Dale A. Devitt

Water Use by Tamarix Ramosissima and Associated Phreatophytes in a Mojave Desert Floodplain

Water use by the introduced shrub Tamarix ramosissima and three co-oc- curring, native phreatophytes was measured in the lower Virgin River floodplain (southern Nevada) using the stem-heat-balance method. During the 1993 growing season, measure- ments were conducted on Tamarix in a closed, monospecific stand and in a mixed community with the native species Pluchea sericea, Prosopis pubescens, and Salix exigua. Our objec- tives were (1) to determine whether leaf-area-based water use of Tamarix is higher than that of co-occurring native riparian taxa, (2) to assess the role of Tamarix stand leaf area index (LAI, leaf area per unit ground area) on Tamarix water loss, and (3) to verify whether Tamarix is capable of using large amounts of water under the extreme evaporative demands that characterize arid environments. Leaf-area-based sap flow rates were comparable in the four species despite large differences in individual leaf area and total water loss. Daily water use of Tamarix (in grams per day) weighted by the daily potential evapotranspirati-on (PET) increased linearly with the total leaf area per plant, suggesting that water uptake was sufficient to compensate for water loss at the leaf level, even during times of extreme atmospheric water demand. Under high PET, maximum sap flow rates of Tamarix on a leaf- area basis were significantly higher at locations where Tamarix LAI was lower, indicating that highly transpiring Tamarix stands may reduce leaf-level evaporative demand. However, daily estimates of transpiration of dense Tamarix stands (LAI up to 3.5 m2/m2) in our study site exceeded PET by a factor ranging from 1.6 to 2.0, which confirms that mature Tamarix stands can lose very high quantities of water due to the maintenance of high leaf area. Results from this and other studies demonstrate that, at least under moderate to high water tables, key variables controlling water use by riparian stands include structural character- istics such as LAI and density. Management practices aimed at conserving water should be geared towards avoiding the development of dense Tamarix thickets along arid water courses, particularly if it is confirmed that these tend to attain higher LAI than native riparian communities.

Invasive capacity of Tamarix ramosissima in a Mojave Desert floodplain: the role of drought

Tamarix ramosissima (Tamaricaceae) is a woody phreatophyte that has invaded thousands of hectares of floodplain habitat in the southwestern U.S. In this study, we examined the response of gas exchange and stem sap flow of Tamarix and three co-occurring native phreatophytes (Pluchea sericea (Asteraceae), Prosopis pubescens (Fabaceae) and Salix exigua (Salicaceae)) to drought conditions in an early successional floodplain community in the Mojave Desert of southern Nevada. In an analysis of a size/age series of each species across the whole floodplain (both mature and successional stands), stem growth rate was lowest for Tamarix. However, along the same successional chronosequence, Tamarix came to dominate the 50+ year old stands with dense thickets of high stem density. Xylem sap flow, when expressed on a sapwood area basis, was highest in Tamarix under early drought conditions, but comparable between the four species toward the end of the summer dry season. Multivariate analysis of the gas exchange data indicated that the four species differentiated based on water use under early drought conditions and separated based on plant water potential and leaf temperature (indices of drought effects) at the end of the summer dry season. This analysis suggests that the invasive Tamarix is the most drought tolerant of the four species, whereas Salix transpires the most water per unit leaf surface area and is the least tolerant of seasonal water stress. Therefore, Salix appears to be well adapted to early successional communities. However, as floodplains in this arid region become more desiccated with age, Tamarix assumes greater dominance due to its superior drought tolerance relative to native phreatophytes and its ability to produce high density stands and high leaf area.

Water relations of riparian plants from warm desert regions

Riparian plants have been classified as “drought avoiders” due to their access to an abundant subsurface water supply. Recent water-relations research that tracks water sources of riparian plants using the stable isotopes of water suggests that many plants of the riparian zone use ground water rather than stream water, and not all riparian plants are obligate phreatophytes (dependent on ground water as a moisture source) but may occasionally be dependent on unsaturated soil moisture sources. A more thorough understanding of riparian plant-water relations must include water-source dynamics and how those dynamics vary over both space and time. Many rivers in the desert, Southwest have been invaded by the exotic shrubTamarix ramosissima (saltcedar). Our studies ofTamarix invasion into habitats formerly dominated by native riparian forests of primarilyPopulus andSalix have shown thatTamarix successfully invades these habitats because of its (1) greater tolerance to water stress and salinity, (2) status, as a facultative, rather than obligate, phreatophyte and, therefore, its ability to recover from droughts and periods of ground-water drawdown, and (3) superior regrowth after fire. Analysis of water-loss rates indicate thatTamarix-dominated stands can have extremely high evapotranspiration rates when water tables are high but not necessarily when water tables are lower.Tamarix has leaf-level transpiration rates that are comparable to native species, whereas sap-flow rates per unit sapwood area are higher than in natives, suggesting thatTamarix maintains higher leaf area than can natives, probably due to its greater water stress tolerance.Tamarix desiccates and salinizes floodplains, due to its salt exudation and high transpiration rates, and may also accelerate fire cycles, thus predisposing these ecosystems to further loss of native taxa. Riparian species on regulated rivers can be exposed to seasonal water stress due to depression of floodplain water tables and elimination of annual floods. This can potentially result in a community shift toward more stress-tolerant taxa, such asTamarix, due to the inability of other riparian species to germinate and establish in the desiccated floodplain environment Management efforts aimed at maintaining native forests on regulated rivers and slowing the spread ofTamarix invasion must include at least partial reintroduction of historical flow, regimes, which favor the recruitment of native riparian species and reverse long-term desiccation of desert floodplain environments.

