Jessica Gurevitch, Samuel M. Scheiner, and Gordon A. Fox - The Ecology of Plants - 2002 / The Ecology of Plants Jessica Gurevitch, Samuel M. Scheiner, and Gordon A. Fox; 2002 / Ch 18

tologists are still refining their understanding of all of its effects. These effects may also be changing due to global warming (see Chapter 22).

El Niño (literally “the boy”) is named for the Spanish term for the infant Jesus, because its effects were initially noticed off the coast of Peru beginning around Christmas. El Niño is only one half of the ENSO; it alternates with La Niña (“the girl”) years, during which conditions are reversed.

The heart El Niño is a cycle of tropical Pacific oceanic and atmospheric conditions. During an ordinary year, the trade winds in both hemispheres blow west across the Pacific, pushing strong ocean currents along in the same direction (Figure 18.15A). The water being pushed away from the western coasts of North and South America is replaced with very cold, nutrient-rich water dragged up from the ocean bottom. This upwelling of cold bottom water in the eastern Pacific supports extremely productive marine food webs as well as affecting global climates. Strong trade winds push warm surface waters into the far western Pacific north of Australia. Warm water has increased evaporation, bringing rainfall to the islands of the western Pacific and to eastern Australia and Asia.

When an El Niño year commences, the trade winds slacken (Figure 18.15B). As a result, upwelling in the eastern Pacific ceases, with huge negative repercussions for fish populations, marine mammals, seabirds, and other organisms that are ultimately dependent on the nutrient-rich bottom water. Because this cold water never makes it to the surface, warm surface water now spreads across the tropical Pacific from east to west. Surface temperatures in the central Pacific Ocean become as much as 5°C warmer than normal, leading to drastic decreases in rainfall in the western and central Pacific.

As an example, the very severe El Niño year of 1997–1998 (Figure 18.16) resulted in extreme drought in Australia, Indonesia, and southern Africa.

Warm, moist air now rises above the eastern Pacific, causing heavy rainfall in ordinarily dry regions such as southern California, northern Chile, and northward to the coasts of Peru and Ecuador. This rain can result in severe flooding and massive landslides. Finally, a complex change in high-altitude winds 12 km over the Pacific forces a jet of air to move eastward, past Central America, and then across the Atlantic to Africa, depressing hurricane activity in the Atlantic and causing other disturbances in normal weather patterns. These changes in upper-level atmospheric winds in the southern Pacific also force changes in the jet stream in northern mid-lat- itudes, spreading El Niño’s effects on weather to a far greater portion of the globe.

Eventually, these conditions reverse. During La Niña years, at the opposite part of the ENSO, strong trade winds push warm surface waters into the far western Pacific north of Australia. Evaporation is intensified in the eastern Pacific, further strengthening the trade winds. Meanwhile, the movement of surface waters eastward causes a strong upwelling of cool, deep waters in the western Pacific along the coast of South America. The Equatorial Countercurrent, which flows counter to the prevailing winds, is relatively weak. The effects on weather patterns are essentially the opposite of those in an El Niño year. At some point, the positive feedback that maintains this system reverses, and El Niño conditions return again.

ENSO is a succession of alternating events. We do not completely understand what triggers the changes between El Niño and La Niña conditions, but the ENSO clearly depends on a complex interplay between ocean

currents and winds. Ocean waters are always exchanging energy (as heat) with the atmosphere, although they hold far more heat, and move heat around the globe more slowly, than do atmospheric currents. This exchange of heat energy with the ocean drives the movement of winds, and the winds drive ocean currents.

How do these complex changes in atmospheric and oceanic interactions affect weather and organisms in different places? In North America, El Niño events may be linked to milder winters along the U.S.-Canada border, increased winter storms along the California coast, floods in the southeastern United States, increased snowfall in the southwestern mountains, and greatly decreased hurricane activity in the Atlantic. La Niña also can have strong effects on weather patterns in North America, including increases in reported tornado strength and frequency in the Midwest, stronger and more frequent hurricanes in the Atlantic, and drought and forest fires in the Southwest.

