Avanços, desafios e diretrizes para a restauração ecológica por semeadura direta de espécies arbóreas
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2023-04-12
Autores
Souza, Diego Cerveira de
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Universidade Estadual Paulista (Unesp)
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Nas próximas décadas, milhões de hectares de florestas tropicais deverão ser restaurados para mitigar os efeitos adversos das mudanças climáticas e da perda da biodiversidade. Para que estas ações de restauração possam ser economicamente viáveis, especialmente em regiões com menos recursos financeiros, métodos de restauração mais baratos precisam ser desenvolvidos. Atualmente, a restauração de florestas tropicais ocorre principalmente por meio do plantio de mudas, que possui custos iniciais elevados, mas a semeadura direta tem se mostrado uma técnica capaz de reduzir estes custos. Vários estudos foram implantados nos últimos anos, no Brasil, para testar o potencial de uso da semeadura direta, mas os resultados destes estudos nunca foram compilados e analisados de maneira conjunta, o que poderia fornecer ferramentas importantes para o aprimoramento da técnica. Uma importante lacuna também é a correta escolha de espécies para serem semeadas, bem como o conhecimento dos atributos funcionais que afetam o desempenho das espécies na semeadura direta. Além disso, os estudos já desenvolvidos no país são predominantemente avaliados somente nos primeiros anos pós-semeadura, sendo raros estudos que acompanham o processo de sucessão destas áreas, a fim de analisar se os objetivos básicos da restauração vêm sendo efetivamente alcançados. Objetivamos, com este trabalho, analisar de forma ampla o uso da semeadura direta para fins de restauração florestal no Brasil, para compreender melhor os desafios e fatores de sucesso, buscando fornecer subsídios para o aprimoramento e a ampliação do uso desta técnica para a restauração das florestas tropicais. Para isso, estruturamos este trabalho em três capítulos. No Capítulo 1, analisamos o estado da arte do uso de técnicas de semeadura direta para restauração das florestas brasileiras, por meio de uma revisão sistemática, visando identificar as tendências, os avanços e as lacunas de conhecimento, bem como fornecer diretrizes para pesquisas futuras. No Capítulo 2, levantamos as espécies arbóreas já testadas em projetos de semeadura direta no Brasil e os seus atributos funcionais, visando verificar quais espécies apresentam bom desempenho e quais atributos funcionais explicam este desempenho. No Capítulo 3, descrevemos as mudanças temporais de um projeto de restauração implantado há 25 anos via semeadura direta, com baixa diversidade de espécies, para verificar se os parâmetros estruturais vêm se assemelhando aos de florestas nativas. Os nossos resultados mostram que (1) os experimentos conduzidos no país são implantados principalmente nos biomas Mata Atlântica e Cerrado, em pastagens abandonadas, no período de chuvas, com a semeadura de um número baixo de espécies e uma quantidade alta de sementes por hectare; (2) as taxas médias de estabelecimento e crescimento são baixas para a maioria dos estudos, mas as taxas médias de sobrevivência são elevadas; (3) poucas espécies mostram-se adaptadas a projetos de semeadura direta; (4) espécies com sementes grandes e cotilédones de armazenamento apresentam maiores taxas de estabelecimento quando diretamente semeadas no campo; e (5) florestas em restauração por semeadura direta com baixa diversidade de espécies conseguem se assemelhar em riqueza, diversidade e área basal a florestas nativas próximas, mas não quanto à estrutura vertical e estratificação, com baixa densidade de indivíduos no estrato intermediário e baixa diversidade nos estratos intermediário e superior. As baixas taxas de estabelecimento e de crescimento verificadas na maioria dos estudos conduzidos no país levariam à necessidade de uso de uma grande quantidade de sementes para o alcance de densidades e coberturas de solo semelhantes aos obtidos pelo plantio de mudas. Atualmente, não há disponibilidade de sementes para suprir esta demanda na maior parte das regiões do país, o que dificulta a aplicação da semeadura direta em larga escala. Poucas espécies conseguem se adaptar ao método, mas a semeadura direta em baixa diversidade pode ser uma alternativa promissora para a restauração de florestas tropicais, desde que haja fragmentos de vegetação nativa próximos, capazes de propiciar a colonização por novas espécies. Neste caso, em situações práticas, a seleção de espécies que apresentam melhor desempenho, bem como daquelas com sementes grandes e cotilédones de armazenamento, pode auxiliar no aumento das taxas médias de estabelecimento das espécies e reduzir as chances de insucesso e o desperdício de sementes. Estudos futuros referentes a ações de implantação e manejo inicial pós-semeadura são essenciais, principalmente para aumentar as taxas médias de estabelecimento das espécies, assim como o número de espécies indicadas para serem utilizadas. Devem ser testadas, em casos de semeadura direta com baixa diversidade, novas combinações de espécies, de diferentes grupos funcionais. Também precisa ser mais bem analisado, neste tipo de projetos, implantados com baixa diversidade, os efeitos do surgimento de pragas e doenças e seus impactos na comunidade; os riscos de monodominância na estrutura horizontal e vertical das florestas, os seus impactos e as ações de manejo indicadas; e, ainda, o papel da matriz na qual as áreas estão inseridas no aumento da riqueza e da diversidade das comunidades.
