2.1 Studied species
Goldenrod species are hemicryptophytes (shoots are annual and newly sprout each spring) with rhizomes; they are insect pollinated and self-incompatible, with inflorescences forming at the top of each shoot which can produce up to 10 000–20 000 wind-dispersed seeds per one ramet (Bielecka, Królak, & Biardzka, 2017; Guzikowa & Maycock, 1986; Moran, Reid, & Levine, 2017; Schmid, Puttick, Burgess, & Bazzaz, 1988). The seeds of S. canadensis and S. gigantea have a high germination percentage (Weber, 2000; Weber & Jakobs, 2005), but in dense, well-established Solidago stands, seed germination and seedling emergence are exceptional. The clone size increases via horizontal rhizomes, and the death of an established genet is a rare event (Meyer & Schmid, 1999a,b).
The native habitats of S. canadensis are tall-grass prairies, infrequently grazed pastures, abandoned farmlands, roadsides, and waste areas in North America (Johnson, 1995; Werner, Bradburyt, & Grossi, 1980). Solidago gigantea prefers moist habitats, such as woods, stream edges, and woodland borders (Johnson, 1995). In Europe, S. gigantea and S. canadensis occupy similar habitats and prefer fallow lands and ruderal habitats on moist to mesic sites, such as abandoned farmlands, scrub, roadsides, forest edges, grasslands, wetlands, and riversides (Szymura & Szymura, 2013; Szymura & Szymura, 2016). Invasive goldenrods are highly competitive for nutrients, water, and space, and they release allelopathic compounds that inhibit growth of other plants (Gusev, 2015; Ledger et al., 2015; Werner et al., 1980; Zhang & Wan, 2017). Due to prolific vegetative propagation, they form dense stands and decrease the biodiversity of plants (Chmura et al., 2016; Ye et al., 2019; Zhang & Wan, 2017); arthropods (de Groot, Kleijn, & Jogan, 2007), including pollinators (e.g. wild bees, hoverflies and butterflies) (Moroń et al., 2009; Moroń, Marjańska, Skórka, Lenda, & Woyciechowski, 2021; Lenda et al., 2020) and ants (Kajzer-Bonk, Szpiłyk, & Woyciechowski, 2016; Lenda, Witek, Skórka, Moroń, & Woyciechowski, 2013); and birds (Skórka et al., 2010).
Solidago canadensis was the first alien Solidago species recorded in Europe, in 1648, while S. gigantea was first recorded in 1758. The species were found in the territory of Poland about 100 years later, S. gigantea in 1853 and S. canadensis in 1872 (Tokarska-Guzik, 2005). After S. canadensis and S. gigantea were introduced into botanical gardens, they were distributed among gardeners. The plants were attractive and easy to grow as ornamental plants, and they were useful for beekeepers (Guzikowa & Maycock, 1986; Roháčová & Drozd, 2009; Weber, 1997; Zihare & Blumberga, 2017). Recently Solidago species have become widely distributed throughout Poland. According to the stages of invasion (Blackburn et al., 2011), S. canadensis and S. giganteaare now fully invasive species, with individuals dispersing, surviving, and reproducing at multiple sites in a wide variation of habitats over an extensive spatial area (E category).