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  • Thumbnail for Microclimatological feedbacks at treeline: Is treeline structure modifying the local microclimate?
    Microclimatological feedbacks at treeline: Is treeline structure modifying the local microclimate? by Dickson, Chris C.

    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.