Recruitment dynamics and population structure of willows in tundra disturbed by retrogressive thaw slump thermokarst on Alaska’s North slope

Abstract Retrogressive thaw slumps (RTS) are large (> 1 ha) depressions of exposed mineral soil on hillslopes caused by the thaw and displacement of ice-rich permafrost soils in high-latitude regions; since the 1980s the number of RTS observed on Alaska’s North Slope has increased by two-thirds. Some RTS in the Toolik Lake area are filled with tall (≥ 0.5 m) willow thickets, likely within decades after disturbance. Tall shrub thickets are different in structure and function from mixed dwarf tundra communities and may have different long-term impacts on ecosystems and wildlife. Currently it is unknown to what degree seedlings versus clonal recruitment contribute to shrub thickets. We assessed size and distribution of clones (modular stems of individuals) using eight microsatellite (SSR) markers to genotype leaf tissue of 223 willow ramets (stems) at two sites: an RTS aged 11–30 years since disturbance, and nearby undisturbed moist acidic tussock (MAT) tundra. Genotypes of known clones from excavated ramets were used to determine the mutation rate of clones. Spatial arrangement of ramets within clones was assessed in 18 × 18 m sampling grids nested at 2 m (“far clones”), 1 m (intermediate), and 0.25 m (“near clones”) between ramets. We identified 121 genotypes including 10 clonal genotypes in the RTS, and 63 genotypes including 11 clonal genotypes in the undisturbed MAT. Percent distinguishable was greater than 76% at both sites. Mean spatial distance between clonal ramets was not different at either site but among far clones, ramets were separated 7–16 m downslope in the RTS. Salix pulchra was the dominant willow at both sites; a third of willow genotypes in the RTS were identified as S. glauca, a disturbance colonizer. Rarer and hybrid species comprised 4–20% of all genotypes but were more abundant in the RTS. At both sites, within-species expected heterozygosity (HT) ranged from 0.49 - 0.85. Our results suggest: 1) sexual recruitment and low clonal expansion likely explain genetic diversity of willows in disturbed and late-successional sites; and 2) downslope separation of far clones in the RTS suggests disturbance effects; and 3) species richness was higher in the disturbed site.