M. Rasoul Sharifi

Comparative Water Relations of Phreatophytes in the Sonoran Desert of California

The seasonal and diurnal water relations were compared among six desert phreatophytes, two evergreen shrubs, and one deciduous shrub. All species were located in one wash woodland in the Sonoran Desert of southern California. There are several mechanisms by which these phreatophytes have adapted to the desert environment. One group of winter-deciduous phreatophytes (Olneya tesota, Prosopis glandulosa, and Acacia greggii) experienced summer midday leaf water potentials below -4.0 MPa. These phreatophytes had a series of physiological mechanisms for tolerating summer water stress, including seasonal and diurnal osmotic adjustment and the maintenance of high leaf conduc- tance at low leaf water potential. Osmotic adjustment of these three phreatophytes was similar to or greater than that of two evergreen species (Larrea tridentata and Simmondsia chinensis). Dalea spinosa, a stem-photosynthetic phreatophyte, avoided water stress by maintaining a very small leaf area. The summer-deciduous phreatophytes (Hyptis emoryi, and Chilopsis linearis) demonstrated mechanisms of drought avoidance such as change in leaf biomass and low summer leaf conductance. Little osmotic adjustment occurred in the summer-deciduous phreatophytes. The phreatophytic species studied in this investigation have evolved adaptations to water stress that are similar to those of deciduous and evergreen shrubs of the Sonoran Desert. Desert phreatophytes are a complex group of species with varied adaptive mechanisms to tolerate or avoid drought and should not be considered simply as a group of species that avoid desert water stress by utilizing deep ground water unavailable to other desert species of drought tolerance and avoidance.

Surface Dust Impacts on Gas Exchange in Mojave Desert Shrubs

Windblown dust, an environmental problem in many disturbed arid lands, has the potential to affect the physiological performance of desert shrubs. Physiological parameters of gas exchange for three species (Larrea tridentata, Hymenoclea salsola and Atriplex canescens) were measured at a Mojave Desert site, at which both undisturbed and heavily dusted individual shrubs occurred. Maximum rates of net photosynthesis (A) of dusted organs were reduced to 21% of those of control plants in resinous leaflets of Larrea, to 44% in resinous leaves and photosynthetic stems of Hymenoclea, and to 58% in non-resinous C 4 leaves of Atriplex, which have vesiculated trichomes. Dusted plants of all three species showed reduced maximum leaf conductance (g s ), transpiration (E) and instantaneous water-use efficiency (A/E). Intrinsic water-use efficiency (A/g s ) was also reduced, except in Atriplex, in which it remained unchanged. Temperatures of dusted leaves and photosynthetic stems were 2-3°C higher than those of control plants, due to greater absorptance of infra-red radiation. Dust also significantly increased photosynthetically active radiation (PAR) reflectance. Heavily dusted shrubs had smaller leaf areas and greater leaf-specific masses, suggesting that the short-term effects of reduced photosynthesis and decreased water-use efficiency may cause lowered primary production in desert plants exposed to dust during seasons when photosynthesis is occurring.

Summer water relations of the desert phreatophyte Prosopis glandulosa in the Sonoran Desert of southern California

SummaryProsopis is a genus of phreatophytic trees inhabiting hot deserts and semiarid grasslands of the world. Although desert trees are exposed to unusual environmental temperature and water stress, few investigations have evaluated their water relations. This is particularly true for Prosopis species growing in areas where a large portion of their water use comes from ground water.Water relations components for Prosopis glandulosa were studied at Harpers Well, near the Salton Sea, California during the summer months of 1980. Maximum temperatures (49° C), irradiance (2,000 μE/m2/sec), and vapor pressure deficit (5.3 kPa) were reached in July. During this time Prosopis glandulosa predawn xylem pressure potentials were below-3.0 MPa. Prosopis glandulosa at Harpers Well is able to maintain open stomata during high temperatures, high vapor pressure deficit and at low estimated turgor pressure. Leaf resistance measurements indicate that stomata are open primarily in the morning, but may reopen in the afternoon in trees with greater water resources. Osmotic potentials of juvenile shoots were higher (-1.0 to-2.5 MPa) than mature shoots (-3.5 MPa). Estimated turgor potential remained low (0.1–0.2 MPa) during the morning and early afternoon. Estimated turgor pressure increased from August to September as temperatures and vapor pressure deficit decreased. Leaf conductance was strongly associated with leaf vapor pressure deficit and estimated turgor potential but poorly associated with xylem pressure potential. Prosopis stomata seem to be uncoupled from tissue water potential until-4.8 MPa is reached.

