Dinâmica espaço-temporal de fragmentos florestais em duas microrregiões do noroeste do Rio Grande do Sul

Abstract

The understanding of landscape dynamics involves understanding the changes in the use and occupation of the land. In this process, the fragmentation of forests is very common. In thus sense, the objective of this study was to evaluate in a multi-temporal perspective the dynamics of forest fragments of the micro regions of Frederico Westphalen and Três Passos - RS, given the circumstances of the rural population and geomorphological aspects. The study period was from 1985 to 2014, covering an area of 9046.9 km² that is located in the Atlantic Forest Biome. Satellite images were acquired from Landsat 5 (Thematic Mapper sensor) and Landsat 8 (Operational Land Imager sensor) of the dates 1985, 1994, 2005 and 2014. The classification of images was given by manual photo interpretation. The forest fragments were mapped and divided into size categories: very small (<5 ha), small (5 - 10 h), medium (10 - 100 h) and large (> 100 ha). In the socioeconomic analysis, rural population data (1980 a 2010) and vegetal production data (wood for charcoal, firewood and logs) were obtained. For the geomorphological analysis images of the ASTER-GDEM sensor were acquired, and three maps generated: slope, hypsometry and slope aspect. For the analysis of the dynamics of the fragments some landscape metrics were inserted. Regarding tree cover, an increase of 50.4% of the area was identified between 1985 and 2014, while the number of forest fragments had a 56.2% increase. Great part of mapped forest fragments have very small size and are located in undulating and strongly wavy land, at elevations between 300 and 450 m and with its dimensions oriented east and south. A gradual decline in rural population indices of all the constituent municipalities of the two micro regions has been identified, which indicates an increase in the rural exodus. The wood production for coal, firewood and logs presented, in both micro regions, an abandonment or reduction in the production of these derivatives by the method of vegetal extraction, however, there was an increase in production using silvicultural techniques. In almost all municipalities, an increase in forest cover between 1985 and 2014 was observed, in higher or a lower proportion. By crossing the data of rural population with the arboreal covering index of the cities, a tendency for increasing forest cover as the rural population decreases was verified. The forest fragmentation transition maps presented a gain of area of 1258.1 km², loss of 433.4 km² and constant fragmentation of areas of 1081.3 km². According to the calculations obtained by the landscape metrics, the number of fragments and their size showed an increase in course of time, increasingly occupying the landscape area. However, the region's landscape has become very fragmented, probably caused by natural regeneration of the forest. In general, the fragments have regular shapes and complex perimeters. According to the results presented, although the region is highly fragmented, it is going through a forest regeneration process, a fact that reflects the abandonment of areas of difficult agricultural cultivation, the actions of encouragement to preserve native forests, planting forests and the formation of ecological corridors.