In response to NASA Carbon Cycle Science Solicitation to “improve understanding of terrestrial carbon storage, "in Arctic/Boreal terrestrial ecosystems that may be approaching a potential tipping point with regard to the release of stored carbon" the proposed study will map and characterize the current Arctic forest-tundra transition zone and reveal its changes during the last several decades with multi-sensor satellite data and field observations. Note that the study will also consider alpine tree line, but only within the Arctic forest tundra ecotone.

Climate change has already affected the Higher Northern Latitudes, altering vegetation productivity, carbon sequestration, and many other processes. Establishing the current boundary between the Forest-Tundra biome is important yet difficult to define because vegetation form throughout the transition varies dramatically from site level dynamics including microclimate, topography, winter snow depth, wind, edaphic conditions, etc. coupled with the long-term site history of their interactions. Those processes have produced many different vegetation structure forms from sporadic forest cover patches to growth-stunted trees that resemble shrubs. Many of these growth forms are not visible from most earth observing satellites. Studies thus far, have found large differences in the location of the ecotone. Our recent study used MODIS 500-m vegetation continuous fields (VCF) data with a limited sample of QuickBird-2 data to define ecotone location but lacked the spatial precision needed to establish a baseline that can be used to monitor climate warming impacts to vegetation cover in the near term. In addition, due to limitations of MODIS coverage at the time, the final product extended only to 70° N. Recent advances using the global archive of Landsat data have provided vegetation continuous fields (VCF) at 30-m, and could help to define the subarctic tree line. Recent studies have found that Arctic greening is associated with densification of shrubs, increasing biomass, and establishment of new shrubs within thawing patterned ground features, but they have not been spatially comprehensive to establish a subarctic tree line. Recently granted federal civilian access to sub-meter commercial satellite data provides the means to establish an improved estimate of the subarctic tree line at an unprecedented scale. We will provide a 30-m arctic-boreal ecotone map using the Landsat based global vegetation continuous field product constrained and validated with commercial sub-meter imagery. The remotely sensing based canopy height, biomass and cover can be used to initialize and run ecosystem models.

The objectives of this proposed work are:

  1. Use recently available Landsat based percent tree cover maps with estimates of uncertainty to improve our prior circumpolar arctic-boreal transition assessment.
  2. Use high-resolution data in intensive study sites along the forest-tundra transition zone and characterize the spatial patterns of tree-tundra mosaic across the boundaries. These data include commercial sub-meter panchromatic and stereo imagery from WorldView-1, WorldView-2, and Quickbird-2.
  3. Estimate forest cover and biomass change in intensive study site transects using high-resolution stereo satellite data, field observations, and allometry.

This proposal directly addresses NASA is interest in Carbon Dynamics in Arctic/Boreal Terrestrial Ecosystems. The implications are significant as a sentinel of climate change and for developing confidence in predictions of changes in carbon balance due to a rapidly changing environment. This proposed work also contributes directly to NASA's interests in characterizing critical ecosystems, especially in high latitudes, and can contribute data and science analysis to the proposed ABoVE experiment.

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