Globally, the El Niño of 1982–1983 caused droughts in Africa, Australia, and South America. Coral reefs off Costa Rica, Panama, Colombia, and the Galápagos Islands suffered drastic losses (50%–97% die-offs) because of the warmer water and loss of nutrients. In that same event, 85% of the seabirds on the Peruvian

coast died or abandoned their nests; the same fate befell almost all of the 17 million seabirds living on tiny Christmas Island in the Pacific.

The El Niño of 1997–1998 was even more extreme, with the resulting drought exacerbating forest fires in Indonesia and Malaysia and harming the already threatened rainforests there; the rainforests of the Amazon basin also suffered from extreme drought. In many places, the unusual severity of this particular El Niño, coupled with other problems facing these tropical forests (such as logging and fragmentation), combined to cause much greater damage than any of these factors would have by themselves (Wuethrich 2000). Other areas affected by the drought included Australia and southern Africa, while severe flooding occurred in southern California and in the coastal deserts of northern Chile and Ecuador.

These cycles also affect plant population dynamics. Because the U.S. Southwest normally has low amounts of precipitation, this region provides a sensitive context for studying these effects. The ENSO effects both precipitation and temperature, as can be seen by looking at the climate record for the past century at Las Cruces, New Mexico (Figure 18.17). Periods of dry winters combined with wet summers in this area are asso-

Figure 18.17

Patterns of temperature and precipitation for the past century at Las Cruces, New Mexico.

(A)Contour map of average monthly temperature (contour interval 5°C). Colder winters are indicated by larger areas within the 5° contour, such as the period from 1928 to 1934. Warmer summers are indicated by larger areas within the 25° contour, such as the period from 1927 to 1937. Arrows followed by solid lines indicate years with large numbers of seedlings of Bouteloua eriopoda (black grama grass, Poaceae).

(B)Contour map of average monthly precipitation (contour interval 10 mm). Wet periods are indicated by the dark triangular-shaped areas, such as the period from 1965 to 1975. (After Neilson 1986.)

ciated with range increases of the native Bouteloua eriopoda (black grama grass, Poaceae; Neilson 1986). Periods with dry spring weather are associated with largescale forest fires (Figure 18.18).

Short-term oscillations such as ENSO are embedded in larger, long-term phenomena. Over the past 5000 years there may have been periods of more frequent El Niño events from 4800 to 3600 years before present (B.P.), around 1000 B.P., and after 500 B.P. These periods also appear to be associated with increased flooding in the southwestern United States (Ely et al. 1993).

Predictability and Long-Term Change

In this chapter we have described patterns of variation in temperature and precipitation. These patterns are generally regular, with some predictability. Random effects act to introduce unpredictability into these patterns. The degree of unpredictability is one determinant of the types of plant communities found at different locations (Figure 18.19). Uncertainty is also an important determinant of some kinds of adaptations. For example, predictable variation is more likely to favor adaptation by phenotypic plasticity (see Chapter 5), while unpredictable variation is more likely to favor adaptation by a jack-of-all-trades strategy.

The distinction between random and predictable patterns of change depends on the life span of the plant relative to the rate at which the climate changes. The ENSO, for example, varies enough in its details (e.g., intensity and length) that for an annual plant it is effectively random, while for a Sequoia sempervirens (redwood, Cupressaceae) that lives for centuries, it forms a predictable cyclic pattern.

At the longest time scale—tens of millions of years— continental drift and tectonic activity play a major role in changing climate. Mountain building, for example, creates rain shadows. The spread of grasslands across the middle of North America is associated with the rise

of the Rocky Mountains during the Eocene 34 million years ago. As continents drift across the face of the Earth, their climates change. For the past 100 million years, Australia has been moving northward; as a result, its climate has changed from mainly temperate and polar to mainly tropical and subtropical. Changes in atmospheric CO2 concentrations, both natural and anthropogenic, also have had large effects on global climate in the past as well as currently. We discuss these long-term patterns in detail in Chapters 21 and 22.

Figure 18.19

Association between the variation and predictability of rainfall and the presence of different types of plant communities in Arizona. Desert areas dominated by creosote bush (lower Sonoran Desert) or desert scrub (upper Sonoran Desert) have low amounts of precipitation and high year-to-year variation. As precipitation amounts increase and variation decreases, plant communities become dominated by grasses and then pine forest. (After Davidowitz 2002.)

