Desert Shrub Species Research Articles

Changes of Soil Moisture and Its Impact on Desert Shrubs in Alashan Desert

Based on 25 years(1981-2005) of continuous positioning observation data on desert shrub in Xilingaole area of Alashan league in Inner Mongolia,by M-K trend method and correlation method,the growing periods and yield trends of the 5 main species of desert shrubs,soil moisture trends,and the relationship between them were analyzed.The results showed that the soil moisture,especially that of 10-20cm soil layer decreased obviously(P0.05).Compared with 10-30cm,the soil moisture of 30-50cm soil layer was relatively stable.During 1981-2005,the reviving date,flowering and withering date of the 5 species of desert shrubs all had a ahead trend,especially for withering,reviving on the second,which made the growing length of the 5 desert shrubs reduced.The fresh weight of the 5 species of desert shrubs was significantly decreased.Compared with dry weight,fresh weight decreased more obvious.Soil water content mainly affected the reviving date of the 5 desert shrubs.The higher soil water content was,the earlier reviving date.The 30-50cm soil layer was the main layer affecting shrubs reviving due to its relatively stable.The 10-20cm soil layer had a significant impact on dry weight and fresh weight of desert shrubs.The correlation of dry weight,fresh weight and average soil moisture of last year was significant than that of the current year,indicating that dry weight and fresh weight of desert shrubs more closely related to pre-soil moisture.

Seasonal variation in plant hydraulic traits of two co‐occurring desert shrubs, Tamarix ramosissima and Haloxylon ammodendron, with different rooting patterns

AbstractEffective hydraulic responses to varying soil moisture and evaporative demand are crucial to plant survival in arid ecosystems. This study was carried out during two growing seasons (2004–2005) on two typical desert shrub species, Tamarix ramosissima and Haloxylon ammodendron, co‐occurring in the Gurbantonggut Desert (Central Asia), to investigate their hydraulic responses to seasonal variations in water availability. The root distribution was studied by excavating the intact root systems. Leaf‐specific apparent hydraulic conductance (Kl) for the two species was calculated based on leaf water potential (ψl) and transpiration rate (Tr), which were monitored during the growing seasons. T. ramosissima had a deeper taproot (3.1 vs. 2.6 m) and a larger root surface area (3.02 vs. 1.28 m2) than H. ammodendron. Combined with a higher ψl, this meant that it maintained a better water status in general. For the deep‐rooting T. ramosissima, the seasonal pattern of its predawn leaf water potential (ψpd) was in high accordance with the seasonal changes in soil moisture at a depth of 2.6–2.8 m, which was largely influenced by the upflow of groundwater through capillary ascent, and barely responded to rain events. For the shallow‐rooting H. ammodendron, the seasonal pattern of ψpd was closely related to soil moisture in the upper layer at a depth of 0–0.2 and 0.6–0.8 m, which was recharged periodically by rain events and responded acutely to rainfall above 5 mm. The two species differed in their maximum transpiration rates (Trmax) and ψpd: in T. ramosissima, Trmax gradually dropped with decreasing ψpd; in H. ammodendron, Trmax showed no significant response to ψpd. For T. ramosissima, the major water resource was groundwater and vadose zone water, and ψl contributed significantly to transpiration regulation. For H. ammodendron, the primary water resource was precipitation input, which was not sufficient to keep the ψl for this species outside the critical range of leaf shedding in summer. Thus, for these two representative species that share the same habitat, contrasting response strategies to water limitation were observed in relation to water acquisition and root distribution characteristics.

