Arctic Alaskan Shrub Growth, Distribution, And Relationships To Landscape Processes And Climate During The 20Th Century

Abstract

The primary change underway in the tundra of Arctic Alaska is the increase in air temperature and expansion of deciduous shrubs since 1980. I explored relationships between shrub expansion and relevant ecosystem properties such as climate, soil characteristics, erosion, and herbivory. Alnus viridis ssp. fruticosa (Siberian alder) shrubs located along streams, rock outcrops, or other features with active disturbance regimes showed a positive correlation between growth ring widths and March through July air temperature. Climate-growth relationships were much weaker for alder in adjacent tussock tundra. Additionally, tussock tundra sites had different vegetation composition, shallower thaw, lower mean annual ground temperature, lower mean growing season temperature, higher soil moisture, more carbon in mineral soil, and higher C:N values in shrub leaves than nearby non-tussock alder. Growth rings and site characteristics imply that preexisting soil conditions predispose alder shrubs growing in non-tussock tundra to respond rapidly to warming. Analysis of temporal series of aerial photography from 1950 and 2000 and of Landsat imagery from 1986 and 2009 showed an increase in percent cover of shrubs, primarily in riparian areas. This increase in shrubs is contemporaneous with a decline in peak discharge events from the Kuparuk River and a lengthening of the growing season since 1980, both of which may have caused the decline in sediment deposition observed in 3 of 4 lake sediment cores dated with lead and cesium isotopes. Both alder shrub growth and erosion are particularly sensitive to runoff dynamics during the snowmelt and green-up period, and these dynamics are affected by spring temperatures. Ptarmigan, moose, and hares forage heavily on shrubs protruding above the deepening snow during the late-winter, and selective browsing on willow vs. alder is likely influencing shrub community composition. The increase in shrubs during the 20th century may represent additional habitat for these herbivores, and herbivore-mediated changes in shrub architecture may have important implications for how shrubs trap snow and ultimately affect surface energy balance. Evidence from this thesis indicates shrub growth and cover have increased in response to persistent warming, particularly in areas where the organic layer is thinner and active layer deeper.