Weed Communities and Their Diversity Depending on the Age of the Plantation and Selected Plant Species Intended for Energy Purposes

In the conducted study, the structure of weed communities and their dynamics were assessed and compared in relation to plantation age and the species of energy crop. Weed diversity within the stands of the investigated energy crops was evaluated in three-year cycles, from 2006 to 2024. The assessment of weed diversity in the stands of the studied energy crops was conducted in three-year cycles, i.e., in the first year of cultivation (2006—the year of plantation establishment), the fourth year of cultivation (2009), the seventh year of cultivation (2012), the tenth year of cultivation (2015), the thirteenth year of cultivation (2018), the sixteenth year of cultivation (2021), and the nineteenth year of cultivation (2024). The species composition of weed communities and the abundance of individual weed species were determined. The diversity and dominance patterns of weed communities occurring in Salix viminalis, Miscanthus × giganteus, and Phalaris arundinacea were described using two indices: the Shannon–Wiener diversity index and the Simpson dominance index. As a result of the conducted observations, it was found that weed abundance, species diversity, and weed infestation dynamics depended on the energy crop species and the age of the plantation. Greater interannual variability was observed in weed abundance, whereas species richness remained relatively more stable between years. The highest species diversity was recorded in the Salix viminalis plantation, where a total of 53 weed species were identified. In contrast, considerably fewer taxa were found in Miscanthus × giganteus (42 species) and Phalaris arundinacea (41 species). Moreover, it was found that regardless of the energy crop species, segetal weeds dominated during the first years of cultivation, i.e., E. crus-galli, A. spica-venti, A. retroflexus, Ch. album, M. inodora, and V. arvensis. In subsequent years of cultivation, however, the dominant species were ruderal weeds, such as A. vulgaris, T. officinale, and U. dioica, as well as invasive species, e.g., S. canadensis and S. gigantea. In subsequent years, ruderal species became dominant. The Shannon–Wiener diversity and Simpson dominance indices indicated differences in species richness and the relative abundance of individual weed species. The highest values of the Shannon–Wiener diversity index and the lowest values of the Simpson dominance index were recorded in weed communities of Salix viminalis plantations aged 7–16 years after establishment. In contrast, the highest Simpson index values, indicating dominance by one or a few weed species, were observed in the first year of cultivation regardless of the energy crop species, as well as in the 19-year-old Miscanthus × giganteus plantation.