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 21

420 Chapter 21

Figure 21.1

The geological time scale (numbers are in millions of years). Although the Cenozoic era was traditionally split into the Tertiary and Quaternary periods, a newer system now divides it into two periods of more equal duration, the Paleogene and the Neogene. (After Stanley 1987.)

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410

Silurian

440

Ordovician

505

Cambrian

544

Precambrian

The Paleozoic Era

Fossil evidence tells us that plants first invaded land in the Paleozoic era, during the late Ordovician through Silurian periods. In Chapter 3 we considered some of the evolutionary challenges they faced in accomplishing this major transition. Land plants are descended from the Charophyta, a group of branched, filamentous green algae. Today many green algae grow along seashores, where they are subjected to daily drying as the tides rise and fall. Undoubtedly these were the conditions under which land plants evolved.

The invasion of plants drastically changed terrestrial environments, setting the stage for two major (and separate) animal invasions (by arthropods and by vertebrates) as well as for all subsequent plant evolution.

During the Silurian and Devonian periods, the terrestrial landscape was completely transformed. This period of time saw a steady increase in the morphological and ecological complexity of both plants and animals and the consequent dramatic radiation of major phyla. At the beginning of this period, the only vegetation consisted of ground-hugging mats in very damp areas, dominated by plants related to liverworts (Figure 21.2). Based on fossil evidence, we can infer that these early plants were mechanically supported by turgor

pressure, that their gametophytes produced both male and female reproductive organs like those of modern bryophytes and ferns, and that they had, at most, only rudimentary roots (Bateman et al. 1998). The forms of their sporophytes were simple, but at least one species,

Figure 21.2

Marchantia is a liverwort, a nonvascular plant. The 3-centimeter tall structures in the center are reproductive organs of the female (left) and male (right) gametophytes. Early terrestrial plants may have had a similar form. (Photograph by S. Scheiner.)

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Figure 21.3

A forest in what is now Colorado as it might have looked 300 million years ago. A shallow sea lies to the east, and rivers choked with gravel flow from the mountains to the sea. The banks of the rivers are colonized by forests of scale trees (Sigillaria, Lycopodophyta) and giant horsetails. The trees form a very minimal canopy with little shade. They have green trunks, and even their surface roots are green; the whole tree can photosynthesize. Forests of primitive conifers grow in the higher terrain of the foothills and mountains. (Painting by Jan Vriesen, courtesy Denver Museum of Nature and Science.)

Rhynia gwynne-vaughanii, had already evolved the ability to spread by clonal growth.

By the end of the Devonian, soils had developed in many habitats, and tall forests rose above primordial swamps dominated by giant lycopods up to 40 meters tall (Figure 21.3). As root systems evolved, land plants diversified and spread to mesic and drier uplands. By the end of the period, many major vascular plant phyla— Psilotophyta (whiskferns), Lycopodophyta (clubmosses; Figure 21.4), Sphenophyta (horsetails) and Pteridophyta (ferns)—had all evolved, as had the seed plants (Bateman et al. 1998). Today the first three of these groups grow only as small understory plants. Many ferns also grow as small plants, but tree ferns are large plants that grow in a number of tropical and subtropical habitats. A consistent trend during this period was the increasing dominance of the sporophyte (diploid) generation. As the sporophyte became larger and longer-lived, the gametophyte became smaller and shorter-lived.

We can speculate about the selective processes that created this diversification. The evolution of vascular tissue allowed plants to increase in height, and this abil-

Paleoecology 421

ity probably triggered an “arms race.” There is generally little advantage to a plant in becoming very tall unless it is likely to be shaded by its neighbors (see Figure 6.6). If one species in a community is taller than all of the others, it will outcompete its neighbors for light. Natural selection will then favor individuals of those other species that can grow even taller. This process continues until other constraints balance selection for height. Selection for increased height, as well as other selective forces, resulted in the evolution of wood, which in turn enabled the great diversity of forms of trees and shrubs to evolve. The evolution of roots, in contrast, was proba-

bly driven by a combination of abiotic factors as well as competition for nutrients and water. As a result of these adaptations, plants were able to spread from wet areas, eventually spreading over nearly the entire surface of Earth that is not covered by water or ice.