Distribution of axis-splitting in Mojave Desert shrub species along an elevational gradient

Although highly branched from the base, all shrubs have short main axes linking canopies to root systems. Main axes become increasingly segmented into independent canopy/stem/root segments as aridity increases across continents. The resulting hydraulic modularity has been proposed as an adaption to low soil moisture that prevents runaway embolism and minimizes risk of hydraulic failure. Here we test the hypotheses that (1) at a regional scale, the importance of axis-splitting species in communities declines with increasing elevation, as a proxy for precipitation, and (2) that this decline is explained by lower occurrence of low-elevation dominant species. We evaluated all species for axis splitting and determined importance values in plots along an elevational transect in the Mojave Desert. As predicted, as elevation increased, the total importance of axis-splitting species declined from 100% at low-elevation sites to 75% at the highest elevation site. However, this decline was not due solely to the decline of the lower elevation dominant species. At the high elevation site, the influx of new species resulted in a six-fold increase in species richness and almost all of the new high elevation woody eudicotyledonous species exhibited axis splitting; non-splitting species were represented by other growth forms.

Hydraulically integrated or modular? Comparing whole‐plant‐level hydraulic systems between two desert shrub species with different growth forms

* Hydraulic systems of shrubs vary between hydraulically integrated and modular architectures; the latter divide the shrub into independent hydraulic units. Hydraulic systems of two common North American desert shrub species, the multi-branched Ambrosia dumosa and the single-stemmed Encelia farinosa (both Asteraceae), were compared to test for division into independent hydraulic units and the implications of such a division for water loss through leaves and roots. * Hydraulic systems of mature shrubs in the field were characterized using dye tracers and by documenting the degree of stem segmentation. Young pot-grown shrubs were subjected to heterogeneous and homogeneous watering. Spatial within-canopy variation of leaf water potentials and stomatal conductances, as well as soil water contents, were measured in response to manipulated soil water heterogeneity. * Results show that young Ambrosia shrubs are divided into independent hydraulic units long before they physically split into separate ramets as mature shrubs, and that young and mature Encelia shrubs possess integrated hydraulic systems. No hydraulic redistribution was detected for eitherspecies. * Our study shows that functional segmentation into independent hydraulic units precedes physical axis splitting, rather than being the consequence of split axes, and suggests that mature shrubs with round basal stems are likely to be hydraulically integrated.

(Re)discovery of a mistletoe infecting the Cardón cactus

85 T here are few known examples of cacti being successfully parasitized by other plants, such as mistletoes. This is surprising, because mistletoes infect numerous species of desert shrubs, many of which grow side by side with cacti. Two cases have been observed in South America: Tristerix aphyllus successfully infects Trichocereus chilensis in an area just north of Santiago, Chile1–3, and Ligaria cuneifolia infects plants of Corryocactus brevistylus growing near Arequipa, Peru4. In its second year, The Cactus and Succulent Journal printed an anonymous note stating that an unnamed parasite infected Echinocereus chloranthus near El Paso, but to our knowledge that has not been verified. The San Diego Natural History Museum has a photograph and two specimens collected by Reid Moran of the mistletoe Phoradendron brachystachym (formerly P. diguetianum) growing on both Pachycereus pringlei and P. pecten-aboriginum. On a field trip to Baja California Sur in 2007, Jon Rebman discovered plants of Pachycereus pringlei infected by large plants of Phoradendron brachystachyum. The infection does not appear to be widespread; only two infected cacti were seen on this field trip, and no infected cacti had been noticed on previous expeditions. This incidence of mistletoe/cactus infection appears to resemble that of the Ligaria/Corryocactus in that the mistletoe has numerous long branches and many green leaves. We do not know of any studies of this case of Phoradendron/Pachycereus parasitism. The phenomenon of mistletoe/cactus parasitism is rare—or at least rarely reported. We encourage anyone who does field work in cactus habitats to report any case they notice.

