Vegetation Zones of the Southwest - Design with the Desert: Conservation and Sustainable Development - page 133

7
Vegetation Zones of the Southwest
Ward W. Brady
CONTENTS
7.1 Introduction ........................................................................................................................ 135
7.2 Desert Plants and Life Histories ...................................................................................... 136
7.3 Desert Ecosystems of the Southwestern Region ........................................................... 137
7.3.1 The Sonoran Desert ............................................................................................... 138
7.3.2 The Chihuahuan Desert ....................................................................................... 141
7.3.3 The Mojave Desert ................................................................................................. 143
7.3.4 The Great Basin Desert ......................................................................................... 145
7.3.5 Correlation of Vegetation and Climate above the Desert Background .......... 146
7.4 Conclusion .......................................................................................................................... 149
References ..................................................................................................................................... 149
7.1 Introduction
Part of the fascination and beauty of the southwestern region of the United States and
northern Mexico arises from the complex mosaic of ecological communities that are
spread across the landscape. While it is apparent to any traveler that the dominant theme
of this mosaic is desert, patches of grassland, chaparral, forest, and other vegetation types
intertwine as elevation changes and watercourses dissect the region. Previous chapters
in this topic (see Chapter 1) have given a general overview of the deserts of the world
and have provided brief discussions of geology, soils, climate, and hydrology specific to
the southwestern arid region (see Chapters 2 through 4). The objective of this chapter is
to discuss how these environmental factors interact with plant strategies to produce the
observed mosaic of communities and to proceed to a broad discussion of the communities
themselves. This then will provide a context for discussion, in subsequent chapters, of
native wildlife populations (see Chapter 9) and anthropogenic changes that are rapidly
changing the fabric of this landscape.
From a landscape ecology perspective, desert communities can be considered the matrix
of the landscape. That is, desert communities are the dominant and background ecological
system across the area. Because this matrix is itself quite complex and variable across the
southwestern region, it is important to define its common features. First, while the term
desert itself is difficult to rigorously define, it nevertheless generally includes (1) the idea
of dryness due to the combination of low precipitation and high temperatures and (2) the
idea of a resultant sparseness of vegetation. UNESCO and UNEP 1,2 formalized the concept
of aridity when they defined arid regions based on the relationship between water input
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