Prosopis Velutina Research Articles

Effects of Fire Frequency and Intensity on Velvet Mesquite in an Arizona Grassland

Increases of velvet mesquite (Prosopis velutina Woot.) in southwestern grasslands might have been caused by livestock consumption of fuels that once burned with sufficient frequency and intensity to kill the trees. However, attempts to control mesquite with fire usually have failed. We measured fire damage and 5 years of postfire recovery for 225 mesquite trees > 1 m tall, following a 2002 wildfire that included grasslands differing in fire history, presence vs. 34-year livestock exclusion, and predominance of native vs. exotic grasses. The fire burned 100% of ground cover in ungrazed areas and 65% on grazed lands. Top-kill was 100% for trees in exotic ungrazed grasslands (the areas with highest fuel loads), 79% for trees in ungrazed native grasslands, and 28% for trees in grazed grasslands. Most top-killed trees produced ground sprouts, so that by 2006 the combined foliage volume from ground sprouts and surviving branches was 78% (± 3.2 SE) of preburn foliage volume in grazed areas, 66% (± 3.3) in ungrazed exotic grasslands, and 57% (± 4.0) in ungrazed native grasslands. Fire damage was greater among surviving trees in ungrazed areas that had burned twice (1987 and 2002) than among those that had burned only once since 1968 (in 2002), especially in native grasslands where postfire foliage recovery for twice-burned trees was only 47% (± 6.3) by 2006. Only 1 of 84 trees died in the area burned once, whereas 12 of 66 (18.2%) died in the area burned twice, including several individuals > 3 m tall. These results suggest that repeated fires likely could have prevented the historic spread of velvet mesquite into southwestern grasslands, but probably could be used to control mesquite today only in areas where abundant herbaceous growth provides sufficient fine fuels.

Intraseasonal Variation in Water and Carbon Dioxide Flux Components in a Semiarid Riparian Woodland

We investigated how the distribution of precipitation over a growing season influences the coupling of carbon and water cycle components in a semiarid floodplain woodland dominated by the deep-rooted velvet mesquite (Prosopis velutina). Gross ecosystem production (GEP) and ecosystem respiration (R eco) were frequently uncoupled because of their different sensitivities to growing season rainfall. Soon after the first monsoon rains, R eco was high and was not proportional to slight increases in GEP. During the wettest month of the growing season (July), the system experienced a net carbon loss equivalent to 46% of the carbon accumulated over the 6-month study period (114 g C m−2; May–October). It appears that a large CO2 efflux and a rapid water loss following precipitation early in the growing season and a later CO2 gain is a defining characteristic of seasonally dry ecosystems. The relative contribution of plant transpiration (T) to total evapotranspiration (ET) (T/ET) was 0.90 for the entire growing season, with T/ET reaching a value of 1 during dry conditions and dropping to as low as 0.65 when the soil surface was wet. The evaporation fraction (E) was equivalent to 31% of the precipitation received during the study period (253 mm) whereas trees and understory vegetation transpired 38 and 31%, respectively, of this water source. The water-use efficiency of the vegetation (GEP/T) was higher later in the growing season when the C4 grassy understory was fully developed. The influence of rain on net ecosystem production (NEP) can be interpreted as the proportion of precipitation that is transpired by the plant community; the water-use efficiency of the vegetation and the precipitation fraction that is lost by evaporation.

Hydrogen isotope fractionation during water uptake by woody xerophytes

Stable isotope measurements are employed extensively in plant–water relations research to investigate physiological and hydrological processes from whole plant to ecosystem scales. Stable isotopes of hydrogen and oxygen are routinely measured to identify plant source water. This application relies on the assumption that no fractionation of oxygen and hydrogen isotopes in water occurs during uptake by roots. However, a large fraction of the water taken up through roots in halophytic and xerophytic plants transverses cell membranes in the endodermis before entering the root xylem. Passage of water through this symplastic pathway has been hypothesized to cause fractionation leading to a decrease in 2H of root xylem water relative to that in the surrounding soil medium. We examined 16 woody halophytic and xerophytic plant species in controlled conditions for evidence of hydrogen isotope fractionation during uptake at the root–soil interface. Isotopic separation (Δ2H = δ2Hsoil water − δ2Hxylem water) ranging from 3‰ to 9‰ was observed in 12 species. A significant positive correlation between salinity tolerance and the magnitude of Δ2H was observed. Water in whole stem segments, sapwood, and roots had significantly lower δ2H values relative to soil water in Prosopis velutina Woot., the species expressing the greatest Δ2H values among the 16 species examined. Pressurized water flow through intact root systems of Artemisia tridentata Nutt. and Atriplex canescens (Pursh) Nutt. caused the δ2H values to decrease as flow rate increased. This relationship was not observed in P. velutina. Destroying the plasma membranes of root cells by excessive heat from boiling did not significantly alter the relationship between δ2H of expressed water and flow rate. In light of these results, care should be taken when using the stable isotope method to examine source-water use in halophytic and xerophytic species.