As plants diversified, biotic interactions probably came to play an increasingly important role in plant evolution. Many modern ecological interactions evolved

Figure 21.4

Lycopodium obscurum (ground pine, Lycopodiaceae) is a clubmoss, a vascular plant approximately 10 centimeters tall. It has a stem, but not true roots or leaves. (Photograph by S. Scheiner.)

Figure 21.5

The positions of the continents over geological time: (A) Late Permian (255 million years ago), when the land masses were joined together into one supercontinent, Pangaea. (B) Middle Cretaceous (94 million years ago), when the northern continent, Laurasia, and the southern continent, Gondwana, had begun to break apart. (C) Middle Eocene (50 million years ago), when the continents were taking on their current configurations. (D) Today.

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very early, including mycorrhizal symbioses and herbivory during the Devonian and animal pollination and seed dispersal during the Carboniferous and Permian periods.

The Carboniferous period is so named because of the enormous amounts of fossil carbon—oil and coal— that were deposited that time. Coal deposits were generated mainly from the remains of wetland plants, including ferns and other spore-producing plants, as well as early gymnosperms (DiMichele et al. 2001). The organic sources of oil are less well understood; they appear to be primarily marine autotrophic and heterotrophic plankton, but may also include wetland plants. As climates changed in the late Carboniferous— mainly by becoming drier—seed plants began to dominate the landscape. This change in the vegetation was rapid, resulting in a nearly complete change in the species present at any given location, with upland plants rapidly replacing those of wet lowlands.

The Mesozoic Era

The Dominance of Gymnosperms

At the beginning of the Mesozoic era, all of the continents were joined together into one supercontinent, Pangaea. They have been carried apart and reshuffled by plate tectonics since that time (Figure 21.5). The coalescence of Pangaea, along with increases in atmospheric CO2 and temperature, led to widespread continental climates, greater seasonality, and widespread aridity. Any productivity benefits of the high CO2 levels may have been more than counterbalanced by the combination of high temperatures (from the greenhouse effect) and low or seasonal rainfall.

By the late Jurassic period, 150 million years ago, Pangaea had separated into two large continents, northern Laurasia and southern Gondwana. These continents, in turn, would break up into our current continents by the end of the Cretaceous period. The Atlantic Ocean was coming into being by the early Cretaceous, first with the separation of North America and Eurasia about 130 million years ago, and then with the separation of South America and Africa about 90 million years ago. As a result of these continental movements, areas that were once inland became coastal. Coastal areas are both wetter and less seasonal than inland regions (see Chapter 18). Thus, overall conditions for plant growth tended to improve throughout the Jurassic and Cretaceous.

We can consider the history of land plants as the successive dominance of three major groups. The pteridophytes (ferns) and pteridosperms (seed ferns) dominated the flora during the late Paleozoic, when climates were warm and wet. Despite the similarity in their names, ferns and seed ferns are not closely related; seed

Paleoecology 423

ferns are an extinct group that superficially resembled ferns, but reproduced through seeds rather than free living gametophytes.

With increasing aridity and continentality—influence of land masses on weather, in this case due to the formation of Pangaea—during the first half of the Mesozoic, the flora became dominated by conifers, cycadophytes, and other gymnosperms, and seed ferns gradually went extinct. Nonflowering families, genera, and species from the Mesozoic which persist today include: Osmunda (a fern genus), Ginkgo biloba (ginkgo, Ginkgoaceae), Sequoia (a conifer genus that today includes the giant sequoia of California), and Araucaria (a conifer genus of the Southern Hemisphere that today includes several important forest trees, such as A. excelsa, the Norfolk Island pine of the South Pacific, and A. araucana, the monkey puzzle tree of the southern Andes of South America). The angiosperms came to dominate the terrestrial fauna during the Cretaceous.

