Direct measurement of denitrification in a Prosopis (Mesquite) dominated Sonoran Desert ecosystem

Summary Denitrification was directly measured using the acetylene inhibition technique in a Sonoran Desert ecosystem dominated by Prosopis glandulosa. Soil under Prosopis and from the unvegetated area between Prosopis was wetted with 50 mm of water and denitrification measured for 48 hours. The mean denitrification rate under Prosopis was 11.6 g N ha^-1h^-1 compared to only 0.2 g N ha^-1h^-1 away from Prosopis. The denitrification response to wetting was rapid and rates peaked about 24 h after water application.The much higher denitrification under Prosopis probably results from high available organic C under Prosopis, but other soil chemical and physical changes effected by Prosopis may influence denitrification rates. About 0.5 kg N ha^-1 of Prosopis cover may be lost from this ecosystem by denitrification after infrequent major rainfalls.     

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

Summary Two shoot populations of the rhizomatous, patchforming herb Solidago canadensis were studied throughout a developmental cycle in two abandoned pasture sites in southern Ontario. The shoot cohorts that emerged in spring dominated the two populations; subsequent recruitment was very low. Shoot mortality was highest in June and was concentrated in the smallest size classes. Both populations showed a pronounced bimodal size structure for most of the growth cycle. Relative growth rate of shoots declined as the growing season progressed and tended to be lowest in the smallest size classes. Inflorescence production depended on shoot size. The calculated relationship between log mean weight and log density of shoots was not constant during the growth cycle and the calculated maximum biomass values do not transgress the ultimate thinning line suggested from previously published data.Address for proofs and present address: Department of Geography, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX. England     

Delivered Biomass Costs of Honey Mesquite (Prosopis glandulosa) for Bioenergy Uses in the South Central USA

Honey mesquite (Prosopis glandulosa Torr.), a multistemmed tree that grows on grasslands and rangelands in the South Central USA (Texas, Oklahoma, and New Mexico), may have potential as a bioenergy feedstock due to a large amount of existing standing biomass and significant regrowth potential following initial harvest. The objective of this research was to determine the cost to harvest, store, and deliver mesquite biomass feedstock to a bioelectricity plant under the assumption that the rights to harvest mesquite could be acquired in long-term leases. The advantage of mesquite and similar rangeland shrubs as bioenergy feedstocks is that they do not grow on land better suited for growing food or fiber and thus will not impact agricultural food markets as corn grain ethanol has done. In addition, there are no cultivation costs. Results indicated that mesquite biomass density (Mg?ha^?1) and harvesting costs are major factors affecting cost of delivered biomass. Annual biomass consumption by the bioelectricity plant and percent of the total system area that contains biomass density that is suitable for harvest significantly affected land- related factors including total system area needed per bioelectricity plant and transport costs. Simulation results based on actual biomass density in Texas showed that higher and more spatially consistent biomass density would be an important factor in selecting a potential location for the bioelectricity plant. Harvesting mesquite has the potential for bioenergy feedstock given certain densities and total land areas since higher harvest and transport costs are offset by essentially no production costs.     

Rhizobial ecology of the woody legumePsorothamnus spinosus in a Sonoran Desert Arroyo

Psorothamnus spinosus seedlings were selected at random and excavated from 10×30 m quadrats along a 270 m transect in a southern Californian desert arroyo. Forty-six of the fifty seedlings excavated (92%) were nodulated. The mean number of nodules/plant was 7.0±3.6. Maximum rooting depth was 40 cm, and most (58.8%) of the nodules were found between 15 and 30 cm depth. The mean root/shoot ratio was 1.4±0.4. The rhizobial population density between 10 to 40 cm depth across all quadrats was non-detectable using a plant infection, most probable number method. All 70 isolates from field nodules were fast-growingRhizobium spp. Two colony morphologies were observed: wet, translucent, and wet, opaque. All isolates tested were effective on bothP. spinosus and another desert woody legume, mesquite (Prosopis glandulosa). Many isolates had a high relative efficiency, and seven of eight efficient isolates had uptake-hydrogenase activity. Root stimulation of the low rhizobial population found in the arroyo and subsequent root nodulation may be an important factor in seedling establishment by woody legumes in N-limited desert arroyo systems.     

