ECOLOGICAL SIGNIFICANCE OF SLOPE ORIENTATION IN THE TIJUCA FOREST ECOSYSTEM (RIO DE JANEIRO, BRAZIL)

Abstract

The Tijuca Mountains run in a northeast-southwest direction, most of the slopes are either north-facing or south-facing, Studies of biotic and abiotic factors on the Morro da Boavista (alt. = 716m) have shed light on the ecological significance of north and south slope orientation. In a total of four 1250 m² quadrats (two on each slope), all trees with dbh > 2,5 cm were identified and floristic similarity was calculated using Jaccard's Index. From September 1987 to August 1990, air temperatures were measured on both slopes using max-min thermometers, and litter-layer moisture was determined by the gravimetric method. Species composition of the two slopes differs: 25 % are common to both slopes, while 34% and 41% are found only on north and south slopes, respectively. Similarity between the two slopes was 49.57 %. Maximum air temperatures always occured en the north slope (100 %); mean difference between the maxima of the two slopes was 3.60e. Litter-layer moisture on the south slope was 42 % greather than on the north slope (3.1 days versus 1.9 days - as measured by 50 % moisture loss). Lower humidity of the litter layer on the north slope causes greather susceptibility to fires which, together with high levels of pollutent deposits, contributes significantly to the progressive degradation of forest cover on this slope.