Box Reconstructing ancient plant ecology

Some of the best evidence about fossil plants is microscopic, and indeed the scanning electron microscope (SEM) has revolutionized the levels of detail that paleobotanists can retrieve. In the tropics today there are often vast wildfires that burn up hundreds of acres of forest. Fires may be started by a carelessly thrown cigarette or a bottle that focuses the rays of the sun, but usually the causes are natural; fallen branches and leaves may just be so dry that a chance lightning strike may spark off a huge conflagration that burns for days or weeks.

Wildfires are not always destructive; indeed, many plants rely on occasional fires to clear old timber and to allow new shoots to grow. And the ash from the fire provides phosphorus and other nutrients. Wildfires were common in the past, and particularly in the tropical belt during the Carboniferous when atmospheric oxygen levels may have been higher. Howard Falcon-Lang of the University of Bristol has studied this phenomenon, and he has shown the remarkable detail that may be observed from ancient charcoal, the burned up remnants of wood. When the charcoal is examined under an SEM, it shows distinctive subcellular pits that allow identification of the precise type of tree caught in the fire (Fig. 18.18a). Fine details such tree rings are also preserved, indicating perhaps a seasonal tropical climate like present-day East Africa (Fig. 18.18b).

Close study of the distribution of charcoal and the sedimentology of typical Carboniferous beds in North America shows that fires were commonest in the higher areas, away from the banks of rivers, where plant debris could become very dry (Fig. 18.18c). The wildfires may have been set off by nearby volcanic eruptions. Careful measurements through the sediments suggest that wildfires may have been very frequent in Carboniferous times. They must have been a regular part of the growth and regrowth of forests, as well as destabilizing hill slopes thus triggering occasional landslides.

Read more about Carboniferous wildfires and plant ecosystems in Falcon-Lang (2000, 2003) and at links listed at http://www.blackwellpublishing.com/paleobiology/. Read about another astonishing discovery in the Carboniferous, a huge rain forest in Illinois, catastrophically buried by a sudden rise in sea level, in DiMichele et al. (2007).

Figure 18.18 Carboniferous wildfires and the use of the SEM: (a) ancient charcoal can reveal spectacular details under the SEM, such as cross-field pitting, which provides evidence for which species of plants burned; and (b) part of a tree-ring. Note the transition from thin-walled "early wood" (left) to thick-walled "late wood" (center). The rings of growth may indicate a seasonal tropical environment like northern Australia or East Africa. Study of these plant remains and the sediments shows that wildfires happened every 3 to 35 years, and especially in drier uplands (c). PDP, poorly-drained coastal plain; WDP, well-drained coastal plain. (Courtesy of Howard Falcon-Lang.)

Figure 18.18 Carboniferous wildfires and the use of the SEM: (a) ancient charcoal can reveal spectacular details under the SEM, such as cross-field pitting, which provides evidence for which species of plants burned; and (b) part of a tree-ring. Note the transition from thin-walled "early wood" (left) to thick-walled "late wood" (center). The rings of growth may indicate a seasonal tropical environment like northern Australia or East Africa. Study of these plant remains and the sediments shows that wildfires happened every 3 to 35 years, and especially in drier uplands (c). PDP, poorly-drained coastal plain; WDP, well-drained coastal plain. (Courtesy of Howard Falcon-Lang.)

Cordaites (burning)

Pteridosperm Sphenopsid

Lycopsid (buried)

Cordaites (burning)

Pteridosperm Sphenopsid

Lycopsid (buried)

Red mudstone | Gray mudstone Sheet sandstone Channel sandstone ("v") Calcrete nodule

Debris flow

Upland fires lead to slope destabilization and debris flows

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