Does spatial genetic structure increase with altitude? An answer from Picea abies in Tyrol, Austria

Harsh environment at high altitude may affect the mating system of plant species, especially those with wide ecological amplitude. Smaller effective neighbourhood size, less pollen and seed production, higher rate of inbreeding and a shift towards vegetative propagation may be involved. These changes can be reflected in spatial genetic structure (SGS). Populations of Norway spruce [Picea abies (L.) Karst.] were analysed along an altitudinal cline to verify whether SGS increases with altitude. Three putatively autochthonous populations in Tyrol (Austria) at 800, 1,200 and 1,600 m above sea level (asl) were studied. Six highly polymorphic DNA markers (expressed sequence tag-derived simple sequence repeats, EST-SSRs) were used to genotype a total of 450 contiguous trees (150 trees per population). Loiselle's kinship coefficient was used to quantify SGS. Against expectation no significant SGS was found in any of the populations, indicating a random spatial pattern. Significant SGS was observed when all populations were treated as a single one conforming to an isolation-by-distance pattern. Nearly identical allelic frequencies were found resulting in very small population differentiation (F (ST) = 0.002). The fixation index decreased with diameter at breast height (a proxy for age) indicating natural selection against inbred trees. The results of this study indicate that seed and pollen dispersal mechanisms in Norway spruce are strongly counteracting spatial aggregation of similar genotypes even at high elevations.