The New Neotropical Companion

used for cattle pasture. Sites had experienced light,

medium, or heavy use for up to 13 years and ranged

from two to eight years in age (since abandonment).

In areas subject to light use, succession was rapid,

resulting in a biomass accumulation of about 10 tons

per hectare annually, or 80 tons after eight years. Tree

species richness was high, and many shade- tolerant

forest species were present. Moderately grazed pastures

also underwent rapid succession when abandoned, but

biomass accumulation was only about half what it was

on lightly grazed sites, and tree species richness was

lower as well. Heavily grazed sites remained essentially

covered in grasses and herbaceous species, with few

trees invading, and had a biomass accumulation of

only about 0.6 tons annually per hectare.

The conclusion is that most Amazonian lands

subjected to light or moderate grazing, once abandoned,

regenerate secondary forest. Succession to secondary

forest was impeded only in areas subject to intensive

grazing for long periods; such areas were estimated to

represent less than 10% of all pastureland in the region

studied. This topic is discussed further in chapter 18.

Fire in the Amazon?

While you are standing in a rain forest experiencing

100% relative humidity, the dew point thoroughly

oppressive, watching in wonder at the intensity of the

rain deluging everything around you, the thought of

the rain forest catching fire and burning seems at best

a fanciful notion. And yet, as evidence assembled by

Chris Uhl and colleagues (1988b) suggests, for the past

few thousand years, fire has been an important natural,

large- scale disturbance factor throughout Amazonia.

There is an abundance of charcoal residue in central

and eastern Amazonian soils, and radiocarbon dating

of the sediments from the Venezuelan Amazon

along the upper Río Negro indicates that during the

past 6,000 years there have been several major fires,

occurring perhaps during periods of extended dryness.

Recall that climatic variability is not unusual over

long periods of time, even in Amazonia. Large- scale

Amazonian fires, even if infrequent, add yet another

disturbance dimension to the dynamics of the region’s

rain forests.

Today human- set fires occur frequently in tropical

forests and are of concern to conservation biologists

(discussed below). There is evidence that frequent fires

may result in the blocking of normal succession and the

conversion of areas of forest into savanna (chapter 18).

Severe fires in the tropics are more frequent in drought

years during severe El Niño/Southern Oscillation events.

When Succession Does Not Succeed

Though uncommon, it is possible for secondary

succession to become arrested and for vegetation

development from an open, disturbed site to

secondary forest to cease. When this happens, forest

is lost indefinitely. Several factors in combination may

act to stop normal succession. One is the presence of

invasive plant species, such as exotic grasses, that grow

densely enough to prevent other species from growing.

Others are frequent fire or heavy grazing by livestock.

Plate 7- 14. Pastures, such as this one in Panama, have the

capacity to undergo secondary succession and return to

forest. Photo by John Kricher.

Plate 7- 13. The former Mayan city of Tikal, in Guatemala, as it

looks today. Photo by John Kricher.

chapter 7 if a tree falls . . . rain forest disturbance dynamics 103