The size and shape of the forest fragments were not associated with the abundance of H. dulcis individuals from all the size classes evaluated (class I, II and III), partially refuting the first hypothesis of the study. However, a greater proportion of individuals occurs at the fragment’s edges, regardless of class, size or shape, partially corroborating the first hypothesis of this study. Fragmentation interference (size and shape of forest fragments) drives the edge effect, resulting in a reduction of the forest through a vulnerability of richness and abundance of adult species and seedlings (Tabanez and Viana 2000; Portela and Santos 2007; Ribeiro et al. 2009).
It is important to note that the shape with value 1 is considered a standard shape and equivalent to a square. Therefore, more irregular forest fragments are more susceptible to show a greater edge effect, especially those with small size due to greater interaction with the surroundings (Saunders et al. 1991; Forman 1997; Nascimento et al 2006; Malinowski et al. 2008; Cemin et al. 2009). The shape criterion of the forest fragments herein is not associated with invasion and abundance of H. dulcis.
But we emphasize that the invasion pattern was observed in the inner-edge direction when the size classes of the H. dulcis individuals were analyzed separately. Furthermore, a greater proportion of adults (81.6%), young (90.4%) and seedlings (83%) was observed at the edges of the forest fragments, suggesting the invasiveness dynamics of this species from the edges. Trees with smaller diameters are generally found in edge areas (Malchow et al. 2006; Oliveira et al. 2008; Silva et al. 2015; Alves et al. 2018), and it is difficult to find large trees (Laurance et al. 2000; Paula et al. 2011). It is also observed that there is high regeneration on the edge, as well as high mortality (Laurance et al. 1998; Harper et al. 2005). Thus, in many cases, the forest is leading to initial succession stages due to these differentiated dynamics in the edge areas (Tabarelli et al. 2008).
It is also noteworthy that the high proportion of individuals from young classes and seedlings may suggest success in recruiting H. dulcis on the edges, meaning that the species is finding favorable environmental conditions for its establishment and development in these environments. Young H. dulcis individuals and seedlings (Classes II and III) still do not form fruit and have not reached the canopy, at which time they will obtain sufficient light for flower and fruit formation, representing an increase in the species’ invasiveness in the forest fragment (Gerber 2018).
Regardless size and shape, the presence of H. dulcis is also observed inside the forest fragments. These results highlight that H. dulcis individuals (mainly adults) are also established in environmental conditions such as the interior forest fragments (more closed canopy). Large and regular forest fragments may be less susceptible to the edge effect and, consequently to biological invasion. In this study, the fragments with shape < 2 can be considered the areas with integral ecological characteristics and priority for biodiversity conservation (Ribeiro et al. 2009). The results observed in this study showed a lower number of H. dulcis individuals in this fragment category.
Some studies have shown evidence that non-native species are not dependent on disturbance and are not restricted to early successional stages (Webb 2000; Martin et al. 2009) or edge areas, showing tolerance to both shade and the ability to invade closed canopy forests (Gilbert and Lechowicz 2005; Martin and Marks 2006; Major et al. 2013). The success in the establishment and development of the study species seems to be associated with its ecological characteristics, such as: its dispersion form, which increases the distance in which the propagules can reach; food source for the fauna increasing the seed dispersion (Padilha et al. 2015); shade and sun tolerance developing well in open areas and with high light incidence (Carvalho 1994; Lorenzi et al. 2003; Dechoum et al. 2015; Gerber 2018). However, the canopy opening was not related to the H. dulcis abundance. The southern region of Brazil has Semi-deciduous Forests, that show 50% leaves drop during May and September months (Klein 1972; IBGE 2012; Vibrans et al. 2012a).
However, this characteristic can be increased due to the high H. dulcis abundance, a species with deciduous phenology (Schmidt et al. 2019). Thus, a more open canopy occurs in these months (autumn). These environmental conditions may enable an increase in the establishment and development rate of non-native species, mainly H. dulcis inside forest fragments. The absence of a relationship between canopy opening and the abundance of H. dulcis may be associated with the species’ plasticity in adapting to different physiological characteristics and the light entry into forest fragments in different seasons (Schmidt et al. 2019).
The landscape elements (canopy opening and slope) were not associated with the abundance of H. dulcis, refuting the second hypothesis. The forests fragments in the study region are concentrated in areas that have a higher slope and are therefore unsuitable for large scale agricultural production. The Federal Forest Code (2012) can be associated as one of the factors responsible for the considerable increase in small fragments, mainly observed in sloping areas, dispersed in an agricultural matrix. These conditions could facilitate the establishment of H. dulcis, mainly due the increased edge effect.
The results of this study indicated that the landscape elements were not associated with the pattern of occurrence of H. dulcis. However, the observed results highlight that the greater H. dulcis abundance occurs mainly at the edge of forest fragments, regardless of the individuals’ size. This pattern of occurrence can be used as an aid in environmental decisions and highlight the potential of H. dulcis for establishing and growth, regardless of the size and shape of the subtropical forest fragments.