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Plant Physiognomy across the Globe

A continent is a complex mosaic of climates and vegetation forms. Across a continent, this variation falls into regular patterns. Here we introduce several of these broad patterns. The first of these is a north-south gradient in vegetation form due to temperature. A second major pattern is variation from west to east in response to changes in average precipitation. Such patterns exist on all continents because of the latitudinal differences in solar radiation and the movement of weather systems described earlier in this chapter. While we concentrate here on North America, and while each continent is unique, the principles behind these patterns can be applied to the other continents as well. This variation will be addressed in greater detail when we describe the world’s biomes in the next chapter.

Forests

Forests are communities dominated by trees whose leaves touch each other, resulting in a closed canopy. If you were to travel southward in eastern North America from northern Canada to the Carolinas, Georgia, and northern Florida, you would pass through a variety of different-looking forests. Forest composition is affected by both climate and soil; to focus on the effects of climate, we confine our discussion to the relatively rich soils found inland (away from the nutrient-poor coastal plain).

Immediately to the south of the tree line you would find taiga or boreal forest, which is dominated by coniferous, evergreen needle-leaved trees. Next you would enter a region dominated by broad-leaved trees—decid- uous angiosperms that drop their leaves in winter— although conifers would still be present. Finally, you would come upon broad-leaved evergreen forests dominated by angiosperms that retain their leaves all year. In summary, there is a transition from evergreen forests to deciduous forests and back to evergreen forests, although the type of evergreen tree changes from nee- dle-leaved to broad-leaved, and from gymnosperms to angiosperms. These changes in tree type with latitude are largely driven by differences in seasonal temperatures. Of course, the description of this imaginary trip is very general; for the moment, we are ignoring many other details to concentrate on temperature effects. Similar patterns are found, for example, in eastern Asia as one travels from Korea and northeastern Russia to the south of China and beyond to Southeast Asia.

To understand why these changes occur, let’s reverse our trip. Beginning in southeastern North America, growing seasons are longest in the broad-leaved evergreen forests found along the Atlantic and Gulf coasts. These areas are classified as subtropical because they rarely experience temperatures below freezing. Tropical

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and subtropical forests can be dominated by either deciduous or evergreen angiosperms.

The critical climatic factor determining which one occurs in a given area is seasonality of precipitation. Areas that receive rain throughout the year are evergreen, while areas with pronounced dry periods are deciduous. As always, there are exceptions to this general pattern. For example, many forests in the southeastern United States are dominated by a particular genus of needle-leaved evergreen gymnosperms: Pinus (pines). These pine forests are typically found on soils that are very low in nutrients and are subject to frequent fires. (The effects of soil nutrients on leaf life span are considered in the context of plant strategies in Chapter 9.)

Moving north, the transition from broad-leaved evergreen to broad-leaved deciduous forests depends on the likelihood of freezing temperatures. Broad-leaved evergreen trees are vulnerable to frost through two of its effects. First, leaf tissues will die if kept frozen. Second, the weight of ice or snow on leaves will cause them to be torn off or, worse, will break branches and even tree trunks. A very unusual October snowstorm in 1997 in the Midwest and Great Plains of the United States, which occurred before the leaves had fallen, caused major damage to deciduous trees as branches and even trunks snapped under the weight of the snow-burdened canopies. Tall, narrow conifers such as spruces (Picea) and firs (Abies) shed snow far more effectively. So it is no surprise, then, that the boundary of broad-leaved evergreen trees corresponds with that of freezing temperatures (Figure 18.20).

Farther north, the transition between broad-leaved deciduous and needle-leaved evergreen forests is related to growing season length. An evergreen tree can take advantage of short periods of favorable conditions as long as there is enough unfrozen moisture in the soil to support transpiration (a major limitation for these trees). Conifers are able to avoid leaf damage due to freezing, by increasing the osmotic concentration of intracellular water (thereby lowering its freezing point). Ice crystals form only between the cells (which does little damage to membranes); this acts to draw water out of cells, which further increases the osmotic concentration inside the cells.

