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Changes in land use can promote the colonization of naturally susceptible environments, such as riparian zones, by invasive alien species, which can delay or prevent the processes of forest regeneration. These species often create conditions that facilitate their persistence and hinder the recovery of the original ecosystem. I present here two papers on the evaluation of 1) the impact of Psidium guajava L. dominance in secondary succession of abandoned pastures in the upper Paraná River-floodplain (UPRF); and 2) the impact of P. guajava clearing on native vegetation recovery in the same sites. In the first paper I compared the tree seedling banks of sites dominated by the exotic P. guajava and sites dominated by native species. In the second paper I evaluated the recruitment of native trees in response to three treatments: cutting all P. guajava individuals + herbicide; cutting all P. guajava individuals; and no action (control). The tree seedling bank in the area dominated by P. guajava had higher density, but lower species richness than in the area with dominance of natives. Species composition also differed between areas. P. guajava clearing led to an increase in trees' density and saplings' diversity, while the seedling bank showed almost no response to treatments. These results indicate that P. guajava control was effective in stimulating the recruitment of native trees. However, without continuous management P. guajava may reestablish and become dominant again. If P. guajava dominance in early succession is let without intervention, the abandoned pastures at the UPRF may turn into forests with low diversity and unrepresentative of the regional biodiversity. Management projects should be designed with an ecosystem scale approach, in which medium and long-term monitoring would certainly be required. Alterações no uso da terra em ambientes naturalmente suscetíveis à invasão por espécies exóticas, como zonas ripárias, podem propiciar a colonização por espécies exóticas invasoras. Muitas vezes, estas espécies podem criar condições que facilitem sua persistência e inibam a recuperação do ecossistema original. Nesse sentido, o objetivo foi avaliar o impacto da dominância de Psidium guajava L. sobre o processo de sucessão secundária em pastagens abandonadas na planície de inundação do alto rio Paraná (PIARP), bem como examinar a viabilidade do controle desta espécie em termos de recuperação da vegetação nativa. Avaliou-se a dominância de P. guajava na sucessão inicial em pastagens abandonadas na PIARP e se pode afetar o recrutamento de espécies arbóreas nativas, por meio da comparação do banco de plântulas arbóreas entre uma área dominada por P. guajava e outra dominada por espécies nativas. Verificou-se se o controle de P. guajava na sucessão inicial em pastagens abandonadas na PIARP poderia favorecer o recrutamento de espécies arbóreas nativas. Para isso, foi realizado um experimento que consistiu em três tratamentos: (CH) corte raso de todos os indivíduos + aplicação de herbicida; (CR) corte raso de todos os indivíduos e; (CT) controle. Apesar do banco de plântulas arbóreas da área dominada por P. guajava apresentar uma maior densidade, a riqueza de espécies foi inferior à da área com dominância de nativas. Além disso, a composição de espécies foi dissimilar. A remoção de P. guajava resultou no aumento da densidade, riqueza e diversidade dos indivíduos arbóreos considerados juvenis, enquanto que o banco de plântulas quase não respondeu aos tratamentos. Desta forma, o controle de P. guajava foi eficiente em estimular o recrutamento de espécies arbóreas nativas. Entretanto, sem manejo contínuo P. guajava pode se restabelecer e se tornar dominante novamente. Caso não ocorra a intervenção, a dominância de P. guajava na sucessão inicial em pastagens abandonadas na PIARP pode resultar em florestas com baixa diversidade e pouco representativas da biodiversidade regional. Porém, os projetos de manejo devem ser desenhados dentro do contexto ecossistêmico, e certamente necessitarão de programas de monitoramento de médio a longo prazo. 87 f

