Previous studies have found treeline dynamics to be related to macroclimatic factors (Harsch et al., 2009) and microclimatic factors such as local snowpack, wind, and air and soil temperature (Germino, Smith, and Resor, 2002; Harsch et al., 2011; Smith et al., 2003). Seedling health has been thought to indicate the future of treeline position (Germino et al., 2002; Smith et al., 2003; Harsch et al., 2011) and has appeared to be highly related to soil temperature (Grace, Berninger, and Nagy, 2002; Harsch et al., 2011; Smith et al., 2003; Körner et al., 2004; Malanson et al., 2011). This study examines the effects of soil temperature on seedling establishment, survival, and growth, and the degree to which soil temperature may influence seedling dynamics relative to other factors. Daytime and nighttime soil temperature, as well as seedling establishment, survival, and growth data, were collected in an abrupt treeline consisting entirely of Engelmann Spruce (Picea engelmannii) on Pikes Peak, Colorado (~3,550 m). The study site was split up into three different microsites with unique microclimates – the Lower Sheltered Zone, Upper Sheltered Zone, and Tundra – where the relationships between seedling dynamics and soil temperatures could be analyzed separately, and post hoc analyses were conducted to compare the broader zonal relationships. Seedling establishment within the Lower Sheltered Zone was generally greater than expected in the warmer areas and less than expected in the colder areas. Establishment in the Tundra, however, followed the opposite pattern. Survival was largely statistically unrelated to soil temperature, unless a certain threshold was surpassed within the Lower Sheltered Zone and Upper Sheltered Zone. Growth was statistically significantly related to daytime soil temperatures within the Lower Sheltered Zone by a second-degree polynomial, with an optimum at 7.87°C (R2 = .39, n = 16, p = 0.04), but statistically unrelated to soil temperatures within the other two zones. Post hoc zonal comparisons of seedling dynamics revealed relatively large statistically significant differences, which suggested that other microclimatic factors were influencing seedling dynamics more than soil temperatures.
Thermal conditions control the elevation to which trees persist in alpine settings. Long-term historical data suggests a correlation between periods of anomalously warm regional temperatures and treeline advance on Pike’s Peak (Southern Rocky Mountains, Colorado, USA) (Kummel et al., W.I.P). Still, treelines do not uniformly respond to warming and treeline form is shown to be an indicator of sensitivity to warming (Harsch & Bader, 2011). This dependence suggests that further investigation of the relationship between climate and treeline movement is warranted. While alpine vegetation are controlled by the climate at treeline, they also interact with the air around them and in this way influence local climate. This report focuses on the microclimatology of air parcels surrounding individual trees and the relationship between microclimatology and tree growth. We found important results that indicate the formation of distinct microclimatological regions around individual trees. Specifically, it seems that trees act as a barrier to upslope airflow and in so doing cause the formation of eddies on the leeward side of trees. The longer residence times of entrained air tends to correspond with elevated temperature and moisture conditions. This microclimate formation suggests that trees process and shape their local climate in interesting ways. Understanding the sensitivity of treeline to climate change will be a question of understanding the interaction of local tree climate with that of the overall treeline.
Vertical growth is an important element to consider when evaluating the movement of an alpine treeline. The vertical growth of trees is decisive in the establishment of trees upslope of the existing treeline, as trees must be able to grow up, mature, and reproduce in order for the treeline to advance. The purpose of this study was to explore the possible causes of, and factors influencing, the vertical growth of trees in a treeline environment, specifically at the alpine treeline of Pike’s Peak, CO. Vertical growth was first studied on an individual scale, specifically investigating the thermal regime of trees and its impact on growth. The air temperature profile showed a nighttime inversion of daytime conditions. During the night there was a lapse rate of approximately 1°C, with the coldest conditions closest to the ground. Thus, the smallest trees were in significantly colder environments during the night than the largest trees. During the day, there was a lapse rate of approximately 3°C per meter, a very high lapse rate, with the warmest conditions occurring closest to the ground. Thus, the smallest trees were in the warmest conditions throughout the day. Additionally, it was found that small trees were coupled to ground conditions during the day as well as the night, and that the taller trees were coupled to atmospheric conditions. Yet, the coupling relationships were not exact, as the tree temperatures never exactly matched the ground or atmospheric temperatures. Finally, I investigated whether daytime or nighttime temperatures impacted growth more closely. It was found that daytime conditions were more important for the growth of trees at the study site on Pike’s Peak. The second part of the study investigated tree growth on a stand-wide scale, considering whether or not there were larger spatial patterns affecting the vertical growth of trees. I found that a shelterbelt-like system was in place at the treeline, the presence of which seemed to be affecting the growth of the trees within its bounds. Specifically, there was depression of growth directly upslope of the trees creating the upper bounds of the treeline, then an area of facilitated growth, ending with a return to normal conditions. Yet, these shelterbelt conditions were only detected for trees one meter or taller. The growth patterns for trees under 1 meter did not correlate to the growth patterns of taller trees. Additionally, the shelterbelt conditions would only be present during the day, which further confirms the importance of daytime conditions found in the first study. This exploratory study was a first look into the drivers of vertical growth of trees at an alpine treeline.
Pike’s Peak Treeline Microclimatology: Our study site in Pike’s Peak is one of the few abrupt treelines that is advancing with recent regional warming. We established that there is most likely an eddy in the lee of timberline during askew flow as evidenced by the increasing size (both length and height) of a slow air bubble from parallel to askew flow. This increased size of the slowed air bubble creates sheltered conditions downwind of the shelterbelt. Shelterbelts are known to ameliorate agricultural health because eddies can create beneficial climatic conditions through decreased wind speeds. However, the eddy created in our study site may not create a better environment for tree growth. Tree establishment above 2H must be inhibited by too high of wind speeds creating high shear and near non-existent snow cover during the winter. The area between timberline and 2H has been slowly filling in with seedlings since the mid/late 1800’s. The trees in this section do not grow into krummholz form. If a seedling can be established it grows into a fully-grown symmetrical tree. It is difficult, but not impossible for seedlings to become established in this zone. Tree establishment is most likely dependent on very specific microsites within this area that have some wind flow, moderate snow cover in the winter, and 40-80% open sky exposure.