The tree line is a climatic boundary, however its ability to respond to changing climate seems to be constrained by the spatial distribution of trees at the leading edge; compared to abrupt or krummholz tree lines, diffuse tree lines are moving upslope much more readily in response to recent anthropogenic warming. Here we report on the micrometeorological processes that result from the diffuse leading edge of a moving tree line on Pikes Peak, Colorado, USA, and on the impacts these processes have on tree temperatures. We focus on the layering and movement of air in the lower 10m of the atmosphere including the height of the displacement of the zero velocity plane. Our experimental design consisted of 300m upslope transects through the tree line into the alpine tundra where we measured: (1) height of the zero plane displacement using handheld anemometers, (2) temperature of 10cm tall seedlings, 3-5m tall trees, and tundra grasses using an IR camera, (3) temperature and relative humidity at 2.5cm an 2m using Kestrel hand held weather stations, (4) the vertical atmospheric profiles using 10m towers equipped with 8 anemometers at 5 different elevations, (5) vertical movement of air using a bubble-blowing machine. Our results show that (1) the zero plane height decreased exponentially with increasing elevation (R2=0.432, N=57, p<0.0005) from approximately 25cm within the tree line to 2.5cm in the tundra above. The spatial variability of the zero plane height also decreased with elevation. (2) The temperature of small seedlings was (3) closely coupled to the ground vegetation (paired t-test t= 2.213, df=10, p=0.051),but seedlings were on average 3.88°C warmer than trees (paired t-test t= 5.808, df=10, p<0.0005), and trees were 6.1°C colder that the tundra (paired t-test t= 6.617, df=10, p<0.0005). (3) Compared to the air at 2m, the air layer at 2cm had higher temperature (+2.5°C, paired t-test t= 7.205, df=19, p<0.0005), and higher relative humidity higher (+29%, paired t-test t= 9.657, df=19, p<0.0005). (4) The vertical wind profile had a simple and smooth slow down to the zero plane at 2.5cm in the alpine tundra. However the profile was complex in all locations where trees were present: It showed an initial slow down to a very low speed at 3-4m, increase in velocity at 2m, and final slow down to the zero plane at 25cm. Qualitative and quantitative analysis of bubble movement (5) showed that the upper boundary layer was turbulent.
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.
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.
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.
Treelines are climatically constrained ecotones existing worldwide. With global warming and climate change, treelines are expected to advance in elevation on a global scale. Previous research has shown that abrupt treeline shapes are advancing at far slower rates than diffuse treeline structures, indicating that temperature increases are not the only factor. Smaller-scale, endogenous factors may be at play including microclimates, tree-to-tree interactions and feedbacks. Our study at an abrupt treeline on Pike’s Peak aims to understand the effects of temperature and smaller-scale factors on seedling growth, in the effort to try and understand the feedbacks involved in treeline movement and formation. Results indicate that this specific abrupt treeline is creating a microclimate that facilitates seedling growth above the historical treeline. Once this new growth of seedlings matures, another abrupt treeline will form and perpetuate the process.
We aimed to find what kinds of microclimates were created by an abrupt treeline and relate those microclimates to the spatial structure of the treeline itself. We specifically wanted to understand how airflow is directly related to air temperature upslope of treeline. To do this, we took data from an abrupt treeline on Pike’s Peak in the Front Range of the Colorado Rocky Mountain Range. Our data was taken in September of 2016, which is representative of the tail-end of the growing season for trees. The wind speed and direction appeared to have a strong relationship with the air temperature, as the daytime uphill anabatic airflow created eddy zones of slow-moving air that were able to warm up from sensible heat dissipated at the ground surface., The nighttime downhill katabatic winds accumulated pockets of slow-moving cold air. This study helped us understand that sheltering with respect to treelines is not the result of single and independent trees, but rather the result of the entire treeline as complete three-dimensional structure. This is important because the effects of sheltering at treeline will vary from location to location based on the shape of the entire spatial structure of the ecotone.