, stem sap flow increased fastest in Fr + WL, with some delay in both NoWL (without WL) treatments. Increased fine root damage was followed by increased compensatory root growth.
Short and long root production and longevity decreased, and mortality increased by soil Fr, while there were no significant effects of WL. The starch content of leaves was temporarily higher in Fr than NoFr in the latter part of the growing season. Physiological activation (dark-acclimated chlorophyll fluorescence and chlorophyll content index) and growth of leaves and shoot elongation and stem diameter growth started earlier in Fr than NoFr (soil not frozen). Using a whole-tree approach, we studied the responses of silver birch ( Betula pendula Roth.) saplings, growing in mineral soil, to 6-week Fr and WL in factorial combinations during dormancy, with accompanying changes in soil gas concentrations. This may expose tree roots to waterlogging (WL) and soil freezing (Fr) more than in the current climate and therefore affect tree growth and survival. Winter precipitation and soil freeze–thaw events have been predicted to increase in boreal regions with climate change.
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