( )Journal of Arid Environments 1998 40: 281]294Article No. ae980443

Growth rates, salt tolerance and water usecharacteristics of native and invasive riparian

plants from the delta of the Colorado River, Mexico

Edward Glenn*‡, Rene Tanner*, Shelby Mendez*,Tamra Kehret*, David Moore*, Jaqueline Garcia*

& Carlos Valdes†

* Environmental Research Laboratory, 2601 E. Airport Drive,Tucson, Arizona 85706, U.S.A.

† Instituo Technologica y de Estudios Superiores de Monterrey r CampusGuaymas, Apdo. Postal 484 Guaymas, Sonora 85400, Mexico

( )Received 23 March 1998, accepted 20 July 1998

Six riparian plant species representing native and invasive species from theColorado River delta in the Sonoran Desert of Mexico were tested for salttolerance and water use characteristics in a greenhouse study in Tucson,Arizona. Negative linear regression equations relating relative growth rates( y1 y1)RGR, g g day of each species to mean root zone salinity had high

2 . . .( )coefficients of determination r s 0 73]0 86, p - 0 001 . Salt tolerancelevels, expressed as % reduction in RGR per g ly1 NaCl in soil solution,varied widely among species: Allenrolfea occidentalis, 0% reduction; Tamarix

. .ramosissima, 1 8% reduction; Pluchea sericea, 3 5% reduction; and Baccharis.( )salicifolia, Salix gooddingii and Populus fremontii, 7]9% reduction p - 0 05 .

Transpiration was proportional to RGR for all species. Contrary to someprevious reports, Tamarix did not have unusually high water use compared tothe other species. Differences in salt tolerance among species determined inthis study support field observations that soil salinity, which can reach highvalues along channelized and flow-regulated stretches of south-western UnitedStates rivers due to lack of overbank flooding, is a major factor in thereplacement of native riparian species by invasive species.

q 1998 Academic Press

Keywords: riparian zone vegetation; salinity tolerance; Colorado River delta;water-use efficiency; desert rivers

Introduction

Historically, the native forest vegetation along most of the lower Colorado River was( )the Populus fremontii]Salix gooddingii cottonwood]willow association with mesic

( )‡ E-mail: eglenn@ag.arizona.edu .

0140]1963r98r030281 q 14 $30.00r0 q 1998 Academic Press

E. GLENN ET AL.282

( ) ( )understory plants such as Baccharis salicifolia seepwillow Szaro, 1989 . Humandisturbance of this desert river system over the past century has profoundly altered the

( )composition of the vegetation Ohmart et al., 1988; Szaro, 1989; Busch & Smith, 1995 .Water diversions, ground-water decline and flow regulation have reduced therecruitment of Populus and other mesic species, while invasive species have proliferated

( )to the point that the Tamarix ramosissima]Pluchea sericea salt cedar]arrowweed scrubassociation, or monotypic stands of Tamarix now dominate most of the former Populus

( )and Salix habitats Szaro, 1989; Busch & Smith, 1995 . The remaining populations ofPopulus and Salix along the river are skewed towards adult and senescent plants

(because seedlings have difficulty in establishing in the altered habitats Stromberg &)Patten, 1991; Busch & Smith, 1995 . Similar changes have occurred on other south-

(western rivers Haase, 1972; Howe & Knopf, 1991; Stromberg & Patten, 1991;)Shafroth et al., 1995; Briggs, 1996; Sala et al., 1996; Stromberg et al., 1996 and in

( )other arid regions of the world Loope et al., 1988; Hart et al., 1990 .Flow-regulated and channelized river stretches can develop saline bankside conditions

because they are no longer subject to periodic overbank flooding which washes salts( )from the soil Ohmart et al., 1988 . Invasive species are hypothesized to succeed in part

( )due to greater salt tolerance. For example, a field study Busch et al., 1992 comparingthe vegetation, hydrology and soil conditions along stretches of the Bill Williams River( ) ( )undisturbed and Colorado River disturbed showed that the disturbed river stretches,dominated by Tamarix and Pluchea, had soil salinity levels 2]3 times higher thanundisturbed stretches that still supported Populus and Salix. By contrast, Shafroth et

