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Tolerance of five riparian plants from the lower Colorado River to salinity drought and inundation
Journal of Arid Environments (2001) 49: 147}159 doi:10.1006/jare.2001.0839, available online at http://www.idealibrary.com on
Tolerance of five riparian plants from the lower Colorado River to salinity drought and inundation
Matthew W. Vandersande*-, Edward P. Glenn*-? & James L. Walworth*A *The University of Arizona, Department of Soil, Water, and Environmental Science, College of Agriculture, Arizona, U.S.A. -Environmental Research Laboratory, 2601 E. Airport Drive, Tucson, Arizona 85706, U.S.A. A 429 Shantz Building, Tucson, Arizona 85721, U.S.A.
Two greenhouse experiments were conducted to compare the effects of salt stress and water stress on four native riparian species and one invasive species collected from the lower Colorado River, Mexico. Within a drying soil at the control salinity level, Populus fremontii, Salix gooddingii and Baccharis salicifolia were able to extract water from the soil equal to that of Tamarix ramosissima and Pluchea sericea. Yet, at elevated salinity levels T. ramosissima and P. sericea exhibited a superior water-use ability. Under flooded conditions all native riparian species outperformed T. ramosissima. The results show that the invasive species T. ramosissima has a competitive advantage over native species mainly with respect to salt tolerance. This suggests that pulse flooding along the river could reduce Tamarix’s competitive advantage by flushing out accumulated salts from the bankside and subjecting T. ramosissima to prolonged inundation. 2001 Academic Press Keywords: Tamarix; saltcedar; Riparian wetlands; Colorado River; Cottonwood-Willow association; salt stress; inundation; drought.
Introduction Riparian ecosystems of the south-western United States and north-west Mexico have undergone a remarkable change in plant community composition over the past 50 years (Busch & Smith, 1995; Stromberg & Patten, 1991). The perennial rivers were originally dominated by gallery forests of native trees such as Populus fremontii and Salix gooddingii and mesophytic understory species such as Baccharis salicifolia (Szaro, 1989). However, the invasive, salt-tolerant Eurasian shrub, saltcedar (Tamarix ramosissima), has replaced much of the native vegetation and has negatively modified the riparian environment (Cleverly et al., 1997). Often growing in monoculture or with arrowweed (Pluchea sericea), the Tamarix–Pluchea association has replaced more than 90% of the original ?Corresponding author. E-mail: [email protected] 0140}1963/01/090147#13 $35.00/0
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gallery forest vegetation in the riparian zone of the lower Colorado River, Mexico (Szaro, 1989). This replacement vegetation reportedly degrades the habitat value of the riparian zone for birds and mammals (Ellis, 1995; Ohmart et al., 1988), while also reducing plant species diversity (Di Tomaso, 1998; Shafroth et al., 1995). The highly managed and altered flow regime of the lower Colorado River has disrupted the conditions needed for growth and survival of the native species (Stromberg, 1998). The lack of overbank flooding along regulated rivers has contributed to the combined stresses of a saline bankside with low available soil moisture (Busch & Smith 1995). These conditions favor the establishment of salt-tolerant species (e.g. T. ramosissima) over the native mesophytic species (Shafroth et al., 1995). Tamarix ramosissima is also thought to have other physiological traits that provide a competitive advantage over native tree species. These include: rapid growth, high water use—which allows it to extract water from the soil faster than native species during establishment (Smith et al., 1998; Anderson, 1982), superior drought tolerance (Cleverly et al., 1997), prolific seed production, and the ability to survive long periods of inundation (Gladwin & Roelle, 1998; Brock, 1994). The competitive advantage of Tamarix over native species is thought to be so great that biological control programs have been recommended as the best means of control (Fornasari, 1997; DeLoach et al., 1996). However, most of the comparative studies between Tamarix and native species have been conducted in the field, and few studies have made direct comparisons among species under controlled growing conditions. In contrast to field results, Glenn et al. (1998) found superior salt tolerance of T. ramosissima and P. sericea compared to Populus, Salix and Baccharis, but little difference in growth rates or water-use characteristics among these species. The purpose of this controlled greenhouse study was to determine the interaction of salt and water stress on the survival, salt tolerance, water-use characteristics, and growth rates of plant species currently found along the Colorado River. The study also aimed to determine the tolerance of the same species to long-term inundation of the root zone. These experiments were intended to simulate two contrasting but common conditions found along western riverbanks in arid environments during times of seedling establishment. The drought experiment was designed to mimic conditions found on sandbars and banksides following a flood event. As floodwaters recede, seedlings must cope with a rapid reduction of available water and elevated salinity levels resulting from capillary rise and evaporation. The inundation experiment, using the same five species, was designed to mimic saturated soil conditions found during a flood event. By conducting the experiments in a greenhouse, we could effectively monitor growth and survival under controlled conditions. By determining the factors