A treeline’s spatial structure is a key predictor to how it will respond to increasing global temperatures. At our fieldsite on Pikes Peak, CO, it has been observed that diffuse treelines are migrating upslope almost 10x faster than abrupt treelines. Local scale climate-modification is likely a key determiner of spatial structure and sensitivity of a treeline to climate change. This study investigated 1) the overall structure of microclimates at the stand level from remotely sensed surface temperatures 2) the ability of individual trees to modify local climate conditions, and 3) the impact of local scale climate modification by trees and saplings on seedling recruitment. Based on prior observations, the ecotone was divided into the forest (FZ), lower shelter (LSZ), upper shelter (USZ), and tundra zones (TZ). An ICI thermal camera attached to a custom built drone was used to capture temperature data at treeline before dawn and at midday. Agisoft Photoscan was used to build orthomosaics of temperature regimes using photogrammetry. Tree heights were obtained through field measurements. Data was visualized and extracted in ArcGIS and statistically analyzed in SPSS.
Results/Conclusions
(1) At night, the area immediately above treeline was the coldest due to cold air damming, and temperatures increased moving uphill. During the day, the overall pattern reversed. It was warmest in the area immediately upslope of the treeline and temperatures decreased with increasing elevation.
(2) At night, this overall pattern was modified by heat islands created underneath individual trees and by extremely cold spots that formed downslope (downwind) of trees above treeline. There was a threshold height at which the trees started creating cold spots. This threshold differed among the zones, with the lowest threshold size in the TZ (h=.75m) and highest threshold size in the USZ (h=2.5m). The tree canopies were significantly warmer than the ground around them. Temperature varied within the canopy with the upwind side being significantly warmer than the downwind side. During the day, trees modified the climate through shadows by retaining night time temperatures in these spots, with a temperature difference of up to 17C.
(3) Fewer seedlings were recruited into shadows than expected by random (𝝌2 =3.0078, df=1, p=0.0397). Seedlings recruited to cold spots as expected on random (𝝌2 =0.270, df=1, p=0.3016).