Abiotic factors
The variables representing abiotic environment (A) are the most
important for model performance for both species; however, the impact of
these variables was more pronounced in the case of S. gigantea ,
compared with S. canadensis .
The distribution of both species was restricted climatically, and their
presence was unlikely in areas with an average yearly temperature below
approximately 5.5°C. The temperature corresponds with the altitudinal
zonation of vegetation in the studied region and relates to a lower
limit of the montane zone, starting from an altitude of approximately
600–850 m a.s.l. in the studied region. The negative effect of cold
climate on the distribution of both Solidago species studied is in
accordance with studies examining their potential distribution in
Europe, which indicated that northern Europe as a region is outside
their climatic requirements (Weber, 2001). Although both species can be
observed sporadically at higher altitudes, their typical upper limit is
1200 m a.s.l. (Moran et al., 2017; Weber & Jakobs, 2005). In the case
of S. gigantea , positive correlations have been found between the
mean temperature and growth parameters, and high spring temperatures
(above 24 °C) are advantageous for germination (for review, see Weber &
Jakobs, 2005). Solidago canadensis plants are taller at lower
attitudes, and at higher altitudes, they are not able to develop seeds
because of the limited length of the vegetation period (Moran et al.,
2017). It should be noted that the data referred to here regarding
altitude come from the central Alps, while the climate in the Carpathian
Mountains is more severe; therefore, the upper limits of the vegetation
zones are at lower altitudes in the Carpathian Mountains comparing with
the Alps (Ellenberg, 1988; Pawłowski, 1972).
The species distributions were also correlated with temperature
seasonality, which in the studied region is also related to the
precipitation pattern (Appendix, Table S.3.). Solidago canadensisis more abundant in the western part of the study region, which has
lower temperature seasonality and higher precipitation, while S.
gigantea avoids the southern part of the region with higher
precipitation and also lower temperature seasonality. Previous studies
examining the potential range of this species in Europe (Weber, 2001)
suggested that these aspects (continentality gradient and precipitation)
did not restrict their distribution in this part of Europe. Therefore,
the extent to which the observed relation is causal is not clear, and
the possibility exists that it reflects a peculiarity of the
distribution in the studied region.
The models did not indicate that soil properties and landrelief features
are among the crucial factors explaining the distributions of the
invaders. Both species are known to have rather broad tolerance to soils
(Szymura & Szymura 2016; Weber & Jakobs, 2005; Werner et al., 1980),
which could explain why soil properties were not relevant in studied
region. Observations from early phase of invasion on studied region, up
to 1989s, underlined the role of river valleys, as a main route of
invasion (Tokarska-Guzik, 2005). The results obtained here show that the
species are broadly widespread and their invasion is no longer related
to watercourses.