Fatores edáficos que limitam a germinação, o estabelecimento e o crescimento de espécies arbóreas em campos úmidos de Cerrado
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2020-09-17
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Universidade Estadual Paulista (Unesp)
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Os campos úmidos são formações herbáceo-arbustivas que ocorrem sobre solos mal drenados e lençol freático superficial, sendo frequentemente encontrados entre formações savânicas e florestais no Cerrado. Há poucas informações sobre como os fatores edáficos interferem no estabelecimento de espécies arbóreas, impedindo sua colonização por árvores das vegetações adjacentes. Assim, o objetivo desta tese foi elucidar a influência de fatores edáficos dos campos úmidos sobre as estratégias de estabelecimento de espécies arbóreas típicas de formações savânicas e florestais do Cerrado, permitindo a manutenção da fisionomia campestre e, portanto, conferindo resiliência aos campos úmidos. Foram realizados experimentos em laboratório e casa de vegetação para avaliar a tolerância de sementes e de plantas jovens ao alagamento, respectivamente. Além disso, conduzimos experimentos em diferentes áreas de campos úmidos na Floresta Estadual de Assis, Assis, SP, onde introduzimos sementes e mudas das espécies Calophyllum brasiliense (espécie da floresta, exclusiva de ambientes úmidos), Hymenaea stigonocarpa (espécie da savana, exclusiva de solos bem drenados), e Tapirira guianensis (espécie generalista, indiferente à umidade do solo). No capítulo 1, avaliamos o grau de tolerância ao alagamento em condições de laboratório utilizando sementes de doze espécies arbóreas, sendo quatro savânicas, quatro florestais e quatro generalistas. Adicionalmente, avaliamos a emergência de plântulas de duas dessas espécies em condições de campo (campos úmidos). Os resultados obtidos sugerem que sementes de espécies típicas da floresta de galeria e espécies generalistas apresentam maior tolerância ao alagamento do que sementes de espécies savânicas. Entretanto, o nível de tolerância e a capacidade de germinar em condições de alagamento dependem da espécie e do tempo de exposição ao estresse. A alta tolerância ao alagamento em algumas espécies pode favorecer a emergência de plântulas em campos úmidos. No capítulo 2, avaliamos, em casa de vegetação, como o encharcamento e o alagamento do solo do campo úmido interferem na fisiologia fotossintética, crescimento e mortalidade de plantas jovens das espécies C. brasiliense, H. stigonocarpa e T. guianensis. O regime de alagamento do solo afetou de maneiras distintas as espécies estudadas. A espécie C. brasiliense apresentou alta tolerância aos regimes de alagamento, sendo capaz de manter sua fisiologia fotossintética e crescimento mesmo nas condições mais severas de alagamento do solo. A espécie T. guianensis apresentou tolerância intermediária, apresentando redução da capacidade fotossintética apenas em condições mais severas de alagamento, porém sem prejuízos ao crescimento. E a espécie savânica H. stigonocarpa foi menos tolerante às condições de encharcamento e alagamento do solo, com acentuada redução da capacidade fotossintética, do crescimento radicular e maior mortalidade. No capítulo 3, avaliamos como o estabelecimento inicial (estágio de plântula) e tardio (mudas) de árvores no campo úmido é afetado pelo tipo funcional das espécies e como fatores edáficos (profundidade mínima do lençol freático, teor de argila e saturação de bases) interferem em sua fisiologia foliar. Os resultados demonstraram que a capacidade de estabelecimento varia de acordo com o tipo funcional, sendo a espécie de floresta e a generalista aquelas que apresentam maior capacidade de estabelecimento, devido à maior capacidade de estabelecimento de plântulas (C. brasiliense) ou sobrevivência das mudas (T. guianensis). Além disso, mudas de C. brasiliense e T. guianensis apresentaram maiores taxas de assimilação de carbono e melhor balanço hídrico, resultando em maiores taxas de crescimento e sobrevivência em comparação com a espécie de savana H. stigonocarpa. Por outro lado, mudas da espécie savânica apresentaram menores taxas de assimilação de carbono e maior déficit hídrico foliar. Como resultado, as mudas não cresceram e apresentaram menores taxas de sobrevivência. O alagamento do solo gerado pelo lençol freático superficial foi o principal fator edáfico limitante da assimilação de carbono e balanço hídrico de espécies arbóreas no campo úmido, impondo restrições ao crescimento e estabelecimento, especialmente para a espécie da savana.