The early Cretaceous was a time of warm temperatures and very little difference among the seasons. CO2 levels were somewhere between 3 and 4 times higher than they are today (Figure 21.6), contributing to warmer temperatures through the greenhouse effect (see Chapter 22). The warm climate and ready availability of CO2 made the early Cretaceous a period generally favorable to plant growth. By the late Cretaceous, however, these favorable conditions had gradually declined, with climates becoming much more seasonal and larger differences in temperature existing between the equator and the poles.

The plant communities of the Mesozoic were similar to those of today in some ways. There were distinct biomes dominated by trees, shrubs, or herbs. However, the trees were predominately conifers; today, tropical forests and many temperate forests are dominated by angiosperms, with coniferous forests mostly found in temperate or boreal regions. Savannas are a particularly arresting example of how ancient biomes differ from those of today. Today’s savannas are dominated by grasses along with scattered angiosperm trees. In the Mesozoic, there were no grasses; instead, ferns provided most of the ground cover, while the trees were largely gymnosperms.

If we consider the modern distribution of ferns, we might think of them as a largely tropical group, as they are most abundant and diverse in the Tropics. Interestingly, most of today’s fern families originated in moist temperate regions (above 30° N and S latitude) during the Mesozoic (Skog 2001). During this time, ferns were noticeably absent from the Tropics. Long-distant dispersal of spores allowed fern families to spread widely in temperate regions, so that during much of the Mesozoic, many families were found in both the Northern and Southern hemispheres. Only later did they spread into the Tropics.

(A)

CO2 concentration (ppm)

Million years ago

Figure 21.6

(A) Estimated concentrations of atmospheric CO2 over the past 500 million years. The shaded areas above and below the solid line are upper and lower limits of the estimate. The black bars indicate periods when Earth's climate was relatively cool; white areas are periods of relative warmth. CO2 levels have mostly decreased for the past 175 million years. (After Rothman 2002.) (B) Estimated concentrations of atmospheric CO2 over the past 450,000 years, measured from trapped air in ice cores from the Antarctic. Over this period CO2 levels have varied, but there has not been a trend either up or down. (After Barnola et al. 1999.)

The Breakup of Pangaea

and the Rise of the Angiosperms

The rise of the angiosperms was a critical event in the history of plants. A few tantalizing fossils suggest an origin as early as the Triassic for this group, but that interpretation is controversial and not well supported by current evidence. What is certain is that by the middle to late Cretaceous, angiosperms were spreading throughout the world. This group now accounts for approximately 75% of all land plant species and a level of plant diversity not seen previously on Earth. The separation of the continents during the Cretaceous not only contributed to climate change, but also helped to accelerate speciation rates in both plants and animals through geographic isolation (see Chapter 6). In this world of newly divided continents the angiosperms spread, proliferated, and diversified. Their rise created a profound change in the structure of communities. First, the nature of animal communities went through a dramatic transformation. Insects began to diversify; today their species numbers vastly overwhelm those of any other group of animals. In some cases, such as the diversification of lepidopterans (butterflies and moths), the timing of insect radiations can be linked directly to the evolutionary radiation of angiosperms.

The evolution of the flower—a key feature of angiosperms—opened possibilities for more specialized

Thousand years ago

relationships between plants and pollinators. The pollination of earlier vascular plant taxa depended mainly on the wind (as in conifers) or on beetles (as in cycads), which mainly consumed pollen and only occasionally transferred some of it to other plants. With the evolutionary diversification of flowers came nectar rewards, and diversification in flower shapes, colors, and scents increased the reliability of insects in moving pollen to receptive stigmas on other flowers of the same species. Today some of the best-known examples of mutualisms are those of flowers and their pollinators. The evolution of fruits similarly opened new possibilities for interactions between plants and their animal seed dispersers (and seed predators).

Another important difference between angiosperms and the groups that preceded them is that angiosperms include many herbaceous and small woody species that often have faster growth and life histories. In conifers, seed maturation can take from six months to three years, while almost all angiosperms can mature seeds in just weeks to months. More generally, angiosperms encompass an extremely wide diversity of plant architectures, life histories, and reproductive modes.

