Woody encroachment due to changes in climate or in the disturbance regimes (fire and herbivory) has been observed throughout the savannah biome over the last century with ecological, hydrological and socioeconomic consequences. We assessed changes in tree density and basal area and estimated changes in rain interception by the canopies across a 5-year period over a biomass gradient in Cerrado vegetation protected from fire. We modelled throughfall, stemflow and net rainfall on the basis of tree basal area (TBA). Tree density increased by an average annual rate of 6.7%, basal area at 5.7% and rain interception by the canopies at 0.6% of the gross rainfall. Independent of the vegetation structure, we found a robust relationship of 0.9% less rainfall reaching the ground as TBA increases by 1 m 2 ha 21 . Increases in tree biomass with woody encroachment may potentially result in less water available for uptake by plants and to recharge rivers and groundwater reserves. Given that water is a seasonally scarce resource in all savannahs, woody encroachment may threaten the ecosystem services related to water resources.This article is part of the themed issue 'Tropical grassy biomes: linking ecology, human use and conservation'.
Stemflow is a hydrological flux that largely influences the input of rainwater to the soil and its spatial distribution. Here, we aimed to identify the factors influencing this hydrological flux by quantifying the stemflow of 129 trees in the Brazilian savannah. The attributes of individual trees were investigated as predictor variables influencing either stemflow volume or tree efficiency in generating stemflow-proportion of rainfall over the canopy flowing down the stem. The volume of stemflow was higher in trees that were large, had perennial leaves, and exhibited monopodial growth. However, the efficiency of a tree in generating stemflow was negatively correlated to its dimensions. The most efficient trees had a smooth bark surface, monopodial growth, and straight stems. There was large variation in efficiency among individuals, which is related to the high diversity of attributes among the studied trees. The small size of Cerrado trees explains the lower stemflow volume in comparison with trees in forest ecosystems. The efficiency observed in the Cerrado trees was also lower when compared to trees from other ecosystems, even in arid environments.
-(Ecosystem restoration and water yield). Although there is a widespread belief that planting forests increases water yield and rainfall, science has demonstrated that, at the scale of a watershed, the proportion of annual rainfall that flows through the river or recharges groundwater reserves is inversely proportional to the tree biomass and that increasing forests does not increases rain in the same watershed. It follows, therefore, that open ecosystems such as grasslands and savannas are more efficient than forests in water yield and also that restoration through planting trees cannot increase the amount of water produced by a watershed, in spite of improving water quality and regulation of hydrological flows. Besides preserving natural ecosystems, adopting soil conservation and suitable cultivation practices that increase the infiltration of rain throughout the catchment area can have greater contribution than planting forests if the goal is to feed the river flow even in dry months, ensuring the ecosystem services depending on that water. Keywords: ecological restoration, ecosystem services, hydrology, land use, water yield RESUMO -(A restauração de ecossistemas e a produção de água). Embora exista uma crença generalizada de que plantar florestas aumenta a água dos rios e a quantidade de chuva, a ciência tem demonstrado que, na escala de microbacia hidrográfica: 1) a proporção da chuva anual que se torna recurso hídrico renovável é inversamente proporcional à biomassa arbórea e 2) mais floresta não significa mais chuva na mesma bacia. Ecossistemas naturais abertos como campos e savanas são, portanto, mais eficientes que florestas como geradores de recurso hídrico renovável e plantar árvores não aumenta a produção de água, embora melhore sua qualidade e contribua para regulação da vazão ao longo do ano. Uma vez preservados os ecossistemas naturais, adotar práticas de cultivo e conservação do solo que aumentem a infiltração da chuva em toda a bacia, trará contribuição maior do que o plantio de florestas para assegurar a vazão dos rios mesmo nos meses de estiagem, proporcionando os serviços ecossistêmicos dependentes dessa água. Palavras-chave: ciclo hidrológico, hidrologia, restauração, serviços ecossistêmicos, uso da terra
1. Forest encroachment into savannas is a widespread phenomenon, the rate of which may depend on soil conditions, species composition or changes in stand structure. As savanna specialist trees are replaced by generalist species, rates of stand development may increase. Because generalists can persist in forests, they are likely to grow more quickly and survive longer in dense stands, compared to savanna specialists. Furthermore, the faster growth rates of generalists may allow them to overtop and outcompete savanna specialists, causing rapid species turnover.2. We measured growth and survival of 6,147 individuals of 112 species of savanna and generalist tree species over a period of 10 years in an ecological reserve in Assis, São Paulo State, Brazil. We modelled growth and mortality as a function of soil texture and nutrients, tree size, competitive neighbourhood, and membership in savanna or generalist (species which can persist in forests and savannas) functional groups.3. Tree growth and survival was strongly influenced by competition, as estimated by the basal area of trees taller than a focal tree. At the stand level, savanna species are unable to contribute basal area growth in closed stands, while generalist species continue to increase in basal area even at high stand basal area. This phenomenon is driven by differences in growth and mortality. Generalists grew faster than savanna species, both in height and diameter. This difference in growth rates led to savanna species becoming suppressed more rapidly than generalists. When suppressed, savanna species were more than twice as likely to die than were generalists. Soils had inconsistent and mostly weak effects which were difficult to separate from gradients of stand structure. Synthesis.We demonstrate that the presence of generalist trees accelerates the rates of basal area accumulation due to their greater growth rates and tolerance of shading. Generalists outcompete savanna trees by growing faster in the open and overtopping savanna specialists. Due to the slow growth and high mortality of savanna species in the shade, they are unable to form closed-canopy stands.Accounting for differences among functional types and development of vegetation structure is critical for modelling forest encroachment.
