Environmental Problems, Their Causes, and Sustainability 1

Figure 6-18 (facing page) shows how climate andvegetation vary with latitude (distance from the equator)and altitude (elevation above sea level). If youclimb a tall mountain from its base to its summit, youcan observe changes in plant life similar to those youwould encounter in traveling from the equator to theearth’s poles.6-4 DESERT BIOMESWhat Are the Major Types of Deserts? Hot,Medium, and ColdDeserts have little precipitation and little vegetationand are found in tropical, temperate, and polar regions.A desert is an area where evaporation exceeds precipitation.Annual precipitation is low and often scatteredunevenly throughout the year. Deserts have sparse,widely spaced, mostly low vegetation.Deserts cover about 30% of the earth’s land surfaceand are found mostly in tropical and subtropicalregions (Figure 6-16). The largest deserts are found inthe interiors of continents, far from moist sea air andmoisture-bearing winds. Other, more local desertsform on the downwind sides of mountain ranges becauseof the rain shadow effect (Figure 6-15).During the day the baking sun warms the groundin the desert. At night, however, most of the heatstored in the ground radiates quickly into the atmosphere.This occurs because desert soils have little vegetationand moisture to help store the heat and theskies are usually clear. This explains why in a desertyou may roast during the day but shiver at night.A combination of low rainfall and different averagetemperatures creates tropical, temperate, and colddeserts (Figures 6-17 and 6-19). Take a close look at thegraphs in Figure 6-19.Tropical deserts (Figure 6-19, left) are hot and drymost of the year. They have few plants and a hard,windblown surface strewn with rocks and some sand.They are the deserts we often see in movies.In temperate deserts, daytime temperatures are highin summer and low in winter and there is more precipitationthan in tropical deserts (Figure 6-19, center).The sparse vegetation consists mostly of widely dispersed,drought-resistant shrubs and cacti or othersucculents adapted to the lack of water and temperaturevariations, as shown in Figure 6-20 (p. 114). Tracehow nutrients and energy flow through the ecosystemin this diagram. In cold deserts, winters are cold, summersare warm or hot, and precipitation is low (Figure6-19, right).In the semiarid zones between deserts and grasslands,we find semidesert. This biome is dominated bythorn trees and shrubs adapted to long dry spells followedby brief, sometimes heavy rains.How Do Desert Plants and AnimalsSurvive? Stay Cool and Get Water Any WayYou CanDesert plants and animals have a number of strategiesfor staying cool and getting enough water to survivein hot and dry climates.Adaptations for survival in the desert have two themes.One is beat the heat and the other is every drop of watercounts. Desert plants exposed to the sunlight must conserveenough water for survival and lose enough heatso they do not overheat and die.Desert plants have evolved a number of strategiesfor doing this. During long hot and dry spells plantsMean monthly temperature°C °F in. mm100 153537530 9014 35025 8013 32520 7012 30015 6011 27510 5010 2505 409 2250 308 200–5 32°F Freezing point207 175–10–15106 150–20 05 125–25 –104 100–30 –203 75–35–40–45–30–40–5021050250JanFebMarAprMayJunJulAugSepOctNovDecMonthsTropical desert(Saudi Arabia)Mean monthly precipitationMean monthly temperature°C °F1003530 9025 8020 7015 6010 505 400 30–5 20–10–1510–20 0–25 –10–30 –20–35 –30–40 –40–45 –5032°F Freezing pointJanFebMarAprMayJunJulAugSepOctNovDecMonthsTemperate desert(Reno, Nevada)in.1514131211109876543210mm3753503253002752502252001751501251007550250Mean monthly precipitationMean monthly temperature°C °F10035309025 8020 7015 6010 505 400 30–5 20–10–1510–20 0–25 –10–30 –20–35 –30–40 –40–45 –5032°F Freezing pointJanFebMarAprMayJunJulAugSepOctNovDecMonthsPolar desert(northwest China)in.1514131211109876543210mm3753503253002752502252001751501251007550250Mean monthly precipitationFigure 6-19 Climate graphs showing typical variations in annual temperature (the graphed line) and precipitation(the shaded area on the graph) in tropical, temperate, and polar (cold) deserts.http://biology.brookscole.com/miller14113