Landscape Heterogeneity of Shrub Expansion in Arctic Alaska
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The expansion of shrubs into tundra areas is a key terrestrial change underway in the Arctic in response to elevated temperatures during the twentieth century. Repeat photography permits a glimpse into greening satellite pixels, and it shows that, since 1950, some shrub patches have increased rapidly (hereafter expanding), while others have increased little or not at all (hereafter stable). We characterized and compared adjacent expanding and stable shrub patches across Arctic Alaska by sampling a wide range of physical and chemical soil and vegetation properties, including shrub growth rings. Expanding patches of Alnus viridis ssp. fruticosa (Siberian alder) contained shrub stems with thicker growth rings than in stable patches. Alder growth in expanding patches also showed strong correlation with spring and summer warming, whereas alder growth in stable patches showed little correlation with temperature. Expanding patches had different vegetation composition, deeper thaw depth, higher mean annual ground temperature, higher mean growing season temperature, lower soil moisture, less carbon in mineral soil, and lower C:N values in soils and shrub leaves. Expanding patches—higher resource environments—were associated with floodplains, stream corridors, and outcrops. Stable patches—lower resource environments—were associated with poorly drained tussock tundra. Collectively, we interpret these differences as implying that preexisting soil conditions predispose parts of the landscape to a rapid response to climate change, and we therefore expect shrub expansion to continue penetrating the landscape via dendritic floodplains, streams, and scattered rock outcrops.
KeywordsArctic Alaska shrub climate dendrochronology soil
Field work was supported by the National Park Service and the University of Alaska Anchorage Environment and Natural Research Institute. Data loggers were supported by NSF-IPY Grant 0732954 and NSF-OPP Grant 0612534, the latter awarded to Jeffrey M. Welker. Martin Hallinger received funding from the Scholarship Program of the German Federal Environmental Foundation. Thanks to Lola Oliver at the University of Alaska Fairbanks Forest Soils Laboratory for her patience and expertise. Thanks especially to the volunteers who participated in at least one of the three river expeditions: Greta Myerchin, Ben Gaglioti, Mark Winterstein, Ty Spaulding, Lisa Garrison, and Greg Goldsmith.
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