Sensitivity of water relations and photosynthesis to summer precipitation pulses for Artemisia tridentata and Purshia tridentata

For much of the western USA, precipitation occurs in pulses, the nature of which determine soil water potential and plant physiological performance. This research utilized three experiments to examine the sensitivity of photosynthesis and water relations for two widespread Great Basin Desert shrub species, Artemisia tridentata (which has both deep and shallow roots) and Purshia tridentata (which reportedly has only deep roots), to (1) variation in pulse magnitude size, (2) the kinetics of responses to pulses, and (3) the relationship between pulse-size and antecedent soil water content. At the study site in the southwestern Great Basin Desert, USA, summer rainfall exhibits a greater frequency of larger-sized events, and longer inter-pulse intervals, compared to annual patterns. Compared to pre-watering values, stem water potential initially increased by about 2.00 MPa for A. tridentata and 1.00 MPa for P. tridentata following watering to simulate an 11.5 mm rainfall pulse. For the same water addition, stomatal conductance increased by 0.3 mol m−2 s−1 and photosynthetic CO2 assimilation increased 8-fold for A. tridentata and 6-fold for P. tridentata. Water potential and photosynthetic gas exchange were maximal for both species 2–3 days following a pulse addition. In comparison to P. tridentata, the increase in photosynthesis for A. tridentata was more pronounced for plants treated incrementally with several small pulses compared to plants treated with one pulse of an equivalent total volume. The results indicate that both species can respond to a range of summer rainfall pulse magnitudes within about 2 days, with A. tridentata generally exhibiting larger responses in comparison to the co-dominant shrub species P. tridentata, which at this study site does indeed have shallow roots. In a future climate, the timing and magnitude of summer rainfall pulses will determine the extent to which these two species undergo changes in water status and photosynthetic carbon uptake, with implications for their fitness.

The effect of spatial resolution on measurement of vegetation cover in three Mojave Desert shrub communities

Moderate resolution remotely sensed imagery (20–30 m) has lacked sufficient spatial resolution to accurately measure total vegetative cover in arid environments because the density of plant cover is generally too low to significantly influence spectral reflectance. New and emerging imagery is now available at significantly higher spatial resolutions, resulting in pixel sizes equal to or smaller than many dominant desert shrubs. This had led to an opposing problem, where desert shrubs now exhibit multi-modal probability functions associated with the individual elements of the plant, such as foliage, woody stems and branches, and shadows within and around shrub canopies. High resolution imagery was collected at three spatial resolutions for study sites in the south-central Mojave Desert of California to (1) determine the effect of spatial resolution on the detection of four common species of shrubs ( Chilopsis linearis, Psorothamnus spinosus, Larrea tridentata, and Ambrosia dumosa), (2) assess the affect of increasing pixel size on the accuracy of area estimates of shrub cover, and (3) assess the accuracy of vegetation cover measurements when compared to vegetation cover measured in the field. Spatial resolution greater than 1.0 m was not capable of discerning a large percentage of the shrubs in this region of the Mojave Desert. Yet, high correlation ( r 2 = 0.71 – 0.93 ) was found between field measurements of percent cover and those derived from imagery at spatial resolutions ranging from 0.6 to 1.0 m. The results indicated that a spatial resolution of 1.0 m or smaller was necessary to estimate percent cover and the area of individual shrubs from high-resolution imagery. Spatial resolution is dependent on the abundance and size of species of desert shrubs within plant communities, however 1.0 m resolution was found to be adequate for this region of the Mojave Desert.

INTERSPECIFIC COMPETITION AND RESOURCE PULSE UTILIZATION IN A COLD DESERT COMMUNITY

In desert ecosystems a large proportion of water and nitrogen is supplied in rain-induced pulses. It has been suggested that competitive interactions among desert plants would be most intense during these pulse periods of high resource availability. We tested this hypothesis with three cold desert shrub species of the Colorado Plateau (Gutierrezia sarothrae, Atriplex confertifolia, and Chrysothamnus nauseosus), which differ in their dis- tribution of functional roots. In a three-year field study we conducted a neighbor removal experiment in conjunction with simulated 25-mm precipitation events and the addition of a nitrogen pulse in either spring or summer. We measured predawn water potential (P), gas exchange, leaf 8'5N, carbon isotope discrimination (A), and growth of target plants for the duration of the study. We found that G. sarothrae used resource pulses to a larger extent than A. confertifolia, which has more functional roots at depth. In all species, the addition of a water or nitrogen pulse did not significantly affect maximal rates of photosynthesis or branch growth. Contrary to our initial hypothesis, we did not find that pulse use was reduced by the presence of neighboring plants. Nevertheless, there was strong evidence for com- petitive interactions, which were more likely mediated by water at depth than by nitrogen. In the more deep-rooted species A. confertifolia, neighbor removal affected P, gas exchange, A, percentage of carbon, and growth. G. sarothrae, which has a much smaller proportion of roots at depth, was less affected by the removal of neighboring shrubs, and not at all when only predominantly shallow-rooted herbaceous species were removed. These results suggest that shrubs in this cold desert community may primarily compete for water in deeper soil layers, where water depletion is slow and dominated by plant water uptake. There appeared to be little competition for water in shallow soil layers, where depletion is fast and dominated by evaporation.