Influences of microclimatic conditions and water relations on seasonal leaf dimorphism of Prosopis glandulosa var. torreyana in the Sonoran Desert,California

Summary Seasonal measurements of microclimatic conditions were compared to seasonal indices of leaf structural components and plant water relations in Prosopis glandulosa var. torryana. P. glandulosa had two short periods of leaf production which resulted in two distinct even aged cohorts of leaves. The two leaf cohorts (summer, winter) were concurrent in the summer and fall, contrasting to previous studies on other species in which one leaf form replaces a previous leaf type. The structural characteristics of these two cohorts differed significantly in two replicate year cycles. The leaves of the spring cohort were larger in weight and area but similar to the summer cohort in specific leaf weight and leaflet number. The second growth period leaves constituted only a small proportion of the total plant leaf area. The dimorphism between the two cohorts was best associated with plant water relations and not energy load. Second growth period leaves maintained turgor to greater water deficits but lost turgor at higher leaf water potentials. Seasonal osmotic adjustment occurred for first growth period leaves but not second growth period leaves. The small leaves produced during the hot climate were most likely the result of low turgor potential during development rather than an adaptation to tolerate stressful environments.     

Seasonal dynamics of nitrogen cycling for a Prosopis woodland in the Sonoran Desert

Prosopis woodlands in the Sonoran Desert have levels of above-ground biomass and productivity much higher than those predicted for desert plant communities with such low levels of precipitation. A stand ofP. glandulosa near the Salton Sea, California, has 13,000 kg ha^?1 aboveground biomass and a productivity of 3700 kg ha^?1 yr^?1. Such a high level of productivity is possible because Prosopis is decoupled from the normal limiting factors of water and nitrogen availability. Soil nitrogen contents for the upper 60 cm of soil beneath Prosopis canopies have 1020 g m^?2 total nitrogen, 25 per cent of which is in the form of nitrate. Such accumulations of nitrogen may be the result of active symbiotic nitrogen fixation. Early estimates suggest that about 25–30 kg N ha^?1 yr^?1 is fixed in these stands. Since Prosopis covers only 34% of the ground surface and its water resources are not limiting, much higher levels of nitrogen fixation and productivity may be possible in managed stands at greater densities.     

Woody plant invasion of grasslands: establishment of honey mesquite (Prosopis glandulosa var.glandulosa) on sites differing in herbaceous biomass and grazing history

Summary Emergence and survival of honey mesquite (Prosopis glandulosa var.glandulosa Torr.) seedlings was quantified on sites with contrasting grazing histories: long-term continuous grazing (LTG) and long-term protection (LTP) from grazing by cattle. On each site, different levels of heroaceous defoliation were imposed at monthly intervals (no defoliation=ND, moderate=MD and heavy=HD). The two weeks following seed dissemination appeared to be the most critical toProsopis establishment on LTP-ND plots. Openings in the herbaceous layer created by moderate defoliation of grasses on the LTP site increased germination and/or survival 7-to 8-fold during this period. However, increasing the degree of defoliation from moderate to heavy did not stimulate additional emergence on either the LTP or LTG site. Emergence from scarified seed placed in cattle dung (17 to 30%) was lower than that of bare seed placements in various microhabitats (43–60%). However, deposition of scarifiedProsopis seed in dung in conjunction with graminoid defoliation may be the most likely combination of events when livestock are present. Emergence from seeds transported into grasslands by other fauna likely would be low, unless seeds were deposited in areas where grasses had been defoliated.Prosopis survival was comparably high in dung and bare seed placements after one growing season. survival of seedlings present two weeks after seed dissemination ranged from 74 to 97% at the end of the second growing season. Seedling survival and shoot development (biomass, leaf area and height) were similar on LTP and LTG sites, regardless of the level of herbaceous defoliation or seed placement. In addition, the magnitude and patterns of net photosynthesis, stomatal conductance and xylem water potential were comparable among one-year-old seedtings on ND, MD and HD plots, even though differences in herbaceous species composition and above- and below-ground biomass between these treatments were substantial. Such data suggest competition for soil resources between grasses andProsopis may be minimal early in the life cycle ofProsopis. High rates ofProsopis emergence and establishment on LTP-MD plots are counter to the widespread assumption that long-term and/or heavy grazing is requisite forProsopis encroachment into grasslands. Results are discussed with regard to factors contributing to the recent, widespread invasion of this woody legume into grasslands of southwestern North America.Abbreviations LTG long-term grazed - LTP long-term protected from grazing - ND non-defoliated - MD moderate defoliation - HD heavy defoliation     

