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Publicação
História da conservação dos solos na Ilha de Porto Santo, Madeira
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Resumo(s)
Os solos são formados ao longo de extensos períodos de tempo, sendo entendidos
como “…um dos recursos mais importantes da Biosfera e do Património Natural para
toda a Humanidade…” (Meneses, 2011, p. 5). Apresentam funções diversas a nível
ambiental, económico, social e cultural, essenciais para a vida. A erosão hídrica é um
dos processos mais importantes de degradação do solo e um risco ambiental crítico na
actualidade a nível mundial. O facto de em Portugal este processo ser muito visível nos
finais do século XIX e início do século XX, fez com que, visando a recuperação dos
solos, fossem levados a cabo uma série estudos e de trabalhos de correcção torrencial,
com o objectivo de travar ou mitigar o fenómeno.
Na ilha de Porto Santo, o processo erosivo é bastante intenso, dado o reduzido
número de áreas com vegetação autóctone e áreas reflorestadas, para além da aridez
do seu clima e a própria natureza dos terrenos, factores responsáveis pela pobreza dos
seus solos. A precipitação é directamente responsável pelo regime torrencial das linhas
de água da ilha e, igualmente, pelos processos de erosão hídrica.
Por outro lado, desde que foi descoberta até meados do século XX foi-se assistindo a
uma destruição progressiva do coberto vegetal das serras do Porto Santo, por ações
antrópicas. Deste modo, a partir de 1952, a Circunscrição Florestal da Madeira
desenvolveu um importante trabalho de promoção e defesa do património natural da
ilha, com vários tipos de intervenção, entre os quais a reflorestação e a realização de
obras de correcção torrencial e de defesa do solo contra a erosão.
As obras de correcção torrencial tiveram uma grande expressão na ilha, tendo sido
efectuadas ao longo da maioria dos seus cursos de água. Foram intervencionados
dezoito ribeiros com a construção de barragens (maioritariamente de alvenaria ou
lajes), estacadas, motas, plantação de árvores, entre outros. Tendo em conta todo este
trabalho, seria de esperar que quase meio século depois, essas obras continuassem a
desempenhar a sua função, notando-se uma melhoria na regularização dos caudais
dos cursos de água e da morfologia das vertentes com a diminuição de formas
resultantes da erosão.
A análise de fotografias do “antes” e do “depois” desses trabalhos, bem como da sua
área envolvente revelaram que apesar de um bom número das barragens construídas
se encontrarem ainda hoje em bom estado de conservação, muitas outras estão a
precisar de manutenção, podendo a sua função estar em causa. A reflorestação,
apesar de aparentemente bem-sucedida na maioria dos Picos, encontra-se ainda em
falta noutros, entre os quais na encosta Oeste do Pico da Ana Ferreira, no Cabeço do
Zimbralinho e na Portela, sendo nestes locais que se observam os melhores exemplos
dos efeitos da erosão hídrica. Deste modo, sugere-se uma manutenção continuada a
estas obras e a continuação do revestimento florestal das áreas mais susceptíveis ao
fenómeno erosivo, para que os solos da ilha possam recuperar efectivamente.
Soils are formed through long periods of time, understood as “one of the most important resources of the Biosphere and the World’s Natural Heritage” (Meneses, 2011, p. 5). It has environmental, economic, social and cultural functions, vital for life. Erosion by water is one of the most important processes of soil degradation and a critical threat to the environment nowadays, worldwide. The great impact of this process in Portugal at the end of the Nineteenth century and the beginning of the Twentieth century led to the development of several studies and torrent control works, with the goal of stopping/mitigating the phenomenon. In the Porto Santo Island the erosive process is very intense due to the reduced number of areas with native vegetation and reforested areas, besides the arity of its climate and the very nature of its land, responsible for the poverty of its soils. The rainfall is directly responsible for the torrential regime of the island's water lines and therefore, for the water erosion processes. On the other hand, since its discovery until the mid-twentieth century a progressive destruction of the vegetation cover of Porto Santo could be seen, by anthropogenic factors. For that reason, from 1952 the Circunscrição Florestal da Madeira developed an important work to promote and defend the natural heritage of the island, with various types of intervention, including reforestation and torrent control and soil defence against erosion works. Torrent control had a great expression on the Island, having been performed throughout the majority of its water courses. Within this, stand out eighteen intervened rivers by building dams (mostly of brick or flagstone), fences and tree planting, among others. Taking into account all this work, one would expect that almost half a century later, these works would continue to perform their function, noting an improvement of the system of the water courses and the morphology of the slopes with a decrease of the resulting forms of erosion. The analysis of current photographs and the comparison of photographs of the "before" and "after" of these works and its surroundings revealed that, although a good number of dams are still in good conditions nowadays, many others are in need of maintenance, with its function in risk of being affected. Reforestation, despite apparently successful in most peaks, is still missing in others, including the West slope of Pico da Ana Ferreira, Cabeço do Zimbralinho and Portela, being these places the ones we can observe the best examples of the effects of water erosion. Thus, it is suggested a continued maintenance of these works and the continuation of the lining of the forest areas most susceptible to the erosion phenomenon, so that the quality of the island's soils can recover effectively.
Soils are formed through long periods of time, understood as “one of the most important resources of the Biosphere and the World’s Natural Heritage” (Meneses, 2011, p. 5). It has environmental, economic, social and cultural functions, vital for life. Erosion by water is one of the most important processes of soil degradation and a critical threat to the environment nowadays, worldwide. The great impact of this process in Portugal at the end of the Nineteenth century and the beginning of the Twentieth century led to the development of several studies and torrent control works, with the goal of stopping/mitigating the phenomenon. In the Porto Santo Island the erosive process is very intense due to the reduced number of areas with native vegetation and reforested areas, besides the arity of its climate and the very nature of its land, responsible for the poverty of its soils. The rainfall is directly responsible for the torrential regime of the island's water lines and therefore, for the water erosion processes. On the other hand, since its discovery until the mid-twentieth century a progressive destruction of the vegetation cover of Porto Santo could be seen, by anthropogenic factors. For that reason, from 1952 the Circunscrição Florestal da Madeira developed an important work to promote and defend the natural heritage of the island, with various types of intervention, including reforestation and torrent control and soil defence against erosion works. Torrent control had a great expression on the Island, having been performed throughout the majority of its water courses. Within this, stand out eighteen intervened rivers by building dams (mostly of brick or flagstone), fences and tree planting, among others. Taking into account all this work, one would expect that almost half a century later, these works would continue to perform their function, noting an improvement of the system of the water courses and the morphology of the slopes with a decrease of the resulting forms of erosion. The analysis of current photographs and the comparison of photographs of the "before" and "after" of these works and its surroundings revealed that, although a good number of dams are still in good conditions nowadays, many others are in need of maintenance, with its function in risk of being affected. Reforestation, despite apparently successful in most peaks, is still missing in others, including the West slope of Pico da Ana Ferreira, Cabeço do Zimbralinho and Portela, being these places the ones we can observe the best examples of the effects of water erosion. Thus, it is suggested a continued maintenance of these works and the continuation of the lining of the forest areas most susceptible to the erosion phenomenon, so that the quality of the island's soils can recover effectively.
Descrição
Palavras-chave
Correcção torrencial Ilha de Porto Santo, Ribeiros, Erosão hídrica Ilha de Porto Santo Ribeiros Torrentes Porto Santo Island Streams Torrents Torrent control Water erosion