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Arctic Climate Change, Substrate, and Vegetation. D. A. Walker, W. A. Gould (UAF), and H. Epstein (University of Virginia). National Science Foundation. $1,212,879. 7/1/99-6/30/03.
Abstract: This study was part of the Arctic Transitions in the Land-Atmosphere System (ATLAS) project, which studied the large-scale variations in trace-gases, energy, and water. Remote sensing is playing a key role in the extrapolation of information from studies on the Arctic Slope and the Seward Peninsula to the circumpolar Arctic. In order to accomplish these extrapolations, it is first necessary to achieve a better understanding of the complex interactions between climate, substrate, and vegetation, and spectral reflectance patterns on remotely-sensed images. This project addressed the effects of climate on vegetation composition and structure, and the interactions between continental-scale climate gradients and regional-scale variation in substrate.
Our study consisted of three major components:
  • Western Alaska Transect: Vegetation is being characterized by ground-based studies at several sites along climate gradients in western Alaska. We are particularly interested in the contrasts between moist acidic and nonacidic systems because of their wide distribution and important differences in ecosystem function.
  • Questions:
    1. What are the composition and structure of the vegetation on different substrates along the climate gradient? (e.g., How does the shrub canopy vary along the climate gradient?
    2. How do differences in substrate (pH and soil texture) modify the zonal patterns? (e.g., What are the causes and distribution of moist nonacidic tundra? Is the distinctive soil-pH boundary in northern Alaska controlled by geology or climate?
    3. Can vegetation information from the Kuparuk River basin be extrapolated to new sites in Alaska based on remotely sensed information?
  • Remote-sensing and modeling: A 10-y time-series of biweekly AVHRR imagery will be used to examine the relationships between climate gradients, interannual weather patterns, NDVI, biomass and phenology within four vegetation types (moist acidic tundra, moist nonacidic tundra, wet tundra, and shrub tundra.
    • Questions:
      1. What are the effects of substrate and vegetation types on seasonal patterns of NDVI?
      2. How do these seasonal patterns of NDVI vary along climate gradients (temperature, precipitation)?
      3. How does interannual weather variability affect seasonal patterns of NDVI?
  • Circumpolar Arctic Vegetation Map (CAVM) (see above): This map is essential for a wide variety of projects that are modeling vegetation-climate interactions. An international group of collaborators is developing several maps and remote-sensing products for the circumpolar region. The maps will provide the first detailed vegetation map of an entire global biome. It will have a wide variety of applications for regional (e.g. Alaskan) and global studies and for educational purposes.
Our Research within the ATLAS Project:
The ATLAS project was developed by the National Science Foundation under LAII to determine the effect of changes in climate on key parameters of Arctic ecosystems (for more information on the entire ATLAS project, see the LAII-ATLAS Home Page). The research focus of our group within the ATLAS project has been to determine the effect that climate change may have on arctic vegetation and closely associated factors. Specifically, we have examined the effects of summer warmth on leaf area index (LAI), total aboveground phytomass, and normalized difference vegetation index (NDVI) across the three arctic bioclimate subzones (Subzones 3-5) in northern Alaska and into the subarctic at Council on the Seward Peninsula. We have also investigated the relationship of these factors with differences to geologic substrate.

We conducted our field research at ATLAS sites within these subzones during the summer months of 1998-2001, from the coastal acidic tundra of Barrow on the Arctic Slope (71 degree N) to the moist acidic low shrub of the Seward Peninsula at Council (64 degree N). Summer warmth, defined as the sum of mean monthly temperatures greater than 0 degees C, was used as a key index (SWI) for comparisons between the sites.

The SWI varies from 9 C at Barrow to 34 C at Council. From Barrow to Quartz Creek (65 N, SWI = 32 C), a 5 degree increase in the SWI correlates with about a 115 g m-2 increase in the aboveground phytomass for zonal vegetation on acidic sites and about 60 g m-2 on nonacidic sites. Between all sites, shrubs account for most of the aboveground phytomass increase on acidic substrates, whereas mosses account for most of the increase on nonacidic soils. LAI is positively correlated with SWI on acidic sites, but on the nonacidic sites the relationship is unclear as the field instrumentation was unable to capture differences other than that of the erect vascular plant component of the plant canopy. The NDVI is positively correlated with SWI on both acidic and nonacidic soils, but on nonacidic parent material the NDVI is consistently lower than that of the acidic substrates. One of the most interesting observations was the large increase in mosses at warmer temperatures in nonacidic environments. The increase in mosses on nonacidic sites could affect the soil surface temperatures and decrease the activity of frost boils, which play an important role in nutrient availability and a variety of other ecosystem properties that maintain the nonacidic ecosystems. The sandy substrates at Atqasuk had the lowest productivity and NDVI of all the mesic sites, despite relatively warm temperatures compared to the coastal sites. Low nutrient availability accounted for low productivity, and relatively high lichen cover, which has low spectral reflectance in the near-infrared channels, accounted for the low NDVI values.

The Quartz Creek site on the Seward Peninsula had SWI of 32 C and demonstrates how a system much like that of the Arctic Foothills in northern Alaska might respond to warming. Shrub biomass in the water tracks is much higher, and the tussock tundra systems display greater tussock height and more sedge biomass. The maritime climate of the southern Seward Peninsula near treeline, represented by Council, supports abundant shrub-tundra plant communities with high biomass, and suggests that a special maritime variant of Subzone 5 is justified. It appears that climate warming will likely result in increased phytomass, LAI, and NDVI on zonal sites. Acidic areas supporting abundant shrub phytomass will likely demonstrate the greatest changes.

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Institute of Arctic Biology University of Alaska Fairbanks Alaska Geobotany Center