Mesquite and grass relationships at two spatial resolutions

Relationships between woody plants and grass are typically described at either the stand (≥100 m2) or plant (≤10 m2) spatial resolutions. Descriptions of the Prosopis velutina (mesquite)–grass relationships in the North American Desert Grassland differ between spatial scales: P. velutina often inhibits grass at the stand scale compared to facilitation or neutral relations at the plant scale. We use simultaneous measures of grass density at both spatial resolutions (made at 3 y intervals from 1997 to 2003) to evaluate the influence of the spatial resolution on detecting P. velutina–grass relationships. We confine our observations to the parsimonious conditions of (1) two distinct P. velutina age classes that are exclusive occupants of study sites and (2) total cover of P. velutina was not different between study sites in 2000 and 2003. Thus, total P. velutina cover is constant in those 2 years but P. velutina ages contributing to that cover differed between the study sites. We confirmed the importance of spatial scale because no grass species was related to P. velutina age at the stand scale, but 2 of 5 species were significantly related at the plant scale. Aristida spp. was greater under young than old P. velutina, and Muhlenbergia porteri was greater under old P. velutina. These results suggest that the influence of old P. velutina on grass abundance is restricted to the area under the canopy and does not extend into inter-canopy areas, and therefore the observations at the stand scale may not detect the P. velutina–grass relationships at the plant scale.

Sensitivity of riparian ecosystems in arid and semiarid environments to moisture pulses

AbstractStructural and functional dynamics of riparian vegetation in arid and semiarid basins are controlled by hydrological processes operating at local, landscape and catchment scales. However, the importance of growing‐season precipitation as a control on evapotranspiration (ET) and carbon cycling in these ecosystems varies considerably across the riparian landscape, depending largely on access to the near‐surface water table. Here we describe key findings from ongoing ecohydrological studies along the Upper San Pedro River (USPR) in semiarid, south‐eastern Arizona, USA. Depth to the water table varies substantially across the riparian landscape along the USPR drainage, and is a key factor controlling the sensitivity of cottonwood (Populus fremontii) water‐source use, transpiration rate and photosynthetic metabolism to inputs of monsoonal moisture during the growing season. Landscape‐scale functional variation in cottonwood forests has been incorporated into spatially explicit ET models for estimating water balance components along the USPR. However, of greater importance for understanding controls on water and carbon exchange processes in the riparian corridor is the conversion of sacaton (Sporobolus spp.) grasslands on pre‐entrenchment floodplain terraces to communities dominated by the deep‐rooted woody legume, mesquite (Prosopis velutina). Mesquite is now the dominant cover in the riparian corridor and has increased by more than 300% in the USPR basin since 1972. The physiognomic shift from grassland to mesquite woodland on pre‐entrenchment floodplain terraces alters the sensitivity of ET and CO2 exchange to inputs of growing‐season precipitation. Because mature mesquite shrubs and trees have greater access to groundwater in these habitats than sacaton, ET and gross ecosystem production (GEP) are relatively decoupled from variation in monsoonal precipitation. However, decomposition of litter and soil organic matter in floodplain ecosystems remains highly coupled to monsoonal moisture inputs after mesquite encroachment. Responses of net ecosystem exchange of CO2 (NEE) to inputs of monsoonal rainfall are therefore not simple, but depend on vegetation composition and the connection of dominant plants to the water table. The heterogeneous vegetation patterns and groundwater depths in the riparian landscape offer unique opportunities for understanding fundamental ecohydrological processes linking carbon and water cycles in both riparian and upland ecosystems. Copyright © 2006 John Wiley & Sons, Ltd.

