title: Bursts of CO2 released during freezing offer a new perspective on avoidance of winter embolism in trees.
abstract: OBJECTIVE
  Woody plants can suffer from winter embolism as gas bubbles are formed in the water-conducting conduits when freezing occurs: gases are not soluble in ice, and the bubbles may expand and fill the conduits with air during thawing. A major assumption usually made in studies of winter embolism formation is that all of the gas dissolved in the xylem sap is trapped within the conduits and forms bubbles during freezing. The current study tested whether this assumption is actually valid, or whether efflux of gases from the stem during freezing reduces the occurrence of embolism.
  
  
  METHODS
  CO2 efflux measurements were conducted during freezing experiments for saplings of three Scots pine (Pinus sylvestris) and three Norway spruce (Picea abies) trees under laboratory conditions, and the magnitudes of the freezing-related bursts of CO2 released from the stems were analysed using a previously published mechanistic model of CO2 production, storage, diffusion and efflux from a tree stem. The freezing-related bursts of CO2 released from a mature Scots pine tree growing in field conditions were also measured and analysed.
  
  
  RESULTS
  Substantial freezing-related bursts of CO2 released from the stem were found to occur during both the laboratory experiments and under field conditions. In the laboratory, the fraction of CO2 released from the stem ranged between 27 and 96 % of the total CO2 content within the stem.
  
  
  CONCLUSIONS
  All gases dissolved in the xylem sap are not trapped within the ice in the stem during freezing, as has previously been assumed, thus adding a new dimension to the understanding of winter embolism formation. The conduit water volume not only determines the volume of bubbles formed during freezing, but also the efficiency of gas efflux out of the conduit during the freezing process.
authors.affiliation: Department of Forest Sciences, University of Helsinki, Post Office Box 27, FI-00014, Helsinki, Finland anna.lintunen@helsinki.fi.
authors.forename: A
authors.initials: A
authors.lastname: Lintunen
authors.affiliation: Department of Forest Sciences, University of Helsinki, Post Office Box 27, FI-00014, Helsinki, Finland Department of Physics, University of Helsinki, Post Office Box 64, FI-00014, Helsinki, Finland.
authors.forename: L
authors.initials: L
authors.lastname: Lindfors
authors.affiliation: Department of Forest Sciences, University of Helsinki, Post Office Box 27, FI-00014, Helsinki, Finland Department of Physics, University of Helsinki, Post Office Box 64, FI-00014, Helsinki, Finland.
authors.forename: P
authors.initials: P
authors.lastname: Kolari
authors.affiliation: Department of Forest Sciences, University of Helsinki, Post Office Box 27, FI-00014, Helsinki, Finland Department of Physics, University of Helsinki, Post Office Box 64, FI-00014, Helsinki, Finland.
authors.forename: E
authors.initials: E
authors.lastname: Juurola
authors.affiliation: Department of Forest Sciences, University of Helsinki, Post Office Box 27, FI-00014, Helsinki, Finland.
authors.forename: E
authors.initials: E
authors.lastname: Nikinmaa
authors.affiliation: Department of Forest Sciences, University of Helsinki, Post Office Box 27, FI-00014, Helsinki, Finland.
authors.forename: T
authors.initials: T
authors.lastname: Hölttä
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D005615:Freezing; D028222:Picea / Q000502:physiology*; D028223:Pinus / Q000502:physiology*; D012621:Seasons; D013997:Time Factors; D014197:Trees / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide
keywords: Bubble formation; CO2 efflux; Norway spruce; Picea abies; Pinus sylvestris; Scots pine; cavitation; freezing propagation; winter embolism; wood respiration