Finding the meeting zone: Where have the northern and southern ranges of Norway spruce overlapped?

The goal of the work was to define where migration routes of Norway spruce running from its Weichselian refuges could have intersected and formed a hybrid zone. Based on available pollen data, we have hypothesized that the central European disjunction, also called the “spruceless zone”, could be an overlapping area of boreal and southern ranges.The refugial origin of 1353 trees from 58 Norway spruce stands was determined using the maternally inherited mitochondrial nad1 b/c marker.Polymerase chain reaction-restriction fragment length polymorphism analysis of the nad1 b/c marker revealed the presence of two mtDNA variants in Poland that we designated as the “northern” and “southern” haplotypes. The geographic distribution of these haplotypes corresponds to particular Norway spruce ranges and their refugial origins. The northern haplotype was mainly found within the boreal part of the range while the southern haplotype was noted in populations from the southern portion. Co-occurrence of the two haplotypes was noted in stands from the “spruceless zone”; however, the frequency of the southern haplotype was significantly higher in those populations.Our results combined with available fossil pollen data allow us to define the area of the Middle-Polish Plains as a hybrid zone for Norway spruce originating from Carpathian and Russian refuges. The domination of Norway spruce of Carpathian origin in the hybrid zone might be attributable to its earlier expansion from this refuge in contrast to the timing of its expansion from the Russian refuge. The findings presented here together with historical sources may also help explain the modern sparse distribution of Norway spruce in the Middle-Polish Plains.