The sensitivity of ecosystem carbon exchange to seasonal precipitation and woody plant encroachment

Ongoing, widespread increases in woody plant abundance in historical grasslands and savannas (woody encroachment) likely will interact with future precipitation variability to influence seasonal patterns of carbon cycling in water-limited regions. To characterize the effects of woody encroachment on the sensitivity of ecosystem carbon exchange to seasonal rainfall in a semi-arid riparian setting we used flux-duration analysis to compare 2003-growing season NEE data from a riparian grassland and shrubland. Though less seasonally variable than the grassland, shrubland NEE was more responsive to monsoon rains than anticipated. During the 2004-growing season we measured leaf gas exchange and collected leaf tissue for delta(13)C and nitrogen content analysis periodically among three size classes of the dominant woody-plant, Prosopis velutina and the dominant understory species, Sporobolus wrightii, a C(4) bunchgrass, present at the shrubland. We observed size-class and plant functional type independent patterns of seasonal plant performance consistent with greater-than-anticipated sensitivity of NEE in the shrubland. This research highlights the complex interaction between growing-season precipitation, plant-available alluvial groundwater and woody plant abundance governing ecosystem carbon balance in this semi-arid watershed.

Factors Affecting the Development of Wood Rot on Lemon Trees Infected with Antrodia sinuosa, Coniophora eremophila, and a Nodulisporium sp.

Brown heartwood rot, which often is found in branches within lemon groves in southwestern Arizona, is caused by two basidiomycete fungi, Antrodia sinuosa and Coniophora eremophila. Another fungus, a species of Nodulisporium, has been recovered from small, dying lemon tree branches with an internal white wood rot. Studies were conducted from 1999 through 2002 to compare the extent of wood decay caused by these fungi (i) on lemon tree branches at different times of the year, (ii) on different types of citrus, (iii) on some desert woody perennial plants, and (iv) on lemon tree branches treated with selected fungicides. The mean length of wood decay columns recorded in lemon tree branches inoculated with A. sinuosa, C. eremophila, and the Nodulisporium sp. during the time periods of November to January, February to April, May to July, and August to October was 2.9, 4.7, 13.3, and 15.2 cm, respectively. There was a significant linear correlation between the length of wood decay columns and air temperature for all three pathogens. The mean length of wood decay columns for all time periods in branches inoculated with A. sinuosa, C. eremophila, and the Nodulisporium sp. was 11.8, 5.8, and 9.6 cm, respectively. In two trials, wood decay columns were significantly greater on Lisbon lemon tree branches inoculated with A. sinuosa compared with those on Marsh grapefruit and Valencia orange trees inoculated with the same pathogen. Wood decay in the presence of the Nodulisporium sp. was greater on branches of lemon compared with grapefruit trees in two trials and on lemon compared with orange trees in one of two trials. With the exception of C. eremophila on creosote bush, each of the three wood rot pathogens caused some wood decay in branches of velvet mesquite, salt cedar, Mexican palo verde, and creosote bush, four common desert perennials found in southwestern Arizona. Compared with nontreated but inoculated lemon trees, the length of wood decay columns in branches inoculated with A. sinuosa, C. eremophila, and the Nodulisporium sp. in the presence of propiconazole was reduced by 79, 94, and 92%, respectively, and, in the presence of azoxystrobin, was suppressed by 71, 80, and 89%, respectively. Current management guidelines focus on minimizing branch fractures and other nonpruning wounds in conjunction with early detection and removal of infected branches before the onset of the increased wood decay development period extending from May to October.

Drainage Size, Stream Intermittency, and Ecosystem Function in a Sonoran Desert Landscape

Understanding the interactions between terrestrial and aquatic ecosystems remains an important research focus in ecology. In arid landscapes, catchments are drained by a channel continuum that represents a potentially important driver of ecological pattern and process in the surrounding terrestrial environment. To better understand the role of drainage networks in arid landscapes, we determined how stream size influences the structure and productivity of riparian vegetation, and the accumulation of organic matter (OM) in soils beneath plants in an upper Sonoran Desert basin. Canopy volume of velvet mesquite (Prosopis velutina), as well as overall plant cover, increased along lateral upland–riparian gradients, and among riparian zones adjacent to increasingly larger streams. Foliar δ13C signatures for P. velutina suggested that landscape patterns in vegetation structure reflect increases in water availability along this arid stream continuum. Leaf litter and annual grass biomass production both increased with canopy volume, and total aboveground litter production ranged from 137 g m−2 y−1 in upland habitat to 446 g m−2 y−1 in the riparian zone of the perennial stream. OM accumulation in soils beneath P. velutina increased with canopy volume across a broad range of drainage sizes; however, in the riparian zone of larger streams, flooding further modified patterns of OM storage. Drainage networks represent important determinants of vegetation structure and function in upper Sonoran Desert basins, and the extent to which streams act as sources of plant-available water and/or agents of fluvial disturbance has implications for material storage in arid soils.

