Under climate change, modifications on plants’ growth are expected to be the strongest at species margins. Therein, tree acclimation could play a key role as migration is predicted to be too slow to track shifts of bioclimatic envelops. A requirement is, however, that intra-population genetic diversity be high enough for allowing such adaptation of tree populations to climate change. In this study, we tested for the existence of relationships between genetic diversity, site environmental conditions, and the response of annual tree growth to climate of Pinus cembra at its southern limit in the Alps. Site-specific climatic and environmental factors predominantly determined the response of trees along the precipitation gradient. The growth-climate interactions were chiefly linked to mean annual precipitation and temperature, slope and tree-size, and less to genetic diversity. We show that genetic background of Pinus cembra has exclusively indirect modulating power with limited effects on tree-ring formation, and within the southern limit in the Alps, genetic variability is not necessarily well expressed in the patterns of annual tree growth. Our results may imply little adaptive capacity of these populations to future changes in the water balance.

Genetic processes shape the modern-day distribution of genetic variation within and between populations and can provide important insights into the underlying mechanisms of evolution. The resulting genetic variation is often unequally partitioned within species’ distribution range and especially large differences can manifest at the range limit, where population fragmentation and isolation play a crucial role in species survival. Despite several molecular studies investigating the genetic diversity and differentiation of European Alpine mountain forests, the climatic and demographic constrains which influence the genetic processes are often unknown. Here, we apply non-coding microsatellite markers to evaluate the sporadic peripheral and continuous populations of cembra pine (Pinus cembra L.), a long-lived conifer species that inhabits the subalpine treeline ecotone in the western Alps to investigate how the genetic processes contribute to the modern-day spatial distribution. Moreover, we corroborate our findings with paleoecological records, micro and macro-remains, to infer the species’ possible glacial refugia and expansion scenarios.

Aim

Distribution?wide trends in climate variability significantly influence the genetic diversity, differentiation and population structure of tree species. This study investigates the effects of disturbances such as fire, fragmentation and climate on modern?day genetic patterns and clonal structures of trembling aspen at the transcontinental scale.

Location

North American boreal zone.

Taxon

Trembling aspen (Populus tremuloides Michx.).

Methods

One thousand two hundred individuals in a 30?site network (40 trees per location) were genotyped with neutral genetic markers and studied in relation to regional differences in climate and surrounding site conditions (aridity, fire cycle, fragmentation). Multiple linear regression models and variance analysis were used to test relationships between genetic indices, structural parameters and the surrounding site factors.

Results

Overall, a high percentage of single ramet clones (SRC) and clonal diversity was detected and assumed to be the consequence of multiple sexual reproduction events that took place at all sites, together with suckering, which shapes the clonal structure of populations. Neutral genetic diversity and clonal structure suggested no substantial differences among sites, which were categorized into climate moisture index (CMI) classes; aspen stands across Canada were highly similar from a genetic point of view. Allelic richness (AR) and the average number of alleles (Na) varied significantly among clonal organization groups, and landscape fragmentation and a higher frequency of fires showed a negative influence on the levels of genetic diversity.

Main conclusions

Our results are inconsistent with the idea that the genotypic diversity of trembling aspen is related to the intensity of disturbance within the boreal forest. It appears that species?specific disturbance responses and post?fire recruitment mechanisms are more important than dominant ecological factors, such as climate and fire regimes, in shaping distribution?wide patterns of neutral genetic variation and clonal structure.