Bowen Ratio estimates of evapotranspiration for Tamarix ramosissima stands on the Virgin River in southern Nevada

A Bowen ratio energy balance was conducted over a Tamarix ramosissima (saltcedar) stand growing in a riparian corridor along the Virgin River in southern Nevada. Measurements in two separate years were compared and contrasted on the basis of changes in growing conditions. In 1994, a drought year, record high temperatures, dry winds, and a falling water table caused partial wilt of outer smaller twigs in the canopy of many trees in the stand around the Bowen tower. Subsequently, evapotranspiration (ET) estimates declined dramatically over a 60-day period (11 mm d−1 to <1 mm d−1). In 1995, the Virgin River at the Bowen tower area changed its course, hydrologically isolating the Tamarix stand in the vicinity of the tower. In 1996, a 25% canopy loss was visually estimated for the Tamarix growing in the area of the tower. Higher soil temperatures relative to air temperatures were recorded in 1996 in response to this loss in canopy. With a more open canopy, thermally induced turbulence was observed in 1996. On day 160 of 1996, a 28°C rise over a 9-hour period was correlated with increased wind speeds of greater than 4 m s−1. Subsequently, higher ET estimates were made in 1996 compared to 1994 (145 cm versus 75 cm). However, the energy balance was dominated by advection in 1996, with latent energy flux exceeding net radiation 65% of the measurement days compared to only 11% in 1994. We believe this advection was on a scale of the floodplain (hundreds of meters) as opposed to regional advection, since the majority of wind (90%) was in a N–S direction along the course of the river, and that a more open canopy allowed the horizontal transfer of energy into the Tamarix stand at the Bowen tower. Our results suggest that Tamarix has the potential to be both a low water user and a high water user, depending on moisture availability, canopy development, and atmospheric demand, and that advection can dominate energy balances and ET in aridland riparian zones such as the Virgin River.

Evaluating temporal variability in the spectral reflectance response of annual ryegrass to changes in nitrogen applications and leaching fractions

A two‐year study was conducted in 2002 and 2003 at the University of Nevada, Las Vegass center for urban water conservation to assess canopy spectral response of annual ryegrass (Lolium multiflorum Lam.) grown under various combinations of N and irrigation (based on leaching fraction: LF) treatments. Multispectral measurements were acquired using a ground‐based spectroradiometer (200–1100 nm) on a biweekly basis during the growing season (October–May) in 2002 and 2003. Multispectral parameters were correlated with soil–plant parameters and temporal variability was investigated. Results showed that the normalized difference vegetation index (NDVI), stress index (SI), photochemical reflectance index (PRI) and canopy reflectance at 693 nm, were highly correlated with tissue N concentration (TN), tissue moisture content (TM), TN×TM and canopy colour, as influenced by N and LF treatment combinations. Coefficients of determination ranged from 0.50 to 0.79 (P<0.001) based on single‐day correlations and correlations established over the entire growing period in 2002 and in 2003. TN was mainly predicted from wavelengths in the VIS portion of the spectrum, while TM was predicted from wavelengths in the VIS and NIR. Correlations were inconsistent between spectral parameters and physiological parameters throughout the study confirming the problem of temporal variation associated with spectral signatures of turfgrass species. However, spectral reflectance showed significant potential for monitoring turfgrass N and moisture status, and was able to capture temporal variability over the same growing period and from one year to another. The results provide a sound basis for future validation of ground‐based remote sensing for turfgrass management on golf courses.

Effect of Salinity and Nitrogen Status on Nitrogen Uptake by Tall Fescue Turf

ABSTRACT Nitrogen (N) absorption is inhibited by root zone salinity, which could result in increased NO3 leaching. Conversely, N absorption is enhanced by moderate N deficiency. Because turfgrasses are grown under N-limiting conditions, it is important to understand the interactive effects of salinity and N deficiency on N uptake. This study examined the effect of N status (replete versus deficient) and salinity on N (15NO3 and 15 NH4) uptake and partitioning by tall fescue (Festuca arundinacea Schreb.). Two cultivars (‘Monarch’ and ‘Finelawn I’) were grown in nutrient solution culture. Treatments included N level (100% or 25% of maximum N demand) and salinity (0, 40, 80, and 120 meq L−1) in a factorial arrangement. Absorption of NO3 and NH4 was greater in low-N than in high-N cultures, but was reduced by salinity under both N treatments. Salinity reduced partitioning of absorbed N to leaves and increased retention in roots. These results suggest that turfgrass managers should consider irrigation water quality when developing their fertilizer program.

