Niche models and paleoecological observations indicate that climate change will alter the geographic distributions of plant species. Climate warming will likely be more pronounced at high elevations due to the strength of snow-albedo feedbacks. An upward shift in the position of alpine treeline would accentuate land surface feedbacks to warming and displace alpine species. We have established a new experiment to investigate potential shifts in the distribution of subalpine tree species, and the alpine species they might replace. We are asking how tree species recruitment and alpine species growth and reproduction vary within their current ranges, and in response to temperature and soil moisture manipulations. We are also examining whether genetic provenance and ecosystem processes constrain seedling and alpine herb responses.

The Alpine Treeline Warming Experiment is located across three sites at Niwot Ridge, CO, ranging from the lower limit of subalpine forest to above treeline. We are using infrared heaters to raise growing season surface soil temperatures by 4-5°C, and to lengthen the growing season. The warming treatment is crossed with a soil moisture manipulation to distinguish effects due to higher temperatures from those due to drier soil. Each plot is a common garden sown with two populations each of Limber pine (Pinus flexilis) and Engelmann spruce (Picea engelmannii). We established an additional set of experimental plots to examine treatment effects on alpine species. As a complement to the field experiment, we are conducting growth chamber experiments with the same species and populations.
Results/Conclusions

To date, we have determined that seedling germination rate, lifespan, and first year survival is higher within the species’ current range than in the alpine, and for Engelmann spruce, higher at the low elevation limit than the high elevation limit under ambient conditions. Source population (low vs high elevation) is a significant factor explaining natural variation in germination rates and timing, seedling development, seedling physiology, and seedling survival. Evidence from growth chamber experiments suggests that warming advances the timing of germination, accelerates seedling development, and lengthens the period of time for seedling carbon gain. Responses to warming in the growth chamber were larger for Limber pine than Engelmann spruce, and for high than low elevation populations. Our initial results suggest that germination and growth of subalpine tree seedlings may be promoted by warmer temperatures. Further, population level differences in seedling response to environmental change are significant.