Collaborative Research: Environmental and biological controls on carbon uptake phenology in permafrost affected boreal forests

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Rapid warming in boreal forests has driven diverse ecosystem changes in recent decades, including permafrost thaw, longer non-frozen seasons, and warmer summers. The impact of these changes on the carbon balance of boreal forests remains highly uncertain. A growing body of evidence indicates that boreal forests are shifting toward being a net carbon dioxide source due to increases in soil respiration. To offset this loss of carbon, ecosystems rely on sustained vegetation uptake and accumulation of carbon through plant photosynthesis. However, it is unclear how environmental conditions, particularly moisture and temperature, affect the timing, duration and magnitude of photosynthetic carbon uptake (i.e., photosynthetic phenology) across boreal plant functional types. To address these important knowledge gaps, this study focuses on a leaf-to-watershed analysis at the Caribou-Poker Creek (BONA) Watershed in Alaska. Specifically, the study seeks to answer the question, “What are the environmental and biological controls of photosynthetic phenology in permafrost-affected boreal forests?” This research will provide training to undergraduate and graduate students in field and laboratory research practices. The project will also partner with science educators within the PolarTREC program to develop informational StoryMaps, educational videos, and learning activities to raise public awareness of the important ecosystem services that boreal forests provide to society. This study will use a novel approach that incorporates high-frequency observations of solar-induced chlorophyll fluorescence (SIF) as an indicator of vegetation gross primary productivity (GPP), and L-band microwave backscattering intensity as an indicator of canopy water content. These measurements will be complemented by a suite of observations including leaf and ecosystem gas exchange, and environmental measurements (e.g., soil temperature, soil moisture, water flow velocity) along a soil-to-vegetation continuum. The observation will be used to identify the key functional relationships between GPP and environmental conditions occurring at the stand and watershed levels through statistical analyses (e.g., multivariate adaptive regression splines). Newly gained insight will be used to optimize a data and process-model integration framework using the remote-sensing driven CARbon Data Model (CARDAMOM) and to extend from the BONA watershed to the boreal forests of Alaska and northwestern Canada. The CARDAMOM framework will allow us to: (1) obtain new, observation-constrained, regional estimates of GPP; (2) assess the regional sensitivity of boreal carbon dioxide uptake under various climate scenarios; and (3) identify the sink potential and trajectory of GPP in boreal forests, given the state of climate warming, a lengthening growing season, and changes in soil water conditions. The study results will be an important step forward in reducing uncertainty in GPP budget estimates for boreal forests, and will help set priorities for future model improvements. This project is co-funded by the OPP Arctic Natural Sciences Program and DEB Ecosystem Science Program.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.