Characteristics of Woodland Rhizobial Populations from Surface- and Deep-Soil Environments of the Sonoran Desert

A collection of 74 rhizobial isolates recovered from nodules of the desert woody legumes Prosopis glandulosa, Psorothamnus spinosus, and Acacia constricta were characterized by using 61 nutritional and biochemical tests. We compared isolates from A. constricta and Prosopis glandulosa and tested the hypothesis that the rhizobia from a deep-phreatic rooting zone of a Prosopis woodland in the Sonoran Desert of southern California were phenetically distinct from rhizobia from surface soils. Cluster analysis identified four major homogeneous groups. The first phenon contained slow-growing (SG) Prosopis rhizobia from surface and deep-phreatic-soil environments. These isolates grew poorly on most of the media used in the study, probably because of their requirement for a high medium pH. The second group of isolates primarily contained SG Prosopis rhizobia from the deep-phreatic rooting environment and included two fast-growing (FG) Psorothamnus rhizobia. These isolates were nutritionally versatile and grew over a broad pH range. The third major phenon was composed mainly of FG Prosopis rhizobia from surface and dry subsurface soils. While these isolates used a restricted range of carbohydrates (including sucrose) as sole carbon sources, they showed better growth on a range of organic acids as sole carbon sources and amino acids as sole carbon and nitrogen sources than did other isolates in the study. They grew better at 36°C than at 26°C. The FG Acacia rhizobia from surface-soil environments formed a final major phenon that was distinct from the Prosopis isolates. They produced very high absorbance readings on all of the carbohydrates tested except sucrose, grew poorly on many of the other substrates tested, and preferred a 36 to a 26°C incubation temperature. The surface populations of Prosopis rhizobia required a higher pH for growth and, under the conditions used in this study, were less tolerant of low solute potential and high growth temperature than were phreatic-soil isolates. SG Prosopis rhizobia from phreatic and surface soils were physiologically distinct, suggesting adaptation to their respective soil environments.     

Effects of Copper Sulfate on Seedlings of Prosopis pubescens (Screwbean Mesquite)

Phytoextraction is an established method of removal of heavy metals from contaminated soils worldwide. Phytoextraction is most efficient if local plants are used in the contaminated site. We propose that Prosopis pubescens (Screw bean mesquite) would be a successful phytoextractor of copper in our local soils. In order to determine the feasibility of using Screw bean mesquite, we utilized inductively-coupled plasma-optical emission spectroscopy (ICP-OES) and elemental analysis to observe the uptake of copper and the effects on macro and micro nutrients within laboratory-grown seedlings. We have previously shown that P. pubescens is a hyperaccumulator of copper in soil-grown seedlings.

Light and transmission electron microscopy demonstrated death of root cells and ultrastructural changes due to the presence of copper from 50 mg/L – 600 mg/L. Ultrastructural changes included plasmolysis, starch accumulation, increased vacuolation and swollen chloroplasts with disarranged thylakoid membranes in cotyledons. Inductively coupled plasma-optical emission spectroscopy analyses of macro- and micro-nutrients revealed that the presence of copper sulfate in the growth medium of Petri-dish grown Prosopis pubescens seedlings resulted in dramatic decreases of magnesium, potassium and phosphorus. At 500-600 mg/L of copper sulfate, a substantial increase of sulfur was present in roots.     

