Contrasting invasion patterns of two closely related Solidago alien species

Aim: New areas are often simultaneously invaded by closely related alien species; however, between-species differences in the course of their invasive spreading due to diverse ecological preferences have rarely been investigated. Here, we aim to study the species-specific spatio-temporal invasion patterns of Solidago canadensis and S. gigantea.Location: Slovakia-the Western Carpathian and adjacent Pannonian regions.Taxon: Invasive alien Solidago canadensis and S. gigantea.Methods: Our study was based on revised herbarium specimens, a recent field survey which included gathering abundance and habitat data, and environmental data from GIS layers. We characterised the environmental niche differentiation between S. canadensis and S. gigantea. Generalised additive models were then used to identify the key drivers of the species' occurrences and to assess their potential current and future distributions.Results: Both species began to spread in Slovakia in the 1850s; in the first 100 years, the number of S. gigantea sites increased exponentially, while that of S. canadensis remained low. Currently, S. canadensis is more widespread in the region. The species have similar habitat preferences. Their environmental niches overlap, but are not identical. The core distribution of S. canadensis is in foothills and valleys that experience lower annual mean temperatures, while that of S. gigantea is in lowlands with higher annual mean temperatures. A large part of their potential distribution areas seems to be already invaded. Possible future climate change could stimulate the spreading of S. canadensis into higher altitudes and S. gigantea farther into lowlands and Carpathian foothills.Main conclusions: The studied invasive Solidago species differed considerably in the early stages of invasion. Their environmental niches, current realised and potential distribution patterns are different. In the studied region, further spreading, with an increase in the number and size of populations, is likely unless effective measures are undertaken.