Microclimate
Even in the complete absence of vegetation, major climatic forces, or macroclimates, are expressed differently at a very local spatial level, which has resulted in the recognition of so-called microclimates. Thus, the surface of the ground undergoes the greatest daily variation in temperature, and daily thermal flux is progressively reduced with both increasing distance above and below ground level (Figure 4.2). During daylight hours the surface intercepts most of the incident solar energy and rapidly heats up, whereas at night this same surface cools more than its surroundings. Such plots of temperature versus height above and below ground are called thermal profiles. An analogous type of graph, called a bathythermograph, is often made for aquatic ecosystems by plotting temperature against depth (see Figure 4.17).
- Figure 4.2. Idealized thermal profile showing temperatures at various distances above and below ground at four different times of day. [After Gates (1962).]
Daily temperature patterns are also modified by topography even in the absence of vegetation. A slope facing the sun intercepts light beams more perpendicularly than does a slope facing away from the sun; as a result, a south-facing slope in the Northern Hemisphere receives more solar energy than a north-facing slope, and the former heats up faster and gets warmer during the day (Figure 4.3). Moreover, such a south-facing slope is typically drier than a north-facing one because it receives more solar energy and therefore more water is evaporated.
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- Figure 4.3. Daily marches of temperature on an exposed south-facing slope (solid line) and on a north-facing slope (dashed line) during late summer in the Northern Hemisphere. [After Smith (1966) after van Eck.]
By orienting themselves either parallel to or at right angles to the sun’s rays, organisms (and parts of organisms such as leaves) may decrease or increase the total amount of solar energy they actually intercept. Leaves in the brightly illuminated canopy often droop during midday, whereas those in the shaded understory typically present their full surface to incoming beams of solar radiation. Similarly, many desert lizards position themselves on the ground perpendicular to the sun’s rays in the early morning when environmental temperatures are low, but during the high temperatures of midday these same animals reduce their heat load by climbing up off the ground into cooler air temperatures and orienting themselves parallel to the sun’s rays by facing into the sun.
- Figure 4.4. Temperature profiles in a growing cornfield at midday, showing the effect of vegetation on thermal microclimate. [After Smith (1966).]
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The major effect of a blanket of vegetation is to moderate most daily climatic changes, such as changes in temperature, humidity, and wind. (However, plants generate daily variations in concentrations of oxygen and carbon dioxide through their photosynthetic and respiratory activities.) Thermal profiles at midday in corn fields at various stages of growth are shown in Figure 4.4, demonstrating the marked reduction in ground temperature due especially to shading. In the mature field, air is warmest at about a meter above ground. Similar vegetational effects on microclimates occur in natural communities. A patch of open sand in a desert might have a daily thermal profile somewhat like that shown in Figure 4.2, whereas temperatures in the litter underneath a nearby dense shrub would vary much less with the daily march of temperature.
Humidities are similarly modified by vegetation, with relative humidities within a dense plant being somewhat greater than those of the air in the open adjacent to the plant. An aphid may spend its entire lifetime in the very thin zone (only about a millimeter thick) of high humidity that surrounds the surface of a leaf. Moisture content is more stable, and therefore more dependable, deeper in the soil than it is at the surface, where high temperatures periodically evaporate water to produce a desiccating effect.
- Figure 4.5. Wind velocities within a forest vary relatively little with changes in the wind velocity above the canopy. [After Smith (1966) after Fons (1940).]
Wind velocities are also reduced sharply by vegetation and are usually lowest near the ground (Figures 4.5 and 4.6). Moving currents of air promote rapid exchange of heat and water; hence an organism cools or warms more rapidly in a wind than it does in a stationary air mass at the same temperature. Likewise, winds often carry away moist air and replace it with drier air, thereby promoting evaporation and water loss. The desiccating effects of such dry winds can be extremely important to an organism’s water balance.
- Figure 4.6. Daily march of average wind velocities during June at various heights inside a coniferous forest in Idaho. [After Smith (1966) after Gisborne.]
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In aquatic systems, water turbulence parallels wind in many ways, and rooted vegetation around the edges of a pond or stream reduces water turbulence. At a more microscopic level, algae and other organisms that attach themselves to underwater surfaces (so-called periphyton) create a thin film of distinctly modified microenvironment in which water turbulence, among other things, is reduced. Localized spatial patches with particular concentrations of hydrogen ions (pH), salts, dissolved nitrogen and phosphorus, and the like, form similar aquatic microhabitats.
By actively or passively selecting such microhabitats, organisms can effectively reduce the overall environmental variation they encounter and enjoy more optimal conditions than they could without microhabitat selection.
Innumerable other microclimatic effects could be cited, but these should serve to illustrate their existence and their significance to plants and animals.
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