lower boundary of the Encinal or forest level is the ratio of evaporation to soil moisture. Soil composition The nutrient content of soils at different elevations further complicates the demarcation of altitudinal zones. Soils with higher nutrient content, due to higher decomposition rates or greater weathering of rocks, better support larger trees and vegetation. The elevation of better soils varies with the particular mountain being studied. For example, for mountains found in the tropical rainforest regions, lower elevations exhibit fewer terrestrial species because of the thick layer of dead fallen leaves covering the forest floor. At this latitude more acidic, humose soils exist at higher elevations in the montane or subalpine levels. In a different example, weathering is hampered by low temperatures at higher elevations in the Rocky Mountain of the western United States, resulting in thin coarse soils. Biological forces In addition to physical forces, biological forces may also produce zonation. For example, a strong competitor can force weaker competitors to higher or lower positions on the elevation gradient. The importance of competition is difficult to assess without experiments, which are expensive and often take many years to complete. However, there is an accumulating body of evidence that competitively dominant plants may seize the preferred locations (warmer sites or deeper soils). Two other biological factors can influence zonation: grazing and mutualism. The relative importance of these factors is also difficult to assess, but the abundance of grazing animals, and the abundance of mycorrhizal associations, suggests that these elements may influence plant distributions in significant ways. Solar radiation Light is another significant factor in the growth of trees and other photosynthetic vegetation. The earth's atmosphere is filled with water vapor, particulate matter, and gases that filter the radiation coming from the sun before reaching the earth's surface. Hence, the summits of mountains and higher elevations receive much more intense radiation than the basal plains. Along with the expected arid conditions at higher elevations, shrubs and grasses tend to thrive because of their small leaves and extensive root systems. However, high elevations also tend to have more frequent cloud cover, which compensates for some of the high intensity radiation. Massenerhebung effect The physical characteristics and relative location of the mountain itself must also be considered in predicting altitudinal zonation patterns. The Massenerhebung effect describes variation in the tree line based on mountain size and location: mountains surrounded by large ranges will