In the coming decades, millions of hectares of tropical forests will have to be restored to mitigate the adverse effects of climate change and the loss of biodiversity. To be economically viable, especially in regions with less financial resources, cost-effective restoration methods need to be developed. Currently, the restoration of tropical forests has been made mainly through the planting of seedlings, with higher implantation costs, where direct seeding is expected to be a technique capable of reducing these costs. Several studies have been implemented in recent years in Brazil to test the potential of direct seeding techniques, but the results have never been compiled and analyzed systematically, which could help improving restoration practice. An important gap is also the correct choice of species to be sown, as well as the knowledge of the functional traits that affect the performance of the species in direct seeding techniques. In addition, most studies predominantly include evaluations only during the first years after sowing, with rare studies accompanying the succession process afterwards, in order to analyze whether the basic objectives of ecological restoration have been effectively achieved. The objective of this study is to make a comprehensive analysis of direct seeding for forest restoration in Brazil, to better understand the challenges and success factors, to provide background for improving and upscaling the use of this technique for ecological restoration of tropical wooded ecosystems. We structured this study in three chapters. In Chapter 1, we analyzed the state of the art of direct seeding techniques for restoring Brazilian forests and wooded savannas, through a systematic review, aiming at to identify trends, advances and knowledge gaps, as well as to provide guidelines for future research. In Chapter 2, we surveyed the tree species already tested in direct seeding projects in Brazil and investigated some of their functional traits, aiming at to verify species with best performance and functional traits explaining it. In Chapter 3, we described the temporal changes in forest structure and diversity of forests restored by direct seeding, implemented 25 years ago with low species diversity, to verify whether how similar the structural parameters are with those of a native forest nearby. Our results showed that (1) most of the experiments carried out in Brazil have been undertaken mainly in the Atlantic Forest and Cerrado biomes, implanted in abandoned pastures, during the rainy season, using a low number of species and a high amount of seeds per hectare; (2) mean establishment and growth rates are low for most studies, but mean survival rates are high; (3) a few species are adapted to direct seeding techniques in different regions of Brazil; (4) species with large seeds and storage cotyledons have higher establishment rates when directly sown in field; and (5) forests restored by direct seeding with low species diversity can resemble native forests in richness, diversity and basal area, but not in canopy layering, with low density of individuals in the intermediate layer and low diversity in the intermediate and upper layer. The low establishment and growth rates verified in most of the studies carried out in the country will demands a large amount of seeds to reach density and soil coverage levels similar to those obtained by planting seedlings. Currently, there is no availability of seeds to meet this demand in most of the country’s regions, and the harvesting of large amount of seeds could generate seed loss, which potentially hampers the upscaling of direct seeding for ecological restoration. Few species are well adapted to this technique, but even with low initial species diversity direct seeding can be a promising alternative for tropical forest restoration, in case that there are native vegetation remnants nearby which could guarantee further colonization by native species. The correct species selection, including those classified as having high potential, as well as those with large seeds and storage cotyledons, can increase average establishment rates and reduce failure and seed waste chances. Future studies addressing implantation and initial postseeding management are essential to increase the average establishment rates, as well as the number of species that can be recommended for direct seeding. New species combinations, including different functional groups, should be tested in direct seeding projects. Pest outbreaks and diseases and their impacts on the community shall be better explored in future studies, as well as the risks of monodominance in n forest structure, , their impacts and the indicated management actions; and the role of landscape in providing propagule sources for further colonization
