Mounting research on alpine treeline advance suggests that global and regional temperatures do not completely explain changes in treeline elevation and distribution. Rather, micrometeorological feedbacks may play an important role in treeline advance by increasing local temperatures. On Pikes Peak, the comparison of a transition zone microclimate at treeline to an adjacent rockslide microclimate at the same elevation showed that the transition zone microclimate heats more quickly and to a higher maximum temperature than the rockslide. Observed differential heating is particularly prevalent in the near-surface soil temperature, an important location for seedling establishment and growth. During the June observation period, daytime temperature maximums in the transition zone soil were 7C warmer on average than in the rockslide. Local warming at the treeline’s leading edge suggests that the presence of trees increases soil heat flux through a variety of mechanisms. Canopy warming, varying soil moisture, and sheltering are each considered independently as possible causes of differential heating. First, I investigate the possibility that heat captured in the canopy warms the transition zone microclimate. However, this theory is unsupported by data showing daytime canopy transpiration and cooling, and infrared photos revealing that the canopy is significantly cooler than the rockslide during the day. Second, I explore whether higher soil moisture in the transition zone is responsible for differential heating via increased conduction. However, soil moistures are actually lower in the treeline microclimate, suggesting that low soil moisture may be a characteristic of warming rather than its cause. Third, I look at the idea that trees shelter the microclimate from wind and hence reduce heat loss. While sheltering effects show some relationship with differential heating, there is no consistent correlation between high wind and differential heating. While this analysis does not offer a clear cause of differential warming, a better understanding of the treeline system is gained, and suggestions are made for how and where to look for warming feedbacks in the future. Thus, while results are inconclusive, warming feedbacks at treeline that increase soil temperatures during the critical growing season should be further considered as factors in treeline advance.
Recent study of altitudinal treeline advance has revealed that increasing seasonal temperatures only partly explain the processes that influence treeline structure and elevation. Microsite modifications, induced by the structure of the treeline, may in fact play a large role in regulating the microclimate, creating more favorable conditions for further seedling establishment and recruitment near the treeline. To explore these modifications, previous research on Pikes Peak has compared heating dynamics within a treeline microclimate to the microclimate of an adjacent rockslide at an identical elevation. Observations indicated that the treeline heats up faster and to a higher maximum temperature than the rockslide nearly every day of the study period (Johnson, 2011). Potential mechanisms for this differential heating were explored, however only the sheltering potential of the trees to reduce winds proved worthy of further investigation (Anderson, 2012). To expand upon these findings, this study aims to verify the presence of differential heating between treeline and rockslide, investigate the role of sheltering to reduce heat loss within treeline, and explore to what extent this sheltering could extend beyond the treeline’s leading edge. First, this study found that temperatures within the treeline were on average ~7C warmer than the rockslide from 15cm above the ground to 10cm deep within the soil, a critical habitat for seedling establishment (Körner, 1998). Furthermore, this study reveals that the magnitude of differential heating increases throughout the growing season, exhibiting larger differences later in the season. These findings indicate that, despite decreasing solar input late in the season, the treeline has a higher capacity to retain heat than the rockslide and prolongs favorable growing conditions later into the summer months. To investigate how sheltering may play a role in holding heat within the treeline, the zero-plane displacement was calculated for the treeline, rockslide, and upper tundra. Results indicate that treeline form shelters a boundary layer of warm air close to the ground that could enable increased heat storage within the treeline’s soil. Furthermore, this sheltering effect extends beyond the treeline’s leading edge and modifies the tundra microclimate by reducing wind effects in lee of the treeline. This mechanism of sheltering could create a positive feedback loop in which microclimatological modifications, induced by the trees presence, allow for continual growth beyond the forest boundary.