The structure of the boundary layer and its effect on tree temperatures at treeline

Background/Question/Methods

The treeline 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: diffuse treelines 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 treeline 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 200m upslope transects through the treeline 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 machine 

Results/Conclusions

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 treeline 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 closely coupled to the ground vegetation (paired t-test t= 2.213, df=10, p=0.051), but seedlings were on average 3.88C warmer than trees (paired t-test t= 5.808, df=10, p<0.0005), and trees were 6.1C 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 (+2.5C, paired t-test t= 7.205, df=19, p<0.0005), and higher RH 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.