Effects of Elevation on Species Diversity

A variety of terms refer to forests of high elevation in the tropics. Cloud forests include all forests in the humid tropics that are frequently covered in clouds or mist, thus receiving humidity through the capture or condensation of water droplets (horizontal precipitation) which influences the hydrological regime, radiation balance, and several other climatic, edaphic, and ecological parameters (Stadtmuller 1987). Cloud forests may include lower montane (about 1,000-2,000 m), montane (2,000-3,000 m), montane thicket, and finally elfin woodland, until the treeline at about 4,000 m, depending on latitude. With higher elevation, the forest decreases in stature and in number of species. For example, in lowland Malaya (about 400 m maximum elevation) there are nine distinct forest types according to the predominance of one species or another; higher up, there are six forest types, and the next zone up has only two forest types (Jacobs 1988).

In general, cloud forests are found at elevations higher than 1,500 m above sea level; however, on some islands like Puerto Rico and Jamaica and in some isolated mountains (e.g. the Macuira Mountain in Colombia, and in the Santa Ana Mountain in Venezuela), cloud forests can be found at a much lower elevation (Grubb 1977). These elevations may vary depending on the "Massenerhebung effect", the compression and lowering of life zones on small land masses as compared to continents (Grubb 1977), due to a faster saturated lapse rate (moist adiabatic rate). The moist adiabatic lapse rate is the rate at which saturated air cools as it is lifted up by the wind or the air currents. As moist air is lifted up and the temperature decreases, the air becomes saturated and cloud formation occurs. The moist adiabatic lapse rate is lower when air encounters large land masses in part because large land masses radiate more heat than small masses. For example, on Caribbean islands, the lapse rate is high because land masses are small. In contrast, along the west coast of South America, air must rise to a higher level before dew point saturation, due to the large mass of the Andes.

On the island of Puerto Rico, a wide variety of ecosystems exist each with distinct species. On an elevational gradient, species diversity is greatest at 700 m, at the transition between the lower and upper montane forests where cloud condensation is common. Diversity decreases with increasing altitude through palm forest to the cloud forests which begin here at only 1,000 m (Lugo and Scatena 1995; Weaver 1995).

Along an elevational gradient beginning on the west coast of South America and up the Andes, we would expect a series of plant communities as follows (van der Hammen 1974): savanna and dry tropical woodland with low species diversity; lower tropical forest with increased precipitation and increased diversity; sub-Andean and Andean forest, where temperature, mois ture, and soils are optimal for plant growth and species diversity is highest; sub-paramo and paramo where temperature limits metabolic rates and lowers decomposition, resulting in decreased nutrient availability, which decreases species diversity; and finally perennial snow where plants are limited to a few species of algae and lichens. Moisture condensation commonly occurs between 2,000 and 3,500 m (Lauer 1993), where cloud forests typically begin. The gradient down the eastern slope is similar, except in the lower reaches where low soil fertility rather than lack of moisture results in lower species diversity (soils in the Amazon basin are highly weathered and low in nutrient elements when compared to younger soils in the Andes). Each forest type along the elevational gradient has its own complement of species, and the nature and the diversity of these species characterize each forest type.

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