Experimental design and site description

We established 80 3-m diameter plots across three sites at Niwot Ridge, Colorado, ranging from 10,000 ft to 11,600 ft in elevation. We established 20 plots near the lower or warm-edge limit of Engelmann spruce and limber pine distributions, in the lower subalpine site (LSA). We placed another 20 plots near the upper or cold-edge limit of these two species, in the upper subalpine site (USA). Above the upper or cold-edge limit of subalpine trees, we placed 40 plots in the alpine site (ALP). We are using half of the ALP plots to study the target tree species, and half to study alpine plant species.

In all sites, we are applying two environmental treatments, heating and watering. Plots are heated with infrared heaters mounted to deliver even heat to the entire plot (Kimball et al. 2007), while water is delivered manually during the growing season (starting 2010) using electronically controlled backpack sprayers. The experimental design is a full factorial, meaning that in each site, one quarter of the plots are heated, one quarter are watered, one quarter receive both heat and water, and one quarter receive no treatment as "controls." The watering treatment is meant to compensate for the effects of soil drying that occur with the heating, helping us distinguish biological responses to warming from responses to soil drying.

Each year, we sow seeds in fall and transplant greenhouse-started seedlings in spring from two distinct populations of limber pine and two distinct populations of Engelmann spruce into the plots in each site. The 20 alpine species research plots in the ALP site receive no outside sources of seed or seedlings. Cages made from hardware cloth are placed over the plots to protect seeds and/or seedlings from being eaten by rodents. Reducing such "biotic effects" is important so that we can differentiate changes in seedling numbers and health caused by climate effects from changes caused by other ecological factors.

LSA Dimmers switchesCore field measurements
Climate in the three sites is measured by recording air temperature, relative humidity, wind speed, and photosynthetically active radiation every 15 minutes. In all of the experimental plots we measure soil microclimate at 5-10cm and 15-20cm depths. We also measure snow, soil and vegetation properties relevant to understanding seedling and alpine plant success.

All trees from which seed is collected are also genotyped using a range of nuclear, chloroplast and mitochondrial DNA markers. This allows us to determine whether there are genetic differences among our populations that correlate with their responses to the experimental treatments.

During the growing season we monitor seedling germination and survival, as well as growth rate, rate of photosynthesis, and water stress. We also follow alpine species phenology (timing of flowering), physiology, and relative abundance. Collaborators and students are pursuing additional projects related to soil and plant microbial communities, winter and spring snow properties, and alpine plant traits.

Growth chambers
In addition to our field study, we have ongoing experiments in controlled-environment growth chambers located at UC Merced. We germinate seeds from the same and a broader set of populations than those used in the field study to determine whether germination and initial growth are responsive to climate warming, and to look more carefully at how seed origin might interact with warming and watering to determine growth patterns.