|Biological Monitoring: Signals from the Environment (GTZ, 1991)|
|Concept for a biological monitoring study:|
|2. Study area: the central Andean plateau|
In contrast to the more strongly dissected southern and northern
Andes, the central Andes are characterized by high plateaus at elevations
hetween 3400 and 4300 m above sea level (GOMEZ MOLINA, LITTLE 1981). This
Altiplano or altoplanicie is bounded on the west and east by cordilleras with
peaks rising to between 5000 and over 6000 m above sea level. Mountain ranges
running in a north-south direction divide the Altiplano up into a number of
basins which, depending on their respective climatic conditions, have either a
lake (e.g. Lake Titicaca, Lake Poop6) or salt marshes (e.g. Salar de Uyuni,
Salinas Grandes de Jujuy) in their middle, if they have no outlet to the sea.
Geologically, the study area forms a trough filled with sedimentary material up
to 15 km thick. Its present topographical structure is only about 4 million
years old. It is believed that the Andes were formed, and continue to be formed,
by underthrusting of the Nazca Plate beneath the South American Plate. This
process began about 200 million years ago, accompanied by violent volcanic
activity. As a result, the Paleozoic schists, sandy shales and sandstones were
joined on the surface by andesitic igneous rock rich in deposits of various ores
(AHLFELD 1,967; PUTZER 1976). The volcanic activity took place in several
phases. It was most intense 50 to 60 million years ago, resulting in formation
of the western flank of the Andes. The western cordillera (Cordillera Real) is
younger than the eastern cordillera (Cordillera Oriental), and its surface
structure is still not fully stabilized, as is evidenced by the continued
presence of active volcanos at a number of points. During the Tertiary (about 15
million years ago), siliceous magma was pressed to the surface in the central
and northern parts of the Altiplano and in northern Chile, thus further
increasing the diversity of rock types and parent materials for soil
Descriptions of the climate of the high-elevation regions of the Andes are contained in PROHASKA (1976) and JOHNSON (1976); climatic diagrams and further references are provided by RUTHSATZ (1977, 1983) and ERIKSEN (1986).
The climatic situation of the Andean zone is determined by its location within the zone of subtropical summer rains, enormous land masses extending to considerable elevations, proximity to the Pacific Ocean, and the Humboldt Current. The combined effect of all of these factors is to give the Altiplano a dry and a wet season, a gradient of increasing overall precipitation and number of months with rainfall (an increasingly pronounced continental climate) moving from the northeast towards the southwest, as well as a succession of different climatic zones depending on altitude and direction of exposure. At elevations of 3700 - 3800 m above sea level, the northern portion of the Andean plateau, in the vicinity of Lake Titicaca, receives annual rainfall of between 600 and over 800 mm, most of which falls during 7 to 9 months of the year (subhumid climate). Extensive periods of complete dryness or drought do not occur during the remaining months of the year, however. The mining city of Oruro, at about the same elevation but located further to the south, receives annual precipitation of just under 400 mm - there,7 months of the year can be classified as dry (semiarid climate). In the southern portion of the northern Argentinean puna, the total annual precipitation of about only 100 mm is concentrated in just 2 months (subarid climale).
The flora and fauna of the Andean plateau must be adapted to periodic lac k of water, as well as to a general lack of warmth and below-freezing temperatures which are of brief duration but occur almost daily. The mean annual temperature in these high-elevation zones rarely exceeds 10 °C. Daily temperature fluctuations between +40 °C and -10 °C at ground level are common, even during the growing season. The average daily minimum temperature in Oruro (3708 m above sea level) is -11 °C in July (during the dry season) and +4 °C in January (during the rainy season). The corresponding daily maxima are 14 °C and 19 °C, respectively.
During the rainy season, easterly and northeasterly winds prevail, which can become quite strong and gusty during thunderstorms. During each dry season, several windstorms sweep in from the west, sometimes continuing unabated for days at a time. They blow dust and sand out of the dry river beds, the vegetation-poor salt marsh landscapes, and degraded pastureland, and can sometimes cause the formation of extensive dune fields. They also pick up dust containing ores from the dried-out decantation basins and waste dumps of mining operations, and disperse them over large distances and extensive areas. On a local scale, mountain and valley wind systems play a major role, especially during the dry season. Following a calm period during the morning hours, by noon at the latest strong winds arise that blow up through the valleys and do not lessen in intensity until late in the evening. The reciprocal katabatic winds during the night are much weaker.
