The effect of precipitation frequency and magnitude on Engelmann Spruce (Picea engelmannii) physiology

Background/Question/Methods

Over the past two decades, over 800,000 hectares of Engelmann spruce (Picea engelmannii) in Utah’s high country have died with little evidence of regeneration. The direct cause of death is the mountain pine beetle, but the extent and magnitude of the die-off is likely amplified by climate change. Discussions on the physiological controls over high elevation forest dynamics have emphasized either adult tree growth or seedling physiology or establishment dynamics. Both of these approaches have emphasized C acquisition and use. Whole plant C physiology studies typically focus on the role of temperature in regulating photosynthesis and C-sinks. However, understanding the impacts of growing season drought, and its interaction with temperature, remains an impediment to fully forecasting how treeline will respond to changing climate. Here we report on a growth chamber and a field study that examines the influence of growing season precipitation amount and timing on photosynthetic rates and dynamics and plant water relations. The question that we addressed is whether a shift to infrequent, intense precipitation will negatively impact spruce seedling C balance and water relations, restricting the regeneration of forests in Utah’s subalpine.

Results/Conclusions

An initial growth chamber experiment showed that maximum photosynthetic rates for P. engelmannii seedlings increased with larger watering events compared to trees experiencing frequent, small events. However, the trees that had larger watering events also had higher respiration rates, resulting in no net difference in production between watering treatments. In the field there were high mortality rates for seedlings, with precipitation amount an important predictor of seedling survival. We observed the highest mortality rates among trees that had a 70% reduction in growing season precipitation. These results indicate that precipitation has the potential to modify seedling dynamics at treeline and it is critical to examine the interaction between temperature and water availability to understand subalpine forest regeneration.