Yellow-cedar and western redcedar ecophysiological response to fall, winter and early spring temperature conditions

Western redcedar (Thuja plicata Donn) and yellow-cedar ( Chamaecyparis nootkatensis (D. Don) Spach) populations originating from an elevation zone where these two species naturally coexist were monitored to define their performance patterns in response to seasonal temperature conditions within the fall, winter and early spring field conditions of the Pacific Northwest coastal forest region. Western redcedar and yellow-cedar populations were measured for changes in growth rhythms, photosynthetic patterns and freezing tolerance. Net photosynthesis (P-n) for both species was directly related to minimum air temperature that occurred during the prior evening, though no population differences were detected within each species. Photosynthesis was greater in western redcedar, than yellow-cedar when minimum air temperature was above freezing. Freezing temperatures from similar to 0 to -5 degrees C caused a greater reduction in photosynthesis for western redcedar, though not a complete cessation of photosynthetic capability in either species. Freezing tolerance increased at a moderate rate in the fall as mean air temperature declined for both species when their shoot systems were still active, with freezing tolerance increasing at a rapid rate when shoot systems showed no mitotic activity. No shoot growth or mitotic activity was detected in shoot tips of both western redcedar and yellow-cedar when mean air temperature decreased to 4 degrees C for the previous week. No population differences, within each species, were detected in the development of fall freezing tolerance. Yellow-cedar obtained a slightly greater level of freezing tolerance when fall temperatures were < 4 degrees C. Both species had a loss of freezing tolerance as mean air temperature increased in late winter. Shoot growth resumed in both species in late winter when mean air temperature increased to 6 to 6.5 degrees C. The resumption of shoot growth resulted in a faster loss of freezing tolerance for western redcedar compared to yellow-cedar.