We aimed to find what kinds of microclimates were created by an abrupt treeline and relate those microclimates to the spatial structure of the treeline itself. We specifically wanted to understand how airflow is directly related to air temperature upslope of treeline. To do this, we took data from an abrupt treeline on Pike’s Peak in the Front Range of the Colorado Rocky Mountain Range. Our data was taken in September of 2016, which is representative of the tail-end of the growing season for trees. The wind speed and direction appeared to have a strong relationship with the air temperature, as the daytime uphill anabatic airflow created eddy zones of slow-moving air that were able to warm up from sensible heat dissipated at the ground surface., The nighttime downhill katabatic winds accumulated pockets of slow-moving cold air. This study helped us understand that sheltering with respect to treelines is not the result of single and independent trees, but rather the result of the entire treeline as complete three-dimensional structure. This is important because the effects of sheltering at treeline will vary from location to location based on the shape of the entire spatial structure of the ecotone.
Treelines are climatically constrained ecotones existing worldwide. With global warming and climate change, treelines are expected to advance in elevation on a global scale. Previous research has shown that abrupt treeline shapes are advancing at far slower rates than diffuse treeline structures, indicating that temperature increases are not the only factor. Smaller-scale, endogenous factors may be at play including microclimates, tree-to-tree interactions and feedbacks. Our study at an abrupt treeline on Pike’s Peak aims to understand the effects of temperature and smaller-scale factors on seedling growth, in the effort to try and understand the feedbacks involved in treeline movement and formation. Results indicate that this specific abrupt treeline is creating a microclimate that facilitates seedling growth above the historical treeline. Once this new growth of seedlings matures, another abrupt treeline will form and perpetuate the process.
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.