Unger Steven

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Unger
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Steven
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  • Preprint
    Short term changes in moisture content drive strong changes in Normalized Difference Vegetation Index and gross primary productivity in four Arctic moss communities
    ( 2018-04) May, Jeremy L. ; Parker, Thomas C. ; Unger, Steven ; Oberbauer, Steven F.
    Climate change is currently altering temperature and precipitation totals and timing in Arctic regions. Moss communities constitute much of the understory in Arctic vegetation, and as poikilohydric plants moss are highly sensitive to timing and duration of moisture levels. Here we investigate the role of moisture content on NDVI, red and near-infrared reflectance, and gross primary productivity (GPP) of two sphagnum and two pleurocarpus moss community types during two separate drying experiments. For both experiments, blocks of moss were collected near Imnavait Creek, Alaska, saturated to full water capacity, and then allowed to air dry before being re-saturated. Drying of blocks was conducted in a translucent outdoor tent during the first experiment and under indoor climate-controlled conditions during the second. Community NDVI (experiment 1 and 2), and GPP (experiment 2) were measured at regular intervals during the dry-down and after rewetting. In both experiments, moss NDVI sharply declined between 80% and 70% moisture content for sphagnum moss communities (NDVI change = -0.17 to -0.2), but less so for pleurocarpus moss communities (NDVI change = -0.06 to -0.12). Changes in NDVI were largely the result of increases in reflectance in red wavelengths. Peak GPP for all community types in the second experiment (1.31 to 2.08 μmol m-2 s-1) occurred at 80% moisture content and declined significantly as moisture content decreased. Rates of GPP continued to decline below 80% moisture content until near zero as moss reached a steady weight (air dry) over a period of 84 hours, while NDVI values declined slowly between 70% hydration and fully air dry. Re-saturation caused NDVI to increase in both sphagnum (NDVI change = +0.18 to +0.23) and pleurocarpus (NDVI change = +0.10 to +0.17) communities. Only sphagnum communities showed GPP resuming (0.824 μmol m-2 s-1) after 24 hours. The strong changes in NDVI and mismatch of moss NDVI values and GPP with moisture content fluctuations indicate that using NDVI as a proxy for productivity in Arctic vegetation communities may be problematic and underscores the need for quantification of moss community coverage, composition, and moisture content.
  • Article
    Intraspecific variation in phenology offers resilience to climate change for Eriophorum vaginatum
    (Canadian Science Publishing, 2021-05-19) Parker, Thomas C. ; Unger, Steven L. ; Moody, Michael L. ; Tang, Jianwu ; Fetcher, Ned
    The phenology of Arctic plants is an important determinant of the pattern of carbon uptake and may be highly sensitive to continued rapid climate change. Eriophorum vaginatum L. (Cyperaceae) has a disproportionate influence over ecosystem processes in moist acidic tundra, but it is unclear whether its growth and phenology will remain competitive in the future. We investigated whether northern tundra ecotypes of E. vaginatum could extend their growing season in response to direct warming and transplanting into southern ecosystems. At the same time, we examined whether southern ecotypes could adjust their growth patterns in order to thrive further north, should they disperse quickly enough. Detailed phenology measurements across three reciprocal transplant gardens over a 2-year period showed that some northern ecotypes were capable of growing for longer when conditions were favourable, but their biomass and growing season length was still shorter than those of the southern ecotype. Southern ecotypes retained large leaf length when transplanted north and mirrored the growing season length better than the others, mainly owing to immediate green-up after snowmelt. All ecotypes retained the same senescence timing, regardless of environment, indicating a strong genetic control. Eriophorum vaginatum may remain competitive in a warming world if southern ecotypes can migrate north.