( )al. 1995 concluded that salinity played only a minor role in the replacement ofPopulus by Tamarix on sand bars along the Rio Grande, since both plants showedequal germination and seedling growth in salt concentrations up to 5 times normal

. y1( )river-water salinity c. 2 5 g l .Tamarix and other invasive species are also hypothesized to have higher consumptive

( )water use than the species they replace e.g. Weeks et al., 1987 , with the result that( )they can desiccate water courses Vitousek, 1990 . However field measurements of

( )transpiration by stomatal conductance Anderson, 1982; Busch & Smith, 1995 or by( )the stem heat balance method Sala et al., 1996 have shown little difference in

transpiration rates between Tamarix and other riparian tree species at the leaf or whole( )branch level of measurement. Water-use efficiency is also an issue; Anderson 1982

regarded Tamarix as a ‘water spender’ with low water-use efficiency based onmeasurements of transpiration and photosynthesis of leaves, but other studies havereported higher water-use efficiency by Tamarix and Pluchea compared to mesic

(riparian species based on carbon stable isotope ratios in plant tissues Busch & Smith,)1993, 1995 .

Most hypothesis testing has been carried out in the field, with relatively few( )controlled growth experiments performed for comparison Briggs, 1996 . Jackson et al.

( )1990 grew Salix, Populus, Pluchea, Tamarix, Allenrolfea and other Colorado Rivery1 ( )species in a greenhouse in solutions ranging from 0 to 60 g l salt NaCl q CaCl .

Salix and Populus did not survive to the end of the experiment on salt concentrations. y1 ( )higher than 1 5 g l the first treatment level above controls , whereas Allenrolfea had

100% survival up to 60 g ly1 salt. Pluchea and Tamarix were intermediate, survivingup to 18 and 36 g ly1, respectively; salinity was therefore identified as potentially a

(major determinant of plant distribution along salinized stream banks Jackson et al.,)1990 . However, this study did not analyse growth rates as a function of salinity, or

( )measure water use. Kleinkopf & Wallace 1974 conducted a greenhouse study in whichTamarix showed a marked diminution in growth in response to salinity but nodiminution in instantaneous rates of transpiration or photosynthesis over a salinitygradient from 0 to 200 mM NaCl. They concluded that growth was negatively affectedby salinity due to diversion of energy for increased respiration and salt pumping.

In this study the salt tolerance and water use parameters of six riparian plants from

RIPARIAN PLANTS IN THE COLORADO RIVER DELTA 283

the Colorado River delta in Mexico under controlled growth conditions in a greenhousewere compared. The study is part of an effort to understand the factors controlling thedistribution of vegetation along the riparian zone of the Colorado River as it approaches

( )the Gulf of California Glenn et al., 1996 . The species distribution is still underinvestigation, but in general the dominant plants are Populus and Salix in the northernpart of the delta, which receives fresh water flows, whereas Tamarix, Pluchea, andAllenrolfea are dominant in the southern part of the delta which receives salineagricultural drain water and tide water. Our objective was to develop predictiveequations relating survival, growth rate, transpiration rate, and water-use efficiency tosubstrate salinity for common tree and understory species of the delta. Our startinghypotheses were that these species would exhibit marked differences in salt tolerance,but less pronounced differences in water use characteristics, in keeping with the most

( )recent field measurements of water use Busch & Smith, 1995; Sala et al., 1996 .

Materials and methods

( ) ( )The experimental designs of Glenn et al. 1995 and Glenn & Brown 1998 were( )modified to compare Allenrolfea occidentalis, Baccharis salicifolia sB. glutinosa ,

( )Pluchea sericea sTessaria sericea , Populus fremontii, Salix gooddingii, and Tamarixramosissima collected from the study site; taxonomic designations follow Felger et al.( )1997 . The experimental method involved growing plants in a well-drained, sandy soilmix in pots irrigated daily with salinized nutrient solution, using a high leachingfraction to maintain the root zone salinity near to the salinity of the irrigation water atall times. Although the soil in the pot was continually moist, excess water drained fromthe pot within 1 h of application, so aerobic conditions were maintained betweenirrigations. The daily irrigation volume and drainage from each pot were quantified andthe soil surface of the pot was covered with plastic to minimize evaporative water loss.The experiment was designed to provide estimates of growth rate, transpiration, and

( )water-use efficiency over the period of the experiment. Sodium chloride NaCl wasused as a single salt addition to the base solution to adjust salinity because it is the salt

(most likely to accumulate to excess levels in lower Colorado River ecosystems Ohmart) q yet al., 1988 , and because Na and Cl are the ions most often associated with the

( )growth reduction of non-halophytes by salts Greenway & Munns, 1980 .

