Cyclic Freezing Effects on Tracheid Bordered Pits


  • Harvey D. Erickson
  • Yang-Hsien Chen


<i>Tsuga heterophylla</i>, sapwood, heartwood, freezing, thawing, bordered pits, pit damage, pit aspiration, microtome-cryostat


Repeated freezing and thawing of wet wood of western hemlock (Tsuga heterophylla (Raf) Sarg.) caused changes in some tracheid bordered pits ranging from partial pit aspiration to tori loose and moved out of the pit cavities. Influencing factors were: lower temperatures, position in the specimen, number of freeze cycles, water content of the wood, and manner of preparation. The most damaging effects by cyclic freezing occurred near the end-grain surface of the blocks. At 1 cm from the end, very little change occurred.

Lowering the freezing temperature and increasing the number of freeze cycles increased the number of pits affected but not in direct ratio. Pit damage near the block ends was slightly greater in green wood that was water-saturated before cyclic freezing. Storing sapwood blocks for 8 months before cycling tended to decrease the number of unaspirated pits. In fresh sapwood, 96% of the bordered pits were unaspirated; in heartwood only 7%. In green heartwood, cyclic freezing deaspirated some of the pits near the block ends. Relationships of the results to natural phenomena are discussed.


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