Abstract
Climate change will increase autumn air temperature, while photoperiod decrease will remain unaffected. We assessed the effect of increased autumn air temperature on timing and development of cold acclimation and freezing resistance in Eastern white pine (EWP, Pinus strobus) under field conditions. For this purpose we simulated projected warmer temperatures for southern Ontario in a Temperature Free-Air-Controlled Enhancement (T-FACE) experiment and exposed EWP seedlings to ambient (Control) or elevated temperature (ET, +1.5°C/+3°C during day/night). Photosynthetic gas exchange, chlorophyll fluorescence, photoprotective pigments, leaf non-structural carbohydrates (NSC), and cold hardiness were assessed over two consecutive autumns. Nighttime temperature below 10°C and photoperiod below 12 h initiated downregulation of assimilation in both treatments. When temperature further decreased to 0°C and photoperiod became shorter than 10 h, downregulation of the light reactions and upregulation of photoprotective mechanisms occurred in both treatments. While ET seedlings did not delay the timing of the downregulation of assimilation, stomatal conductance in ET seedlings was decreased by 20–30% between August and early October. In both treatments leaf NSC composition changed considerably during autumn but differences between Control and ET seedlings were not significant. Similarly, development of freezing resistance was induced by exposure to low temperature during autumn, but the timing was not delayed in ET seedlings compared to Control seedlings. Our results indicate that EWP is most sensitive to temperature changes during October and November when downregulation of photosynthesis, enhancement of photoprotection, synthesis of cold-associated NSCs and development of freezing resistance occur. However, we also conclude that the timing of the development of freezing resistance in EWP seedlings is not affected by moderate temperature increases used in our field experiments.
Highlights
Global land-surface temperatures are increasing, in northern latitudes and during winter months (Intergovernmental Panel on Climate Change, 2007)
We explored the effect of a moderate increase of air temperature by +1.5◦C during the day and +3◦C during the night on the development of cold acclimation by assessing photosynthesis, photoprotective nonphotochemical quenching (NPQ) and pigments, carbohydrate metabolism and freezing tolerance in Eastern white pine in a field experiment
In contrast to our initial hypotheses, we did not observe a significant delay in the downregulation of photosynthesis or cold hardening when seedlings were exposed to elevated temperature in heated plots, nor did these seedlings exhibit altered carbohydrate metabolism or impaired cold hardiness
Summary
Global land-surface temperatures are increasing, in northern latitudes and during winter months (Intergovernmental Panel on Climate Change, 2007). Records collected since the mid-twentieth century describe a delay in the onset of dormancy and an increase in growing season length in temperate and boreal forest regions across the northern hemisphere, in North America (Boisvenue and Running, 2006; Piao et al, 2007; McMahon et al, 2010). The environmental signals used by trees to sense seasonality and trigger dormancy and development of cold acclimation are the decrease in temperature and the length of photoperiod during the autumn (Welling et al, 2004). Increasing temperatures, as projected by climate change models, will delay the low temperature signal while photoperiod will remain unaffected. Asynchronous phasing of temperature and photoperiod is expected to impact the onset and development of cold acclimation during autumn. More recent studies identified differences in the sensitivity of various evergreen conifer species to photoperiod and temperature during autumn bud dormancy (Olsen, 2010; Cooke et al, 2012)
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