Experimental design and procedures

( ) ( )Seeds A. occidentalis or branch tip cuttings all other species were collected fromplants growing in the middle zone of the Colorado River delta near Ejido Oviedo Mota,

( )Mexico 328 209 N, 1158 109 W , and were propagated in a greenhouse in Tucson,Arizona, U.S.A. Plants were grown in germinating trays in 8 cm3 inserts containing a

( )mixture of sand and potting medium 1:1 vrv . After plants were well established theywere transplanted from the trays into treatment pots, keeping the root ball intact. One

. (plant each was transplanted into 15 2-1 capacity plastic pots 25 cm diameter, 23 cm.)deep containing 12 3 kg of dry, medium texture, sieved river sand, which was soaked

with nutrient solution prior to receiving the transplant; after draining, pots heldapproximately 2 l of solution. The soil filled the pot to within 2 cm of the top, allowingroom for the addition of irrigation solution. Each pot was covered with a plastic filmpressed tightly to the soil and brought over the sides of the pot to act as a vapor barrier;the seedling or cutting was planted into the pot through a small opening in the film atthe center of the pot. The film was then gathered tightly around the base of thetransplant to minimize evaporative water loss from the pot.

E. GLENN ET AL.284

.( )Two semicircles of styrofoam 2 5 cm thick , cut to fit the inside diameter of the pot,were placed over the film to act as an additional vapor barrier and to reduce solarheating of the soil surface. The styrofoam pieces were notched at the center where theplant emerged from the pot to allow room for stem expansion during growth. Pots werespaced 20 cm apart on a bench made of wooden slats; the space between pots preventedthe plant canopies from growing together. Each pot was equipped with a bottom draintube that passed between the slats and discharged into a 4-1 capacity collecting bottleset on the greenhouse floor under the pot.

All plants, except Populus, were cut to 10 cm height after transplanting so that theystarted the experiment at approximately the same size. Populus cuttings were nottrimmed because most of the new growth was clustered near the apex and would have

.been removed; the mean starting size of these plants was 25 9 cm. The salinity trials( )were initiated by irrigating the pots with salinized treatment solutions see below . This

resulted in a gradual replacement of non-saline water in the pot with salinized media;the leachate salinity equalled the salinity of the irrigation solutions after 5]7 days. Eachday during the trial, the volume and salinity of drainage water from each pot wasmeasured, drainage bottles were emptied and replaced, and pots were irrigated with 1or 2 l of treatment solution.

Each species trial was continued until the plants of the fastest growing treatment hady1 (evapo-transpiration rates of more than 1 l day 50% of the water-holding capacity of

)the pot , by which time roots had proliferated to fill the pots and plants were up to 1 min height. All plants of that species were then harvested on the same day. The days toharvest varied among species because they had different growth rates and weretherefore not uniform in the time required to reach the endpoint determined by waterconsumption.

The trials were conducted in two experiments which varied slightly in design andexecution. Allenrolfea and Tamarix were grown in the first experiment fromJune]August 1997. Each species was grown in three replicates on five salinity treatments

y1 (containing 0, 4, 8, 16, or 32 g l added NaCl 2 species = 5 treatments = 3. . .)replicate s 30 pots . These treatment levels are equivalent to 0, 70 4, 140 8, 281 6, and

. y3563 2 mol m NaCl, respectively. The salinity treatments were designed to span therange of salinities to which these species are exposed in the Colorado River delta, but tocover the low end of the salinity range more closely than the high end. Three blankpots, prepared and irrigated with non-saline nutrient solution in the same manner astest pots but not containing transplants, were included to correct for water loss from

( )pots in the absence of plants Glenn & Brown, 1998 . Pots were placed on thegreenhouse bench in a random design. Initially, all pots were irrigated with 1 l of testsolution per day, but as plants grew it became apparent that they required more waterand daily irrigation was increased to 2 l per day for all pots 4 weeks into theexperiment, and was maintained at that level for the remainder of the experiment.

