Date of Degree


Document Type


Degree Name



Earth & Environmental Sciences


Kyle C. McDonald

Committee Members

Peter Groffman

Wenge Ni-Meister

Reiner Zimmerman

Subject Categories

Biological and Chemical Physics | Environmental Indicators and Impact Assessment | Environmental Monitoring | Multivariate Analysis | Optics | Other Environmental Sciences | Other Physics | Other Plant Sciences | Soil Science


Ecophysiology, Alaska, freeze/thaw, phenology, tundra, remote sensing


This dissertation examined the seasonal freeze/thaw activity in boreal-Arctic soils and vegetation physiology in Alaska, USA and Alberta, Canada, using in situ environmental measurements and passive microwave satellite observations. The boreal-Arctic high-latitudes have been experiencing ecosystem changes more rapidly in comparison to the rest of Earth due to the presently warming climatic conditions having a magnified effect over Polar Regions. Currently, the boreal-Arctic is a carbon sink; however, recent studies indicate a shift over the next century to become a carbon source. High-latitude vegetation and cold soil dynamics are influenced by climatic shifts and are largely responsible for the regions atmospheric carbon fluxes. Under a warming climate, soils are thawing for extended periods of time, allowing for heightened aerobic decomposition of organic matter in the soil, increasing soil carbon emissions. Simultaneously, vegetation performs photosynthesis longer, resulting in increased sequestering of atmospheric carbon. Regional and global climate affect one another through land-atmosphere carbon feedbacks.

The strength, and contribution, of this study lies in high quality fine-scale in situ datasets day-of-year occurrences for soil state transitions and vegetation growth phenophase activity at site-specific locations. Findings include (1) a more thermally variable active layer in dry tundra compared to wet tundra; (2) active layer isothermal conditions are established rapidly in the fall ( ~ 2 days) but not in the spring (~ 11 days); (3) boreal willow shrubs (Salix Spp.) have the shortest exiting dormancy period (11 days) yet the longest active above ground stem growth (61 days); (4) moist bog type environments were shown to be the optimal spruce (Picea Spp.) growth environments (active trunk growth duration of ~ 55 days); and (5) AMSR satellite data were shown to preemptively estimate land surface condition change compared to in-situ measurements during the spring transition for both tundra types while lagging during the fall transition and freeze-up periods. These results elucidate the need for further field work campaigns collecting active soil layer measurements in order to precisely gauge the seasonally thawed and active windows for soil and vegetation. Outcomes of this research include increased availability of quantified soil and vegetation activity windows. Conclusions include prospects which are valuable for studies attempting to optimize carbon flux estimations using freeze/thaw microwave satellite datasets.