Warming Nights and Increased Precipitation Event Size Decrease Picea engelmannii Productivity

Climate change has the potential to change the distribution of upper treeline in alpine systems. Two elements of climate change—rising low temperatures and changes in precipitation event size--are likely to influence the physiological ecology of treeline species. Rising low temperatures will influence plant carbon status, likely increasing respiration but reducing low-temperature stress. Changes in precipitation event size and frequency may influence the duration of dry periods between rain events. This growth-chamber study was designed to observe the physiological changes to Picea engelmannii (Engelman Spruce), a dominant treeline species in the subalpine, as the seedlings were subjected to warmer nighttime temperatures and less frequent but heavier waterings. The hypothesis was that warm nights were going to increase respiration rates, which would therefore decrease the available carbon and reduce productivity. The changes in water were expected to increase the time that the P. engelmannii were dry with the additional water not compensating for the time between events. Two growth chambers were programmed to the same daytime temperatures, but with one set three degrees warmer to imitate nighttime warming. Daylength was set to mimic daily patterns in the subalpine of central Utah, USA. Half of the seedlings received a watering schedule similar to its indigenous location, and the other half received an increase of 50% in water, but 50% more time between watering. There was a significant interaction between temperature and watering frequency. Maximum photosynthetic rates were sensitive to watering frequency in ambient conditions, but watering frequency had no influence under elevated nighttime temperature. Results indicated that the plants with longer periods between watering had higher glucose levels than the more frequently watered plants. Overall, the trees grown at ambient temperature with the more frequent watering were more productive than all other treatments, which supports our hypothesis. Contrary to conventional wisdom, this research suggests that warming temperatures will not necessarily increase productivity of treeline seedlings.