The rapid warming of the Arctic is leading to impactful consequences for the environment and ecosystems, such as the growing permafrost degradation and accelerating greenhouse gas emissions. This collaborative research investigates the eco-hydrologic changes associated with tree and shrub encroachment into tundra areas of the Polar Urals in the Western Siberian Plain. The specific goals are to find out whether forest encroachment (a) has changed the surface energy budget and led to higher evapotranspiration, (b) whether these changes are detectable and quantifiable at regional scales, and (c) whether tree encroachment has deepened the seasonal thaw layer caused by higher thermal energy flux into the soil. Answering these questions requires long-term, year-round field observations of hydrometeorological conditions such as solar radiation, rain and snowfall, air temperature and humidity, and soil temperature and moisture profiles deep in remote areas. The change of vegetation in the study region to detect the forest encroachment relies on remote sensing observations. Advanced mathematical models of thaw layer dynamics and surface energy budgets are essential for modeling key processes responding to the Arctic warming. 

This project designed and implemented a unique field monitoring program that has produced three and a half years of hydrometeorological data ? including winter season data that are rarely available for remote Arctic regions. The tree-scale field data on forest expansion into tundra has informed interpretation of observations from remote, space-based platforms. These unique data sets were used for the development and validation of models describing evapotranspiration and seasonal evolution of freeze and thaw layer thickness. The integration of models and data has allowed the team to address the project goals. Our findings that represent the intellectual merit outcomes of this project reveal a positive feedback between the climate warming, tree/shrub encroachment, and enhanced heat surplus stored belowground, leading to increased evapotranspiration and thaw layer thickness. Specifically, spatial distribution of encroached trees  shows their tendency to grow on both upward and laterally convex surfaces, indicating preference for new establishment in well-drained locations. A clustering analysis indicates that the tree location patterns are linked to snow distribution. The continuous monitoring at seven plots with recently encroached trees and four tundra plots in their immediate proximity, shows that tundra has higher albedo, and larger diurnal variability of net energy exchange and surface temperature. Tundra plots also show faster rates of soil freezing and higher total soil heat loss during fall-winter period, likely because of the lower snowpack retention capacity of a smoother tundra surface. This implies soil energy heat surplus in areas with encroached vegetation and therefore conditions conducive to permafrost degradation. Further, remote sensing analysis of the larger region reveals increasing evapotranspiration and net vegetation biomass productivity associated with increasing tree cover fraction and advancing tree-line. Other outcomes of the project are new methods for estimating surface topography and vegetation canopy height from satellite observations and innovative models of thaw layer thickness and snow surface temperature suitable for the data-sparse Arctic regions. They can potentially improve the physical representation of land-surface processes used by the Earth System Models to study global climate change. The team also developed complex models of surface energy and processes of subsurface heat and water redistribution, relying on physical relationships between temperature, unfrozen water content, ice content, and water pressure. Combined with an advanced uncertainty quantification machinery, these models can further our predictive understanding of how the slow process of tundra replacement by forest can impact the fate of the underlying permafrost at century- to millennial scales. 

Broader impacts of this project included several elements. Over the project duration, its activities have offered research and outreach opportunities for many undergraduate (7) and graduate (14) students, and a postdoctoral scientist, including minority and female students. These research efforts introduced students to the impacts of climate change on the Arctic, while offering opportunities for independent and creative research and development of technical skills in data analysis and modeling. Students and project personnel shared insights through conference presentations and peer-reviewed publications. Unique monitoring data collected over the course of the project will contribute to future research. The team developed media products targeted toward broad audiences: a film ?Cultures of Ice? tying together Arctic science and Michigan winter outdoor life was developed as a ?teach-out? activity for the 2020 Earth Day: https://www.youtube.com/watch?v=0YY7J5Uj56s. We worked with media specialists to develop a physical exhibit at the University of Michigan Museum of Natural History (2019-2021) and  produce a series of documentaries highlighting this research an online format: https://lsa.umich.edu/ummnh/visitors/exhibits/virtual-research-stations/valeriy-ivanov--the-warming-arctic.html. Project personnel was also engaged in public discussion panels and invited presentation at Museum.

Last Modified: 12/30/2022
Modified by: Desheng Liu