Neotropical wet grasslands from the Brazilian Cerrado are herbaceous-shrub vegetation, which occur on poorly drained soils with surface groundwater, and frequently found between savanna and forest vegetation. Little is known about how edaphic factors regulate wet grasslands and constrain colonization of tree species from the adjacent arboreal vegetation. Therefore, this thesis aimed to elucidate the effects of edaphic variables of wet grasslands on the establishment of typical tree species from savanna and forest vegetation. Experiments were carried out in the laboratory and greenhouse to assess seed and seedling tolerance to flooding in tree species. In addition, we performed field experiments in different areas of wet grasslands at Assis State Forest, Assis, SP, Brazil, where we introduced seeds and saplings of the species Calophyllum brasiliense (a gallery forest species, typical of humid soils), Hymenaea stigonocarpa (a savanna species, typical of well-drained soils), and Tapirira guianensis (generalist species, indifferent to soil humidity). In chapter 1, we evaluated seed tolerance to flooding of savanna (4 species), forest (4 species) and generalist (4 species) tree species. Additionally, we evaluated seedlings’ establishment of two from these species under field conditions (wet grasslands). Our results suggest that the flood tolerance was low or absent in seeds of the savanna species, while it tended to increase among the generalist and gallery forest species, which were able to keep the germinability capacity after been submerged in water during different periods. Seedling emergence over wet grasslands was, in general, higher for the forest species. However, the level of tolerance and the ability to germinate while submerged depended on the species and time of exposure to stress. The high seed tolerance to flooding favors seedling emergence in wet grasslands. In chapter 2, we evaluated in semi-controlled experiments how soil waterlogging and flooding of grasslands affect photosynthetic physiology, growth and mortality of C. brasiliense, H. stigonocarpa and T. guianensis tree saplings. Soil waterlogging and flooding differentially affected the species under study. The species C. brasiliense presented high tolerance to soil waterlogging and flooding, being able to maintain its photosynthetic physiology and growth even under flooding conditions. The species T. guianensis presented an intermediate pattern of tolerance, exhibiting reduction in photosynthetic capacity only under flooding conditions, but without impacts on growth. The savanna species H. stigonocarpa presented low tolerance to soil waterlogging and flooding; in these conditions, their saplings presented reduction in photosynthetic capacity and root growth, and higher mortality. In chapter 3, we evaluated how tree establishment is affected by the functional type of species in wet grasslands and how edaphic factors interfere in leaf physiology of the species. The results showed that tree establishment varies according to the type of species, with the species C. brasiliense and T. guianensis exhibiting highest establishment potential, due to the greater ability of seedling establishment (C. brasiliense) or sapling growth and survival (T. guianensis). In addition, saplings of C. brasiliense and T. guianensis showed higher carbon assimilation rates and better water balance in plants, resulting in higher saplings growth and survival rates than observed for H. stigonocarpa. On the other hand, saplings of H. stigonocarpa exhibited lower carbon assimilation rates and greater leaf water deficit, leading to inhibition of sapling growth and lower survival rates. Soil waterlogging produced by surface groundwater was the main edaphic factor limiting leaf carbon assimilation and water balance of saplings in wet grasslands, imposing restrictions on plant establishment, especially for the savanna species.
Neotropical wet grasslands from the Brazilian Cerrado are herbaceous-shrub vegetation, which occur on poorly drained soils with surface groundwater, and frequently found between savanna and forest vegetation. Little is known about how edaphic factors regulate wet grasslands and constrain colonization of tree species from the adjacent arboreal vegetation. Therefore, this thesis aimed to elucidate the effects of edaphic variables of wet grasslands on the establishment of typical tree species from savanna and forest vegetation. Experiments were carried out in the laboratory and greenhouse to assess seed and seedling tolerance to flooding in tree species. In addition, we performed field experiments in different areas of wet grasslands at Assis State Forest, Assis, SP, Brazil, where we introduced seeds and saplings of the species Calophyllum brasiliense (a gallery forest species, typical of humid soils), Hymenaea stigonocarpa (a savanna species, typical of well-drained soils), and Tapirira guianensis (generalist species, indifferent to soil humidity). In chapter 1, we evaluated seed tolerance to flooding of savanna (4 species), forest (4 species) and generalist (4 species) tree species. Additionally, we evaluated seedlings’ establishment of two from these species under field conditions (wet grasslands). Our results suggest that the flood tolerance was low or absent in seeds of the savanna species, while it tended to increase among the generalist and gallery forest species, which were able to keep the germinability capacity after been submerged in water during different periods. Seedling emergence over wet grasslands was, in general, higher for the forest species. However, the level of tolerance and the ability to germinate while submerged depended on the species and time of exposure to stress. The high seed tolerance to flooding favors seedling emergence in wet grasslands. In chapter 2, we evaluated in semi-controlled experiments how soil waterlogging and flooding of grasslands affect photosynthetic physiology, growth and mortality of C. brasiliense, H. stigonocarpa and T. guianensis tree saplings. Soil waterlogging and flooding differentially affected the species under study. The species C. brasiliense presented high tolerance to soil waterlogging and flooding, being able to maintain its photosynthetic physiology and growth even under flooding conditions. The species T. guianensis presented an intermediate pattern of tolerance, exhibiting reduction in photosynthetic capacity only under flooding conditions, but without impacts on growth. The savanna species H. stigonocarpa presented low tolerance to soil waterlogging and flooding; in these conditions, their saplings presented reduction in photosynthetic capacity and root growth, and higher mortality. In chapter 3, we evaluated how tree establishment is affected by the functional type of species in wet grasslands and how edaphic factors interfere in leaf physiology of the species. The results showed that tree establishment varies according to the type of species, with the species C. brasiliense and T. guianensis exhibiting highest establishment potential, due to the greater ability of seedling establishment (C. brasiliense) or sapling growth and survival (T. guianensis). In addition, saplings of C. brasiliense and T. guianensis showed higher carbon assimilation rates and better water balance in plants, resulting in higher saplings growth and survival rates than observed for H. stigonocarpa. On the other hand, saplings of H. stigonocarpa exhibited lower carbon assimilation rates and greater leaf water deficit, leading to inhibition of sapling growth and lower survival rates. Soil waterlogging produced by surface groundwater was the main edaphic factor limiting leaf carbon assimilation and water balance of saplings in wet grasslands, imposing restrictions on plant establishment, especially for the savanna species.
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Palavras-chave
Alagamento, Germinação de sementes, Fisiologia fotossintética, Assimilação de carbono, Emergência de plântulas, Flooding, Seed germination, Photosynthetic physiology, Carbon assimilation, Seedling emergence