Competition for soil water is one of the major processes that drive the assembly of plant communities, particularly in regions subjected to long dry seasons. The relationship between plant density and soil water competition has strong, but poorly understood, practical implications for population dynamics, restoration and conservation. We planted seedlings of the pioneer legume tree Senegalia polyphylla in containers and assessed the effects of plant density on their resistance to water stress. We completely eliminated the above-ground parts (stem and shoots) of randomly selected plants at 60-days post-planting in order to create three different densities of seedlings. We saturated the soil of the remaining seedlings and then deprived them of additional water until all seedlings died. We assessed the time elapsed from water suppression to wilting and death of individual plants. We found a strong negative relationship between plant density and time to wilting or death. Seedlings survived water stress 80% longer when density was reduced by two-thirds. Planting in lower densities or reducing density by thinning should be considered as strategies to enhance resistance to water stress in drought-prone regions.
Agradeço, encarecidamente, à orientadora, Giselda Durigan, pelo incentivo, confiança, sugestões, conselhos e, acima de tudo, encorajamento e apoio.Ao Instituto Florestal, pela oportunidade.Aos funcionários da Floresta Estadual de Assis, seu atual chefe, Osmar Vilas Boas e exchefe, José Carlos Molina Max, que, apesar dos recursos sempre escassos, nunca negaram apoio a esta pesquisa.Ao Professor Walter de Paula Lima, um agradecimento muito especial por ter sido o grande mestre de gerações de hidrólogos no fabuloso mundo da Hidrologia Florestal.Ao Programa de Pós Graduação da Escola de Engenharia da USP de São Carlos e aos secretários Claudete, Nelsinho e José Luiz, sempre solícitos, pelo enorme apoio com os trâmites burocráticos.Aos membros da banca examinadora, pela disposição em contribuir na avaliação desta tese.Ao esposo Wilson, que me ensinou que as barreiras existem para serem vencidas. Aos colaboradores incansáveis Edison 2013Apesar da grande área ocupada pelo Cerrado no território brasileiro e de sua importância ecológica e hidrológica, ainda é desconhecida a repartição da água das chuvas no gradiente fisionômico da vegetação nesse bioma. No presente estudo quantificamos a interceptação da água das chuvas e a umidade do solo, para estabelecer correlações entre as características estruturais da vegetação, representadas principalmente pela área basal, e os dados hidrológicos. Para tanto, quantificamos a transprecipitação e o escoamento pelo tronco das árvores, em 15 parcelas representativas do gradiente fisionômico entre o cerrado típico e o cerradão, localizados na Estação Ecológica de Assis (EEcA), SP, para obtenção da proporção da precipitação efetiva que atravessa o dossel e chega à superfície do solo, em comparação com a precipitação total, quantificada a céu aberto. Os indivíduos lenhosos em cada parcela com DAP ≥ 5 cm foram identificados e medidos. A quantidade de transprecipitação foi tanto maior quanto menos complexa a estrutura da vegetação, sendo também influenciada pelas características da chuva incidente. A proporção da transprecipitação também foi fortemente influenciada pela estrutura da vegetação, porém, não foi detectada influência das características das chuvas. Assim, foi possível concluir que, em ambiente de Cerrado, é a estrutura da vegetação que influencia de forma decisiva a proporção de água que alcançará o solo sob forma difusa. A facilidade em captar a água da chuva e escoá-la pelo tronco variou em função de atributos da árvore, sendo que árvores mais altas e com copa volumosa escoaram maior quantidade, enquanto árvores de crescimento monopodial, tronco ereto e reto e copa compacta têm maior eficiência de captação. O agrupamento das parcelas em diferentes fisionomias resultou em diferenças significativas nas taxas de precipitação efetiva e interceptação, indicando que elas devem ser caracterizadas hidrologicamente como ambientes distintos, onde há gradação na quantidade de água que chega ao solo em função do gradiente de biomassa. A transição da entrada de água ao solo no gradiente...
Cartas da rede hidrográfica elaboradas por instituições governamentais têm sido utilizadas para delimitação de Áreas de Preservação Permanente (APPs) no Brasil, mas são frequentes os problemas decorrentes de discrepâncias entre essas cartas e a verdade terrestre. Para caracterizar e quantificar essas discrepâncias, quer sejam metodológicas ou decorrentes da escala dos mapas ou de mudanças temporais, mapeamos as nascentes e cursos d’água em cinco imóveis rurais (totalizando 10.860 ha, nos municípios de Brotas e Ribeirão Bonito, SP) e comparamos com a rede hidrográfica regional mapeada pelo IBGE e pelo IGC. Verificamos, para a região de estudo, superdimensionamento de 157% no número de nascentes e de 83% na extensão dos cursos d'água pelo IBGE e de 209% no número de nascentes e de 84% na extensão dos cursos d'água pelo IGC. Consequentemente, ambos os mapas resultam em superdimensionamento também das APPs. Consideramos, portanto, que tais cartas podem ser utilizadas como referência aproximada da rede hidrográfica, mas não podem oferecer embasamento para nenhuma decisão jurídica ou de planejamento ambiental sem que sejam devidamente corrigidas por uma verificação em campo.
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