Ecogeographical Analysis of the Heracleum persicum , H . mantegazzianum , and H . sosnowskyi Distribution at the Northern Limit of Their Secondary Ranges in Europe

This paper analyzes the distribution of Heracleum persicum , H. mantegazzianum , and H. sosnowskyi at the northern limit of their secondary ranges in Europe based on original and published data. The northernmost H. persicum occurrences are localized in coastal regions in the northern and central parts of the Scandinavian Peninsula (up to 71° N). In Fennoscandia, H. mantegazzianum and H. sosnowskyi were noted up to 69° N. The gradient of climatic resources in the subarctic and temperate zones made it possible to identify factors limiting further expansion of these species in northern Europe and determine their tolerance limits to these factors. The duration of the frostless season at the northern boundary of the secondary range of invasive Heracleum species is 80–150 days. The probability of cold damage to vegetating plants in areas located north of 66° N (i.e., in the northern part of Finland and in the continental northern part of European Russia) is high. The biological minimum for the sum of active air temperatures ≥5°C is 1150°C, while the minimum requirement of the studied plants for the sum of active temperatures ≥10°C is over 450°C. If available thermal resources exceed this level, then invasive Heracleum species can naturalize in climatic conditions of subarctic Europe. The presence of a sustainable snow cover more than 25 cm depth in areas where the minimum winter temperatures are less than –30°C prevents the freezing of renewal buds and seedlings, thus, preserving the viability of plants. It is proposed to use climatic indices computed as ratios between the air temperature and the snow cover depth (or the precipitation amount in winter) as climatic markers characterizing wintering conditions for plants. The main factors limiting the spread and naturalization of invasive Heracleum species in northern Europe are lack of heat, soil freezing to temperatures critical for hibernating plant organs, and early autumn frosts.