Altitudinal zonation

Altitudinal zonation (or elevational zonation) in mountainous regions describes the natural layering of ecosystems that occurs at distinct elevations due to varying environmental conditions. Temperature, humidity, soil composition, and solar radiation are important factors in determining altitudinal zones, which consequently support different vegetation and animal species. Altitudinal zonation was first hypothesized by geographer Alexander von Humboldt who noticed that temperature drops with increasing elevation. Zonation also occurs in intertidal and marine environments, as well as on shorelines and in wetlands. Scientist C. Hart Merriam observed that changes in vegetation and animals in altitudinal zones map onto changes expected with increased latitude in his concept of life zones. Today, altitudinal zonation represents a core concept in mountain research. Altitudinal zonation (or elevational zonation) in mountainous regions describes the natural layering of ecosystems that occurs at distinct elevations due to varying environmental conditions. Temperature, humidity, soil composition, and solar radiation are important factors in determining altitudinal zones, which consequently support different vegetation and animal species. Altitudinal zonation was first hypothesized by geographer Alexander von Humboldt who noticed that temperature drops with increasing elevation. Zonation also occurs in intertidal and marine environments, as well as on shorelines and in wetlands. Scientist C. Hart Merriam observed that changes in vegetation and animals in altitudinal zones map onto changes expected with increased latitude in his concept of life zones. Today, altitudinal zonation represents a core concept in mountain research. A variety of environmental factors determines the boundaries of altitudinal zones found on mountains, ranging from direct effects of temperature and precipitation to indirect characteristics of the mountain itself, as well as biological interactions of the species. The cause of zonation is complex, due to many possible interactions and overlapping species ranges. Careful measurements and statistical tests are required prove the existence of discrete communities along an elevation gradient, as opposed to uncorrelated species ranges. Decreasing air temperature usually coincides with increasing elevation, which directly influences the length the growing season at different elevations of the mountain. For mountains located in deserts, extreme high temperatures also limit the ability of large deciduous or coniferous trees to grow near the base of mountains. In addition, plants can be especially sensitive to soil temperatures and can have specific elevation ranges that support healthy growth. The humidity of certain zones, including precipitation levels, atmospheric humidity, and potential for evapotranspiration, varies with elevation and is a significant factor in determining altitudinal zonation. The most important variable is precipitation at various elevations. As warm, moist air rises up the windward side of a mountain, the air temperature cools and loses its capacity to hold moisture. Thus, the greatest amount of rainfall is expected at mid-latitudes and can support deciduous forest development. Above a certain elevation the rising air becomes too dry and cold, and thus discourages tree growth. Even though rainfall may not be a significant factor for some mountains, atmospheric humidity or aridity can be more important climatic stresses that affect altitudinal zones. Both overall levels of precipitation and humidity influence soil moisture as well. One of the most important factors that control the lower boundary of the Encinal or forest level is the ratio of evaporation to soil moisture. 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 subapline 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. 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. 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. 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. This effect predicts that zonation of rain forests on lower mountains may mirror the zonation expected on high mountains, but the belts occur at lower elevations. A similar effect is exhibited in the Santa Catalina Mountains of Arizona, where the basal elevation and the total elevation influence the elevation of vertical zones of vegetation. In addition to the factors described above, there are a host of other properties that can confound predictions of altitudinal zonations. These include: frequency of disturbance (such as fire or monsoons), wind velocity, type of rock, topography, nearness to streams or rivers, history of tectonic activity, and latitude.