Conopy architecture of Larrea tridentata (DC.) Cov., a desert shrub: foliage orientation and direct beam radiation interception

SummaryAt sites in the United States, creosote bushes (Larrea tridentata (DC.) Cov.) orient foliage clusters predominantly toward the southeast. Foliage of bushes at the southernmost distribution extreme in Mexico shows no predominant orientation. Clusters at all sites are inclined between 33° and 71° from the horizontal. Inclinations are steeper in the drier and hotter Mojave Desert than in the Chihuahuan Desert. Individual leaflets, though not measured, appear more randomly oriented than foliage clusters. In several populations studied, branches were shorter in the southeastern sectors of the crown, reducing self-shading early in the morning. Measurements of direct beam radiation interception by detached branches, using digital image processing, indicated that foliage clusters oriented toward the southeast exhibited less self-shading during spring mornings than clusters oriented northeast. This effect was not apparent at the summer solstice. This type of canopy architecture may tend to minimize self-shading during the morning hours when conditions are more favorable for photosynthesis, resulting in an improved daily water use efficiency.

Photosynthesis of cryptobiotic crusts in a seasonally inundated system of pans and dunes at Edwards Air Force Base, western Mojave Desert, California: Laboratory studies

Summary Cryptobiotic crusts (biological soil crusts) are a dominant component of a unique system of dunes and intermittently inundated pans and playas situated on Pleistocene Lake Thompson bed at Edwards Air Force Base, California. Previous work reported on basic ecological parameters including distribution, abundance, and species composition of these algal (cyanobacterial)-dominated crusts.As a step toward building an ecological model in support of the military installation natural resources management plan, we determined the photosynthetic rate of the algal crusts under controlled laboratory conditions using a portable photosynthesis system. We examined the relation between the environmental conditions of light intensity, crust moisture content, antecedent crust moisture content, and atmospheric CO 2 concentration, on photosynthetic rates. Maximal photosynthetic rates were 4.97 and 7.75 μmol m −2 s −1 for cryptobiotic crusts collected from dunes (upland crusts) and from pans (aquatic remnant crusts), respectively, which corresponds well with the upper range of previous reports for such algal crusts. Light saturation levels were about 1500 and 1500–2000 μmol m −2 s −1 for cryptobiotic crusts from both the dunes and the pans respectively. Optimal moisture content of the crusts for photosynthesis was 15–30% and 90–100% for crusts from dunes and pans respectively; both lower and higher moisture contents resulted in decreased rates of photosynthesis. With increasing atmospheric CO 2 concentrations up to at least 1000 ppm, photosynthesis of both sorts of crusts increased linearly to rates several times that of rates at natural CO 2 concentrations.

Structural and physiological adaptation to light environments in neotropical Heliconia (Heliconiaceae)

Influence of habitat on physiological and structural characteristics was investigated for broad-leaved tropical monocotyledons in the genus Heliconia (Heliconiaceae). Seven species were selected from three different light regimes, enabling an analysis of the extent to which this genus has adapted its photosynthetic strategies and morphological characteristics to different daily photon flux densities (PFD). Predictably, light response curves showed a clear gradient with respect to light saturation and rates of maximum net assimilation ( A max ). Heliconia latispatha , an open site species, showed saturation at higher PFD (1400 μmol m −2 s −1 ) and higher A max (14–16 μmol m −2 s −1 ) than H. mathiasiae , a forest edge species (PFD 1000 μmol m −2 s −1 ; A max 7.5–8.5 μmol m −2 s −1 ) and H. irrasa of deep-shade forest understorey (PFD 250 μmol m −2 s −1 ; A max 3.5 mol m −2 s −1 ). Leaf blade areas were largest in open sites, and leaf specific mass was also significantly higher, but leaf support efficiency was highest in understorey species. Species in open sites had thicker leaves with more chlorenchyma, whereas deep-shade species had very thin leaves and low stomatal densities. These rapidly growing herbaceous perennials appear to allocate much of their above-ground biomass to leaf tissues and have a relatively low investment in support tissues. This contrasts with understorey palms, in which leaf form and structural investment has been interpreted as a trade-off between economy and protection against tissue loss from falling branches. Presence of below-ground rhizomes in Heliconia may provide the key to this difference.