At the end of the Cretaceous, tropical forests were found in what is now southeastern North America. These forests had open canopies with relatively tall conifers similar to Sequoia and Metasequoia (Taxodiaceae), with an understory consisting of a variety of angiosperms, including rosette palms. At this time, the middle of North America was covered by a vast shallow sea. The areas west of the sea were covered by a broad-leaved evergreen community with a variety of species like those in the southeast. Temperate forests, dominated by broad-leaved evergreen conifers, cycads, and ginkgos, covered what is today the northwestern United States and southwestern Canada. In those forests, the angiosperms were gen-

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erally deciduous species growing along streams and in other disturbed habitats. The far northwestern areas of North America—the area above 60° N latitude—were covered by broad-leaved deciduous forests with a mixture of conifers (such as Taxodiaceae and Gingko) and angiosperms, including species related to birches (Betulaceae) and elms (Ulmaceae). The milder climates of the Cretaceous allowed these broad-leaved communities to exist where now there is taiga and tundra.

The Cretaceous-Tertiary (K-T) Boundary

The Mesozoic era came to an end 65 million years ago. There is strong evidence that a large asteroid (10 ± 4 kilometers in diameter) slammed into what is now the Caribbean Sea near the current Yucatán Peninsula. The asteroid struck from the southeast, throwing vast amounts of debris into the atmosphere to the north. This debris eventually settled to the ground, forming a distinct geological layer marking the transition between the Cretaceous and Tertiary periods, known as the K-T boundary.

How this asteroid impact led to massive global change—including the extinction of the dinosaurs—is still an area of active research and controversy. Under one hypothesis, the debris thrown up by the impact may have been sufficiently dense to darken the skies for long enough that photosynthesis was largely shut off. Other evidence suggests that the impact radiated enough heat to start massive and widespread fires that lasted for several years. The debris remaining in the atmosphere from the impact and the fires is likely to have led to a drastic cooling in the world’s temperatures for a period of years to decades. It has also been hypothesized that the disruption in the global carbon cycle caused by the impact led to major oscillations in global climates for the next million years. It is now generally accepted that this event is tied to the extinction of the dinosaurs, along with 70% of the marine species that existed at the time.

In North America, the flora of the Paleocene epoch contained many fewer species than the flora of the late Cretaceous. In the southern part of the continent, almost 80% of existing plant species became extinct, while polar forests had a 25% extinction rate. Even as far away from the impact as New Zealand, a recent study of fossil pollen and spores found that a diverse flora consisting of gymnosperms, angiosperms, and ferns was abruptly replaced with one consisting of a small number of fern species (Vajda et al. 2001). Recovery occurred only gradually over more than a million years, with the eventual evolution of many new species, particularly angiosperms.

The Cenozoic Era

In the early Eocene epoch, global temperatures began to fall, following falling global CO2 levels. In North America,

Paleoecology 425

the Rocky Mountains and Sierra Nevada continued rising, creating a mid-continent rain shadow (see Chapter 18). Along with continued mountain building in the Himalayas, these geological changes led to a cooling in the global climate. The cooling was strongest in the Southern Hemisphere: the South Pole was covered by ice during the Oligocene epoch, but the North Pole was not. This cooling may have been reinforced by another effect. Massive mountain building during this period resulted in the weathering of large amounts of rock: chemical reactions during weathering remove CO2 from the atmosphere (Ruddiman and Kutzback 1989). Temperatures remained high enough, though, that throughout the Miocene epoch the vegetation was tropical even at high latitudes.

These changes in climate had important ecological consequences. By the Oligocene epoch, broad-leaved evergreen forests in the middle of North America had thinned out to tropical woodlands and savannas. Grasses, which originated in the Paleocene, began to spread widely during the Miocene. Grass eaters evolved from ancestors that ate leaves from trees or shrubs. Most notably, horses evolved in North America during this time. (Eventually horses migrated to Eurasia over the land bridge between Alaska and Siberia, and went extinct in North America 10,000 years ago. Today’s wild herds in North America are descendants of horses brought by the Spanish conquistadores just 500 years ago.)

