In evergreen coniferous forests experiencing strong seasonality, primary metabolism is greatly suppressed by low temperatures and trees enter a state of sustained photosynthetic downregulation, despite maintaining their leafy crowns over winter. The resumption of photosynthetic activity in spring, its timing and extent, may be influenced by exogenous (e.g., temperature [air and soil], photoperiod, or soil moisture availability) and endogenous factors (e.g., dehardening of photosynthesis or the availability of liquid water in the xylem) to degrees that remain poorly understood. Interannual variability in ecosystem CO2 exchange may be disproportionately influenced by activity during periods of transition out of and into winter, lending greater importance to a deeper understanding of the annual springtime resumption of photosynthetic activity. Furthermore, global climate change alters potentially regulatory environmental factors in complex manners; mechanistic understanding of seasonal transitions should lead to better predictions of ecosystem CO2 exchange under future climate conditions. We examined environmental conditions and physiological characteristics of three co-dominant coniferous tree species in the subalpine coniferous forest at Niwot Ridge in the Colorado Rocky Mountains. We focused specifically on whether appreciable photosynthesis could be observed during occasional warm and sunny days and the exogenous and endogenous factors associated with photosynthesis during the seasonal transition.
No appreciable photosynthetic CO2 assimilation was observed in winter, even towards the end of the season, during periods when foliage occasionally reached near optimum temperatures. During these periods, bole temperatures remained below freezing, suggesting that the inhibition of water conduction caused by frozen xylem contributed to the absence of photosynthesis. Over winter, needles of all three conifer species increased their total carotenoid to chlorophyll ratios and xanthophyll cycle pool sizes. The resumption of photosynthesis correlated with the reappearance of lower xanthophyll cycle conversion states (i.e., lower amounts of xanthophylls associated with thermal energy dissipation), after a winter period where only very high conversion states were observed. This suggests that the transition from a seasonally downregulated state of sustained, photoprotective thermal energy dissipation to a state of photosynthetic competency may partially control the resumption of CO2 assimilation in spring. Thus, there may be coordination between the dehardening of photosynthesis and the first appearance of liquid water in the boles of trees. Some of the transitions in leaf pigment composition of the sort we describe may be observable via remote sensing, which thus may serve as a means of estimating the timing of the resumption of photosynthetic activity across large spatial scales.