Influence of soil texture on hydraulic properties and water relations of a dominant warm-desert phreatophyte

We investigated hydraulic constraints on water uptake by velvet mesquite (Prosopis velutina Woot.) at a site with sandy-loam soil and at a site with loamy-clay soil in southeastern Arizona, USA. We predicted that trees on sandy-loam soil have less negative xylem and soil water potentials during drought and a lower resistance to xylem cavitation, and reach E(crit) (the maximum steady-state transpiration rate without hydraulic failure) at higher soil water potentials than trees on loamy-clay soil. However, minimum predawn leaf xylem water potentials measured during the height of summer drought were significantly lower at the sandy-loam site (-3.5 +/- 0.1 MPa; all errors are 95% confidence limits) than at the loamy-clay site (-2.9 +/- 0.1 MPa). Minimum midday xylem water potentials also were lower at the sandy-loam site (-4.5 +/- 0.1 MPa) than at the loamy-clay site (-4.0 +/- 0.1 MPa). Despite the differences in leaf water potentials, there were no significant differences in either root or stem xylem embolism, mean cavitation pressure or Psi(95) (xylem water potential causing 95% cavitation) between trees at the two sites. A soil-plant hydraulic model parameterized with the field data predicted that E(crit) approaches zero at a substantially higher bulk soil water potential (Psi(s)) on sandy-loam soil than on loamy-clay soil, because of limiting rhizosphere conductance. The model predicted that transpiration at the sandy-loam site is limited by E(crit) and is tightly coupled to Psi(s) over much of the growing season, suggesting that seasonal transpiration fluxes at the sandy-loam site are strongly linked to intra-annual precipitation pulses. Conversely, the model predicted that trees on loamy-clay soil operate below E(crit) throughout the growing season, suggesting that fluxes on fine-textured soils are closely coupled to inter-annual changes in precipitation. Information on the combined importance of xylem and rhizosphere constraints to leaf water supply across soil texture gradients provides insight into processes controlling plant water balance and larger scale hydrologic processes.

Nitrogen Form and Concentration Affect Nitrogen Leaching and Seedling Growth of Prosopis velutina

Velvet mesquite [Prosopis velutina Woot., Syn.: P. juliflora (Swartz) DC. var. velutina (Woot.) Sarg.] has become more popular in arid landscapes of the southwestern U.S., but little information on N requirements during the seedling stage is available. In addition to optimize growth of seedlings, minimizing N in runoff during production is an important consideration. Experiments were conducted to determine how biomass production and N leaching were affected first by different ratios of ammonium and nitrate N in sand culture and second by different N concentrations when seedlings were grown in two substrates. Mesquite seedlings produced the greatest biomass after 120 days when fertigated with a solution of 33 NO3–: 67 NH4+. Loss of N through leachate was 40% greater when NH + 4 comprised two thirds or more compared to one third or none in the fertigation solution. Nitrogen in leachate was highest after 16 weeks of treatment, coinciding with the reduced growth rate of seedlings. The second experiment utilized either sand or commercial growing media and a fertigation solution of 33 NO3–: 67 NH4+. Fertigation with 200 mg·L–1 N after 60 days in either substrate produced greatest biomass, while rates of 25, 50, or 100 mg·L–1 N produced about half of that biomass. With few exceptions, less N in either form was found in leachate when seedlings were grown in media and were fertigated with the two higher N rates compared to seedlings grown in sand at the two higher N rates. Plant morphology, biomass accumulation, photosynthate allocation, and the fate of N in the growing substrate and in leachate were strongly affected by the choice of growing substrate.