The drop in CO2 levels may have had a second important effect on plant ecology beyond the change in temperature. Studies of the isotopic signatures of fossil plants show that C4 angiosperms first evolved from C3 ancestors during the Oligocene. C4 plants are more efficient than C3 plants at CO2 uptake (see Chapter 2), especially at low CO2 concentrations. During the late Miocene, tropical and subtropical grasslands dominated by C4 grasses spread widely, especially between 7 and 8 million years ago. Their spread has often been explained as a response to the drop in CO2 concentrations. However, the drop in CO2 had already become substantial by the early Miocene, some 16 million years earlier. So, the delay in the spread of C4 grasslands is unexplained. Researchers have suggested that climate change, rather than changes in global CO2 concentration, might be the most important factor in the rise of C4 grasslands (Pagani et al. 1999).

We can look at changes in the distribution of C3 and C4 grasses in response to much more recent climate changes in order to understand what might have happened during the Miocene. Studies of fossils from 10,000 years ago in two lake beds in Mexico and Guatemala support the hypothesis that climatic drying was responsible for the local spread of C4 grasses (Huang et al. 2001). Atmospheric CO2 concentrations were the same at both sites, and thus can be ruled out as a cause of differences in the vegetation. At the Mexican site, C3 grasses spread

426 Chapter 21

as the local climate become rainier at the end of the most recent glaciation; at the Guatemalan site, C4 grasses spread as the local climate become drier. Studies of grasslands in southern Africa also suggest that the relative proportion of C3 to C4 plants varies with climate (Scott 2002).

How will the Earth’s vegetation change in response to the current rise in global CO2 levels? Will it favor the spread of C3 plants over C4 plants (see Chapter 22)? It seems likely that the answer will depend on how changes in the abundance of CO2 affect regional climates.

Paleoecology Methods

How do we know so much about the history of plant communities? Studies of the past use a variety of techniques. Of primary importance is the study of fossils. From these remains of leaves, stems, flowers, and other plant parts, we can reconstruct past communities and climates (Prentice et al. 1991).

The sources of fossils depend on the habitat where they were formed. Because most organic material decomposes quickly, anoxic—oxygen-free—conditions are ideal for its preservation. In nonmarine habitats, these conditions are most often found at the bottoms of lakes and bogs. Sediments formed in lakes and bogs are the primary source of macrofossils, such as leaves, flowers, stems, and seeds. For information from hundreds of thousands or millions of years ago, we rely on hardrock fossils, in which the organic material has been replaced by nonorganic material. Material from more recent periods may still be organic. Bogs, which have highly acidic waters that slows decay, are especially useful in this regard.

Fossils can be retrieved from lakes and bogs by removing a core from the sediments at the bottom. A long tube or pipe, open on the bottom, is pushed down into the sediments, and the core is then carefully drawn up. Thin slices of the core are examined; fossils, if present, are identified, and the slices are aged using isotopic dating.

In addition to macrofossils, these cores yield pollen grains (Figure 21.7) and other microfossils. Palynology is the study of spores and pollen. Wind-borne pollen often lands in a bog or lake and settles to the bottom. Because pollen can travel long distances, cores from lake or bog sediments provide a profile of the community over a wide area around the body of water. In contrast, macrofossils usually provide a picture only of the plants growing immediately adjacent to the area studied.

Palynology has strengths and weaknesses as a method of studying the past. One strength is that pollen is readily preserved, and large amounts of it can usually be found. A weakness is that not all species are present or equally represented, or even identifiable. Only the pollen of wind-pollinated species makes it into lakes and bogs in quantity; animal-borne pollen is largely missing. Thus, sediment core samples are biased toward trees—

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because most temperate-zone trees are wind-pollinat- ed—and wind-pollinated herbaceous groups, especially grasses and sedges.

The level to which pollen can be identified varies among taxa. Pollen from some trees can be identified to species. In many genera, however, only subgenera can be distinguished. For example, among pines, the pollen of Pinus resinosa (red pine, Pinaceae) and P. banksiana (jack pine) cannot be told apart, although they can be differentiated from that of P. strobus (white pine). In some cases, such as the grasses, pollen can be identified only to the family. Despite these limitations, palynology has been key to reconstructing communities and migration patterns in temperate regions.

