suppl1_Environment drives spatio-temporal patterns of clonality in white spruce (Picea glauca) in Alaska.xlsx

Cloning is a strategy used by many plant species to reproduce under environmental conditions unfavorable for sexual reproduction. To test the alternative hypothesis whether cloning is an "exit strategy" or caused by selection, clonal growth in white spruce (Picea glauca (Moench) Voss) was investigated in three pairs of edge (treeline) and core (contiguous forest) stands in Alaska. Latitudinal and elevational treeline at a bluff in the dry Interior served as edge populations. A microsatellite assay with 11 markers was employed for a massive genotyping effort, resulting in a total of 2782 and 2571 trees that were mapped and genotyped, respectively. the lowest proportion of clonal trees was found in the Interior (4.5%) followed by stands from the elevational (9.0%) and latitudinal treeline (21.7%). At the two latter sites, clonal growth was more frequent in the edge population. Based on 960 aged trees an overall trend towards a higher proportion of clonal trees with age emerged, indicating that clonal growth becomes more likely with increasing age continues over the life span of a tree. The proportion of clonal trees was inversely correlated with the height of the lowermost branches and mean annual temperature; a positive correlation was found with mean annual rainfall. Most (80 of 96) clones were limited to distances less than 20 m, which indicates layering or uprooted trunks as the most likely mechanisms. Neither the FST values, a PCA of the microsatellite multi-locus genotypes nor an AMOVA revealed any genetic predisposition for cloning. The degree of clonality at a site seems to be determined by environmental conditions.
Synthesis: The study demonstrates that clonal growth in Picea glauca is most likely the result of plant structure and dependent on environmental conditions. Since performance of trees, and therefor likely plant reproductive success, is lower in populations with a high proportion of clones, selection for clonal growth seems to be highly unlikely for white spruce.