||Shifting distributions of subalpine species, and consequent reduction in alpine habitat would have direct consequences for regional climate, many species, and for water resources in the Western U.S. While models suggest that treeline will rise due to global warming, the probabilities, rates, and extents of such changes have not been experimentally determined, at either the upper or lower range limits of treeline species. To experimentally test projections of subalpine tree speciesí uphill migration with climate change, we established the Alpine Treeline Warming Experiment (ATWE) at Niwot Ridge, CO. Common gardens subject to warming and watering treatments are replicated at three sites: near the lower limit of subalpine forest (Forest), within the alpine-treeline ecotone (Treeline), and in the alpine tundra, beyond the current elevation ranges of the species (Alpine). Seeds of three subalpine tree conifers are sown each year to quantify germination, survival, and physiological function and their dependence on climatic factors.
Timing of seed germination appears largely controlled by cumulative temperature, with earlier germination in the Forest and in heated plots. However in 2010, limber pine seedling survival decreased in plots with longer, warmer growing seasons and with a greater number of days at very low soil water content (below 8% by volume). This soil moisture threshold corresponded with shutdown of stomatal conductance and photosynthesis in first year seedlings, suggesting shallow soil water availability is critical to seedling establishment within and beyond this speciesí range. Results from 2011 and Engelmann spruce also appear to support the importance of soil water availability to seedling establishment. Thus, emerging results from the ATWE indicate that contrary to expectations, warming may not promote seedling establishment at Treeline and in the Alpine if it leads to growing seasons having extended periods with low soil moisture. Consistent with expectations, warming may reduce seedling establishment in lower elevation subalpine Forest. Additional years of study are required to understand the role of temperature in seedling establishment and to identify robust relationships due to substantial interannual variation in date of snowmelt, summer precipitation and temperature, and seed quality.