Dry Tropical Forests

 
People are often surprised to find that tropical forests extend well into North America.   These are, in fact, important corridors of northward dispersal for many plants and animals with tropical ancestry.

The dry tropical forests exist as two basic types, thornscrub and deciduous forest, distinguished chiefly by annual rainfall, frost severity, and altitude.

Dry tropical forests generally have summer and winter rainy seasons interspersed with long drought periods when little or no rain falls.

 The tropical thornscrub biome occurs at lower elevations where rainfall rarely exceeds 15-20 inches per year.  It is characterized by mixed thorny shrubs and trees and columnar cacti such as cardon hecho (Pachycereus pectin-aboriginum), organ pipe cactus (Stenocereus thurberi) and others.  The columnar cacti frequently penetrate the relatively-open thornscrub canopy.

The tropical deciduous forest biome occurs at moderate elevations where rainfall typically exceeds 20-25 inches per year. The trees are considerably taller here than in thornscrub and the columnar cacti rarely penetrate the canopy. The plants and animals in this community are sensitive to cold temperatures near freezing which defines the limits of northward dispersal for many tropical species of plants and animals.

There are, of course, ecotones (transition zones) between these biomes and the pine-oak woodland above them, where the distinguishing characteristics are less obvious.

These environments are home to many species of tropical plants and animals.