Baccharis, Pluchea, Populus, and Salix were grown in the second experiment fromAugust]October 1997, immediately after harvest of the first experiment. Salinity levels

y1 . . . . y3( )were 0, 2, 4, 8, and 16 g l 0, 35 2, 70 4, 140 8, and 281 6 mol m NaCl , lower thanin the first experiment because this set of plants is putatively less salt tolerant than the

( )first set Jackson et al., 1990 . Methods were the same as for the first experimentexcept that pots were placed on tables in a complete block design to minimize the

(effects of environmental gradients in the greenhouse, and all pots 4 species = 5)treatments = 3 replicates q 3 blanks s 63 were irrigated with 2 l of test solution per

day throughout the experiment. At the beginning of the experiment some of theBaccharis plants exhibited yellowing of new growth, which was alleviated by sprayingall plants in the experiment once per week throughout the experiment with Fe-

. y1( )Sequestrene chelated iron solution at 2 5 mg l . Day temperatures over the twoexperiments ranged from 258C to 358C, light transmission through the greenhouseglazing was approximately 65% of ambient and ranged from 1000]1300 mmol my2 sy1

RIPARIAN PLANTS IN THE COLORADO RIVER DELTA 285

(at midday, and relative humidity was 60]70% the greenhouse was evaporatively)cooled through the trials .

Composition of treatment solutions, measurement of drainage volume and salinity,and other analytical methods

Treatment solutions were prepared using the municipal water supply plus NaCl to givethe desired treatment salinity. The municipal water supply contained 310 mg ly1 total

q . y3 2q .( ) (dissolved solids, of which the principal cations were Na 2 2 mol m and Ca 1 6y3 y . y3 2y . y3) ( ) ( )mol m , and the principal anions were Cl 1 7 mol m and SO 1 0 mol m ;4 . y1carbonate was not determined. Nutrients were added to all solutions as 0 25 g l

. y3 .( )soluble fertilizer Peter’s Complete Soil Test Fertilizer containing 3 6 mol m N, 1 6y1 . y2mol m P, 1 3 mol m K, and micronutrients. Drainage volumes were measured by

decanting water from collection bottles into a 2-l capacity graduated cylinder, andsalinity was measured by the refractive index of the drain water using a hand-held

( ) y1refractometer American Optical Special Scale Model which was scaled in g l , andwhich was calibrated using NaCl solutions. Plant height was measured at the end ofeach trial before plants were harvested. Plants were separated into roots and shoots andoven-dried to constant weight at 608C to determine dry matter production.Unfortunately, roots proved to be difficult to recover completely from the soil andresults were variable among replicates, so only shoot data are reported. Ash content wasdetermined by combustion of ground, 1 g samples of dry shoot material for 4 h at5008C. Ash content was subtracted from dry matter content to determine ash-free dry

( )matter production i.e. organic matter on which growth rates and water-use efficiencywere based. Cation content of shoots was determined by flame emission spectroscopy atthe Soil, Water and Plant Analytical Laboratory, Department of Soil, Water andEnvironmental Science, University of Arizona, Tucson.

Calculation of growth rate, evapo-transpiration, and water-use efficiency

( )Relative growth rates RGR were calculated from the initial and final ash-free dryy1 y1 [ ( )weights of shoots according to the formula: RGR in g g day s ln final wt. y

( )]ln initial wt. rdays growth. The initial shoot weights of the plants used in theexperiment could not be determined without destroying the plants, therefore, initialplant weights were estimated by determining the mean ash-free dry weight of 5 or 10additional shoots of each species selected from the nursery stocks and trimmed to thesame initial size of the test plantings; this mean value was used as a constant in thecalculation of RGR for each species. Estimates of mean initial weight ranged from. . y1 .0 18]0 26 g plant for all species except Populus, which was 0 75 g. Relative salt

( y1 )tolerance % growth reduction per g NaCl l was calculated by converting RGRvalues to % RGR relative to the mean RGR on the fastest growing treatment for eachspecies, then determining the slope of the growth response to salinity by linearregression analysis.