Woody invasion of grasslands: evidence that CO2 enrichment indirectly promotes establishment of Prosopis glandulosa

Grasslands worldwide have been invaded by woody species during the last200 years. Atmospheric CO₂ enrichment may indirectly havefacilitatedinvasion by reducing soil water depletion by grasses. We used a two-stepcorrelative approach to test this hypothesis with the invasive and native shrubhoney mesquite (Prosopis glandulosa Torr. var.glandulosa). 1) Water content to 0.15 m depthwas measured in grassland exposed to a CO₂ gradient from 200 to 550mol/mol to evaluate the prediction that CO₂enrichment lessens soil water depletion by grasses. 2) Soil water content andemergence and survival of mesquite seedlings were measured in adjacentgrasslandplots from which grass roots were excluded to 0.15 m depth toreduce water depletion or that were irrigated to increase soil water levels.With these measurements, we tested the hypothesis that mesquite establishmentislimited by water.Excluding grass roots doubled emergence of mesquite and almost tripledthe fraction of emergent seedlings that survived for 12 weeks following thefirst of two plantings. Seedlings were taller, heavier, and had greater leafarea when grown without grass roots. Root exclusion did not measurably affectsoil water during the 3-week period of seedling emergence, but soilwatercontent over the 12 weeks that seedling survival was studied was higher inplotsfrom which grass roots were excluded and following an April than May planting.Survivorship of mesquite seedlings correlated positively with soil watercontent. Percentage survival of seedlings increased from 1.5% to15% and 28% at the soil water content measured in grasslandexposed to CO₂ concentrations of 270 (preindustrial), 360 (current),and 550 mol/mol (future), respectively. We infer thatrecent and projected increases in atmospheric CO₂ concentration maybe large enough to increase establishment of invading mesquite seedlings ingrasslands that are severely water-limited.     

Seasonal Acclimation of Stem Photosynthesis in Woody Legume Species from the Mojave and Sonoran Deserts of California

Photosynthesis (Pn) was measured in stems of two desert legumes, Caesalpinia virgata at a low elevation site (118 m) in the Sonoran Desert and Senna armata at a higher elevation (950 m) in the Mojave Desert. The lower elevation site experienced higher spring and summer temperatures than the higher elevation site, but the air vapor pressure, irradiance, and rainfall patterns were similar. Mid-morning maximum stem Pn was highest in May for C. virgata (7.8 [mu]mol m-2 s-1) and in July for S. armata (5.8 [mu]mol m-2 s-1). The seasonal variation in maximum stem Pn was not associated with changes in bulk tissue water potential or chlorenchyma tissue nitrogen concentration. The main environmental regulators of seasonal stem Pn were temperature and leaf to air vapor pressure gradient. Light-response curves indicated no major differences in apparent quantum yield or light compensation point between the spring and summer, but light-saturated stem Pn at ambient temperature decreased for C. virgata between these seasons. The optimal temperature for stem Pn remained the same for both species between the spring and the summer. However, stem Pn of both species increased at all temperatures between the spring and summer. Potential stem Pn under optimal conditions and CO2-saturated stem Pn increased for both species between spring and summer. The increase in stem Pn potential allowed these species to maintain stem Pn during the summer even though stem Pn responses to temperature and vapor pressure did not acclimate to seasonal climatic conditions.     