Positive and negative plant interactions contribute to a north-south-patterned association between two desert shrub species.

Abiotic factors are often thought to be the predominant forces shaping desert plant communities. But both positive and negative interactions between plants are frequently observed in deserts, and it is an open question whether they can strongly affect the spatial structure of a desert community. The goal of this study was to answer this question for a plant community in the North American Mojave Desert. Two semi-shrub species, Ambrosia dumosa and Acamptopappus sphaerocephalus, were the focus of this study. At the study site, seedlings emerged predominantly on the northern side of shrubs, indicating positive effects of canopy shading on emergence, but survival of Ambrosia seedlings was much higher in open areas than at the edge of conspecific shrubs. Negative intraspecific interactions also affected Ambrosia shrubs, which did not increase in size over a 4-year period unless the nearest conspecific neighbor had been removed. These negative intraspecific interactions among different life stages of Ambrosia appear to contribute to spatial segregation observed among shrubs of this species. In contrast, Acamptopappus shrubs and their seedlings were aggregated with Ambrosia shrubs, and occurred more often on the northern side of Ambrosia than expected by chance. Removal of Ambrosia neighbors positively affected growth of Acamptopappus, but only when the neighbor was removed on the northern side. For Acamptopappus, an Ambrosia neighbor on the southern side may have some positive effects, which appear to neutralize the negative effects found for northern neighbors. These positive effects were likely at least partly due to shading. Removal of Ambrosia neighbors negatively affected predawn xylem pressure potentials of Acamptopappus, but this effect was only found during one growing season and was briefly reversed during the next. In summary, negative intraspecific interactions appear to cause spatial segregation of Ambrosia shrubs, while a combination of positive and negative interactions apparently contribute to the directional association between Ambrosia and Acamptopappus. Thus plant interactions in this desert appear to shape community structure in at least two dimensions by influencing the distances and in which directions to their neighbors plants can grow and survive.

Different Patterns of Gas Exchange and Photochemical Efficiency in Three Desert Shrub Species Under Two Natural Temperatures and Irradiances in Mu Us Sandy Area of China

Field studies of gas exchange and chlorophyll fluorescence of three desert shrub species, Hedysarum fruticosum var. mongolicum, Artemisia ordosia, and Salix pasmmophylla, showed different patterns under different leaf temperature (T1) and incident photosynthetic photon flux density (PPFD). H. fruticosum var. mongolicum and A. ordosia exhibited higher PN and gs than S. pasmmophylla, especially under very high T1 (>46 °C) and high PPFD (>2 100 µmol m-2 s-1) in hot summer. The decreases of PN with the diurnal course were due mainly to stomata closure. However, PN of S. pasmmophylla was seriously depressed by very high temperature from midday to evening as shown by the negative PN in hot summer, whereas none of such depression was found in spring. Maximal spring photochemical efficiency of photosystem 2 (PS2), i.e, Fv/Fm, was the lowest at 16:00, indicating the injury of PS2 by heat at this stage. In hot summer again, all the three shrubs underwent pronounced midday depression of PN and gs, while in spring they showed a one-peak response. And the first peak appeared 2 h earlier in hot summer than in spring for all the three shrubs. It was the high temperature that led to the different patterns of gas exchange and the serious depression of PN in S. pasmmophylla. H. fruticosum var. mongolicum and A. ordosia were much more tolerant to heat and high irradiance than S. pasmmophylla, which fixed most of CO2 at the fast growing stage in spring. Nevertheless, in hot summer it had to survive the severe hot environment through strong respiration and storage of CO2 only in the early morning.