Response of Cynodon dactylon to prolonged water deficits under saline conditions

Competition for available water resources in both arid and semi-arid locations has led to greater scrutiny of turfgrass irrigation. Irrigation management strategies, including deficit irrigation, need further investigation. The objective of this research was to determine the physiological response of bermudagrass (Cynodon dactylon (L.) Pers.) to prolonged water deficits under saline conditions. Bermudagrass was grown in large columns packed with three different soil types (sandy loam, silt loam and clay). Synthesized saline irrigation water was applied at three different salinity levels (1.5, 3.0 and 6.0 dS/m). Two previous experiments that were conducted with these columns over a 3.5 year period led to differential profile salinization in all 27 columns. At the end of this 3.5 year period, all irrigations were terminated and plant growth and water status were monitored over a 95 day dry-down period. Midday stomatal conductance, leaf water potential, canopy temperature, soil water in storage and stolon elongation were measured over the experimental period. On day 95, above ground tissue was harvested for dry weight and elemental tissue analysis. Midday stomatal conductance decreased around day 30 in all columns regardless of soil salinity. This decrease was not associated with a threshold leaf water potential, as midday leaf water potentials remained constant over a 60 day period. Stolon elongation also ceased before any deviation in the midday leaf water potential occurred. A concomitant reduction in evapo ranspiration was not associated with the measured decrease in stomatal conductance. This would suggest that bermudagrass may have regulated stomatal activity to compensate for lower conductances during periods of greater stress and/or that measured midday stomatal conductances cannot always be directly coupled to extended time evapotranspiration measurements.

Impact of potassium, sodium, and salinity on the protein-and free amino acid content of wheat grain

A lysimeter study was conducted on Cajeme wheat (Triticum aestivum L.) to investigate the impact of salinity on protein and free amino acid content of the grain. Cross correlations were obtained between 16 different soil-plant-water based parameters and the concentration and total accumulation of amino acids.The results indicated that after 3 years of irrigation, the majority of protein bound and free amino acids increased in concentration in the grain. However, both free tryptophan and free proline revealed decreasing concentrations with increasing salinity. Free tryptophan showed a synergism between total accumulation, yield and concentration. Free proline concentrations decreased in association with increasing protein concentrations.Cross correlations of the 16 soil-plant-water based parameters with free and protein bound amino acids revealed significant correlations for free aspartic acid and glycine with total accumulation but not with concentrations. Only methionine plus cystine was lower than suggested FAO levels for essential amino acids and was lower in the third year than in the first year.

Effect of Nitrogen Status on Salinity Tolerance of Tall Fescue Turf

ABSTRACT Turfgrass salinity tolerance is usually studied under conditions of non-limiting nutrition, even though most turfgrasses are managed with growth-limiting levels of nitrogen (N). This study examined the effect of N status (replete versus deficient) on salinity tolerance in tall fescue (Festuca arundinacea Schreb.). Additionally, the interactive effects of N status and salinity on tissue ion concentrations were determined. Two cultivars (‘Monarch’ and ‘Finelawn I’) were grown in nutrient solution culture. Treatments included N level (100% or 25% of maximum N demand) and salinity (0, 40, 80, 120 meq L−1). Salinity reduced leaf growth under high-N conditions, but much less so under low-N conditions. Concentrations of potassium (K), sodium (Na), and chloride (Cl) in the leaf sap were significantly higher in low-N than in high-N plants, indicating that increased salinity tolerance in low-N turf was not due to ion exclusion. These results suggest that efforts to screen turfgrasses for salt tolerance should be conducted using realistic N-fertility levels.

Residential Assessment and Perception Toward Biosolids Compost Use in an Urban Setting

A survey of Nevada residents in the Las Vegas Valley was conducted to determine perceptions and attitudes as they may contribute to the marketing potential of composted biosolids for residential use. Currently only 5 percent of Nevadas sludge is being composted, 15 percent is land applied while 80 percent is being sent to landfills. Sludge generated in the Las Vegas Valley is extremely low in heavy metals due to the low presence of industries that are characterized with the release of heavy metals. A residential survey was developed and mailed to over one percent of the valleys households resulting in a 21.5 percent return. Seventy-six percent of the respondents rated recycling as important or very important and 74 percent indicated that it is important or very important to find uses specifically for sewage sludge. A large percentage of residents were not aware of how sewage sludge is disposed (73.7%), but felt the community should be examining sewage sludge recycling options (75.1%). After a descriptive paragraph defining and describing composted sludge, there was a high level of support for the use of composted sludge on golf courses and public landscapes but a much smaller level of support for its use on vegetable gardens and houseplants. Respondents were more concerned about safety and health than they were about the cost of the product. Respondents looked to public health officials and University scientists for information concerning its safety. Backward multiple regression analysis indicated that males, individuals with higher incomes and higher education were more supportive of the use of composted sludge. Comparing responses from two different cities within the Las Vegas Valley, it was found that differences in demographics between communities led to differences in response. In particular, differences in the level of education, income, sex and ethnicity influenced responses from the two communities. Evaluation of the data suggests that based on a communitys demographics, different educational and marketing approaches may need to be considered.