Stem flow and porometer measurements of transpiration from honey mesquite (Prosopis glandulosa)

The objective of this study was to compare stem flow and porometermethods of measuring transpiration of honey mesquite (Prosopisglandulosa) trees on a semiarid site. Stem flow was measuredusing heat balance stem flow gauges. Porometer measurementsof leaf stomatal conductance (g_s) were made within foliage layersof each stem and scaled to transpiration values for the entirestem (E_stem) using stem leaf area. Simultaneous measurementsusing both methods were made diurnally and under artificiallyimposed stem shading or defoliation in June and October 1990.Stem flow and E_stem had similar diumal patterns except on 2d in June when E_stem increased during the afternoon while stemflow declined relative to midday values. During October, E_stemwas greater than stem flow throughout the day. This was attributedto sampling error in which only undamaged leaves were used forporometer measurements yet, by this time in the growing season,many leaves on each stem were damaged from insects or wind andlikely had lower transpiration rates. A regression coefficientbetween E_stem and stem flow of 0.79 in June and 0.91 in Octobersuggested the two methods were comparable, but there was considerablevariation between methods during peak transpiration rates. Bothtechniques detected that artificial shading or defoliation causedsimilar relative declines in transpiration. Results imply thatestimates of stem transpiration can be obtained by scaling porometermeasurements of leaves but accuracy declines at higher transpirationrates. Key words: Sap flow, evapotranspiration, stomatal conductance, scaling, water relations     

Effects of plant size on photosynthesis and water relations in the desert shrub Prosopis glandulosa (Fabaceae)

The Jornada del Muerto basin of the Chihuahuan Desert of southern New Mexico, USA, has undergone a marked transition of plant communities. Shrubs such as mesquite (Prosopis glandulosa) have greatly increased or now dominate in areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete directly with similar-sized grass plants. This is followed by a phase in which large, established shrubs sequester nutrients and water within their biomass and alter soil resources directly under their canopy, creating “islands” of fertility. We hypothesized that these two phases were associated with shrubs having different physiological response capacities related to their age or size and the resource structure of the environment. As a corollary, we hypothesized that responses of small shrubs would be more tightly coupled to variation in soil moisture availability compared to large shrubs. To test these hypotheses, we studied gas exchange and water relations of small (establishing) and large (established) shrubs growing in the Jornada del Muerto as a function of varying soil moisture during the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrubs following high summer rainfall in July, and highest seasonal soil moisture at 0.3 m. High rates of carbon assimilation and water use would be an advantage for small shrubs competing with grasses when shallow soil moisture was plentiful. Large shrubs had greater net assimilation and water-use efficiency, and lower xylem water potential than small shrubs following a dry period in September, when soil moisture at 0.3 m was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water as soil moisture is depleted. Although the study provides evidence of differences in physiological responses of different-sized shrubs, there was not support for the hypothesis that small shrubs are more closely coupled to variation in soil moisture availability than large shrubs. Small shrubs may actually be less coupled to soil moisture than large shrubs, and thus avoid conditions when continued transpiration could not be matched by equivalent water uptake.     

The demographic costs of nectar production in the desert perennial Prosopis glandulosa (Mimosoideae): a modular approach

Nectar production in angiosperms is considered to represent a reproductive cost, and has been associated with a decrease in fruit set or an overall decrease in the energetic budget of the plant. Populations of Prosopis glandulosa var. torreyana (honey mesquite) are a suitable system to evaluate the demographic costs of nectar production, as populations are composed of a 1:1 proportion of nectarful to nectarless individuals. The study was carried out in a population of 404 individuals of Prosopis g1andulosa var. torreyana found in an area with differing water availability in the Southern Chihuahuan Desert. The possible costs of nectar production were assessed on 1212 shoots of the honey mesquite that were tagged in 1994 and followed until 1998. We used two methods of analysis to describe the effect of nectar production on modular population dynamics: matrix analysis and log-xlinear models. Water availability and the varying environmental conditions affected plant growth, but nectar production did not have an effect on the demographic parameters we measured. The values of λ did not differ between nectar morphs and the only important effects we detected were the year to year variation in precipitation and microclimate differences at each site. Furthermore, the elasticity of each demographic process (growth, fecundity, retrogression and stasis) between nectar morphs did not differ. The log-linear models suggested a similar pattern but could discriminate the importance of each factor (nectar morph, year and site) on module fate. We were not able to detect a demographic cost of nectar production in the honey mesquite. The absence of a demographic response could be due to the negligible cost of producing nectar for this species or that the resources allocated for growth are different from those allocated for reproduction. Our results suggest that the modular fates of mesquites are mainly determined by environmental factors. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nutritive composition of green and ripe pods of honey mesquite (Prosopis glandulosa,fabaceae)