Characteristics of seed germination in five non-halophytic Chinese desert shrub species

The effects of temperature, NaCl, and polyethylene glycol (PEG)-6000 on the seed germination of five non-halophytic Chinese desert shrub species were investigated. The minimum temperature of germination was 10°C for all species, and the maximum temperature varied among species from 25 to 35°C. Isotonic solutions of NaCl and PEG caused different effects on seed germination in all five species. When seeds were moistened with −5·0 MPa NaCl solution for 5 days, the seeds of three species completely lost their germinability, while the seed germinability of the other two species was much less affected. These results are discussed with regard to the rehabilitation of desertified lands in China by seed dispersal.

Phosphorus acquisition by Bromus madritensis ssp. rubens from soil interspaces shared with Mojave Desert shrubs

Abstract Bromus madritensis ssp. rubens (L.) Husnot (Red Brome) is an invasive annual grass that is associated with increases in fire frequency and decreases in perennial plant diversity. The success of Red Brome in the Mojave Desert has been attributed to its competitive ability, but competition between Red Brome and native shrubs for below‐ground resources has not been investigated previously. In this study we present a modification of previous dual‐isotope methods that assesses competitive interactions and responses among plants under field conditions. We then use this method to (i) determine if direct competition for phosphorus (P) occurs between Red Brome and native shrubs, and (ii) evaluate the effectiveness of phosphate acquisition by Red Brome from soil interspaces shared with different Mojave Desert shrub species. Clipping Red Brome to remove ≈85% of its foliage on the day prior to labelling soil interspaces with P isotopes did not have the desired effect of inhibiting phosphorus uptake by Red Brome. Therefore we were unable to verify that direct competition for P occurred between Red Brome and native shrubs. Nonetheless, by sampling plant tissue from unclipped strips of Red Brome that were centred between two shrubs, we were able to evaluate the effectiveness of phosphate acquisition by Red Brome from interspaces shared with different shrubs. There were no differences in the amount of phosphorus acquired by Red Brome from soil interspaces shared with the evergreen shrub Larreatridentata (D.C.) Cov. versus soil interspaces shared with the drought deciduous shrub Lycium pallidum Miers. However, Red Brome acquired more than six times as much P from soil interspaces shared with Larrea than from soil interspaces shared with the drought deciduous shrub Ambrosia dumosa Payne. Less P uptake by Red Brome from soil interspaces shared with Ambrosia suggests greater effectiveness of Ambrosia compared with Larrea to deplete soil P.

Clonal splitting in desert shrubs

Axis splitting is a widespread phenomenon in desert shrubs, and has been reported for shrubs from several plant families, both in old- and new-world deserts. It is so common in dwarf shrubs of arid environments as to be a defining characteristic of this growth form. Although anatomists described this phenomenon several decades ago, there has been only one ecological study of one species, Ambrosia dumosa. The anatomical nature of the various splitting mechanisms that have been found suggests axis splitting to be an extreme form of hydraulic segmentation. The adaptive advantage of clonal splitting in desert shrubs has yet to be determined, but it appears to be largely a risk-spreading mechanism that enables independent mortality of integrated hydraulic units (IHUs) or ramets. This should be especially advantageous in heterogeneous, water-limited environments, where soil water occurs in pockets too small to support a large shrub-genet. Clonal splitting may cause an increase in intraclonal competition among ramets, but there are also indications that at least some species possess mechanisms to reduce competition by minimizing root system overlap among ramets. Many desert shrub species that undergo clonal splitting maintain a dense clump growth form, possibly because such a growth form has positive effects on water and nutrient status of the soil and long-term effects on other soil properties.

