Background/Question/Methods   Treeline advance due to climate change could have important impacts on regional C dynamics and alpine biological diversity. Theoretically, warming at the low end of forecast temperatures could cause treeline advance into herbaceous subalpine vegetation.  However, there continues to be controversy concerning the mechanisms controlling treeline and the physiological responses to rising temperature. Of particular interest is the disproportionate impact of rising nighttime temperatures on tree physiology.  This study focused on changes in the low temperatures that Engelmann spruce seedlings might encounter under the forecast warming.  We hypothesized that an increase greater than 4 °C in low temperature will impair physiological functioning.  We also hypothesized that seedlings grown under simulated “herbaceous cover” will have a higher growth/survival rate than those grown without cover.  Apical bud break, needle growth, and biomass were measured as growth indicators. Non-structural carbohydrates and gas exchange were also measured as indicators of physiological response.  We grew Engelmann spruce seedlings in growth chambers for three months.  Minimum temperature of the control chamber was 9.5°C, with the other two chambers having low temperatures at 13.5°C and 17.5°C. 
Results/Conclusions   Seedlings grown under warmer low temperatures had an earlier bud break (p=0.019).  By the end of 3 months, average needle elongation was highest in the control treatment (p=0.020 for temperature, p=0.026 for cover).  There was no statistical significance (for temperature and cover) in non-structural carbohydrates or assimilation, conductance or internal CO₂ concentrations.  Seedlings grown in the highest low temperature treatment had 30% lower root:shoot ratio, 21% lower RMR and 16% higher SMR than the control seedlings (p=0.001 for all) while cover had no affect.  Our data suggest that subalpine tree seedlings may have a warming temperature threshold, above which growth diminishes. Increases over 4 °C appear to decrease average needle growth and reduce root biomass production.  Lower root:shoot ratios may inhibit a seedlings ability to compete for water with tree/plant species better adapted to warmer temperatures.  Overall, we found that increases in nighttime temperature did not produce a linear response in growth or physiology indicating that there are some extreme temperatures beyond which Engelmann spruce fitness decreases.