Breeding Biology and Nest-Site Selection of Red-tailed Hawks in an Altered Desert Grassland

ABSTRACT Red-tailed Hawks (Buteo jamaicensis) have expanded their range as trees have invaded formerly-open grasslands. Desert grasslands of southern Arizona have been invaded by mesquite trees (Prosopis velutina) since Anglo-American settlement and now support a large population of Red-tailed Hawks. We studied a population of Red-tailed Hawks in an altered desert grassland in southern Arizona. Our objectives were to determine what environmental characteristics influence Red-tailed Hawk habitat selection in mesquite-invaded desert grasslands and to evaluate the habitat quality of these grasslands for Red-tailed Hawks based on nesting density, nest success, and productivity. Red-tailed Hawks had 86% (95% C.I. = 73–99) nest success and 1.82 young per breeding pair (95% C.I. = 1.41–2.23). Nesting density was 0.15 (95% CI = 0.08–0.21) breeding pairs/km2 and the mean nearest-neighbor distance was 1.95 km (95% C.I. = 1.74–2.16). Red-tailed Hawks selected nest-sites with taller nest-trees and greater tre...

Evapotranspiration on western U.S. rivers estimated using the Enhanced Vegetation Index from MODIS and data from eddy covariance and Bowen ratio flux towers

We combined remote sensing and in-situ measurements to estimate evapotranspiration (ET) from riparian vegetation over large reaches of western U.S. rivers and ET by individual plant types. ET measured from nine flux towers (eddy covariance and Bowen ratio) established in plant communities dominated by five major plant types on the Middle Rio Grande, Upper San Pedro River, and Lower Colorado River was strongly correlated with Enhanced Vegetation Index (EVI) values from the Moderate Resolution Imaging Spectrometer (MODIS) sensor on the NASA Terra satellite. The inclusion of maximum daily air temperatures ( T a) measured at the tower sites further improved this relationship. Sixteen-day composite values of EVI and T a were combined to predict ET across species and tower sites ( r 2 = 0.74); the regression equation was used to scale ET for 2000–2004 over large river reaches with T a from meteorological stations. Measured and estimated ET values for these river segments were moderate when compared to historical, and often indirect, estimates and ranged from 851–874 mm yr − 1 . ET of individual plant communities ranged more widely. Cottonwood ( Populus spp.) and willow ( Salix spp.) stands generally had the highest annual ET rates (1100–1300 mm yr − 1 ), while mesquite ( Prosopis velutina) (400–1100 mm yr − 1 ) and saltcedar ( Tamarix ramosissima) (300–1300 mm yr − 1 ) were intermediate, and giant sacaton ( Sporobolus wrightii) (500–800 mm yr − 1 ) and arrowweed ( Pluchea sericea) (300–700 mm yr − 1 ) were the lowest. ET rates estimated from the flux towers and by remote sensing in this study were much lower than values estimated for riparian water budgets using crop coefficient methods for the Middle Rio Grande and Lower Colorado River.

Response of semi‐desert grasslands invaded by non‐native grasses to altered disturbance regimes

AbstractAim Using a long‐term data set we investigated the response of semi‐desert grasslands to altered disturbance regimes in conjunction with climate patterns. Specifically, we were interested in the response of a non‐native grass (Eragrostis lehmanniana), mesquite (Prosopis velutina), and native species to the reintroduction of fire and removal of livestock.Location The study site is located on the 45,360‐ha Buenos Aires National Wildlife Refuge (31°32′ N, 110°30′ W) in southern Arizona, USA. In 1985, livestock were removed and prescribed fires were reintroduced to this semi‐desert grassland dominated by non‐native grasses and encroaching mesquite trees.Methods Plant species cover was monitored along 38, 30‐m transects five times over a period of 15 years. Data were analysed using principal components analysis on the variance–covariance and correlation matrix, multivariate analysis of variance for changes over time in relation to environmental data, and analysis of variance for altered disturbance regimes.Results Reintroduction of fire and removal of livestock have not led to an increase in native species diversity or a decrease in non‐native grasses or mesquite. The cover of non‐native grass was influenced by soil type in 1993.Main conclusions Variability of plant community richness, diversity, and cover over time appear to be most closely linked to fluctuations in precipitation rather than human‐altered disturbance regimes. The effects of altered grazing and fire regimes are likely confounded by complex interactions with climatic factors in systems significantly altered from their original physiognomy.