Exploring Cost Constraints on Stem Elongation in Plants Using Phenotypic Manipulation

Exploring Cost Constraints on Stem Elongation in Plants Using Phenotypic Manipulation

High Foliar Nitrogen in Desert Shrubs: An Important Ecosystem Trait or Defective Desert Doctrine?

Nitrogen concentrations in green and senesced leaves of perennial desert shrubs were compiled from a worldwide literature search to test the validity of the doctrine that desert shrubs produce foliage and leaf litter much richer in nitrogen than that in the foliage of plants from more mesic environments. Mean nitrogen concentration in the green leaves of 78 species of shrubs growing in 11 deserts on five continents (2.2%) was not different from that in 67 species of trees and shrubs growing in deciduous, and mixed deciduous forests (2.2%), and only slightly higher than that in overstory (2.0%) and understory (2.1%) plants growing in tropical wet forest. Mean nitrogen concentration in the green leaves of a ubiquitous shrub that dominates large areas of desert in the United States (Larrea tridentata, 2.1%), and in the green—stem tissues of leafless desert shrubs (2.1%) were also similar to that in plants from mesic environments. Mean green—leaf nitrogen was similar in shrubs growing in different deserts. Mean nitrogen concentration in leaf litter was 1.1% for 11 species of desert shrubs, and 1.0% for the 10 species of this group that were not capable of symbiotic nitrogen fixation. Both concentrations were lower than those routinely provided to describe nitrogen in the litter of desert shrubs (1.5—1.7%), and only slightly higher than the mean nitrogen concentration in 77 species of woody perennials growing in a wide variety of environments worldwide (0.9%). Nitrogen in the leaf litter of one desert shrub (0.4%, Brickellia laciniata) was nearly as low as the lowest leaf—litter nitrogen concentration known for any woody species (0.3%). Because nitrogen concentrations in the foliage of desert shrubs are not higher than those in plants growing in more mesic environments, the tenet that desert shrubs support extraordinarily nitrogen—rich foliage can no longer be supported.

Effects of grazing and drought on population dynamics of salt desert shrub species on the Desert Experimental Range, Utah

We examined natality, mortality, and population turnover of dominant salt desert shrub species under different seasons (winter vs. spring) and intensities (light vs. heavy) of grazing during a drought period on the USDA Forest Service Desert Experimental Range in southwestern Utah. Species responses were more predictable from their life history and physiological traits than from past responses to grazing alone. Heavy or spring grazing increased mortality of Artemisia spinescens, a cool-season shrub susceptible to past grazing, and of Sporobolus cryptandrus and Atriplex confertifolia , a C4 grass and shrub, respectively, that had increased under this grazing regime in the past. Light or winter grazing during this period increased survival and natality of S. cryptandrus, and of Ceratoides lanata, a shrub that had decreased in density but increased in cover under past grazing. Population turnover rates were generally positive for A. spinescens, but were highly negative for A. confertifolia in all but the heavy spring grazing treatment. A. confertifolia had exhibited high mortality during past droughts. C. lanata exhibited little population change reflecting past trends. Generally positive rates of turnover for the two grasses, S. cryptandrus and O. hymenoides, paralleled past trends, except in the spring-heavy treatment which had highly negative turnover rates. In a comparison of grass vs. shrub dominated vegetation types, C. lanata had higher mortality in grass dominated plots; O. hymenoides had higher mortality in shrub dominated plots. Both S. cryptandrus and O. hymenoides exhibited low or negative turnover rates for grazed plots within the shrub dominated type. Overall, light to moderate grazing and the removal of livestock before active physiological growth of cool season species had the least negative effects on population dynamics during a two-year drought period. This grazing regime increased survival or natality of certain species.