( y1 )Evapo-transpiration ET, 1 plant was calculated by subtracting the volume ofleachate water recovered over the experiment by the volume added as irrigation water;

( y1 )transpiration rate T, l d was estimated by subtracting the mean value of water lossfrom the blank pots up to the day of harvest from cumulative evapo-transpiration, then

. y1dividing by days of growth. Evaporation from blank pots averaged 0 045 l day.( )SE s 0 002 , only 10% of the ET rate of plants on rapidly-growing treatments, but

)50% of the rate from plants on the highest salt levels. Hence, estimates of T tendedto be inaccurate on the highest salinity treatments due to non-transpirational water

E. GLENN ET AL.286

( )losses from the pots Glenn & Brown, 1998 . The transpiration rate of control plants( y1 y1)over the 24 h prior to harvest 1 g day was calculated by dividing T over the final

24 h by the shoot fresh weight at harvest.( y1 )Water-use efficiency WUE, g l was calculated by dividing ash-free dry matter

production of a plant by its cumulative transpiration over the growth period. Thisexpression for whole-plant WUE is similar to units of WUE calculated at the leaf levelby dividing short-term rates of photosynthetic carbon dioxide fixation by the rate of

( )transpiration measured simultaneously Osmond et al., 1980; Anderson, 1982 . Valuescalculated for the highest salinity treatments, which produced negligible growth or

( )water use above blank pots, tended to be inaccurate Glenn & Brown, 1998 . The meansalinity in the root zone of the pot was estimated as the arithmetic average of applied

( )salinity and leachate salinity over the experiment Miyamoto et al., 1996 ; rootzone salinities were 10]15% higher than the salinity of the irrigation solution in thefastest growing treatments due to the rapid extraction of water by the plants.

Statistical analyses

( )Relationships between dependant variables RGR, T, and WUE and mean root zone( )salinity were tested by Model I regression analysis of the data Sokal & Rohlf, 1981 .

Although means and standard errors of treatments are shown in the figures, values forindividual replicates were used in the regressions. Slopes of regression equations ofrelative salt tolerance on salinity were accepted as significantly different if their 95%comparison intervals based on variance of the slopes did not overlap, using the method

(for unplanned comparisons among a set of regression coefficients Sokal & Rohlf,)1981 . Means of WUEs pooled across salinity treatments, and of transpiration rates of

plants during the final 24 h of the experiment were compared among species using the.Student-Newman-Keuhls test at p - 0 05. Some species did not have complete

survival on the highest salinity treatments. If two of the three replicates survived, thespecies was considered to have survived the treatment and values from the survivingreplicates were used in the analyses. If fewer than two replicates survived, the specieswas scored as dead on that treatment level and statistical analyses were based on datafrom the remaining treatment levels for which replicate measurements were available tocalculate the error term.

Results

The overall performance of each species is summarized in Table 1. The growth period( ) ( )ranged from 31 days Tamarix to 56 days Baccharis , reflecting different growth rates

among species. Three species approached 1 l dy1 water use at about the same time andwere harvested on the same day. Allenrolfea had optimal growth on 8 g ly1 NaCl,

(while all other species grew best on control solution. Heights of control plants ory1 )plants on 8 g l NaCl in the case of Allenrolfea at harvest ranged from 39]100 cm

and shoot ash-free dry weights ranged from 20]33 g among species. The maximum saltlevel tolerated ranged from 8 g ly1 for Baccharis, Populus, and Salix to 32 g ly1 forTamarix and Allenrolfea, with Pluchea intermediate at 16 g ly1. Allenrolfea plants had

. y1( )significantly p - 0 01 higher ash-free dry weights and RGRs on the 8 g l NaCltreatment compared to the other treatment levels but differences in T and WUE werenot significant across salinity levels. Transpiration, ash-free dry weight, and RGR of all

.( )other species were significantly reduced by salinity p - 0 05 . WUE was not. .( ) ( )significantly affected by salinity p ) 0 05 except for Salix p - 0 01 and this was

. y1(due to a low value on the highest salinity treatment only 0 75 g l compared to a

RIPARIAN PLANTS IN THE COLORADO RIVER DELTA 287

Table 1. Effect of salinity on growth and water use of riparian species from the( )Colorado River delta, showing days of treatment days ; mean shoot height and