Pods from Prosopis glandulosa (honey mesquite) were harvested at seven different growth stages. Protein content (N × 6.25) of the whole pod decreased with maturation with small differences occurring after stage 3. Seed contained about 82% of the total pod protein. Carbohydrate accumulated primarily in the pericarp, and ripe pods (stage 7) contained about 80% total carbohydrate. Fat content of the pod increased slightly with maturation while total ash and fiber (ADF) decreased. Small differences existed between stages 4 and 7 in the essential amino acid pattern of whole pods. The total sulfur-containing amino-acids were the most limiting in both growth stages; the pericarp protein had a relatively better amino-acid pattern than did the seed protein. Both green and ripe honey mesquite pods provide similar amounts of micro- and macro-minerals.     

豆科植物凝集素及其对根瘤菌的识别作用

本文讨论了豆科植物凝集素的性质、分布、基因及其表达;近年来研究表明识别根瘤菌的因子是豆科植物根上的凝集素。将一种豆科植物的凝集素基因转化到另一种豆科植物后,再接种前一种豆科植物的根瘤菌,可以使其被侵染和结瘤。由此人们提出了扩大根瘤菌宿主范围到非豆科植物,特别是粮食作物范围的可能性。     

Defoliation and woody plant (shape Prosopis glandulosa) seedling regeneration: potential vs realized herbivory tolerance

Herbivory by rodents, lagomorphs and insects may locally constrain woody plant seedling establishment and stand development. Recruitment may therefore depend either upon plant tolerance of herbivory, or low herbivore abundance, during seedling establishment. We tested potential herbivory tolerance by quantifying growth, biomass allocation, and survival of defoliated Prosopis glandulosa seedlings under optimal abiotic conditions in the absence of competition. Realized tolerance was assessed by clipping seedlings of known age grown in the field with and without herbaceous competition.At 18-d (= 'young') or 33-d (= 'old') of age, seedlings in the growth chamber were clipped just above the first (cotyledonary) node, above the fourth node, or were retained as non-clipped controls. Potential tolerance to defoliation was high and neither cohort showed evidence of meristematic limitations to regeneration. Clipping markedly reduced biomass production relative to controls, especially belowground, but survival of seedlings defoliated 5 times was still 75%. Contrary to expectations, survival of seedlings defoliated above the cotyledonary node 10 times was greater (P<0.10) for 'young' (75%) than 'old' (38%) seedlings. Under field conditions, survival of defoliated 11-month-old P. glandulosa seedlings was 59% after one defoliation and only 13% after six defoliations.Results indicate P. glandulosa is potentially tolerant of repeated shoot removal early in its life cycle. Seedling tolerance to defoliation under field conditions therefore appears dependent upon abiotic stresses or resource limitations rather than a lack of intrinsic adaptations for shoot replacement or a depletion of the seedlings' bud bank. Curtailment of root growth, a consequence of top removal observed in the growth chamber experiment, may reduce the capacity of P. glandulosa seedlings to acquire soil resources needed for meristem activation and shoot growth under field conditions. The importance of resource availability to post-defoliation regeneration was implicated in the field experiment, where survival, shoot elongation, and aboveground productivity of clipped seedlings was greatest in plots without herbaceous interference.In light of the marked increases in P. glandulosa abundance in grasslands in recent history, our results suggest that (1) utilization of Prosopis seedlings by herbivores may be infrequent or sporadic, (2) periodic episodes of seed production and germination may satiate herbivore populations, and/or (3) Prosopis seedling establishment occurs during periods of low herbivore density.