In contrast, a deciduous tree requires time to produce new leaves in the spring before it can begin photosynthesizing. When the new leaves are emerging, they are particularly vulnerable to injury from freezing. Thus, leaf emergence does not occur until the chance of a late frost is very small. Following leaf emergence, the growing season must be long enough for the tree to accumulate enough carbon for its maintenance, growth, and reproduction. Thus, deciduous trees are able to survive only in regions with a relatively long growing season.

The slow leafing out of deciduous trees in the spring has another effect on the form of deciduous forests: These communities are much more complex in form and higher in species diversity than boreal forests. Because the canopy is deciduous, there are times of year when large amounts of light penetrate to the forest floor. Consequently, there can be up to four layers of vegetation in deciduous forests: herbaceous species along the ground, small shrubs, small trees and large shrubs, and canopy trees. In the spring, the forests turn green from the ground up, with each layer leafing out slightly later. This ground-up effect occurs because the air is warmer near the ground, especially early in the season before the canopy leafs out and reduces convective cooling. The diversity of species and form in deciduous forests, therefore, depends on seasonal variation in temperature.

The evergreen needle-leaved trees that dominate the northern coniferous forests pay a cost for their ability to withstand freezing temperatures: they grow much more slowly than broad-leaved deciduous trees. Once the growing season is sufficiently long, deciduous trees outcompete needle-leaved evergreen trees. The border between the deciduous forest and the needle-leaved evergreen forest corresponds to a temperature line north of which the minimum temperature during the winter falls below –40°C (Arris and Eagleson 1989; compare Figures 18.20 and 19.1). Above this temperature and

below 0°C, plant tissues can supercool, meaning that they can cool without ice formation. It is the formation of ice crystals in plant and animal tissues that is the primary source of damage and injury from freezing.

Tree Line

Temperature has a direct effect on the types of vegetation that can grow in a region. One of the most striking examples of this effect is the alpine tree line, an often abrupt boundary where trees are replaced by lowgrowing vegetation (Figure 18.21). Tree line is most obvious in high mountain ranges, such as the Sierra Nevada and Rockies in North America, the Alps in Europe, the Andes in South America, or the Himalayas in Asia, all of which have forests that stop below the summit. If you were to hike up such mountains, you would reach a point where the forest would thin and the trees would get shorter. At some point, the trees would disappear entirely. A similar sight would greet you at the Arctic or Antarctic tree line on a hike toward the North Pole in North America or Eurasia, or toward the southern tip of South America.

Tree lines are caused by a complex of several factors, including the limits to supercooling mentioned above and others detailed in Chapter 19. In addition, wood will form only when temperatures are above 10°C. Microenvironmental differences in temperature can lead to inter-

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Figure 18.21

Tree line at about 1200 m in Rocky Mountain National Park, Colorado. (Photograph by S. Scheiner.)

esting effects, such as the formation of krummholz (Figure 18.22), a peculiar tree form found at tree line on mountains in different parts of the world.

Above tree line on Mount Washington, New Hamp- shire—the highest peak in eastern North America—you might notice an odd-looking plant. It looks like a horizontal woody vine growing along the ground (Figure 18.23). If you examine it closely, however, you will find that it is Picea mariana (black spruce, Pinaceae), the dominant tree species in the nearby forest. Seeds taken from these plants, if planted farther down the mountain, will grow into tall trees! The krummholz form of the plant comes about because the temperature right at ground level is just a little bit warmer than the air above it—just enough above 10°C for wood formation (Teeri 1969). The warmer temperatures near the ground are caused by reduced convective cooling and longwave radiation by the ground of the energy it absorbs from sunlight.

Grasslands and Woodlands

Traveling from west to east, starting in the rain shadow on the eastern face of the Rocky Mountains and traversing the center of the United States, you will see another gradual transition in the physiognomy of the vegetation. Your journey begins in short-grass prairie, in the midst of low-growing clumps of grasses, with bare patches interspersed between the clumps. As you travel eastward, the vegetation becomes taller and thicker as you reach the midgrass prairie. Here you may notice fewer grasses growing in clumps and more rhizomatous grasses, as well as many dicots.