( )mean shoot ash-free dry weight AFDW of plants on the fastest growing treatment(which was the controls for all species except Allenrolfea , which had optimal

y1 ) ( )growth on 8 g l NaCl ; the highest salt level tolerated Salt Max. ; and( ) ( )F-ratios of the regression of transpiration T , water-use efficiency WUE , and( )relative growth rate RGR on salinity

SaltHeight AFDW Max. F†

y1( ) ( ) ( )Species Days cm g g l T WUE RGR N

. . . . .A. occidentalis 56 38 8 32 5 32 0 8 1 9 -0 1 15NS NS NS

. . . . .B. salicifolia 46 99 7 30 6 8 6 1 -0 1 33 7 12* NS ***

. . . . .P. fremontii 34 88 0 19 5 8 9 2 0 7 24 5 11* NS ***

. . . . .P. sericea 34 73 3 29 9 16 20 7 -0 1 37 7 14*** NS ***

. . . . .T. ramosissima 31 64 0 20 9 32 28 3 0 8 66 0 15*** NS ***

. . . . .S. gooddingii 34 85 3 32 2 8 10 4 4 9 72 4 12** ** ***

. .( ) ( ) ( )† Significance level is denoted as NS not significant ; * p - 0 05 ; ** p - 0 01 ;.( )*** p - 0 001 ; N s number of samples in the analysis.

. y1 )mean value of 1 88 g l across treatments . Although the plants at harvest were smallcompared to field specimens, Tamarix, Pluchea, and Salix were flowering by the endof the experiment on the fastest growing treatments.

Relative growth rates

Graphs and linear regression equations of RGR vs. salinity are shown in Fig. 1. The.( )regression of RGR on salinity for Allenrolfea was non-significant p ) 0 05 but all

2 . .other species had r values in the range 0 73]0 86 for linear equations relating RGR tosalinity. Higher order polynomial equations did not significantly improve r 2 for anyspecies. RGRs for Allenrolfea were low across the salinity range, whereas maximum

. . y1 y1RGRs for the other species were 0 10]0 16 g g day for the control treatment;these RGRs correspond to doubling times of 7 days or less. Slopes of relative salt

.( ) ( )tolerance differed significantly p - 0 05 among species Fig. 2 . Salt tolerance( )rankings were: Allenrolfea ) Tamarix ) Pluchea ) Baccharis s Salix s Populus

.( )p - 0 05 .

Transpiration and water-use efficiency

Over the experiment, T decreased linearly with salinity for all species except Allenfolfea,y1 ( ) (which peaked at 8 g l NaCl Fig. 3 . Transpiration rates of control plants or plants

y1 )on 8 g l NaCl in the case of Allenrolfea were compared over the final 24 h of the( )experiment, by which time T had reached the maximum rate Table 2 . Allenrolfea had

. y1 y1( )the lowest transpiration rate water loss was 3 99 g g day on a fresh weight basis ,

E. GLENN ET AL.288

( )Figure 1. Relative growth rates RGR of riparian species from the Colorado River delta,Mexico, as affected by root zone salinity for plants grown in pots in a greenhouse study. Error

( )bars are mean standard errors N s 3 . Asterisks denote significance level of the linear regression. . .equation at: * p - 0 05, ** p - 0 01 or *** p - 0 001; the regression of RGR on salinity for

.( )Allenrolfea was non-significant p ) 0 05 . Note that different salinity ranges were used for( ) ( ) ( ) ( )different species. a Allenrolfea occidentalis; b Tamarix ramosissima; c Salix goodingii; d( ) ( )Populus fremontii; e Pluchea sericea; f Baccharis salicifolia.

. . y1 y1whereas the other species ranged from 8 25]13 05 g H O g day , with Tamarix2

.( )and Populus having significantly p - 0 05 higher rates than Baccharis. Water-use.( )efficiency calculated over the experiment was highest for Allenrolfea p - 0 05 which

achieved 3 g ly1, whereas the other species were not significantly different from eachy1 ( )other and averaged approximately 2 g l Fig. 4 .

RIPARIAN PLANTS IN THE COLORADO RIVER DELTA 289

Figure 2. Slopes of salt tolerance of riparian species from the Colorado River delta, Mexico,based on the percentage reduction in maximum relative growth rates per g ly1 NaCl in the

( )growth medium. Error bars are standard errors of the slope N s 1]15 ; different letters over.data points denote significant differences in slopes at p - 0 05.

