The recruitment and spread of an arctic deciduous shrub: Sex or no sex?
The rapidity of recent warming in the Arctic has led to pronounced localized changes in vegetation, and experimental and observational studies have shown the expansion of shrubs to be strongly associated with this warming trend. Multiple biophysical mechanisms behind this expansion have been explored, but the biological means by which shrubs increase in cover on landscapes is poorly understood. Although these plants can increase in abundance through sexual recruitment as well as asexual growth and spread, the relative contribution of these two distinctive modes to the expansion of shrubs remains unknown. The primary goal of this study was to assess reproduction in the deciduous shrub Salix glauca (gray-leaf willow) along an 11 °C summer and -20 °C winter climatic gradient from coastal maritime to inland continental sites in West Greenland. We used microsatellite markers to genotype neighboring stems (ramets) to delineate genetically distinct individuals (genets) at scales relevant to shrub expansion in locations with apparent recent increases in shrub cover. We are thus able to infer the proportion of shrubs derived from clonal expansion versus sexual reproduction.
Preliminary results suggest that there are numerous ramets that share identical alleles across this entire set of microsatellite loci at scales of two to several meters; however, these clonal plants represent the minority of individuals sampled across all study sites. Moreover, these results reveal that the proportion of ramets that represent the same genet is more than twice as high in sites at the maritime end of this environmental gradient. The more clonal maritime sites had 46% larger mean stem diameters than the most continental sites (10.04 mm vs. 6.90 mm) while only being 8% taller (30.97 cm vs. 28.58 cm). This suggests that these sites have either older ramets on average given similar constraints on plant height or clonal ramets exhibit greater secondary growth than sexually recruited ones. Additionally, a higher proportion of unrelated ramets in sites with continental climates would suggest that summer establishment is more limiting than winter survivorship for new sexual recruits. Results from this study are germane to which landscapes are likely to see an expansion of S. glauca populations as well as the genetic diversity of present and future populations.