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By the time you have reached Iowa, you are in tallgrass prairie. In addition to taller vegetation, these communities have greater biomass; they are also more productive and more diverse. Besides the obvious grasses, there are many dicot forbs whose stature matches that of the grasses. This shift in plant stature and diversity closely follows a pattern of increasing rainfall.

As you continue eastward, more and more trees are evident. In the areas dominated by grasslands, trees are clustered along streams. Farther east, they begin to dot the countryside, becoming more dense. These areas are woodlands, dominated by trees, but without a closed canopy. Eventually, at about the Illinois/Indiana border, you reach forests.

This transition from grassland to woodland to forest is a function of changes in both the amount and seasonality of precipitation. Annual rainfall, for example, is about 80 cm both in the middle of Kansas and to the northeast in the middle of Minnesota, yet the former is in the center of the tallgrass prairie, while the latter is forested. The difference is that in Kansas, the rainfall is much more seasonal, with more of it occurring in spring and fall, the temperature is warmer, and the chance of drought is greater.

This seasonality of precipitation and propensity for drought leads to another key factor: fire (see Chapter 13). Under natural circumstances, prairies experience fires

Figure 18.22

Krummholz vegetation at treeline in Rocky Mountain National Park, Colorado. Note the twisted trunk of the tree. (Photograph by S. Scheiner.)

every 3–5 years, depending on climatic and landform conditions. Each year dry grasses and other plant material accumulate until there is enough fuel to sustain a conflagration, and lightning (or human activity) ignites a fire. Prairie plants are adapted to these frequent fires. Their vulnerable meristems are located at or below ground level. A fast-moving fire will burn off the tops of the plants, but they quickly resprout. Trees, however, have their meristems at the tips of their branches. Seedlings and young saplings are particularly vulnerable to fire. Grasslands begin to give way to woodlands where the frequency of fire drops low enough that some trees have sufficient time between fires to grow tall enough to become resistant to fire. Even so, these woodlands are dominated by species that have thick bark and are tolerant of fire. The woodlands become closedcanopy forests where precipitation levels are high enough that fire frequency becomes low.

Shrublands and Deserts

Shrublands are another widespread physiognomic form dominated by low-growing woody plants with multiple stems. The locations of these communities are determined by a combination of temperature and precipitation effects. They are primarily found in dry to very dry regions. Areas with mediterranean climates, for example, contain a mixture of shrublands and woodlands. Which type of vegetation predominates in an area

depends on disturbance. Fire often favors shrubs, which can resprout readily after being burned, over trees. Annuals are also very common in shrublands and are especially prevalent in the years immediately following a fire. Other disturbances can also favor shrubs over trees. Today grazing by domesticated animals is the most important factor spreading and maintaining shrublands in subtropical regions of the world.

The other major climatic regions with extensive shrublands are deserts, which are regions where potential evapotranspiration exceeds actual evapotranspiration. The amount of precipitation and its seasonality are among the most important factors in determining what kinds of plants thrive in very dry environments (Figure 18.24). Summer and winter precipitation represent very different sources of water for plants. At summer temperatures, rain only wets the soil surface, often for only a few days. At winter temperatures, precipitation can infiltrate deeper soil layers and remain available there throughout the year.

Wet winters with dry summers favor deep-rooted, long-lived plants with large leaf areas, such as trees. These plants use almost no summer rainwater, even when it is available. As the amount of winter water declines, plants with greater shoot biomass relative to the amount of root biomass, such as shrubs, are favored. As winter water further decreases, summer water becomes increasingly important, favoring plants with

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shallower root systems and even less root material, such as herbaceous annuals and perennials. These plants are capable of extracting surface water quickly, before it is lost to evaporation.

When there is no winter water stored deep in the soil, only succulent plants such as cacti, which store water internally and lose it slowly, can survive between summer rain events. The more often it rains in summer, the less need to store water, and the community becomes increasingly more herbaceous. If wet summers are accompanied by moderately dry winters, herbaceous, shallow-rooted plants, such as grasses or summer annuals, are favored.