Precipitation pulse use by an invasive woody legume: the role of soil texture and pulse size

Plant metabolic activity in arid and semi-arid environments is largely tied to episodic precipitation events or "pulses". The ability of plants to take up and utilize rain pulses during the growing season in these water-limited ecosystems is determined in part by pulse timing, intensity and amount, and by hydrological properties of the soil that translate precipitation into plant-available soil moisture. We assessed the sensitivity of an invasive woody plant, velvet mesquite (Prosopis velutina Woot.), to large (35 mm) and small (10 mm) isotopically labeled irrigation pulses on two contrasting soil textures (sandy-loam vs. loamy-clay) in semi-desert grassland in southeastern Arizona, USA. Predawn leaf water potential (psi(pd)), the isotopic abundance of deuterium in stem water (deltaD), the abundance of 13C in soluble leaf sugar (delta13C), and percent volumetric soil water content (theta(v)) were measured prior to irrigation and repeatedly for 2 weeks following irrigation. Plant water potential and the percent of pulse water present in the stem xylem indicated that although mesquite trees on both coarse- and fine-textured soils quickly responded to the large irrigation pulse, the magnitude and duration of this response substantially differed between soil textures. After reaching a maximum 4 days after the irrigation, the fraction of pulse water in stem xylem decreased more rapidly on the loamy-clay soil than the sandy-loam soil. Similarly, on both soil textures mesquite significantly responded to the 10-mm pulse. However, the magnitude of this response was substantially greater for mesquite on the sandy-loam soil compared to loamy-clay soil. The relationship between psi(pd) and delta13C of leaf-soluble carbohydrates over the pulse period did not differ between plants at the two sites, indicating that differences in photosynthetic response of mesquite trees to the moisture pulses was a function of soil water availability within the rooting zone rather than differences in plant biochemical or physiological constraints. Patterns of resource acquisition by mesquite during the dynamic wetting-drying cycle following rainfall pulses is controlled by a complex interaction between pulse size and soil hydraulic properties. A better understanding of how this interaction affects plant water availability and photosynthetic response is needed to predict how grassland structure and function will respond to climate change.

Spatial and temporal variation in islands of fertility in the Sonoran Desert

In many arid and semi-arid ecosystems, canopy trees and shrubs have a strong positive influence on soil moisture and nutrient availability, creating islands of fertility where organic matter and nutrients are high relative to areas outside the canopy. Previous studies of canopy effects on soil processes have rarely considered how landscape context may modulate these effects. We measured the effects of velvet mesquite trees (Prosopis velutina) on soil moisture and the biogeochemistry of nitrogen at different positions along a topographic gradient from upland desert to riparian zone in the Sonoran Desert of central Arizona. We also examined how landscape position and patterns of precipitation interact to determine the influence of P. velutina on soil moisture, N availability assessed using ion exchange resins, net N mineralization and net nitrification, and microbial biomass C and N. P. velutina clearly created islands of fertility with higher soil organic matter, net N mineralization and net nitrification rates, and microbial biomass under mesquite canopies. These effects were consistent across the landscape and showed little temporal variability. Magnitude and direction of effect of mesquite on soil moisture changed with landscape position, from positive in the upland to negative in the terrace, but only when soil moisture was >4%. Resin N showed responses to mesquite that depended on precipitation and topographic position, with highest values during wet seasons and under mesquite on terraces. We suggest changes in proximity of P. velutina to groundwater lead to shifts in biogeochemical processes and species interactions with change in landscape position along a topographic gradient.

Wildfire effects and post‐fire responses of an invasive mesquite population: the interactive importance of grazing and non‐native herbaceous species invasion

