The Ecological Effects of Microclimates of Mountain Slopes, High Altitude, Valley and City


Essay on The Ecological Effects of Microclimates of Mountain Slopes, High Altitude, Valley and City !

The influence of different ecolo­gical media and of different abiotic environmental factors on the living organisms. All these factors form the macroclimate which is measured by the meterologists. But the climate in which animals actually live is very much different from the gross environmental conditions.

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This immediate environment of animal is called effective climate or microclimate and relates to such of the habitats of ani­mals as those living in the soil within crevices of barks of trees, inside sheaths of grasses, within burrows, in ant nests, beneath logs of wood or rock, within caves, on vegetation in sand dunes, with­in plant galls and a variety of habitats which are occupied by endoparasites.

Woodlice, centipedes and millipedes, for example, re­main restricted to dark microhabitats during the daytime to pre­vent rapid water loss or transpiration. When temperature falls and relative humidity increases at night, these animals emerge of the daytime microhabitats.

Causes of Fluctuations of Environmental Factors in Microclimates:

Thus heat, moisture, air movement and light, all vary in microclimates of a given area. Such microclimatic variations result from differences in slop, soil and vegetation. On a summer after­noon the temperature under calm and clear sky may be 82°F at six feet, the standard level of temperature recording.

But on or near the ground—at the 2-inch level—the temperature may be 5° lower than the standard level. The main reason for the great differences between the ground and six-foot level is solar radiation. During the day the soil, the active surface, absorbs solar radiation, which comes in short waves as light and radiates back as long waves to heat a thin layer above.

Since air flow at ground level is almost non-existent, the heat radiated from the surface remains close to the ground. Temperature decreases sharply in the air above this layer and in the soil below. Thus, on a sunny but chilly spring or late winter day, one can walk in muddy ground while the air about him is cold.


Fig. 11.22. Idealized temperature profiles in the ground and air for various times of day and the transport of heat by convection (after Smith, 1977).

Further, the heat absorbed by the ground during the day is reradiated by the ground at night. This heat is partly absorbed by the water vapour in the air above. The drier the air, the greater the outgoing heat and stranger the cooling of the surface of the ground and the vegetation. Consequently the ground and the vege­tation are cooled to the dew point, and water vapour in the air may condense as dew. After a heavy dew a thin layer of chilled air lies over the surface, the result of rapid absorption of heat in the evaporation of dew.

Likewise, vegetation influences the microclimate of an area, especially near the grout d by altering wind movement, evaporation, and moisture and soil temperatures. Temperatures at the ground level under the shade are lower than in places exposed to sun and wind. On fair summer days a dense forest cover can reduce the daily range of temperatures at one inch by 20°F to 30°F, compared with the temperatures in the soils of bare field.

Vegetation also reduces the steepness of the temperature gradient and influences the height of the active surface, the area that intercepts the maximum quanti­ty of solar insolation. In the absence of or in the presence of very thin vegetation, temperature increases sharply near the soil; but as the plant cover increases in height and density; the leaves of the plants intercept more solar radiation (Fig. 11.23). The plant crown then become the active surface and raise it above the ground. Conse­quently daytime temperatures are higher just above the dense crown surface and lowest at the surface of the ground.


Fig. 11.23. Vertical temperature gradients at mid-day in a cornfield from the time of seeding to the time of harvest. Note the increasing height of the active surface (after Smith, 1977).

Air movements too are reduced to convection and diffusion within dense vegetation. There exists complete calm at ground level in dense grass and low plant cover. This calm is an outstan­ding feature of the microclimate near the ground, since it influen­ces both temperature and humidity and creates a favourable envi­ronment for insects and other animals.

Even humidity differs greatly from the ground top. Since evaporation takes place at the surface of the soil or at the active surface of plant cover, the vapour content (absolute humidity) decreases rapidly from a maximum at the bottom to atmospheric equilibrium above.

Relative humidity increases above the surface, since actual vapour content increases only slowly during the day, while the capacity of the heated air over the surface to hold mois­ture increases rather rapidly.

Topography and Microclimate:

Topography and microclimate bear intimate inter-relation- ship. There exist great microclimatic differences between eastern and western or northern and southern slopes of the mountains or hills. For example, south-facing slopes receive the most solar energy, which is maximal when the slope grade equals the sun’s angle from the zenith point. North-facing slopes receive the least energy, especially when the slope grade equals or exceeds the angle of sun ray inflection.

At latitude North 41° mid-day insolation on a 20° slope is on the average, 40 per cent greater on south slopes than on north slopes during all seasons. This has a marked effect on the moisture and heat budget of the two sites. High tempera­tures and associated low vapour pressures induce evapotranspiration of moisture from the soil and plants.

The evaporation rate is often 50 per cent higher, the average temperature higher, the soil moisture lower and the extremes more variable on south slopes. Thus the microclimate ranges from warm, xeric conditions with wide extremes on the south slope to cooler, less variable, more mesic conditions on the North Slope.

Consequently, western or southern and eastern or northern slopes exhibit different distribu­tion pattern of plants as exhibited by Himalaya. The intensity of slopes also cause into microclimate variations.

