It is widely recognized that average global temperatures have risen significantly within the past century, and more sharply within the past decade.  A unique element of rising temperatures that confirms the role of greenhouse gasses is that the average difference between day and night temperatures has become smaller.  Since daily thermal amplitude is often a limiting factor in tree growth and the location of tree line transitions globally, it is likely that the growth patterns of trees will be affected by changing day and night temperatures. Another aspect of predicted climate change that was integrated into our study is that precipitation events are predicted to become larger and less frequent. In a full factorial experiment we measured growth of Engelmann spruce seedlings over a typical growing season under elevated and ambient temperatures and with changes in watering frequency.  In our study seedlings were grown with temperatures set to mimic growing conditions with daily temperatures and length mimicking 30 year averages from the Wasatch Plateau from June 24-September 1. The control group was set to follow the long-term average nighttime temperatures. The “warm-night” treatment was 3 ^oC warmer at night than the control plots.   We assigned 56 Engelmann spruce (Picea engelmannii) seedlings to four different treatments.  The factorial treatments were temperature (ambient and warm nights) and watering frequency (frequent/small and infrequent/large).  
Results/Conclusions

We found small differences in the timing of bud break. Enzyme kinetics was assessed using A-Ci and A-light curves and we found seasonal changes in enzyme kinetics and maximum photosynthetic rates. The most consistent impact of warmer temperatures was observed in nonstructural carbohydrate pools. Photosynthesis and respiration rates were measured over the course of the study with a Li-Cor 6400.  Stomatal conductance was measured periodically throughout the study before and after watering events and showed little response to watering indicating that our timing and amount of water rarely created water stress in these plants. We can conclude based on our study that nighttime temperatures have the ability to influence plant carbon status and have the potential to influence subalpine treeline dynamics.