Vegetation Zones of the Southwest - Design with the Desert: Conservation and Sustainable Development - page 134

to and potential water loss from the ecosystem. The aridity of an ecosystem increases as
water gain from precipitation falls below potential water loss from evaporation and tran-
spiration. The desert regions of the southwest span the hyperarid to semiarid range under
the UNESCO classification and generally have precipitation/evapotranspiration (P/ET)
ratios of less than 0.50.
Gleason and Cronquist 3 focus on the amount of precipitation relative to demand as well
as the seasonal distribution of precipitation when looking at major patterns of correla-
tion between vegetation and climate. Their Desert type occurs in climates where water
is a limiting factor during both the summer and winter seasons (summer and winter are
here used in the context of the northern hemisphere temperate zone). As will be discussed
in more detail subsequently, the distribution of precipitation does vary by season within
deserts, but in all cases low year-round availability of water severely limits vegetation
pr o duc t iv it y.
While the foundational signature of deserts is aridity as defined by the P/ET ratios,
the precipitation that does occur in them typically arrives in scattered and unpredictable
episodes. While this is true to varying extents in all ecosystems, the aridity of deserts
amplifies its importance. Thus, Noy Mier 4 notes that, in addition to being characterized
by aridity, deserts typically (1) have precipitation that is highly variable throughout the
year and which often occurs in spatially and temporally intermittent events and (2) have
rainfall totals that from year to year are highly unpredictable. Thus, to the foundational
aridity component, Noy Mier adds an uncertainty component. Deserts are then described
as “water controlled ecosystems with infrequent, discrete, and largely unpredictable water
inputs.” Like other ecosystems, but very noticeably in desert ecosystems, production is
triggered by a rainfall event and the size of the subsequent production pulse is dependent
on the magnitude and seasonal timing of the pulse. The biomass produced by this pulse
then is either lost to mortality and consumption or put into a reserve such as seeds or
energy stores in roots and stems. 5
7.2 Desert Plants and Life Histories
The species that populate desert ecosystems were described by Noy Mier 4 as arido-passive .
These are annual and perennial plant species that are dependent on rainfall events to
trigger growth and reproduction and which then pass into a resistant or dormant stage
during dry periods. Noy Mier refers to this life history pattern as the pulse-reserve paradigm.
Grime, from a slightly different perspective, describes such plants as exhibiting either
stress-tolerant or ruderal primary strategies. Primary strategies here refer to similarities in
genetic characteristics that recur widely among species and that cause them to exhibit
similarities in ecology. 6
The concept of stress tolerant and ruderal strategies can best be understood relative
to the competitive strategy. Competition between plant species occurs in all ecosystems;
however, in ecosystems with abundant resources, the primary strategic response to
resource limitation is morphological (see Chapter 8). For instance, if plants with a
competitive strategy are shaded, they initiate shoot growth in an attempt to maximize the
capture of photons. Likewise, the competitive response to water stress is to produce new
roots. 6 In desert ecosystems, a morphological response to water shortage is not always
appropriate because soil moisture is often simply not available in quantities that would