The Future of the Forest-Tundra Ecotone: A Synthesis that adds Interactions among Snow, Vegetation, and Wildlife to the Equation

DESCRIPTION: The boundary between the circumpolar Boreal forest and Arctic tundra is one of the world's most extensive ecotones, with a distance spanning 13,400 km across the Northern Hemisphere. As such, the Forest-Tundra Ecotone (FTE) plays a critical role in global biogeochemical cycling and energy balance, biodiversity, and ecosystem services and socioeconomics of Arctic-Boreal regions (ABR). Ongoing warming has led to the common assumption of progressive northward advance of the entire FTE. Importantly, however, findings of observational studies do not show a spatially consistent FTE response to climate warming. In their circum-Arctic survey, Rees et al. (2020) found that during the 20th century only 52% of study sites have advanced northward, while 46% remained stationary and 2% retreated. They also found that overall, sites that advanced northward did so two orders of magnitude more slowly than expected if vegetation distribution remained in equilibrium with climate, and that rates of advancement varied substantially by region. There are several reasons for these discrepancies, including the possibility that changes in FTE biophysical structure may be too subtle to detect using traditional approaches, and time-lags between quantifiable FTE responses and the recent warming trend in ABRs. Further, herbivory is often overlooked despite the fact that trees and woody shrubs are an important food source for many FTE dwelling herbivores, suggesting that herbivory likely inhibits northward advance of portions of the FTE. Moreover, despite the fact that herbivore pressure varies strongly as a function of snow depth, snow-herbivore interactions have yet to be explicitly included in ecosystem models that predict changes in the position of the FTE. When combined with our own field based observations at the Alaskan FTE during phase 1 of ABoVE, these lines of evidence strongly suggest that critical interactions among local abiotic and biotic factors must be explicitly considered in any assessment of how climate change will alter not only the position and extent of the FTE - hereafter referred to as 'FTE behavior' - but also the implications for its ability to store carbon (C) and provide wildlife habitat. This project's overarching science goal is to include the largely unconsidered role of interactions among snow, key herbivores, and vegetation in determining spatial and temporal dynamics observed in FTE behavior, and in turn how these dynamics impact both the amount of C stored and availability of wildlife habitat within the circum-Arctic FTE. Our general approach is to synthesize observations, knowledge, and models from multiple studies made during ABoVE Phases 1 and 2 to build and validate a new, quantitative ecosystem modeling tool, FTE - Snow+Vegetation+Herbivory (FTE-SVH), that represents what we currently understand are key interactions controlling FTE behavior, C storage, and wildlife habitat (Obj. 1). We will use published studies and biotic and abiotic data collected from 1980-present to guide FTE-SVH development and validation. These will include a combination of NASA, ABoVE specific, and other sources of accumulated knowledge and understanding, as well as previously developed remotely sensed products, ground-based observations, and numerical models. This modeling system will be capable of reproducing observed dynamics in FTE behavior over the last four decades. We will then determine FTE-SVH model sensitivities via five experiments: increased air temperature (Expt. 1) and atmospheric CO2 (Expt. 2); changes in snowscape properties (Expt. 3) and herbivore pressure dynamics (Expt. 4); and circum-Arctic FTE simulations driven with future scenario data for climate and herbivore pressure (Expt. 5) (Obj. 2).