Geoscience Reference
In-Depth Information
Figure 5.4 Forest structure within a Neotropical seasonal forest.
(Illustration by Jeff
Dixon.)
50 ft (15 m) or more in height, often with one dominant species. In the next layer,
various trees about 40 ft (12 m) tall co-dominate. Many are broadleaved with
smooth, thin bark. Lianas, palms, and some cacti also reach the canopy layer.
Woody vines and lianas can be abundant. Both trees and woody vines tend to have
conspicuous flowers that appear at the start of the dry season. The majority of for-
est tree and vine species have seeds that are wind dispersed.
Beneath the canopy, tall shrubs and small trees up to 16 ft (5 m) in height form
the tallest understory layer. Shrubs up to 3 ft (1 m) in height typically form the next
layer. The lowest layer is composed of ground-level herbs that generally are present
only during the summer rainy season. Epiphytes, with the exception of cacti and
bromeliads, are typically sparse or absent in these forests.
In drier forests, the canopy is much lower with fewer layers. Dry evergreen for-
est trees in the upper canopy can be as tall as 23 ft (7 m), with a lower canopy at
10 ft (3 m). Woody vines increase as the forest gets drier. Depending on the forest,
the understory can be dense or quite open, as in woodland areas. The ground layer
is also varied, although there tends to be less vegetation on the forest floor.
Considerable underground biomass exists in seasonal forests because trees de-
velop extensive root systems to store and retrieve water and nutrients during the
dry season. Most trees and some vines have mycorrhizal relationships that facili-
tate nutrient and water absorption.
Many of the plant families in the seasonal forests are found in the rainforest,
although the species are different. Diversity is about 50 percent of that in the Neo-
tropical rainforest. Unexpectedly, the diversity within the dry forest region
increases farther away from the wet forest. For example, the dry forests of Mexico
tend to be the most diverse of the Neotropical seasonal forests, mostly due to their
unique species composition. The legume (Fabaceae) family is by far the most spe-
cies-rich family in all areas with the exception of the Caribbean where the myrtle
(Myrtaceae) family predominates. Other common families found in Neotropical
seasonal forests are the trumpet vine (Bignoniaceae), coffee (Rubiaceae), spurge
(Euphorbiaceae), vochysia (Vochysiaceae), soapberry (Sapindaceae), and caper (Cap-
paridaceae) families. Plants in the cactus (Cactaceae) and coca (Erythroxylaceae)
