Variation in Gas Exchange Characteristics Among Desert Plants
363
experiences both winter and summer precipitation regimes; since the Sonoran
Desert is a low-elevation desert, plants may be active at any time of the year
when precipitation is received.
Species and life-form abundance vary substantially along elevation clines
(Hastings and Turner 1965b), reflecting both changes in precipitation and
temperature. Whittaker and Niering (1965) described a progression from
predominance of trees to increased abundance of suffrutescent shrubs as
precipitation decreases. In particular, they noted a higher species diversity
and greater life-form diversity in the kinds of leaf and stem morphologies
of plants in drier desert locations. Similar trends in the Sonoran Desert
vegetation have also been described by Phillips and MacMahon (1978) and
Bowers and Lowe (1985). As first discussed by Schulze (1982), these patterns
relate to variation in gas exchange characteristics in terms of both photosynthetic pathways and seasonality of carbon gain activities.
Within the Sonoran Desert, elevation-related transects are usually also
related to substrate variation (Fig. 18.1). Upper elevations of a range typically have shallow, coarse soils; the gradient from upper rocky slopes along
a bajada and ending up at a playa is associated with an increase in the fine
particle structure of soils. At various places along a bajada, washes erode
through the alluvium, creating microhabitats that are coarser in structure
and typically have greater soil moisture availability.
Of interest to this discussion is that a dominant life form will often persist
over this entire transect, including wash and slope microhabitats. One
species is typically dominant at a site and there is usually a single-species
replacement pattern along these elevation gradients. For instance, droughtdeciduous shrubs are a key component of virtually all Sonoran Desert plant
communities. In most situations, there is just one dominant species at a
location (Table 18.1), but in both wash and bajada slope microhabitats,
there is a continual replacement of species with elevation. Encelia virginensis
may dominate slope bajada sites at upper elevations, but it is replaced first
by Ambrosia dumosa, and then by E. farinosa at lower elevation sites (Fig.
18.1). Similarly, while Viguiera multiflora predominates at upper elevation
wash sites, E. frutescens dominates the lower elevation sites.
18.3 Variation in Moisture and Temperature as Selective Forces
for Photosynthetic Variation
Availability of moisture is the primary feature influencing plant productivity
and overall performance. To better understand the possible benefits and
disadvantages of different gas exchange characteristics, it is useful to know
the absolute moisture inputs and the variability of those inputs. The predictability of moisture inputs on both a seasonal and an interannual basis is
363
experiences both winter and summer precipitation regimes; since the Sonoran
Desert is a low-elevation desert, plants may be active at any time of the year
when precipitation is received.
Species and life-form abundance vary substantially along elevation clines
(Hastings and Turner 1965b), reflecting both changes in precipitation and
temperature. Whittaker and Niering (1965) described a progression from
predominance of trees to increased abundance of suffrutescent shrubs as
precipitation decreases. In particular, they noted a higher species diversity
and greater life-form diversity in the kinds of leaf and stem morphologies
of plants in drier desert locations. Similar trends in the Sonoran Desert
vegetation have also been described by Phillips and MacMahon (1978) and
Bowers and Lowe (1985). As first discussed by Schulze (1982), these patterns
relate to variation in gas exchange characteristics in terms of both photosynthetic pathways and seasonality of carbon gain activities.
Within the Sonoran Desert, elevation-related transects are usually also
related to substrate variation (Fig. 18.1). Upper elevations of a range typically have shallow, coarse soils; the gradient from upper rocky slopes along
a bajada and ending up at a playa is associated with an increase in the fine
particle structure of soils. At various places along a bajada, washes erode
through the alluvium, creating microhabitats that are coarser in structure
and typically have greater soil moisture availability.
Of interest to this discussion is that a dominant life form will often persist
over this entire transect, including wash and slope microhabitats. One
species is typically dominant at a site and there is usually a single-species
replacement pattern along these elevation gradients. For instance, droughtdeciduous shrubs are a key component of virtually all Sonoran Desert plant
communities. In most situations, there is just one dominant species at a
location (Table 18.1), but in both wash and bajada slope microhabitats,
there is a continual replacement of species with elevation. Encelia virginensis
may dominate slope bajada sites at upper elevations, but it is replaced first
by Ambrosia dumosa, and then by E. farinosa at lower elevation sites (Fig.
18.1). Similarly, while Viguiera multiflora predominates at upper elevation
wash sites, E. frutescens dominates the lower elevation sites.
18.3 Variation in Moisture and Temperature as Selective Forces
for Photosynthetic Variation
Availability of moisture is the primary feature influencing plant productivity
and overall performance. To better understand the possible benefits and
disadvantages of different gas exchange characteristics, it is useful to know
the absolute moisture inputs and the variability of those inputs. The predictability of moisture inputs on both a seasonal and an interannual basis is