Discussion

Controlled growth experiments such as this one cannot be used to directly predict theperformance of a plant in the field because the experimental conditions are simplifiedcompared to field conditions. In the present case, plants were grown on nutrientsolution so that salinity would be the main factor limiting growth, yet riparian plantsare often nutrient-limited in natural ecosystems. We used NaCl as a single salt additionto avoid confounding the interpretation of results which would result from presentingmultiple salts, yet riparian soils often contain complex mixtures of salts. Furthermore,the greenhouse environment is not the same as the outdoor environment, and plantswere grown in separate pots isolated from one another rather than in mixed stands asoccurs in nature. On the other hand, experiments such as this, in which plants aregrown under uniform conditions, can be used as a test of hypotheses that emerge fromfield studies where growth conditions cannot be controlled.

These species exhibited large differences in salt tolerance, similar to the findings of

E. GLENN ET AL.290

Figure 3. Transpiration rates of riparian species from the Colorado River delta, Mexico, asaffected by root zone salinity for plants grown in pots in a greenhouse study. Error bars are mean

( )standard errors N s 3 . Asterisks denote significance level of the linear regression equation at:. . .* p - 0 05, ** p - 0 01 or *** p - 0 001; the regression of RGR on salinity for A. occidentalis

.( )was non-significant p ) 0 05 . Note that different salinity ranges were used for different( ) ( ) ( ) ( )species. a Allenrolfea occidentalis; b Tamarix ramosissima; c Salix goodingii; d Populus

( ) ( )fremontii; e Pluchea sericea; f Baccharis salicifolia.

( )Jackson et al. 1990 . However, their conclusion that Salix and Populus cannot survive. y1 y1beyond 1 5 g l NaCl was not supported, as all species survived up to 8 g l NaCl.

Except for the succulent halophyte Allenrolfea, which had maximum growth at 8 g ly1

NaCl, salinity caused an apparent linear reduction in RGR with slopes ranging from2% reduction in RGR for each g ly1 NaCl for Tamarix to 7]9% for each g ly1 NaClfor Populus, Salix, and Baccharis. The latter three are mesic species and exhibited 50%

RIPARIAN PLANTS IN THE COLORADO RIVER DELTA 291

(Table 2. Mean transpiration rates of control plants or plants on they1 )8 g l NaCl treatment in the case of Allenrolfea , based on

water loss per gram fresh weight of shoots during the 24 h priorto harvest. The number in parenthesis is the standard error of the

( )mean N s 3

Transpiration*y1 y1( )Plant g g day

. .( )Allenrolfea occidentalis 3 99 0 90 a

. .( )Baccharis salicifolia 8 25 0 83 b

. .( )Pluchea sericea 9 39 0 20 bc

. .( )Populus fremontii 13 05 0 57 c

. .( )Salix goodingii 10 55 3 82 bc

. .( )Tamarix ramosissima 12 86 0 67 c

* Means followed by a different letter are significantly different at p -.0 05.

y1 ( )growth reduction at soil salinities of 5]7 g l NaCl. Busch et al. 1992 reported meansoil salinity levels of 1]3 g ly1 NaCl along the Bill Williams river, where Populus andSalix compete successfully with Tamarix and Pluchea, compared to levels of 6]8g ly1 NaCl along the Colorado River where Tamarix and Pluchea were overwhelmingly

( y1dominant. We converted their units of dS m measured by the electrical conductivityy1 . ) ( )of saturated extracts to g l using a conversion factor of 0 6 Richards, 1954 . Thus,

the greenhouse and field results support the conclusion that Populus, Salix, andBaccharis are not competitive with Tamarix and Pluchea above about 4 g ly1 NaCl due

( )Figure 4. Water-use efficiency WUE of riparian species from the Colorado River delta,( )Mexico. Data are pooled across salinity treatments N s 11]15 ; error bars show mean standard

.errors. Different letters over data points denote significantly different means at p - 0 05.

