High elevation sites in the low mountain ranges in Germany are naturally covered by Norway spruce (Picea abies (Karst.) L.) stands. Historically, large scale anthropogenic range expansion starting in the mid to late 18th century had a huge impact on the forest composition throughout Germany. Utilisation and exploitation often led to artificial regeneration, mostly carried out using seeds from allochthonous provenances. Usually, autochthonous (natural) high elevation Norway spruce trees have narrow crown phenotypes, whereas lowland trees have broader crowns. Narrow crown phenotypes are likely the result of adaptation to heavy snow loads combined with high wind speeds. In the present study, neighbouring stand pairs of putative autochthonous and allochthonous origin with contrasting phenotypes in high elevation sites were investigated with 200 samples each. These stands are located in the Ore Mountains, the Thuringian Forest, and the Harz Mountains. Additionally, a relict population with the typical narrow high elevation phenotypes was sampled in Thuringia, known as “Schlossbergfichte”. The objective of the study was to quantify supposedly adaptive phenotypic differences in crown architecture and the genetic differentiation of 11 putatively neutral nuclear microsatellite markers (i.e., simple sequence repeats (nSSRs)). The high differentiation of morphological traits (PST = 0.952–0.989) between the neighbouring autochthonous and allochthonous stands of similar age contrasts with the very low neutral genetic differentiation (FST = 0.002–0.007; G″ST = 0.002–0.030), suggesting that directional selection at adaptive gene loci was involved in phenotypic differentiation. Comparing the regions, a small isolation by distance effect for the Harz Mountains was detected, suggesting landscape resistance restricting gene flow. Finally, the differentiation of the very old autochthonous (up to 250 years) stand “Schlossbergfichte” with typical high elevation phenotypes could cohere with the sampling of a relict genepool.
Species boundaries in oaks are often not clear-cut, which is potentially a result of interspecific hybridization with trait introgression and phenotypic plasticity. Quercus rubra L. and Quercus ellipsoidalis E.J. Hill are two interfertile partially sympatric red oak species (section Lobatae) with different adaptations to drought. Quercus ellipsoidalis is the most drought tolerant of the North American red oak species and is characterized by deep tap roots, a shrubby growth and by deeply dissected leaves. Genetic differentiation between species is low for most molecular markers. However, one genic microsatellite in a CONSTANS-like (COL) gene, FIR013, was previously identified as outlier locus under strong divergent selection between species. In this study, we analyzed leaf morphometric traits in neighboring (parapatric) Q. rubra/Q. ellipsoidalis populations and in one sympatric population from the same region along an environmental gradient. Using multivariate statistics of leaf traits both species showed distinct bimodal frequency distributions for the first canonical discriminant function with some overlap in the phenotypic extremes, especially in the sympatric population. Leaf dissection traits showed strong and consistent differentiation between species in sympatric and parapatric populations, while differentiation for leaf size was lower in the sympatric population under more similar environmental conditions. Leaf phenotypes in F1 hybrids and introgressive forms suggested maternal effects and introgression of leaf traits between species. The association of outlier gene copy number at FIR013 with species-discriminating leaf traits in Quercus rubra can be a reflection of population differences since outlier gene copy number and population membership show significant collinearity. Similar environmental selection pressures on outlier alleles and leaf shape could also have resulted in this association. In future studies, segregating full-sib families could be used to test whether outlier alleles and associated genomic regions are indeed associated with leaf traits or other species-discriminating characters.
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