Kinetics of recovery from winter stress in eastern white pine (Pinus strobus) and white spruce (Picea glauca)

Background/Question/Methods

Evergreens undergo reductions in maximal photochemical efficiency (F_v/F_m) during winter, due to increases in a sustained form of thermal energy dissipation. Upon removing winter-stressed leaves to room temperature and low light, F_v/F_m recovers over several days and can include both a rapid (reversing in minutes) and a slow phase (reversing over days). A previous study demonstrated that species differ in recovery kinetics upon experimental warming in the lab. Our goal was to monitor recovery kinetics of two conifers species, both in the lab upon experimental warming during winter, and in the field during recovery in the spring, in order to determine if artificial recovery experiments are a useful measure of recovery in the field. A second goal was to determine if the rapid component of recovery occurs on cold nights in early spring in conifers. Two species of conifers [eastern white pine (Pinus strobus L.) and white spruce (Picea glauca L.)] were monitored in January of 2013 and monthly until full recovery occurred in the field in early May. F_v/F_m was measured on dark acclimated needles in the field, branches were collected, brought indoors and maintained at room temperature and low light where F_v/F_m was monitored for eight days.

Results/Conclusions

The half time of recovery was determined for each species on a single date in each of January, February, March and April. The half recovery time for pine was significantly longer than that of spruce in each month with an average half recovery time of about 75 hours for pine and 25 hours for spruce. The half recovery time was consistent in both species from January through March, but was shortened to an average of 42 hours for pine in April. Field measurements of Fv/Fm showed that spruce recovered significantly earlier than pine, suggesting that the artificial recovery experiments are representative of recovery occurring in the field. An examination of recovery upon immediate warming of needles was analyzed in order to determine if the rapid component to recovery was present. The results demonstrated that the rapid component to recovery was present during January in the spruce needles, but not in pine. Additionally, the rapid component did not occur in any other month that recovery was monitored. A measurement on a day in March when air temperature was -5˚C showed no rapid component in either species, suggesting that the rapid component may not occur during spring.