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  • Thumbnail for Micrometeorological feedbacks at treeline : are the trees at the leading edge responsible for increased local temperatures?
    Micrometeorological feedbacks at treeline : are the trees at the leading edge responsible for increased local temperatures? by Anderson, Joshua

    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.