E. GLENN ET AL.292

.to differences in salt tolerance. Lower Colorado River water contains approximately 0 8g ly1 of salts, hence a five-fold evapo-concentration of salts in the riverbank soil will besufficient to inhibit the mesic species. Pluchea combined high RGR with high salttolerance, which may explain its success in a variety of riparian habitats, whereasBaccharis combined low RGR with low salt tolerance, helping to explain its relativescarcity under disturbed conditions or when challenged by invasive species. Althoughhighly salt tolerant, Allenrolfea, a leafless succulent, had the lowest RGR of all speciesin the greenhouse. Consistent with this finding, it is not considered an invasive speciesand is usually confined to very saline soils in riparian habitats.

In contrast to salt tolerance, differences in water use between mesic and salt tolerantforms were not pronounced, except for the low transpiration rate of Allenrolfea. Thetranspiration rates measured during the final 24 h of our experiment can be comparedto field measurements made on Tamarix, Prosopis, and Salix using the stem heat

( )balance method on intact branches 1]4 cm in diameter Sala et al., 1996 , within thesize range of our plants at harvest. The field transpiration rates, expressed as water loss

. . y1 y1per g fresh weight of leaves, ranged narrowly from 2 1]2 3 g g h among species. Ifwe assume that transpiration proceeded over 12 h of daylight but was minimal at night

. . y1( )Anderson, 1982 , our rates for Tamarix, Populus, and Salix convert to 0 8]1 1 g ghy1 based on the fresh weight of shoots, and if the shoots are assumed to be

( )approximately 50% leaves, the rates are similar to those measured by Sala et al. 1996 .( )Anderson 1982 , using the leaf chamber method in the field, measured somewhat

. . y1 y1lower rates of 1 1 and 1 2 g g h on a leaf basis for Tamarix and Populus,respectively. Despite differences in methodology, all three studies support theconclusion that the riparian tree species exhibit similar rates of transpiration underboth field and greenhouse conditions, contrary to earlier reports that Tamarix has

(unusually high water use compared to native vegetation Weeks et al., 1987; Vitousek,) ( )1990 . In contrast to Kleinkopf & Wallace 1974 , we found that transpiration of

Tamarix decreased with salinity in proportion to the reduction of RGR.Our whole-plant estimates of WUE were measured in the same units as field

measurements of WUE based on the ratio of net photosynthesis to transpiration using. . y1( )leaf chamber methods. Anderson 1982 reported WUEs of 4 3 and 6 8 g l for

Populus and Tamarix, respectively, and concluded that Tamarix was less efficient thanPopulus in water use. Our estimates of WUE are lower than the leaf-level measurements.This is expected since whole plant estimates of WUE are reduced by carbon lostthrough plant respiration at night and through respiration by non-photosynthetic

( )tissues during the day Osmond et al., 1980 . In general, WUE decreases as the level ofmeasurement proceeds from the leaf to the whole plant and then to the plant stand, andWUE is also dependent upon atmospheric moisture content and other factors. However,when plants are measured under common conditions, differences in intrinsic WUEamong species tend to be preserved at different levels and conditions of measurement( )Osmond et al., 1980 . Hence, we can compare our results with field measurementswhich used the same set of species.

.( )Allenrolfea, a C species, had a significantly p - 0 05 higher WUE than the other4( )species, which had similar values. Contrary to Anderson 1982 , who reported a low

WUE for Tamarix, in our experiment Tamarix had about the same WUE as Salix and( )Populus. Busch & Smith 1993 reported that Tamarix shoots resprouting after fire had

higher WUE than shoots which had not burned, and hypothesized that increased saltin the soil after burning triggered the adaptive response. However, in our experiment

.( )Tamarix WUE was not significantly p - 0 05 affected by salinity, so it is possiblesome other factor influenced WUE in the burned stands. In general, our resultsindicate little variability in intrinsic WUE among the tree species.

( )The study supports the conclusion of Busch & Smith 1995 that salinity is a majordeterminant of plant distribution along disturbed stretches of rivers in the aridsouth-western United States. Restoration methods, including deliberate flooding of the

RIPARIAN PLANTS IN THE COLORADO RIVER DELTA 293

riparian zone, have been developed to mitigate some of the anthropogenic sources of( )salt increase Briggs, 1996 . The present results can aid restoration efforts by providing

quantitative predictors of plant performance as affected by salinity.

( )We thank the National Wetlands Council U.S. Fish & Wildlife Service for partial financialsupport.

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