The other typical characteristics of high-altitude climates
include increased insolation, especially of the shorter wave lengths
(ultraviolet), reduced air pressure, and thus less available oxygen, carbon
dioxide and nitrogen than in lowland areas.
A detailed discussion of Andean soils in Bolivia is given by COCHRANE (1973), and specific aspects of the soil nutrient cycle are dealt with by SALM (1983a, 1983b). All of the other publications which have appeared on the soils of the central Andes are very general treatises.
The principal parent materials for soil formation in the Andes are volcanic ash and metamorphic and igneous rock. A typical attribute of Andean soils is their short developmental history. A number of factors have a negative effect on soil development, including extreme temperature fluctuations, lengthy dry periods, high wind velocities, and an almost complete lack of closed vegetative ground cover. The diversity of soil types occurring in the region under study is further enhanced by the wide range of elevations and climatic zones. On a regional scale, the relief (affecting water-induced erosion and sedimentation) and the degree of exposure both play an important role.
In very broad terms, the western slopes of the central Andes are covered with desertic soils (Regosols, saline soils), while on the eastern slopes the moister climate has permitted the development of Brown Forest soils. The central portion of the Altiplano exhibits soils which are transitional between these two types. Deposits of volcanic ash are superimposed on the soils of the southern, semiarid part of the central Andes. Azonal soils occur in dune, salt marsh, alluvial plain and bog areas.
The arid climate inhibits leaching of the soils. The result is generally a good supply of bases (WINTERHALDER, THOMAS 1978). Soil humus content is frequently low, however, and the production of inorganic nitrogen and phosphorus compounds available to plants is moderate to low.
Degradation caused by overgrazing has been going on for centuries
everywhere on the Andean plateau. In addition, in arable areas the extensive
fallows contribute to loss of topsoil by sheet erosion.
Scientific descriptions of the vegetation of all or parts of the Andes exist dating back to the beginning of the century. A bibliography of references for the Andean plateau is provided by RUTHSATZ (1983).
In general, the vegetation formations of the Andean plateau can be described as montane semideserts in which the plant cover rarely grows higher than 40 to 60 cm, usually with ground coverage of less than 50% (Fig. 3). With increasing overall precipitation and shorter dry seasons the proportion of bunch grasses and various herbaceous plants grows. With increasing aridity, there is a greater percentage of mostly evergreen, sclerophyllous, small-leaved shrubs. In extremely arid areas there is a conspicuously high proportion of low thorny shrubs, which at the same time usually have a greater tolerance of saline soils.
Above 4500 m, the prevalence of lou'shrahs rapidly decreases. At even higher elevations, the mountain slopes are generally covered with hunch-grass steppes, which are more or less open depending on the supply of water. Above about 4900 m the plant cover becomes patchy, with cushion plants and specially adapted hardy herbaceous plants inhabiting more favorable local sites where warmer temperatures prevail.
Azonal alluvial land and flood plains have a dense gramineous cover which often merges directly into the halophytic plant communities of the extensive, more arid basin landscapes. Dune areas can, depending on their salinity levels, be populated by shrubs or distinctly sclerophyllous grasses. In depressions and on moist slopes which derive their water from springs or seepage, so-called hard-cushion bogs have developed, which are composed not of mosses like many bogs of the temperate zones, but instead of stunted, cushion-forming sedges, rushes and herbaceous plants.
Taller shrubs, trees or even forests are very rare today, but originally were probably much more widely distributed on the semihumid northern plateau and on well-protected sites in the central and southern regions. They have fallen prey to logging and extensive livestock grazing practices. The extent to which anthropogenic influences have shaped the current vegetative cover of the Andean plateau is greatly underestimated by foreign visitors and the population of the region itself, occasionally even being denied completely.
Figure 3: Semidesert brush formations in the puna near Abra Pampa (in the Argentinian province of Jujuy). The dominant plant is the shrub Fubiana densa. In between grow various perennial grasses and other shrubs.
As a consequence of the relatively unfavorable climatic and soil conditions, as well as overutilization of the vegetation by humans, the plants which predominate today exhibit a high degree of resistance against:
- Periods of dryness.
- Low nutrient availability (N, P).
- General lack of warmth.
- The ground frosts of brief duration which occur throughout the year.
- Soil salinity.
- Browsing by animals.
As a consequence of their morphological (small, rolled and folded leaves, low and stunted growth, low shoot/root ratio, etc.), anatomical (leaves with thick cuticles, multiple-layer palisade parenchyma, high stomatal density per unit of surface area, pilosity, etc.) and ecophysiological adaptations, most of the dominant plant species can be described as highly xeromorphic and/or sclerophyllous.