AbstractAim To determine how responses of an established velvet mesquite (Prosopis velutina Woot.) population to a 2002 wildfire were shaped by grazing and non‐native herbaceous species invasions, both of which influenced fire behaviour.Location The study was conducted on contiguous ranches (one actively grazed by cattle, one that had not been grazed since 1968) in the Sonoita Valley of southern Arizona. Plant communities on both ranches were comprised of Chihuahuan semi‐desert grassland, savanna, and Madrean evergreen woodland ecosystems, but large areas were dominated by Lehmann and Boer lovegrass, African grass species that were introduced more than 50 years ago.Methods We selected 243 individuals that had been defoliated and bark scorched during the fire using a stratified random design based on pre‐fire grazing status and dominant grass cover. After the start of the 2003 growing season, we recorded individual tree characteristics, fire damage, and measures of post‐fire response, and tested for relationships among classes of: grazing status, bark damage, dominant grass cover type, abundance of live and dead aboveground branches, flowering status, and sprout number and size. Analyses of fire damage and post‐fire response were interpreted with respect to values of fireline intensity, scorch height and energy release that were projected by a fire behaviour model, nexus.Results Nearly all of the trees on grazed areas suffered low levels of fire damage, while a majority on ungrazed areas suffered moderate to severe damage. Trees on grazed areas consequently had significantly more leaf‐bearing twigs and branches in 2003 but a very low number of root sprouts, while individuals on ungrazed areas had a greater density of root sprouts but little post‐fire dead branching and almost no living branches. Among the ungrazed grassland types, more than 75% of the trees on Boer lovegrass plots suffered moderate to severe damage, while a similar percentage of trees in native grass areas suffered low damage. These differences were: (1) attributed to variations in fire characteristics that were caused by differences in litter production and removal, and (2) ecologically significant because trees in the severe damage class showed almost no aboveground post‐fire branching, either live or dead in 2003, while trees in the low damage class exhibited a greater amount of both.Main conclusions Our results affirm the notion that effective management of western grasslands where mesquite encroachment has or will become a problem requires a better understanding of how interactions among key ecosystem influences (e.g. fire, grazing, non‐native species) affect not only mesquite seedlings and saplings but also larger, established individuals and thereby the long‐term structure and functioning of semi‐desert grassland ecosystems. As managers shift their focus from eradication to management of mesquite in western grasslands and savannas, our results provide insights into how prescribed fires (and their effects on mesquite populations) differ from wildfires and how such effects may be mediated by the altered land uses and ecosystem characteristics that now exist in many western ecosystems.

Soil nutrient distributions of mesquite-dominated desert grasslands: changes in time and space

Mesquite trees in desert grasslands create localized distributions of soil N, C, and P, through N fixation, nutrient uptake, and subsequent litterfall. Fires may prevent or reverse this accumulation, but the results of this process are poorly understood. Our objectives were to (1) characterize temporal and spatial variation in velvet mesquite litter mass and C, N, and P concentration; (2) characterize temporal and spatial variation of soil inorganic N in the upper 10 cm, and (3) characterize spatial variation in total soil C, total N, and available P concentrations (0–10 cm) relative to velvet mesquite locations on two sites of contrasting fire frequency. In 1997 and 1998, we sampled velvet mesquite litter monthly and soil seasonally in two adjacent sites with similar physical characteristics but different fire histories at Woodcutter's Canyon, Fort Huachuca, in southeastern Arizona. No fires had occurred in more than 50 years on one site (Site A), and the other site (Site B) had five fires within the 20 years prior to our study. Significantly more litter accumulated under trees at Site A, and most litterfall occurred during the winter months. For both sites and years, litter accumulation was highest during December. In 1997, the maximum average monthly litterfall/tree (total dry mass collected/total trap surface area) was 18.16±13.65 g/m 2 on Site A, and 5.43±2.98 g/m 2 on Site B. In 1998, the peak average litterfall/tree was 24.10±6.52 g/m 2 on Site A, and 9.32±6.85 g/m 2 on Site B. Litterfall C, N, and P concentrations did not differ significantly between locations. Litterfall C concentrations averaged 46%, and did not vary significantly over time. During both years, summer litter contained significantly higher concentrations of N and P than winter litter. Each year, litter C/N ratios ranged from 12:1 (summer) to 23:1 (winter). Soil inorganic N varied significantly over time and was significantly higher (56%) at Site A. Soil inorganic N, total N, total C, and available P concentrations were significantly higher under mesquite canopies compared to intercanopy zones at Site A. Available P concentrations were significantly higher in the intercanopy zones of Site B (6 mg/kg) compared to Site A (4 mg/kg), indicating that it was less localized. Overall, the soil nutrient distribution was more uniform on Site B. Litter mass and N, C, and P concentrations varied significantly over time, but were not correlated with soil inorganic N concentrations. Our study supports the concept that differences in mesquite stand development due to fire directly influences soil nutrient distributions in this plant community.