In the coming decades, millions of hectares of tropical forests will have to be restored to mitigate the adverse effects of climate change and the loss of biodiversity. To be economically viable, especially in regions with less financial resources, cost-effective restoration methods need to be developed. Currently, the restoration of tropical forests has been made mainly through the planting of seedlings, with higher implantation costs, where direct seeding is expected to be a technique capable of reducing these costs. Several studies have been implemented in recent years in Brazil to test the potential of direct seeding techniques, but the results have never been compiled and analyzed systematically, which could help improving restoration practice. An important gap is also the correct choice of species to be sown, as well as the knowledge of the functional traits that affect the performance of the species in direct seeding techniques. In addition, most studies predominantly include evaluations only during the first years after sowing, with rare studies accompanying the succession process afterwards, in order to analyze whether the basic objectives of ecological restoration have been effectively achieved. The objective of this study is to make a comprehensive analysis of direct seeding for forest restoration in Brazil, to better understand the challenges and success factors, to provide background for improving and upscaling the use of this technique for ecological restoration of tropical wooded ecosystems. We structured this study in three chapters. In Chapter 1, we analyzed the state of the art of direct seeding techniques for restoring Brazilian forests and wooded savannas, through a systematic review, aiming at to identify trends, advances and knowledge gaps, as well as to provide guidelines for future research. In Chapter 2, we surveyed the tree species already tested in direct seeding projects in Brazil and investigated some of their functional traits, aiming at to verify species with best performance and functional traits explaining it. In Chapter 3, we described the temporal changes in forest structure and diversity of forests restored by direct seeding, implemented 25 years ago with low species diversity, to verify whether how similar the structural parameters are with those of a native forest nearby. Our results showed that (1) most of the experiments carried out in Brazil have been undertaken mainly in the Atlantic Forest and Cerrado biomes, implanted in abandoned pastures, during the rainy season, using a low number of species and a high amount of seeds per hectare; (2) mean establishment and growth rates are low for most studies, but mean survival rates are high; (3) a few species are adapted to direct seeding techniques in different regions of Brazil; (4) species with large seeds and storage cotyledons have higher establishment rates when directly sown in field; and (5) forests restored by direct seeding with low species diversity can resemble native forests in richness, diversity and basal area, but not in canopy layering, with low density of individuals in the intermediate layer and low diversity in the intermediate and upper layer. The low establishment and growth rates verified in most of the studies carried out in the country will demands a large amount of seeds to reach density and soil coverage levels similar to those obtained by planting seedlings. Currently, there is no availability of seeds to meet this demand in most of the country’s regions, and the harvesting of large amount of seeds could generate seed loss, which potentially hampers the upscaling of direct seeding for ecological restoration. Few species are well adapted to this technique, but even with low initial species diversity direct seeding can be a promising alternative for tropical forest restoration, in case that there are native vegetation remnants nearby which could guarantee further colonization by native species. The correct species selection, including those classified as having high potential, as well as those with large seeds and storage cotyledons, can increase average establishment rates and reduce failure and seed waste chances. Future studies addressing implantation and initial postseeding management are essential to increase the average establishment rates, as well as the number of species that can be recommended for direct seeding. New species combinations, including different functional groups, should be tested in direct seeding projects. Pest outbreaks and diseases and their impacts on the community shall be better explored in future studies, as well as the risks of monodominance in n forest structure, , their impacts and the indicated management actions; and the role of landscape in providing propagule sources for further colonization
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Palavras-chave
Restauração ecológica, Revisão sistemática, Atributos funcionais, Trajetórias sucessionais, Riqueza de espécies, Ecological restoration, Systematic review, Functional traits, Successional trajectories, Species richness