Riqueza, estrutura e composição de espécies arbóreas em floresta secundária invadida por hovenia dulcis thunb, caracterização do seu nicho de regeneração e efeitos alelopáticos

Abstract

The impact of invasive species on native species has been widely recognized as an important cause of the changes in the global diversity. Invading species are able to affect the structure and function of invaded ecosystems. Among the exotic plant species that invade forests in Brazil, Hovenia dulcis is frequent in areas of secondary vegetation. Thus, the first part of this work compares the phytosociological parameters among areas of secondary vegetation invaded by H. dulcis and areas without this exotic species. H. dulcis presented one of the biggest values of absolute density and basal area, demonstrating its importance for the structure of the areas where it occurs. However, the structural variables and the species richness had not been modified in function of the presence of the exotic species. The species composition was altered by the presence of the invading species when the effects caused by unmeasured variables that influence the dynamics of forest succession were controlled, indicating that variations in the composition among areas are in good part determined by differences in abiotic conditions and history of each area, and secondarily by the presence of H. dulcis. The abundance and distribution of the plant species are determined in part during the initial periods of regeneration, when they are more vulnerable to its immediate environment. Changes in require of biotic factors in the different periods of phases of ontogenetic development of the plant can reveal change of the niche of regeneration of this. The availability of light, the fall and accumulation of litter are factors that affect the structure and dynamics of the plant community in different ways. Thus, the second part of this study intends to evaluate the ambient variables around individuals of H. dulcis during the phases of ontogenetic development from young to old saplings and adults, evaluating changes in the regeneration niche in relation to the ambient variables, the ontogenetic period and the seasonal deciduousness of H. dulcis. The analyses indicated differences among the ontogenetic phases only in canopy opening and herbaceous covering. The species composition around H. dulcis individuals also varied among the ontogenetic phases. The percentage of canopy opening differed between adult individuals in relation to young saplings only in the non deciduous period, probably due to the dominance of the exotic species in the canopy. Thus, a change in the regeneration niche in function of the canopy opening is observed. The low density of the tree crowns of H. dulcis exacerbates the effect of the canopy opening in points where adults are established in relation to young saplings. The litter depth during the deciduous period was higher around adults and young saplings. The deciduousness and consequent increase in the leaf litter can favor the survival and growth of the most light demanding species, such as those typical of the initial stages of forest succession, as a result of the increase in the amount of light that arrives at the understory, which, however, can negatively affects seedling that are intolerant to elevated luminosity. The allelopathic compounds produced by plants come from the secondary metabolism and may be present in different plant tissues, being released to the environment in different ways. The allelopaty has been recognized as an important ecological mechanism that influences the dominance in plant communities, being able to determine the structure, composition and the dynamics of such communities. H. dulcis presents among others phenolic compounds, in its leaves that can cause changes during the process of seed germination of native species. Therefore, the third part of this work tests the potential allelopathic effects of leaf extracts of H. dulcis on the germination and growth of the root and aerial parts of seedlings of Casearia sylvestris. The watery leaf extracts of H. dulcis present allelopathic effect in seeds and seedlings of C. sylvestris. The leaf extracts in all concentrations reduced the germination percentage, and lead to a great inibitory effect in root development that can hinder the future establishment of the plants, affecting the uptake of vital resources. Seeds in contact with the leaf extracts presented a reduction in the germination speed. Thus, seedlings of C. sylvestris exposed to alleloquimic compounds of H. dulcis leaves had its initial development delayed. Thus, one concludes that the great density of individuals of H. dulcis corroborates the invasion for this species that modifies the composition of species in the forest in regeneration. H.dulcis modifies its niche of regeneration and thus it influences the niche of the regenerant native species and still it presents inibitiry allelophatic effect on the germination and development of seedlings of the native species.