These effects of the amount and seasonality of precipitation in deserts explain the differences in the physiognomy of the hot deserts of western North America: the Mojave, the Sonoran, and the Chihuahuan. The Mojave Desert (mainly in California) gets almost exclusively winter precipitation. The Chihuahuan Desert to the southeast (mainly in the Mexican states of Chihuahua and Coahuila) gets almost exclusively summer precipitation, whereas the Sonoran Desert between the two in Arizona and Sonora gets both summer and winter precipitation. As a result, while all three regions have

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many shrubs, the Mojave Desert is dominated by lowgrowing shrubs (many of which are deciduous) and has a very large number of annual plant species, the Chihuahuan Desert is dominated by evergreen shrubs and grasses, and the Sonoran Desert has a mixture of many growth forms. In all of these deserts, however, several growth forms coexist; no one growth form can preempt all of the available water.

Summary

The climate of an area is determined by the mean, variability, and seasonality of temperature and precipitation. Mean temperature is determined by the amount of incoming solar radiation, which varies daily, seasonally, yearly, and over the course of centuries. At the equator solar radiation inputs are generally high and even throughout the year; in contrast, solar radiation inputs at high latitudes vary greatly over the year. At much longer time scales—tens to hundreds of thousands of years— changes in Earth’s orbit around the sun and in the tilt of Earth’s axis also create variation in solar radiation.

Patterns of precipitation are caused by differential heating and cooling of the atmosphere at different lati-

Succulents (e.g., cacti)

Shrubs

Herbaceous perennials

Herbaceous annuals

Trees

High

Summer precipitation

Winter precipitation

tudes. At the equator, warm rising air creates a region of high precipitation. That air tends to descend at 30° north and south of the equator, creating regions of low precipitation there. A second area of low precipitation is found near the poles. In addition, the prevailing winds push masses of water, creating ocean currents that carry heat energy to and away from the areas they traverse.

A second pattern is caused by the flow of air across continents and its interaction with mountain ranges. Areas on the upwind (western) sides of mountain ranges tend to be wet, while the downwind (eastern) sides of mountain ranges tend to be dry. The north-south–run- ning mountain ranges of North America create a distinct pattern of alternating wet and dry areas across the continent. Variation in precipitation occurs on time scales of days, seasons, years, and decades. This variation also contains an element of unpredictability that is especially important in extreme climates.

Additional Readings

Classic References

Beard, J. S. 1955. The classification of tropical American vegetation-types. Ecology 36: 89–100.

Gates, D. 1962. Energy Exchange in the Biosphere. Harper & Row, New York.

Holdridge, L. R. 1947. Determination of world plant formations from simple climatic data. Science 105: 367–368.

Mather, L. R. and G. A. Yoshioka. 1968. The role of climate in the distribution of vegetation. Ann. Assoc. Am. Geog. 58: 29–41.

Contemporary References

Ji, J. 1995. A climate-vegetation interaction model: simulating physical and biological processes at the surface. J. Biogeog. 22: 445–451.

It is the combination of the mean climate of a region, the variability of that climate, and the predictability of that variation that determines the types of plants that will grow there. Trees can grow only where temperatures exceed 10°C for sufficient periods of time. Warm temperatures and long growing seasons favor broad-leaved deciduous trees over needle-leaved evergreen trees. Trees require high amounts of rainfall, however. Areas of lower rainfall are dominated by shrubs or grasses. Succulents are favored in areas with unpredictable summer rainfall. The border between woodlands, shrublands, and grasslands is often controlled by rates of disturbance, especially the frequency of fire. Fire frequency, in turn, is controlled by patterns of temperature and precipitation. Thus, an understanding of climatic patterns across the globe provides the basis for understanding patterns of vegetation. We explore those patterns in more detail in the next chapter.

Neilson, R. P. 1986. High-resolution climatic analysis and southwest biogeography. Science 232: 27–34.

Schwinning, S. and J. R. Ehleringer. 2001. Water-use tradeoffs and optimal adaptations to pulse-driven arid ecosystems. J. Ecol. 89: 464–480.

Additional Resources

Heinrich, W. 1973. Vegetation of the Earth in Relation to Climate and the Eco-Physiological Conditions. Springer-Verlag, New York.

Rumney, G. R. 1968. Climatology and the World’s Climates. Macmillan, New York.