Dependence of Weed Composition on Cultivated Plant Species and Varieties in Energy-Tree and -Grass Plantations

Energy plantations create new habitats in agricultural landscapes with species compositions different from those in forests or farmlands. The purpose of our nine-year research project (2010–2018) was to evaluate the dependence of weed-species richness and their selected ecological aspects on stands of energy-plant species, and varieties in energy-tree and -grass plantations in conditions of Central Europe, on the basis of a case study. The permanent research plots were established in plantations containing two varieties of willow (Tordis and Inger), one poplar variety (Pegaso), and one clone of Miscanthus × giganteus. This evaluation included the species composition of understory flora, habitat preferences of different species, life cycle, life forms, ecological demands, and the harmfulness of these weed-species. The ground flora of energy plantations is predominantly composed of synanthropic plants of a weedy character with differences in species composition among different energy-tree and -grass species and varieties. The total number of vascular plant species was 98. The highest number of species (58) was recorded in the Tordis and Inger willow varieties, and the lowest was observed in the Pegaso poplar variety (45). Perennial species prevailed by their share, 10 of which were found in all four research plots. Therophytes and hemicryptophytes prevailed. Most species have high light requirements and are typical for mild-to-warm suboceanic areas, demanding freshly moist alkalic soils that are medium-to-rich in mineral nitrogen. Fifty percent of all observed species are considered weeds in Slovakia. The “very dangerous” category represented 46.94% of weeds, the “less dangerous” category 51.02%, and the “nondangerous” category represented 2.04% out of 49 species. The biggest share of “very dangerous weeds” was found in the poplar stand (38.78%), less in willow (32.65% and 28.57%), and the least in miscanthus stands (26.53%). The weeds of the Tordis variety were relatively poorly influenced by specific environmental conditions, and the weeds of the Inger variety were mainly defined by the soil reaction. Weeds in the undergrowth of both Miscanthus × giganteus and poplar trees (Pegaso) had the greatest affinity to mineral nitrogen content and temperature requirements.