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Warm air in combination with frozen soil is a major cause of wintertime drought damage in evergreen plants in subalpine and boreal environments. We analyzed diurnal tree stem diameter variation (SDV), which reflects soil water uptake, canopy-level water vapor flux ( F  w ), stand photosynthesis ( P  s ), photosynthetically active radiation (PAR), soil and air temperatures ( T  s  and  T  air , respectively) and soil liquid water content (θ) to determine under what conditions photosynthesis is possible in wintertime and how crucial water uptake from soil is for photosynthesis. Measurements were made under field conditions in a Scots pine forest in southern Finland during winter 2002–2003. We found four wintertime periods when there was measurable  P  s  and SDV, the latter always starting 2–7 days after photosynthesis and both usually ending on the same day. Stand photosynthesis began when  T  air  reached 3–4 °C and ended when  T  air  dropped below –7 °C. The trees appeared to rely on stored stem water first and started taking up water from the soil a few days later, when the transpirational demand became strong enough. The more difficult it was to access soil water because of low  T  s  or low θ, the longer the trees used water stored in their stems. Even partial stem freezing did not prevent photosynthesis or soil water uptake.

Wintertime photosynthesis and water uptake in a boreal forest