Open Access

Global climate change is occurring all over the world, but in the Arctic the climate is changing more rapidly and drastically than in many other parts of our planet. Many species that are already at their climatic limit need to adapt to recent climate conditions or migrate in order to not go extinct. The possibilities of adaption include phenotypic plasticity and adaptation to various extents. This is also the case for white spruce P. glauca, which belongs to the conifers and thus in the largest group of gymnosperms still living today. Among the approx. 600 extant conifer species white spruce is one of the most widespread trees in North American boreal forests. Its range extends from 69° N in the Canadian Northwest Territories to the Great Lakes at about 44° N, where it occurs from sea level to an altitude of about 1520 m (Burns and Honkala, 1990). Site related, climate-dependent differences in white spruce reproduction can be seen as a strategy to survive under the harsh climatic conditions at Alaska's treelines: Besides sexual reproduction, the vegetative propagation occurs in the white spruce as an additional reproductive mechanism. This can be realized by "layering" when the lower branches of the tree crown touch the ground and develop roots to later grow as a separate individual with or without a connection to the mother tree. Known as other mechanisms of vegetative propagation are also the rooting of fallen trees which were not completely uprooted, and the "root suckering", in which new shoots sprout from the roots of the tree. However, the latter was not yet observed in the genus Picea. With the help of short, repetitive, non-coding sequences in the genome, which are therefore not subject to selection and are called microsatellites, these clones can be determined by genotyping. For this purpose, using different polymorphic microsatellites, an individual multilocus genotype is created for each tree, by means of which it can be compared with all other trees of the same species. In the first part of this work (article I), the occurrence of clones in three study areas at Alaskan treelines are examined and the reasons for their appearance in variable numbers are discussed. For this purpose, 2571 white spruces (P. glauca) were genotyped and their position was determined via differential GPS in the field. The percentage of clonal trees is higher in areas with harsh climatic conditions and correlates with the height of the lowest branches of the tree crown. This suggests that the vegetative propagation of white spruce is a backup strategy for times when climatic conditions hamper sexual reproduction. The correlation between clone numbers and tree crown height suggests "layering" as the main mechanism for cloning whereas selection for vegetative reproduction seems to be very unlikely shown by the results for genetic differentiation between the clonal and the singleton trees in this study. In the second part of this work (articles II and III), the influence of environmental factors and phenotypic traits on the mycobiome of the needles (including all fungi living on (epiphytic) and in (endophytic) the needles) in our study areas in Alaska was investigated. The mycobiome of the white spruce needles was chosen as a proxy for the parasite infection rate by fungi and thus serves as a fitness parameter. For this purpose, all epiphytic and endophytic fungal species were analyzed by a metabarcoding analysis. In article II, 48 trees of one study area at Alaska’s northern treeline (Brooks Range) were examined for differences in mycobiome due to genetic differentiation, phenotypic characteristics and / or habitat characteristics. The trees used for this study were sampled from two adjacent plots on a south-facing mountain slope with an elevation gradient from 875 to 950 meters above sea level. It could be shown that, in contrast to the trees genotype, the height above sea level, the mountain slope, as well as the height and age of the trees have a significant impact on the mycobiome. The genetic differentiation between the tree individuals, however, showed no significant effect. Based on article II we examined the mycobiome composition of a total of 96 trees in 2 plots (16 trees each) at three sites in Alaska over a distance of 500 kilometers. Additionally, we sampled needles of two different ages for each tree (current year and three years old needles) summing up to 192 samples in total. The incentive of this study (article III) was to investigate the influence of origin and age of spruce needles on their mycobiome and if there is a genetic predisposition that is related to the fungal species community. In addition, the sampling design was improved by collecting needles from all four orientations (North, South, East and West) and sampling trees at a standardized distance to each other to avoid systematic errors. Comparable to article II the influence of the trees genetics on the species community of the epiphytic and endophytic fungi of the white spruce needles seems to be very unlikely. In contrast, a significant influence of the geographic origin and the needle age on the species structure of the needle inhabiting fungal species was found. The phenotypic tree traits height and dbh (diameter at breast height) had only minor influence and did in fact explain less than 2% of the mycobiome variance. Using Illumina sequencing, 10.2 million reads from the nucleotide sequence between the internal transcribed spacer (ITS) genes could be obtained, which yielded in 1575 ribotypes (called operational taxonomic unit, OTU) for the fungi. These were compared with a reference database to compare and assign them to known fungal species. For example, 942 OTUs with >95% similarity could be identified as known species, with 1975 samples identified on genus level and 2683 when determined to family level. The most pronounced difference between the two studies (article II and III) were due to the fungal species of the class of Pucciniomycetes, more specifically the genus Chrysomyxa which belongs to the rust fungi and is plant pathogenic. In the study of article II (sampling in 2012), Pucciniomycetes accounted for only a minor portion of the assigned DNA sequences. In the second study (article III, sampling in 2015) they accounted for more than half of all basidiomycetes found, which in turn contain 20.0% of all DNA sequences, the second largest phylum found beside Ascomycetes (51.4%).