Alpine treeline is a valuable indicator of climate change because of its sensitivity to temperature. On Pikes Peak (Southern Rocky Mountains, Colorado), tree density and elevation in the forest-tundra ecotone has increased in the last century, corresponding with a 2°C increase in regional growing-season temperature. The purpose of this study was to provide a detailed analysis of the process of treeline advancement. Spatial clustering within age classes and elevational bands was used to identify harsh environments and track the upper climatic boundary of tree establishment. Overall, clustering (Ripley’s K, p < 0.01, based on boot-strapping) was more prominent at lower elevations and for older cohorts, indicating the upward migration of the climatic boundary. However, the climatic boundary may be advancing more quickly than treeline as the moving edge changed from a clustered to a randomly dispersed distribution over time: from 1868-1940 the moving edge was clearly clustered, from 1941-1976 it showed mixed results, and from 1977-2010 it displayed a random spatial pattern. Treeline advancement also demonstrated a reach-and-fill pattern, with sudden advancement of treeline, followed by a few decades of infill at lower elevations. The reach-and-fill pattern repeated three times in the last 120 years, with exponential increases in tree density, especially in the last 40 years. The recent explosion of growth and the quickly advancing climatic boundary match temporally with a shift from an abrupt to a diffuse edge typology. To my knowledge, this is the first study that examines in detail the process of changing treeline typology of an advancing treeline.
The tree line is a climatic boundary, however its ability to respond to changing climate seems to be constrained by the spatial distribution of trees at the leading edge; compared to abrupt or krummholz tree lines, diffuse tree lines are moving upslope much more readily in response to recent anthropogenic warming. Here we report on the micrometeorological processes that result from the diffuse leading edge of a moving tree line on Pikes Peak, Colorado, USA, and on the impacts these processes have on tree temperatures. We focus on the layering and movement of air in the lower 10m of the atmosphere including the height of the displacement of the zero velocity plane. Our experimental design consisted of 300m upslope transects through the tree line into the alpine tundra where we measured: (1) height of the zero plane displacement using handheld anemometers, (2) temperature of 10cm tall seedlings, 3-5m tall trees, and tundra grasses using an IR camera, (3) temperature and relative humidity at 2.5cm an 2m using Kestrel hand held weather stations, (4) the vertical atmospheric profiles using 10m towers equipped with 8 anemometers at 5 different elevations, (5) vertical movement of air using a bubble-blowing machine. Our results show that (1) the zero plane height decreased exponentially with increasing elevation (R2=0.432, N=57, p<0.0005) from approximately 25cm within the tree line to 2.5cm in the tundra above. The spatial variability of the zero plane height also decreased with elevation. (2) The temperature of small seedlings was (3) closely coupled to the ground vegetation (paired t-test t= 2.213, df=10, p=0.051),but seedlings were on average 3.88°C warmer than trees (paired t-test t= 5.808, df=10, p<0.0005), and trees were 6.1°C colder that the tundra (paired t-test t= 6.617, df=10, p<0.0005). (3) Compared to the air at 2m, the air layer at 2cm had higher temperature (+2.5°C, paired t-test t= 7.205, df=19, p<0.0005), and higher relative humidity higher (+29%, paired t-test t= 9.657, df=19, p<0.0005). (4) The vertical wind profile had a simple and smooth slow down to the zero plane at 2.5cm in the alpine tundra. However the profile was complex in all locations where trees were present: It showed an initial slow down to a very low speed at 3-4m, increase in velocity at 2m, and final slow down to the zero plane at 25cm. Qualitative and quantitative analysis of bubble movement (5) showed that the upper boundary layer was turbulent.