Resprout characteristics of creosote bush (Larrea tridentata) when subjected to repeated vehicle damage

Abstract Studies were conducted in the central Mojave Desert to quantify how creosote bushes (Larrea tridentata) respond to physical damage during large-scale military training exercises. Creosote bush possesses a resilient growth form that recovers from repeated physical damage via resprouts arising from meristems in stem bark below severed or crushed canopy units. At high levels of disturbance by heavy vehicles, nearly all individuals showed measurable breakage, but without additional damage each plant can regain a full canopy within 5 years under arid field conditions. Resprouts exhibited more vigorous growth and doubled the biomass accumulation stimulated by high rainfall of 1998, an El Nino year, vs. a normal year. New shoots of resprouted individuals were markedly different in morphological traits than canopy old growth and had slightly higher predawn shoot water potentials. The natural ability of this evergreen species to recover from cutting and crushing bodes well for re-establishment of creosote bush desert scrub communities following episodes of severe damage by vehicles.

Carbon isotope discrimination and resource availability in the desert shrub Larrea tridentata.

Publisher Summary Carbon isotope ratios (δ 13 C) in plant leaf tissues provide a potentially valuable means of estimating the assimilation weighted mean daytime concentration of intercellular CO 2 ( c i ) at the phenological stage when that foliar carbon was fixed. This chapter illustrates the application of carbon isotope techniques to understand the mode of physiological adaptation used by Larrea tridentate—an ecologically important warm-desert shrub—in response to broad environmental gradients of water and nutrient availability. The chapter explores the genetic and phenotypic components of variation of δ 13 C within field populations of L. tridentata. Experimental manipulations of water and nutrient availability are described as an approach to assess the phenotypic plasticity in photosynthetic capacity (A) and leaf conductance (g), as well as δ 13 C under conditions of differential resource availability and changing environmental conditions. The patterns of distribution of δ l3 C in leaf tissues of L. tridentata demonstrate a strong genetic component to the control of gas exchange parameters and thus transpiration efficiency (W) in this species.

Plant Functional Groups in Alpine Fellfield Habitats of the White Mountains, California

Abstract An alpine fellfield community on granite substrate (elevation 3750 m) near the University of California Barcroft Laboratory in the White Mountains of eastern California was studied during the 2000 growing season to determine whether classic perennial life forms can be treated as plant functional groups. A series of 1-m2 quadrat samples were measured to determine common species. The four species with greatest cover were Penstemon heterodoxus var. heterodoxus, Trifolium andersonii var. beatleyae, Poa glauca subsp. rupicola, and Eriogonum ovalifolium var. nivale. These and four additional common perennial species were selected for ecophysiological studies representing four distinct ecological life forms: chamaephytes, cushion plants (including mat formers), herbaceous dicot perennials, and graminoid perennials. Summer midday leaf temperatures for species with foliage held close to ground surface were up to 20°C higher than air temperatures, whereas on upright species, leaves away from the ground surface closely matched ambient temperatures. For the eight species, peak values of mean maximum photosynthetic rates ranged from 11.5–25.5 μmol CO2 m−2 s−1, typical of published values, although chamaephytes in the study showed higher rates comparable to herbaceous perennials. Water-use efficiency, as estimated by a ratio of internal to ambient CO2, was relatively high (ci:ca ratios of 0.43–0.59) compared to published data. During the stressful end of the growing season, neither predawn nor midday shoot water potentials ever reached low levels, presenting conflicting evidence for the role of soil moisture as a limiting factor. Overall, the data on plant functional attributes showed no strong patterns of differences between categories of life forms in the fellfield community, suggesting that classical life forms in this habitat do not represent plant functional groups.

THE ALPINE FLORA OF THE WHITE MOUNTAINS, CALIFORNIA

Abstract The alpine zone of the White Mountains of California, defined as non-forested areas above 3500 m, includes 163 native species of vascular plants in an area of 106 km2. No invasive species have become well established. Nearly two-thirds of the native species occur in just seven families, led by the Asteraceae with 30 species. Six genera have five or more species, led by Carex with 14 species. Life forms of the flora are heavily dominated by broad-leaved herbaceous perennials (53%), followed in importance by graminoid perennials (22%) and mats and cushions (11%). Woody shrubs, chamaephytes (low subshrubs), and annuals are relatively few in number, and those species present are generally more characteristic of lower elevation communities. Fellfields form the characteristic habitat for 41% of the flora, while moist meadows and open slopes habitats characterize 24 and 22% of the flora, respectively. Only 31% of the flora is restricted in the White Mountains to the alpine zone, while nearly a third of the alpine flora has a range extending to lower elevations of the montane or cold desert zones below 2900 m. The alpine flora of the White Mountains shares over 70% of its species with the Sierra Nevada. Only three species are endemic to the alpine zone of the White Mountains: Draba californica, D. monoensis, and Potentilla morefieldii.