Structure of Cenopopulations of Heracleum sosnowskyi and Mechanisms for Maintaining Their Stability under the North Conditions

The growth and productivity, rhythm of development, and ontogenetic and spatial structure of cenopopulations of the Sosnowsky’s hogweed (Heracleum sosnowskyi Manden.) on the territory of the Komi Republic were studied. The duration of the growing season of the H. sosnowskyi was 180–190 days due to the early spring germination of seeds and the long autumn growing season of pregenerative individual plants. The studied H. sosnowskyi CPs were characterized by left-sided ontogenetic spectra and high recovery indices. These properties characterized them as young and self-renewing plants. The greatest and the lowest numbers per the unit area had the juvenile and the generative plants (710–1700 and 1–3 individuals/m², respectively). The average density of the immature and virginile individuals was 4–7 pcs/m² and 12–16 pcs/m², respectively. The absence of seedlings during the flowering period of the plants was discovered, resulting from of the synchronous germination of mericarps and the rapid passage of the post-emergence development stage. Plants began to flower at the age of 2–6 full years, and the maximum lifespan of the individuals was 7 years. H. sosnowskyi plants were characterized by high productivity. They formed up to 15 kg/m² of wet phytomass, half of which was produced by generative individuals. The main part of the leaf area of the generative individuals was located in the upper layers (similarly to an “inverted pyramid”) and absorbed about 70% of the incoming PAR. The identified mechanisms of the CP self-maintenance mediate the spread and retention of territories which are occupied by H. sosnowskyi plants on the northern border of the invaded range.