Dinâmica de habitats na região subtropical da América do Sul: determinantes socioeconômicos e padrões de paisagem em um ecótono floresta-campo

We investigated landscape structure and spatial patterns of natural grasslands and Atlantic Forests in southern Brazil in 1984, 1994, and 2005. The studied region is a highland grassland disjunct region (campos de cima da serra) surrounded by forests. We asked: (a) was there an intensification of habitat loss, fragmentation, and isolation of the Atlantic forest or grassland, within the highland ecotonalzone? (b) What was the degree of connectivity/isolation of the patches? (c) Was there any pattern of grassland loss related to moisture/microtopography? (d) Did agroeconomic activities influence landscape patterns and land use change? We analyzed Landsat remotely sensed information to characterize the land use and land cover, also considering subdivision into eight agroeconomic regions. Results showed that the area occupied by agriculture suffered a strong expansion along with exotic tree plantations, at the expense of grasslands, which suffered marked loss and fragmentation. Grassland destruction was biased towards humid areas in valleys between relief undulations. Forests expanded as a network of small patches in close contact with other land uses. Land cover differed significantly among agroeconomic regions, suggesting that conservation measures need to contemplate both forest and grassland ecosystems and socioeconomic distinctions as well.Keywords: Araucaria forest, fragmentation, Pampa.Investigamos a estrutura da paisagem e padrões espaciais de campos nativos e de florestas Atlânticas no sul do Brasil em 1984, 1994 e 2005. A região estudada é um enclavede campo cercado por florestas. Perguntamos: (a) houve intensificação da perda de habitat, fragmentação e isolamento da floresta Atlântica ou do campo? (b) Qual foi o grau de conectividade/ isolamento dos fragmentos? (c) Houve algum padrão de perda de campo relacionada à umidade/microtopografia? (d) O padrão de atividades agroeconômicas influencia a estrutura da paisagem e mudanças no uso do solo? Analisamos imagens Landsat de sensoriamento remoto para caracterizar o uso e cobertura do solo, considerando também a subdivisão da paisagem em oito regiões agroeconômicas. Os resultados mostraram que a área ocupada pela agricultura sofreu forte expansão, juntamente com plantações de árvores exóticas, em detrimento do campo, que sofreu perda e fragmentação acentuada. A destruição do campo foi mais acentuada em áreas úmidas situadas em vales entre ondulações do terreno. As florestas sofreram expansão como uma rede de pequenas manchas em estreito contato com outros usos do solo. A cobertura do solo diferiu significativamente entre as regiões agroeconômicas, sugerindo que medidas de conservação precisam contemplar ambos os ecossistemas, assim como distinções socioeconômicas.Palavras-chave: floresta com araucárias, fragmentação, Pampa

The structuring role of submerged macrophytes in a large subtropical shallow lake : Clear effects on water chemistry and phytoplankton structure community along a vegetated-pelagic gradient

It is well known that submerged macrophytes exert positive feedback effects that enhance the water transparency, stabilizing the clear-water state in shallow temperate lakes. However, the structuring effect of macrophytes on the food web of subtropical and tropical ecosystems is still poorly understood. In this study we investigated the influence of dense submerged vegetation beds on the water chemistry and phytoplankton structure along a littoral-pelagic gradient of large subtropical shallow lake in southern Brazil. Seasonal monitoring was carried throughout one year following along a submerged vegetated-pelagic transect in order to analyze the effects of macrophyte's coverage (percentage of volume infested- PVI) on the water chemistry and phytoplankton community structure. Clear variations on nutrient concentration and phytoplankton biomass/composition could be observed permanently along the transect. Nutrients as orto-phosphate (PO4 −) and bicarbonate increased linearly towards the pelagic zone, whereas dissolved organic carbon and humic substances decreased linearly as PVI decreased. Concomitantly, a significant increase in the phytoplankton biomass was observed outwards from the submerged vegetation bed. In the vegetated area, small species (C-R strategists), unicellular flagellates were selected; whereas in the pelagic zone, larger (K-selected) species of cyanobacteria occurred, especially representatives of the functional groups M, LO, SN, S1 and K. Such results indicate that the macrophytes and inherent metabolism, such as potential excretion of dissolved organic compounds with allelochemicals and nutrient uptake from water column influence the structure of the phytoplankton community reducing also significantly the biomass of cyanobacteria within the dense submerged vegetated zone. Because of the continuous growth of macrophytes over the year in low latitude systems, their feed-back effect pattern tends to also dictate a different role in ecosystem dynamics and structure of the food web. These findings contribute to the management and conservation of subtropical and tropical lakes