Like the mountain slopes, the large scale altitudinal differences greatly affect the distribution of both plants and the animals. The effect of altitude is due to change in climatic conditions. With the rise in the altitude, the temperature decreases, rainfall increases and wind velocity also increases and all of these affect the deve­lopment of soil and vegetation. Due to low temperature and high rainfall the organic matter content of the soil increases at higher altitudes, with an increase in soil nitrogen and a decrease in its pH values.

Generally, xerophytes are more common at lower alti­tudes and chamaeophytes occur at higher altitudes. At higher altitudes the freezing temperature results in a perennial snow covers. These micro-climatic changes are reflected in altitudinai zonation of plants and animals such as in Himalaya Moun­tain the successive zones of plants from base upwards are tropical and subtropical, temperate, and alpine types of vegetation.

The altitude at which the tree growth passes into shrub phase is called timber line or tree line. The tree line zone is char­acterized by following aspects which affect vegetation: lack of soil, desiccation of leaf in jolt weather, short growing season, winter drying, excessive snow, high wind velocity, rapid heat loss at night, and drought and high soil temperatures in the day.

Similar to vegetation zonation, zonation of animals is also observed with the altitude. Yaks and hares live at altitudes upto 5500 metres and insect dominates still higher altitude. Other ani­mals of high altitude include the pelobatid frog, snow partridge, snow leopard, vole, Tibetan sheep, ibex, wild goat and large variety of insect’s mostly coleopterans.

The environment of high altitude is also peculiar in having low air density, low oxygen content, high ozone content, greater insolation and high intensity of ultra-violet radiations. These environmental aspects make the life difficult. The ultra-violet solar radiations are largely responsible for the dwarfing of shrubs.

The high humidity, lower temperature and the ultra-violet radi­ations result in dark pigmentation of high altitude insects (Mani, 1968). The high wind velocity at high altitudes is responsible for the dwarfing of ties and sometimes the formation of flag trees. As the winds blow in one direction, the growth of branches of trees on the side facing the wind is stopped and the development of branches only on one side gives the flag appearance of the trees.

Microclimate of Valleys:

The widest climatic extremes occur in valleys and pockets, areas of convex slopes and low concave surfaces. All these places have much lower temperatures at night, especially in winter, and much higher temperatures dung the day, especially in summer and a higher relative humidity.

Protected from the circulating influences of the wind, the air of these places becomes stagnant. It is heated by insolation and cooled by terrestrial radiation, in sharp contrast to the wind-exposed, well-mixed air layers of the upper slopes. In the evening cool air from the uplands flows down the slope into the pockets and valleys to form a lake of cool air.

Often when the warm air in the valley comes in contact with the inflowing cold air, the moisture in the warm air may condense as valley fog (Fig 11.24). Such a distinct microclimate of valleys influence their plants and animals.

Microclimate of the City:

The climate of rural areas markedly differs with the climate of the urban area or city (Table 11.6). The urban microclimate is a product of the morphology of the city and the density and activity of its occupants. In the urban complex stone, asphalt and concrete Pavements and buildings with their high capacity for absorbing and reradiating heat replace natural vegetation with its low heat con­ductivity.

Rainfall on impervious surfaces of city is drained away as fast as possible, reducing evaporation. Metabolic heat from masses of people and their domesticated animals and waste heat from buildings, industrial combustion and vehicles raise the temper­ature of the surrounding air. Industrial activities, building cons­truction, power production and vehicles pour water vapour, gases and particulate matter into the atmosphere of the city in great quantities.

The effect of this storage and reradiating of heat is the formation of a heat island above cities, large and small in which the temperature may be 6° to 8°C higher than the surrounding country­side (Landsberg, 1970).


Fig. 11.24. Topography plays an important role in the formation and intensity of night-time inversions At night air cools next to the ground, forming a weak surface inversion in which temperature increases with height. As the cooling continues during the night, the layer of cool air gradually deepens. Simultaneously cool air descends down-slope. Both of these cause the inversion to become deeper and stronger. In moun­tain areas the top of the night inversion is usually below the main ridge. If air is cool and moist, fog may form in the valley. Smoke released in such inversions will rise only until its temperature equals that of the surrounding air. Then smoke flattens out and spreads horizontally just below the thermal belt (after Smith, 1974).

The heat islands are characterized by high temperature gradi­ents about the city. The highest temperatures are associated with areas of highest density and activity, while temperatures decline markedly toward the periphery of the city.

Such heat islands are common during the summer and early winter and are more noticeable at night than during the day, when heat stored by pavements and buildings is reradiated to air. The magnitude of the heat island is influenced strongly by local climatic conditions such as wind and cloud cover.

Table 11.6. Comparision of climates of urban and rural areas (Smith, 1977):

Elements Comparison with rural environment
1. Condensation nuclei and particles 10 times more
2. Gaseous admixtures 5-25 times more
3. Cloud cover 5-10 per cent more
4. Winter fog 100 per cent more
5. Summer fog 30 per cent more
6, Total precipitation 5-10 per cent more
7. Relative humidity, winter 2 per cent less
8. Relative humidity, summer 8 per cent less
9. Radiation, global 15-20 per cent less
10. Duration of sunshine 5-15 per cent less
11. Annual mean temperature 0-5-1 ‘0*C more
12. Annual mean wind speed 20-50 per cent less
13. Calms 5-20 per cent more