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species in European woodlands, was found almost twice as frequently as P. serotina. Current spatial occurrence of alien invasive plants is determined by species’ eco-physiological requirements, by biotic interactions, particularly with native vegetation, and by local disturbance, which in our study, is mainly human-induced. The latter two, native vegetation and local disturbance, are key factors for the level of invasion: the actual number or proportion of alien species in a habitat (Chytry et al., 2008 and references therein). The findings of our study concerning the establishment of P. serotina and I. parviflora in these forest patches show that factors predicting their occurrence differed between the two species. We detected different significant determinants for habitats invaded by P. serotina or I. parviflora, even though we limited our research to forest patches.
Our models predicted that P. serotina was significantly more likely to grow in poor sites (low-light and low-nutrient conditions indicated by Ellenberg’s value for native species) and in patches with regular shapes (low perimeter – area ratio). This pattern is consistent with the study of P. serotina seedling survival in temperate forests (Dyderski and Jagodziński, 2019; Jagodziński et al., 2019; Kawaletz et al., 2013). Seedling survival was positively affected by low-light, understorey cover and pH, but negatively affected by increasing canopy openness and soil moisture. On the other hand, greater light availability was positively associated with P. serotina biomass (Dyderski and Jagodziński, 2018a). These results suggest that P. serotina saplings are able to survive in an environment with low resource availability, where competition is low, while later on they need more resources to increase their growth (Vanhellemont et al., 2008). On our study site, the majority of the P. serotina plants were indeed saplings no higher than 1 meter, while trees were found mainly on forest edges. The edaphic niche of P. serotina tends to constitute sites with low levels of nutrients or at least levels similar to those of non-invaded sites in nitrogen values; however, phosphorus values might be higher (Chabrerie et al., 2008). Given the preference for dry soil and a closed canopy, we can expect further spread of P. serotina in our study area and in the vicinity because suitable habitats are available and have not yet been invaded by P. serotina. Among these, Pinus sylvestris L. dominated stands are frequent, and those are habitats where P. serotina is known to establish abundantly (Dyderski and Jagodziński, 2019a).
On the other hand, according to our models, I. parviflora was more likely to occur in habitats with native flora exhibiting high mean Ellenberg’s soil moisture indicator value and high perimeter – area ratio, a metric indicating increased length of forest edge. This is in accordance with I. parviflora habitat preferences as it mainly grow in human disturbed forests and forests edges (CABI, 2020b). In the literature, the enormous success of I. parviflora in colonizing European woodlands is often explained by its broad habitat niche (Quinet et al., 2015), although its edaphic niche is still not well understood, since the literature reports a preference for both nutrient-rich and nutrient-poor sites (Wagner et al., 2017). Our models showed that study site nutrients obtained from Ellenberg’s indicator value for native flora were not a significant predictor. The opposite was true for soil moisture. The model corresponds well with the facts that annual I. parviflora germinates in the first year after seeding, that it is only slightly limited by low soil nitrogen, and that it does not form a persistent seed bank (Skálová et al., 2019). Florianová and Münzbergová (2018) argue that factors affecting I. parviflora success differ depending on the life stage. For example, a dense understorey and low soil moisture repress seedling emergence, while a dense understorey has little effect after seedling establishment.
Our results indicate the importance of several habitat characteristics for susceptibility to invasion. In our studied habitats, it could be that habitat characteristics are better determinants of the level of invasion than propagule pressure or climate, as was stressed by Chytry et al. (2008). Moreover, we recognized one additional, important patch characteristic: human pressure, which we can expect to increase worldwide (Wang et al., 2021). Prunus serotina was connected to less disturbed forest patches, while I. parviflora was also frequent in patches with greater disturbance and more disturbance-related changes, which affected not the patch alone but the adjacent surroundings as well. Just as the traits of successful invaders are specific and context-dependent, and cannot be united into one general trait (Sol et al., 2012), this seems to be true also of habitat characteristics defining susceptibility to invasion.
What we found very concerning is that our models for I. parviflora occurrence indicate that several human-related activities, like the presence of waste in the forest and the high frequency of roads in the surrounding landscape, do coincide with this species. The altered landscapes surrounding habitat fragments are rarely static, but instead undergo continuous changes in land use and succession (Laurance, 2002; Laurance et al., 2007). Therefore, we can probably expect the existing dynamics to change eventually, thus influencing the forest remnants in new or additional and more severe ways. As an example, according to a survey, 12% of garden owners in Slovenia do occasionally deposit green garden waste in nearby forests, unaware that they could thus be introducing invasive alien species into nature (Šipek and Šajna, 2020). We further hypothesize that P. serotina was less common in disturbed patches dissected by walking paths because frequent human presence might have a negative impact on birds, which disperse P. serotina seeds; however, further observations are needed, since the results of studying disturbance effects on P. serotina are contradictory. In a study by Dyderski and Jagodziński (2018b), disturbance was of little importance for the ecological success of P. serotina, while, in contrast, some earlier studies found a positive effect (Closset-Kopp et al., 2007; Chabrerie et al., 2008). It is worrying