Pinus contorta

(lodgepole pine, Pinaceae)

Acer rubrum

(red maple, Sapindaceae)

Quercus garryana

(garry oak, Fagaceae)

Chenopodium album

(lamb’s quarters, Amaranthaceae)

Ipomoea wolcottiana

(bindweed,

Convolvulaceae)

10 m

Figure 21.7

Pollen grains preserved in sediments can be used to identify plant species present at some time in the past. (After Pielou 1991.)

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Another way in which plant material can be preserved is by drying. This means of preservation has provided macrofossils in arid regions of North America where packrats (Neotoma) live. As their name implies, these rodents accumulate large amounts of plant material, which they store in their dens. They urinate on the plant material, caking it with mineral salts. Urination provides substantial evaporative cooling for the den and, incidentally, provides excellent conditions for preservation of the plant material. The accumulations of packrats are referred to as middens. A given den site may be used for centuries by successive generations of packrats, with its middens growing to several meters across. A fossilized midden may be preserved for thousands of years in a dry climate. Samples from packrat middens can be radiocarbon-dated and the plant material identified, allowing researchers to reconstruct changes in the plant community surrounding the den site (Cole 1985; Betancourt et al. 1990). In Arabia and Africa, the rock hyrax creates a similar structure, called a hyraceum.

The Recent Past

We now leave the distant past and jump ahead to a point just 20,000 years before the present, when much of the Northern Hemisphere was covered by glaciers. Over the previous 2 million years, during the Pleistocene epoch, Earth had experienced a period of major glaciations. Starting about 850,000 years ago, there were gradual periods of glacial buildup, followed by abrupt switches to warmer weather and short periods of glacial retreat (called interglacials) (Figure 21.8). Each of these periods of alternating cold and warm weather lasted about 100,000 years. These cycles were caused mainly by changes in the tilt and precession of Earth’s orbit (see Chapter 18). The various glacial advances and retreats are known by various names. In North America, the most recent glacial advance is called the Wisconsin glaciation, named for one of the locations of its southernmost extent (Figure 21.9). Because each glacial advance destroyed most of the traces of previous ones, we focus here on the Wisconsin glaciation, for which the best data exist.

The last warming period began about 12,000 years ago. Since that time, the average global temperature rose, peaked about 7000 years ago, and then declined (Figure 21.8B,C). If not for human activities bringing on a new, rapid period of global warming, we would likely be seeing the growth of glaciers in the next few thousand years.

Figure 21.8

Mean global temperatures over geological time for various periods of time: (A) the past 850,000 years; (B) the past 140,000 years; (C) the past 10,000 years; (D) the past 1,000 years. (After Gates 1993.)

Paleoecology 427

Thousand years ago

(B)

2Sangamonian interglacial

—2

—4

428 Chapter 21

What has all of this climate change meant for plants? As glaciers advanced and retreated, plants in northern latitudes migrated across continents, following moving climate zones. In the Tropics, these climate changes resulted in the contraction of forests and the expansion of grasslands. Our study of these changes in plant distributions has led to new understanding of the causes of diversity patterns across the globe, as well as new insights into the nature of plant communities.

At the Glacial Maximum

At the maximum of the Wisconsin glaciation, large continental glaciers extended as far south as 40° N latitude in North America and 50° N in Eurasia, while smaller

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glaciers were found in mountainous regions such as the Rocky Mountains in North America, the Himalayas in Asia, and the Alps in Europe. Few glaciers formed in the Southern Hemisphere, excepting Antarctica, because there is little continental land area at high latitudes, although some glaciers existed in the Andes mountain range of South America. Regional climatic conditions maintained some glacier-free regions even at high latitudes. For example, some coastal regions remained free of glaciers in Canada and Alaska.

With so much water tied up in glaciers, sea levels were lower than today’s by 85 to 130 meters, resulting in coastal areas that were substantially broader than at present. Of particular importance to plants were the

Figure 21.9

The greatest extent of continental glaciers during the most recent glaciation in (A) North America and (B) Eurasia. (After Mayewski 1981 and Siegert et al. 2002.)

St. Petersburg

France