© 2006 Heron Publishing—Victoria, Canada
Wintertime photosynthesis and water uptake in a boreal forest
Sanna Sevanto (1, 4, 5), Tanja Suni (2), Jukka Pumpanen (3), Tiia Grönholm (1), Pasi Kolari (3), Eero Nikinmaa (3), Pertti Hari (3) and Timo Vesala (1)
1. Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland / 2. CSIRO Atmospheric Sciences, Pye Laboratory, GPO Box 1666, Canberra, ACT 2601, Australia / 3. Department of Forest Ecology, P.O. Box 27, 00014 University of Helsinki, Finland / 4. Department of Organismic and Evolutionary Biology, Harvard University, 3105 Biological Laboratories, 16 Divinity Avenue, Cambridge,
MA 02138, USA / 5. Corresponding author (ssevanto@oeb.harvard.edu) / Received May 9, 2005; accepted September 8, 2005; published online March 1, 2006
Summary
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 (Fw), stand photosynthesis (Ps), photosynthetically active radiation (PAR), soil and air temperatures (Ts and Tair, 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 Ps and SDV, the latter always starting 2–7 days after photosynthesis and both usually ending on the same day. Stand photosynthesis
began when Tair reached 3–4 °C and ended when Tair 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 Ts 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.
Keywords:
CO2 flux, eddy-covariance, evapotranspiration, freezing, latent heat release, Pinus sylvestris, sap flow, soil moisture, soil temperature, stem diameter variation.
