Abstract
A warming Arctic has been associated with increases in aboveground plant biomass, specifically shrubs, and changes in vegetation cover. However, the magnitude and direction of changes in NDVI have not been consistent across different tundra types. Here we examine the responsiveness of fine-scale NDVI values to experimental warming at eight sites in northern Alaska, United States. Warming in our eight sites ranged in duration from 2‑23 seasons. Dry, wet and moist tundra communities were monitored for canopy surface temperatures and NDVI in ambient and experimentally-warmed plots at near-daily frequencies during the summer of 2017 to assess the impact of the warming treatment on the magnitude and timing of greening. Experimental warming increased canopy-level surface temperatures across all sites (+0.47 to +3.14˚C), with the strongest warming effect occurring during June and July and for the southernmost sites. Green-up was accelerated by warming at six sites, and autumn senescence was delayed at five sites. Warming increased the magnitude of peak NDVI values at five sites, decreased it at one site, and at two sites it did not change. Warming resulted in earlier peak NDVI at three sites and no significant change in the other sites. Shrub and graminoid cover was positively correlated with the magnitude of peak NDVI (r=0.37 to 0.60) while cryptogam influence was mixed. The magnitude and timing of peak NDVI showed considerable variability across sites. Warming extended the duration of the summer green season at most sites due to accelerated greening in the spring and delayed senescence in the autumn. We show that in a warmer Arctic (as simulated by our experiment) the timing and total period of carbon gain may change. Our results suggest these changes are dependent on community composition and abundance of specific growth forms and therefore will likely impact net primary productivity and trophic interactions.
Highlights
Warming in the Arctic has been accelerating in recent decades, and the Arctic is experiencing more pronounced temperature increases compared with lower latitudes (ACIA, 2005; Serreze et al, 2009; Stocker et al, 2013)
Full-season average canopy surface temperatures varied along the latitudinal gradient of study sites, with the effectiveness of the open-top chambers (OTCs) warming greater at the southernmost sites compared to the Control Warmed p-value Control Warmed p-value Control Warmed p-value Control Warmed p-value
Full-season warming values from OTCs ranged between +0.47 to +3.14°C, which were similar to those previously reported (Hollister et al, 2006; Bokhorst et al, 2013)
Summary
Warming in the Arctic has been accelerating in recent decades, and the Arctic is experiencing more pronounced temperature increases compared with lower latitudes (ACIA, 2005; Serreze et al, 2009; Stocker et al, 2013). Measurable changes in Arctic plant growth and community dominance in response to warming have been well documented (Arft et al, 1999; Oberbauer et al, 2013; Khorsand Rosa et al, 2015; Bjorkman et al, 2015; Elmendorf et al, 2015; Bjorkman et al, 2018) These responses have not been uniform across the Arctic and are likely associated with local climate conditions and ecohydrology including snow and its role in growing season length and tundra plant ecophysiology (Elmendorf et al, 2012a; May et al, 2017; Prevéy et al, 2017; Jespersen et al, 2018). This includes how these changes are manifested in the seasonality of growth (Welker et al, 1997; Pearson et al, 2013) and the degree to which the abundance of vegetation may be expressed in spectral properties that can be measured at plot and landscape scales (Reidel et al, 2005; Raynolds et al, 2008; Gamon et al, 2013)
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