Topographic and soil surface effects on gypsophile plant community patterns in central Mexico

Abstract. Semiarid gypsum karstlands in north central Mexico are characterized by a mosaic of shrubland and endemic gypsophile grassland community types. Proximal physical factors affecting community patterns on a local scale were investigated at an undisturbed site south of Matehuala, San Luis Potosi. Soil surface characteristics associated with depth of calcareous alluvium overlying gypsum base material were strongly correlated with distribution of desert shrub and endemic grass species. Muhlenbergia purpusii was dominant on bare gypsum soils with indurated crusts, while Bouteloua chasei was dominant where a thin veneer of alluvium permitted development of a cryptogamic crust. A high‐diversity mixed shrub‐succulent community occupied gypsum overlain by 5 ‐ 20 cm of alluvium, whereas Larrea tridentata was the principal species on gypsum overlain by > 20 cm of alluvium. Open sink walls were occupied mostly by species of the Muhlenbergia grassland, while closed sinks supported more mesophytic mixed shrubland vegetation.Physical factors operating mostly at the establishment stage are probably more important in mediating local species patterns on gypsum than factors associated with mineral nutrition. Gypsum material was much less fertile than alluvial material but did not exclude widely distributed species unless exposed at the surface.

Comparative germination of seeds, matured during winter or summer, of some bi-seasonal flowering perennial desert Aiozaceae

Shrub species of Aiozaceae were introduced from South Africa to Israel as seeds, and grew in the field for about 5 years at Sede Boker in the Negev Desert Highlands. In the three species of bi-seasonal flowering perennial desert shrubs tested, it was found that seeds matured during winter (under natural short days and low temperatures) and harvested after winter, differed in their ability to germinate from seeds from the same mother plant that matured and were harvested in summer under natural long days and high temperatures. Different rates and levels of germination were also found in the same harvest of seeds, when these imbibed in water at different temperatures in the light, dark or in the dark with brief illumination. The influence of the environmental conditions during winter and summer maturation (which result in the differential ability of the seeds to germinate) is discussed.

The Effect of CaCO3on the Uptake of Phosphorous by Two Desert Shrub Species, Larrea tridentata (DC.) Cov. and Parthenium incanum H. B. K.

In a phytotron growth study, the effects of soil CaCO3 on the uptake of P by two desert shrub species were species specific. Larrea tridentata seedlings responded to soil CaCO3 with increased root: shoot ratios, increased N:P ratios in tissues, and decreased specific absorption rates of P, indicating that CaCO3 directly inhibited the uptake or the availability of P to roots. Parthenium incanum seedlings responded negatively to CaCO3, but this response was not specific to P uptake and did not appear to be caused by a reduction in P availability. Pedogenic carbonates, present in many arid-land soils of the world, might greatly affect both P availability and the biogeochemical cycle of P in desert ecosystems.

Seasonal patterns of canopy development and carbon gain in nineteen warm desert shrub species.

Canopy development and photosynthetic rate were measured at monthly intervals over a period of one year in 19 shrub and subshrub species of the Mojave and upper Sonoran Deserts. Thirteen of these species realized a substantial fraction of their total net carbon assimilation via twig photosynthesis. The twig contribution to whole plant yearly carbon gain reached a maximum of 83% in species such as Thamnosma montana, Salizaria mexicana, and Baccharis brachyphylla. This large contribution by twigs was due to both low levels of leaf production and the greater longevity of twig tissues. In some other species, however, leaf and twig organs had similar lifespans. During the year of this study (which had an unusually warm, mild winter), no species showed a pattern of winter deciduousness. The reduction in total photosynthetic area between maximal spring canopy development and mid August summer dormancy ranged from 32 to 94%. Some herbaceous perennial species died back to the ground, but none of the woody shrubs were totally without green canopy area at any time of the year. No species studied were capable of high rates of photosynthesis at low plant water potentials in July and August, but, in those species which maintained a substantial canopy area through the drought period, previously stressed tissues showed substantial recovery after fall rains. Photosynthetic rate was significantly correlated with both plant water potential and tissue nitrogen content over the entire year, but only weakly so. This is due in part to the winter months when plant water potentials and tissue nitrogen contents were high, but photosynthetic rates were often low.