Assessing recruitment age and climate sensitivity of deciduous shrubs in tundra disturbed by retrogressive thaw slump thermokarst in the Alaskan Low Arctic

Abstract:

Since the 1980’s, Alaska’s North Slope has experienced a 67% increase in thermokarst activity, including retrogressive thaw slumps (RTS). RTS are large (≥1 ha) de-vegetated depressions caused by ice-rich permafrost soil thaw and mass displacement. Within decades, several RTS near Toolik Lake have been colonized by deciduous shrubs ≥ 0.5 m in height. RTS may feature microsites that support greater plant recruitment and growth than surrounding tundra. By incorporating the climate signal into their annual growth rings, woody shrubs record the magnitude of climate effects on their environment and are thus extremely useful for investigating change in sensitive environments. We used dendrochronology methods on 182 stem cross sections (4,848 rings) of dominant deciduous shrub species: willows (Salix pulchra and Salix glauca) and dwarf birch (Betula nana), to quantify climate effects on shrub secondary growth in RTS-disturbed and undisturbed moist acidic tussock (MAT) tundra at two lakes on Alaska’s North Slope. We expected shrub age, determined from growth ring chronologies, to positively correlate with RTS time since disturbance, wider growth rings in RTS relative to undisturbed MAT, and a magnified climate sensitivity in RTS growth rings. The study revealed that shrub age was a reliable predictor of time since disturbance in young RTS (≤ 1 decade since disturbance) but not in older RTS. Most shrubs in RTS across species had wider rings than shrubs in undisturbed MAT. A 1 °C June mean temperature increase was uniformly associated with a 2-6% increase in secondary growth of most shrubs, but an amplified response to warming was not found in most RTS shrubs. Previous year September precipitation showed a stronger association with increased secondary growth in most RTS shrubs than in MAT, suggesting protective effects of early snowfall accumulations in RTS may be greater than in open tundra. Most shrubs in RTS and undisturbed sites showed decelerating growth with increasing June mean temperature, supporting previous studies of moisture sensitivity in tundra shrubs.