Life Zones at Mount Roraima

Abstract

The object of the following remarks is to present as complete a picture as possible of the general ecological aspects of the territory in the vicinity of Mount Roraima from data gathered during a few months visit. Since the topography of the area has been dealt with in some detail in another paper (Tate, '30), only a few cursory remarks in that respect are now necessary. Mount Roraima, standing at the junction of the three states, Brazil, Venezuela and British Guiana, is the largest of a group of table mountains derived from identical sandstone and igneous formations. Completely encircled by sheer cliffs twelve hundred feet in height, this imposing block of sandstone reaches an altitude of 86oo feet above sea-level. The area of its rough but approximately level summit, which is almost entirely devoid of soil and vegetation, is some twenty-five square miles. The remnants of a volcanic sill, whose slopes reach outwards and downwards for from five to ten miles, support the sandstone beds of the upper, precipitous part of the mass. On the northeastern side the slopes merge with the forests of the Mazaruni drainage, and to the southwest with upland savannas. Vegetation is rather markedly divisible into groups which frequent summit, slopes and savannas, the distinctions being accentuated by temperature and rain-fall contrasts. The chief environmental divisions and the major factors governing them are shown as follows in order of altitude: Laterite savannas and forests.-The country rock is chiefly composed of iron laterites, the end-product of weathering of diabase. The climate is subtropical to tropical (6 a.m. temperature average 640), with well-marked wet and dry seasons, the latter showery during afternoons. The altitude is from 4000 feet (Paulo and Arabupu) to 4500 feet. Diabase slopes.-The country rock is chiefly diabase, but includes a local sandstone exposure and scattered sandstone talus. The climate is sub-tropical (6 a.m. temperature average 59? at Rondon Camp and 6j? at Philipp Camp), with intense to moderate rain-fall. The altitude at Rondon Camp is 6900 feet, and at Philipp Camp is 5200 feet (the top of the talus slope at the foot of the cliffs is 7400 feet). Plateau.-The country rock is sandstone, mainly without soil. The climate is sub-tropical (morning temperature average 52?), with intense rainfall. The altitude is 86oo feet. The first two zones provide wide ranges of climatic variation which result in a number of sub-regions, often conditioned by exposure, which are dealt with in the course of the paper.