Effects of history of use on secondary forest regeneration in the Upper Parana Atlantic Forest (Misiones, Argentina)

The Upper Parana Atlantic Forest, as with other areas, has been modified due to anthropogenic activities, generating a mosaic of agricultural fields and forests of varying ages and levels of conservation. The objective of this study was to identify structural and floristic patterns in secondary forests generated in abandoned fields with varying historic uses. Changes in vegetation were surveyed using chronosequences from 2 to 70 years. Stands with different land use histories in three areas of northern Misiones, Argentina, were considered. Adults (DBH>=10cm), saplings (3>=DBH<10cm), seedlings (DBH<3cm and height>=50cm) and non-arboreous vegetation (herbaceous plants and shrubs) were surveyed. Data was analyzed using multivariate techniques (TWINSPAN, Detrended Correspondence Analysis, Cluster Analysis, and Kruskal-Wallis test). In the floristic analysis, two main forest groups were identified: one made up of primary forest and oldest secondary forest (>20 years), and another of the youngest secondary forests (<20 years). Basal area and density of individuals (seedlings, saplings, and adults) rapidly increased during the first two decades, and subsequently the adult category reached values comparable to those of mature forests (32m²/ha and 850individuals/ha). During the early years of succession, the forests showed floristic and structural differences related to history of use previous to abandonment; abandoned annual crops showed the greatest densities of saplings and seedlings, while abandoned pastures presented only individuals in the seedling category, and at very low densities. In stands analyzed since the beginning of succession, species of several ecological groups were present (pioneer, initial secondary, late secondary), while climax species appeared only in the seedling category, and in abandoned pastures no species of the later groups were recorded. Our results suggest that differences recorded during the early years of regeneration in sites with varying historic uses may be generated mainly by remnant vegetation at the time of abandonment of the fields (grasses and remnant monoculture plants). After 20 years of succession, it was no longer possible to differentiate the forests structurally or floristically, regardless of age or historic use of each stand. Due to their rapid regeneration capacity, the forests studied play an important role from the start of the successional process in heterogeneous anthropogenic landscapes such as that of Misiones, and contribute to maintaining biodiversity while providing numerous environmental services.