Front Cover: Macromol. Biosci. 9/2004

Front Cover: The photograph shows the main trunk of a Prosopis velutina tree bearing vitrified nodules of mesquite gum at the bark, a common phenomenon during springtime in the Desert of Sonora. The left top figure displays hydrophobic chromatography elution patterns of mesquite gum and gum arabic. Mesquite gum has been considered a potential substitute of gum arabic. The bottom figure shows Langmuir isotherms of whole and individual mesquite gum fractions of varying hydrophobicity forming monolayer films at the air/water interface.Further details can be found in the Full Paper by Y. L. López‐Franco, M. A. Valdez,* J. Hernández, A. M. Calderón de la Barca, M. Rinaudo, and F. M. Goycoolea on page 865.

Macromolecular dimensions and mechanical properties of monolayer films of Sonorean mesquite gum.

Mesquite gum sourced from Prosopis velutina trees and gum arabic (Acacia spp.) were characterized using light scattering and Langmuir isotherms. Both gum materials were fractionated by hydrophobic interaction chromatography, yielding four fractions for both gums: FI, FIIa, FIIb and FIII in mesquite gum and FI, FII, FIIIa and FIIIb in gum arabic. In mesquite gum, the obtained fractions had different protein content (7.18-38.60 wt.-%) and macromolecular dimensions (M approximately 3.89 x 10(5)-8.06 x 10(5) g.mol(-1), RG approximately 48.83-71.11 nm, RH approximately 9.61-24.06 nm) and architecture given by the structure factor (RG/RH ratio approximately 2.96-5.27). The mechanical properties of Langmuir monolayers at the air-water interface were very different on each gum and their fractions. For mesquite gum, the most active species at the interface were those comprised in Fractions IIa and IIb and III, while Fraction I the pi/A isotherm lied below that of the whole gum. In gum arabic only Fraction III developed greater surface pressure at the same surface per milligram of material than whole gum. This is rationalized in terms of structural differences in both materials. Mesquite gum tertiary structure seems to fit best with an elongated polydisperse macrocoil in agreement with the "twisted hairy rope" proposal for arabinogalactan proteoglycans.

Hydraulic redistribution by a dominant, warm‐desert phreatophyte: seasonal patterns and response to precipitation pulses

Summary Hydraulic redistribution may have important consequences for ecosystem water balance where plant root systems span large gradients in soil water potential. To assess seasonal patterns of hydraulic redistribution, we measured the direction and rate of sap flow in tap‐roots, lateral roots and main stems of three mature Prosopis velutina Woot. trees occurring on a floodplain terrace in semiarid south‐eastern Arizona, USA. Sap‐flow measurements on two of the trees were initiated before the end of the winter dormancy period, prior to leaf flush. Despite the absence of crown transpiration during the dormant season, sap flow was detected in lateral roots and tap‐roots of P. velutina. Reverse flow (away from the stem) in the lateral root and positive flow (towards the stem) in the tap‐root was observed in one tree, indicating the presence of hydraulic lift. Conversely, reverse flow in the tap‐root and positive flow in the lateral root was observed in the second tree, indicating hydraulic descent. Hydraulic descent was induced in the roots of the former tree by wetting the rooting zone in the upper 70 cm of the soil surface with 50 mm of irrigation. Patterns and rates of nocturnal sap flow in roots of a third tree measured during the growing season were similar to those observed during the dormant season. Nocturnal reverse flow in the lateral root and positive flow in the tap‐root was observed prior to the onset of the summer monsoon. Hydraulic descent commenced immediately following the first large monsoon rain event, and continued after subsequent rain events. After adjusting for differences in sapwood area, maximum diurnal rates of hydraulic descent in the tap‐roots of trees instrumented during the dormant season were 73 and 69% of the maximum night‐time rate of hydraulic descent observed during the growing season. Despite very limited potential for direct infiltration, volumetric soil moisture content in deep soil layers (1·5–9·5 m) increased 2–8% by the end of the monsoon (late September), indicating that plant roots were redistributing non‐trivial amounts of water to deep soil layers. Roots of P. velutina apparently redistribute significant amounts of soil water during the growing season, but also during periods of crown dormancy in winter. In arid regions dormant‐season hydraulic descent may buffer plants from water and nutrient deficits during initial stages of the growing season by transferring soil water derived from winter precipitation to deep soil layers and away from zones of evaporation in surface layers and shallow‐rooted herbaceous plants.