Background/Question/Methods Evergreens undergo a dramatic reduction in their maximal photochemical efficiency (measured as F_v/F_m) during winter, which is largely 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 phase (reversing in minutes) and a slow phase (reversing over days). Both phases are associated with reversal of sustained energy dissipation. Preliminary examination of recovery of evergreens monitored in January in Minnesota showed an absence of the rapid component to recovery. Our goal was to monitor recovery kinetics of sun and shade evergreens in Minnesota throughout winter in order to assess whether the rapid phase of recovery exists early in the winter and converts to the slowly reversible form as winter progresses. Four species of conifers (sun and shade needles) were monitored during the winter of 2007/08: eastern white pine (Pinus strobus L.), balsam fir [Abies balsamea (L.) P. Mill], Taxus cuspidata (L.) and blue spruce (Picea pungens Engelm.). F_v/F_m was measured on dark acclimated needles in the field, twigs were collected, brought indoors and maintained at room temperature and low light where F_v/F_m was monitored for six days.

Results/Conclusions The results demonstrated that all species, and both sun and shade needles, showed a rapidly reversible component to recovery in early winter (November). In the sun needles this component was rarely present later in the season, while in the shade needles it was present (although only a small fraction of the total sustained energy dissipation) on most days monitored during winter. The slowly reversible component to sustained energy dissipation was present in both sun and shade needles of all species beginning in November. In all cases, shade needles recovered significantly faster than sun needles. There was a significant slowing of recovery (the slowly reversible component) as winter progressed in both sun and shade needles, and significant differences between species in their recovery response. The results indicate a relatively small contribution of the rapidly reversible component of sustained energy dissipation compared with earlier studies on evergreens growing in the milder winter conditions of Colorado. The results also provide evidence that the rapid component to recovery diminishes as the season progresses, particularly in needles growing in full sun where the slowly reversible component of sustained energy dissipation accounts for most or all of the observed sustained energy dissipation.