FIG. 27-2 Tropical rain forest on slopes of Mt. Aoyo, Republic 

 of the Congo (courtesy S. Glidden Baldwin). 



of the savanna in Africa probably represents a cli- 

 matic climax, but in those areas with a high pre- 

 cipitation, the savanna may be subclimax due to fire, 

 biotic, or edaphic conditions. This latter is the com- 

 mon situation in South America (Beard 1955). 

 Savannas are communities having a dominant stra- 

 tum oj more or less xeromorphic herbaceous plants, 

 of which grasses and sedges are the principal com- 

 ponents, and with scattered shrubs, trees or palms 

 sometimes present (Beard 1953). The grasses in 

 the savanna may be tall, mid, or short and related 

 taxonomically to those in temperate regions. The 

 grasses and forbs die down in the dry season at the 

 same time that the trees shed their leaves. Sedges 

 are more common than grasses where there is more 

 rainfall. The trees may form a dense narrow stand 

 along rivers — the so-called gallery forest — or may be 

 more or less uniformly scattered through the grass- 

 land to give the appearance of a park or orchard 

 (Burtt 1942). The trees are often thorny or xerophi- 

 lous, crooked in growth, and seldom over 20 m high. 

 There is very little shrubby undergrowth ; lianas 

 and epiphytes are scarce. Probably savanna is in- 

 creasing in extent at the present time because of 

 Human influence in destroying or opening up closed 

 stands of the deciduous and evergreen forests. 



Tropical deciduous forest, including monsoon for- 

 est, is more or less leafless during the dry season, is 

 less lofty than the broad-leaved evergreen forest, but 

 has a higher and more continuous canopy than sa- 

 vanna forest. The forest is rich in woody lianas and 

 iierbaceous epiphytes, but not in woody epiphytes. 

 There are usually two tree strata ; in the upper stra- 

 tum the trees are scattered and strictly deciduous. A 

 proportion of the trees in the lower stratum is ever- 

 green, and the number of these broad-leaved ever- 

 green trees increases in both strata as the climate be- 

 comes more humid and less seasonal. The deciduous 

 forest is less susceptible to burning than is the sa- 

 vanna, while the evergreen forest is practically im- 

 mune. 



At its height, the tropical broad-leaved ever- 

 green forest is nearly completely evergreen, hy- 

 grophilous, 36-55 m (100-180 ft) high, and rich in 

 thick-stemmed woody lianas and in both woody and 

 herbaceous epiphytes. It is commonly called a rain 

 forest because of the continuous high humidity. 

 Seasonal changes are minimal, the aspect being per- 

 petually that of mid-summer. There are usually three 

 strata of trees, one of shrubs and giant herbs, and 

 one of low herbs and undershrubs. 



The trees are extremely varied in size, but the 

 dominants tend to be tall, slender, and unbranched 

 except at tops. The bark is thin, smooth, light- 

 colored, and often covered with lichens. Tree bases 

 are commonly provided with plank buttresses or 

 stilts. Palms and tree ferns may be frequent. There 

 is an extreme variety of species ; for instance, there 

 are seldom less than 15 and sometimes over 30 spe- 

 cies of trees over 30 cm diameter in a single hectare. 

 The Indo-Malayan rain forest is richer in species 

 than either tropical America or Africa ; the African 

 forest, the poorest and most uniform in flora. 



The undergrowth is not a thick jungle as is 

 popularly supposed. Because of the dense shade cast 

 by the several tree strata, shrubs and herbs are scat- 

 tered, there is little or no moss on the forest floor, and 

 one can walk through this forest as easily as through 

 one in a temperate climate. It is only where trees are 

 blown down or the forest is undergoing secondary 

 succession that increased light reaches the ground 

 and jungle growth develops. 



At Barro Colorado Island in the Panama Canal 

 Zone, light intensity under the dense forest canopy is 

 less than one per cent of the direct rays of the sun, 

 but there are numerous sun-flecks where the intensity 

 is greater. Full intensity of sunlight may reach 

 20,000 foot-candles. The temperature near the forest 

 floor out of the sun-flecks is nearly constant. In the 

 tree-tops, temperature rises rapidly from a nightly 

 minimum not very different from that on the forest 

 floor to a maximum in the early afteroon. Air move- 

 ments within the forest are almost nil, and evapora- 



342 Geographic distribution of communities 



