Harsch (2009, 2011) determined that diffuse tree lines are advancing globally as a response to warmer growing season temperatures. However, few have studied the local micrometeorological processes that change local climatic conditions for trees. This thesis aims to understand local airflow patterns and heat distribution within a diffuse tree line on Pikes Peak (Teller County, CO). Previous research identified that the study site represents an advancing tree line that has transitioned from an abrupt to a diffuse structure within the last century (Kummel et al. 2009-present). The mountain location of the study site governs large-scale wind patterns as an anabatic-katabatic wind system. On a more local scale, surface interactions at the diffuse tree line alter daytime flows, resulting in the formation of a low level jet. This jet alters climatic conditions by creating three vertical sub-layers with different atmospheric properties. A preliminary heat budget analysis of the diffuse tree line confirms the findings of jet characteristics while also determining that the low level jet is capable of altering vertical heat distribution and depositing heat as flows move uphill. This heat deposition is likely connected to growth patterns studied by Marks (2014) and overall tree line dynamics (Elwood et al. 2012).
The advancement of global tree lines in response to climate change has raised questions among researchers about tree recruitment at elevations beyond tree line. This study aims to help understand this process by examining the progression of an abrupt tree line of engelmann spruce on the western slope of Pikes Peak, in Colorado Springs, Colorado. Methodology for this study includes drone photography, GIS mapping, dendrochronology, tree growth measurements, and soil moisture measurements. The results of our examination suggest that the three main mechanisms controlling advancement at our tree line include a leeward eddy when upslope winds interact with the tree line like a shelterbelt, a spiral eddy when winds are parallel to tree line, and cold air damming of katabatic winds against the tree line at night. Our examination of the vegetative response of trees at our tree line suggests that the most healthy recruitment is occurring on the southern edge of our transect and at the upper extent of the area expected to be protected by the tree line. We have found that trees and limbs that exist within the cold air dam at tree line have experienced decreased growth compared to trees outside of this layer of cold air.
Throughout the past century, there has been a global shift in climate. Temperatures have been rising, and while precipitation has been fluctuating, it has exhibited not obvious trends. This change in climate has led to global treeline advancement, and has presented ecological, economic, and social implications. Two of the most relevant implications, especially within the context of the western United States, are changing ecosystem dynamics and water yields. Therefore this study aims to explore the effects of climate change at treeline throughout the Colorado Rockies, with the objective to use simple meteorological data to explain and predict radial tree growth. Data was collected at ten individual mountains in five mountain ranges throughout the state. The subsequent dendrochronologies for each mountain were correlated with time, local and regional meteorology, and the other nine sites. The correlation between sites was compared to the distance between sites. Chronologies were also compared to regional wind and storm patterns. Ultimately, no significant climatic trends appeared to influence individual tree growth on a regional scale throughout the Colorado Rockies. In some sites, such as those bordering the western Colorado deserts, increasing precipitation led to increased radial growth. At a small number of sites in the Front Range and the Sawatch Range, increased summer and annual temperatures led to increased radial growth as well. The remaining sites showed no connection between radial tree growth and simple local and regional meteorological data. The dendrochronologies between most mountains were significantly correlated; the correlations ranged from 0.93 to 0.25, with most of the sites correlated at 0.6 and above. Surprisingly, the correlation coefficients between sites did not respond to the distance between mountains in a statistically significant way. Based on an analysis between site correlations, three groups emerged with inter-site correlation at 0.7 and above: west of the Continental Divide, Front Range and Central Rockies, and along the Continental Divide. In general, these groups showed a southwest to northeast orientation. Storm patterns that flow from the southwest to the northeast throughout the state act as the central variable in correlating chronologies between sites. Conclusively this study does not support the hypotheses that claim climate significantly affects radial growth, but instead provides important information that can be used to further understand the implications of climate on treeline dynamics in the Colorado Rockies.
Alpine treelines are very unique ecotones which are visibly responding to climate change worldwide. As global climate change persists alpine treelines are expected to migrate into higher elevations. Not all alpine treelines however are uniform and the microclimates created at the surface have seen to be essential for seedling establishment. The particular microclimate along treeline will dictate how heat is distributed which will ultimately control tree survival. This study discusses how wind interacts with treelines when coming from different directions and where areas of sheltering are created. An area of interest was created along treeline on Pikes Peak in Colorado where wind speeds were measured along a transect moving from the forest up to the tundra above. As expected it was found that a large sheltered area of slow air was created when wind moves uphill over the forest. Downhill moving wind shrinks this sheltered area especially during periods of faster wind. Wind parallel to treeline was found to be more turbulent and sensitive to the local spatial structure. As climate change intensifies it is expected that these sheltered zones created by treeline structure will be altered and become more essential for seedling establishment.
Alpine treeline is a valuable indicator of climate change because of its sensitivity to temperature. On Pikes Peak (Southern Rocky Mountains, Colorado), tree density and elevation in the forest-tundra ecotone has increased in the last century, corresponding with a 2°C increase in regional growing-season temperature. The purpose of this study was to provide a detailed analysis of the process of treeline advancement. Spatial clustering within age classes and elevational bands was used to identify harsh environments and track the upper climatic boundary of tree establishment. Overall, clustering (Ripley’s K, p < 0.01, based on boot-strapping) was more prominent at lower elevations and for older cohorts, indicating the upward migration of the climatic boundary. However, the climatic boundary may be advancing more quickly than treeline as the moving edge changed from a clustered to a randomly dispersed distribution over time: from 1868-1940 the moving edge was clearly clustered, from 1941-1976 it showed mixed results, and from 1977-2010 it displayed a random spatial pattern. Treeline advancement also demonstrated a reach-and-fill pattern, with sudden advancement of treeline, followed by a few decades of infill at lower elevations. The reach-and-fill pattern repeated three times in the last 120 years, with exponential increases in tree density, especially in the last 40 years. The recent explosion of growth and the quickly advancing climatic boundary match temporally with a shift from an abrupt to a diffuse edge typology. To my knowledge, this is the first study that examines in detail the process of changing treeline typology of an advancing treeline.
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.