We evaluated density and species composition of the soil seed bank in active pasture and in secondary forest on a 10 year-old abandoned pasture to identify changes in density, richness, diversity and species composition during secondary succession of abandoned pastures. The implications of those changes for the forest recovery process were also considered. Soil samples were collected at Porto Rico island, state of Paraná, in 2007. The seedling emergence method was used. Data on active pasture collected in 1996, published by Campos and Souza (2003) were used for comparative analysis. No evidence was found of a pattern of changes in density of the soil seed bank during succession of abandoned pastures. We observed increases in richness and diversity, in the contribution of tree and shrub species and dominance of herb species for the seed bank during the first 10 years of abandonment of pastures in riparian forests. At the end of succession, the soil used as pasture can result in systems that are different from the original environment, due to seed bank impoverishment and presence of exotic species. We evaluated density and species composition of the soil seed bank in active pasture and in secondary forest on a 10 year-old abandoned pasture to identify changes in density, richness, diversity and species composition during secondary succession of abandoned pastures. The implications of those changes for the forest recovery process were also considered. Soil samples were collected at Porto Rico island, state of Paraná, in 2007. The seedling emergence method was used. Data on active pasture collected in 1996, published by Campos and Souza (2003) were used for comparative analysis. No evidence was found of a pattern of changes in density of the soil seed bank during succession of abandoned pastures. We observed increases in richness and diversity, in the contribution of tree and shrub species and dominance of herb species for the seed bank during the first 10 years of abandonment of pastures in riparian forests. At the end of succession, the soil used as pasture can result in systems that are different from the original environment, due to seed bank impoverishment and presence of exotic species.

In the tropics, large areas originally under forest systems have been converted into pasture. Recovering such environments takes longer than forest recovery following other kinds of disturbance. This study had three goals. First, to find a potential seed bank of woody species able to colonize abandoned pasture in the Upper Paraná River Floodplain. Second, to assess the impact of soil compaction on forest recovery under pasture. Third, to determine the most efficient soil treatment to recruit woody species seedlings from the pasture seed bank. Density and composition of woody species in the soil seed bank were measured using the seedling emergence method. Soil compaction was determined by resistance to penetration. An experiment in the pasture with four treatments was also conducted to assess the recruitment of woody species in the seed bank. The treatments were: the control; mechanical mowing and burning; mechanical mowing and manual weeding with removal of stolons; and mechanical mowing, manual weeding with removal of stolons and soil tillage. A considerable density of woody species was found in the seed bank, indicating a potential for pasture colonization. Soil compaction can not be considered a limitation to forest recovery in the study area. The removal of Brachiaria humidicola (Rendle) Schweick. and soil tillage can enhance the emergence of woody species seedlings in the seed bank. Nos trópicos, áreas extensas de sistemas florestais têm sido convertidas em pastagens. A restauração destes ambientes é mais lenta comparada com outros distúrbios. O presente estudo teve como propósito responder: Existe banco de sementes potencial de espécies lenhosas, capazes de colonizar uma pastagem abandonada na planície de inundação do alto rio Paraná? A compactação do solo na área de estudo pode ser considerada um fator limitante para a recuperação da floresta sobre a pastagem? e Qual tratamento do solo é mais eficaz no recrutamento de plântulas de espécies lenhosas do banco de sementes em pastagem? A densidade e composição de espécies lenhosas do banco de sementes do solo foi avaliada utilizando-se o método de emergência de plântulas e a compactação do solo foi determinada através da resistência à penetração. Além disso, foi aplicado um experimento com quatro tratamentos: controle; roçada mecânica e queima; roçada mecânica e capina com retirada de estolões; e roçada mecânica, capina com retirada de estolões e revolvimento do solo, a fim de verificar o recrutamento de espécies lenhosas do banco de sementes. Foi constatada a presença de espécies lenhosas no banco de sementes em considerável densidade, indicando potencial de colonização da pastagem. A compactação do solo não pode ser considerada um fator limitante para a recuperação da floresta na área de estudo. A retirada do capim e o revolvimento do solo podem facilitar a emergência de plântulas de espécies lenhosas a partir do banco de sementes. 30 f