Vegetation Zones of the Southwest - Design with the Desert: Conservation and Sustainable Development - page 135

reward growth. Survival in desert ecosystems, therefore, requires different responses to
resource limitations.
Plants having stress tolerance as their primary strategy show an amazing variety of adap-
tations that allow them to endure unfavorable conditions. Walter 7-9 reviewed the diversity
of adaptations to aridity that have evolved and these are further discussed in Chapter 8.
For our purposes, it is enough to emphasize that the common factor among plants hav-
ing the stress-tolerant primary strategy is that they cease active growth and reproduction
when environmental conditions become unfavorable and then endure until conditions
again become suitable. It is their ability to endure during times of resource limitation and
then quickly respond when resources become available that characterizes this strategy.
Grime 6 lists common attributes of stress tolerators found in arid regions as long lived, slow
growing, evergreen, and having mechanisms allowing rapid uptake of resources when
they are available. Thus, unlike plants having the competitive strategy, the response of
stress tolerators to environmental variation is physiological rather than morphological.
Plants that have the ruderal strategy accommodate resource limitations in a fundamentally
different manner. In general, the ruderal strategy is considered to be an adaptation to
disturbances that limit plant biomass by partial or total destruction. 6 Numerous ephemeral
desert plants have developed life histories that tolerate the “destruction” of vegetative
biomass through desiccation and which then endure between suitable rainfall events as
seeds. Ruderals differ from both competitors and stress tolerators in that they tend to be
short lived, fast growing, and, in most cases, seed production is followed immediately by
the death of the parent plant.
Across the southwestern deserts, species composition varies but plant strategies of
the component species are remarkably consistent. Desert ecosystems are dominated by
stress tolerators and ruderals. As the P/ET ratio increases in the less arid communities
(such as those found at higher elevations), one begins to observe plant life histories that
increasingly incorporate aspects of the competitive strategy (resulting in life histories
that are compromises between the dominant stress-tolerant and ruderal strategies and
the competitive strategy—secondary strategies in Grime's system 6 ). Thinking about plants
from the perspective of what strategies best allow them to cope with the environmental
constraints they experience allows one to see fundamental patterns that knit together the
diverse mosaic of ecosystems covering the landscape.
7.3 Desert Ecosystems of the Southwestern Region
A vegetation formation is a large unit of vegetation that has similar form and structure
( physiognomy ) of the most conspicuous plants. 10 Brown et al. 11 describe four major
subdivisions within the Desertscrub Formation of the western United States and northern
Mexico. Here, I shorten Brown's classification of “desertscrub” to the more common
“desert.” It is important to note that this common physiognomy, which characterizes all
four North American deserts, reflects the common stress-tolerant life history shared by
dominant species in all deserts. The Desertscrub Formation is subdivided into four major
subdivisions based on climate and dominant species. 5,11 These are the Great Basin Desert
(roughly centered in Nevada, Utah, and northeastern Arizona), the Mojave Desert (centered
in southeastern California and southern Nevada), the Chihuahuan Desert (centered in
north-central Mexico but extending into southern Arizona and southwestern Texas),