that control the distribution of species along the river, we can better manage and help restore these deteriorating ecosystems. Materials and methods Two greenhouse experiments were conducted to compare the responses of five riparian plants to stresses commonly found along the Colorado River in Mexico. The first experiment, using modified designs of Glenn & Brown (1998) and Glenn et al. (1998), compared the responses of S. gooddingii, P. fremontii, B. salicifolia ("B. glutinosa), P. sericea, and T. ramosissima in a drying soil, and with varying levels of salinity. The second experiment compared the responses of the same five species to saturated soil conditions. Collection and propagation of plant materials and greenhouse conditions Branch tip cuttings of each species were collected from plants growing along the Colorado River in Mexico (32315 N; 11535 W). The cuttings were propagated in a greenhouse in Tucson, Arizona, U.S.A. in germinating trays containing a mixture of
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washed river sand and potting medium (3 : 1 v/v). Mean temperatures during the two experiments ranged from 21)73C in February to 27)33C in May. Light transmission through the clear Plexiglas greenhouse was approximately 74% of ambient level and averaged 100,000 lux. The relative humidity in this evaporatively cooled greenhouse was about 60% for the duration of the experiment. Drought experiment Sixteen rooted cuttings of each species were selected for uniformity from the stock for the experiment. An additional three cuttings of each species were harvested to determine the approximate initial biomass. Each cutting was transplanted, root ball intact, into 10)9-l pots (26 cm diameter, 23 cm deep), without drain holes, and containing 10 kg of soil mixture. Pots were irrigated only once with 3 l of treatment solution. Each pot was then covered with a vapor barrier to minimize evaporative losses. The vapor barrier consisted of a plastic film pressed onto the soil surface, styrofoam inserts, and perlite. The plastic film had a small slit cut at the center through which the plant emerged. Each film was tied tightly around the pot to prevent moisture loss. Styrofoam semicircles (2)5 cm thick) were fitted over the plastic liner and inside the rim of the pot for insulation. Finally, perlite was poured between the plant stem and gaps in the Styrofoam to further reduce solar heating. Pots were placed in a randomized block design on a greenhouse bench with one replicate of each species, for each treatment, per block. The four blocks were situated along the bench to run along the temperature gradient of the greenhouse. Four control pots, prepared in the same manner as each treatment pots but without plants, were used to determine the evaporative loss in the absence of a plant (four repetitions;five species;four salt treatments#four controls"84 pots). Treatment solutions were prepared using Tucson municipal water and NaCl. The municipal water supply contained 423 mg l!1 of dissolved solids, of which the principal cations were Na# (45 mg l!1) and Ca2# (64 mg l!1), and the principal anions were Cl! (12 mg l!1), SO2! (114 mg l!1), and carbonates were unreported. Nutrients were 4 added to each treatment as 0)12 g l!1 (Peter’s Professional 20-20-20—water soluble fertilizer). Salinity treatments contained 0, 0)68, 1)68, and 3)68 g l!1 added NaCl. Therefore, the four treatments contained a total dissolved solids of approximately 500, 1000, 2000, and 4000 mg l!1 plus the nutrient solution. NaCl was added as a single-salt addition to solution treatments to avoid confounding the results with multiple salts, even though natural river water contains a mixture of salts. Plant height was measured weekly up until the plant was harvested. Each pot was weighed weekly, on a temperature compensated spring scale, until the plant exhibited visible signs of water stress, at which time the pot was weighed every day. Signs of water stress were first seen as yellowing or a loss of lower leaves and eventually wilting of most leaves. Since these endpoints are subjective and differed among species, plants were harvested individually when transpiration was zero (no change in pot weight) for a 2-day period. Each plant was sorted into roots, leaves, and stems at harvest. These plant parts were then oven-dried to constant weight at 603C to obtain dry biomass weights. A 30-g sub-sample of soil was oven-dried at 1053 for 24 h in order to calculate the gravimetric soil moisture content. Air dried soil subsamples were used to measure soil electrical conductivity in a fixed ratio (1: 5 soil : water) dilution (YSI Model 32 Conductance Meter) (Artiola 1997). Osmotic and matric potentials were measured using a pyschrometer equipped with Peltier and Richards thermocouples (Decagon True Psi model SC10X). Inundation experiment The inundation experiment used the same five species as the drought experiment, with four replicates, and at the control (unamended) salinity level of the drought
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experiment (five species;four replicates;one salinity"20 pots). Four cuttings of each species were selected for uniformity from the same stock and transplanted into pots 15 cm diameter by 17 cm deep. The pots were placed in a randomized block design along the temperature gradient of the same greenhouse. After 4 weeks of growth, each plant was transplanted into a 20-l pot (28 cm in diameter, 36 cm deep) with a drain hole and valve. Once in the 20-l pots, the plants were grown under unsaturated soil conditions for four weeks to allow for root establishment, then the drains were closed and the pots were flooded with Tucson municipal water having a TDS of 500 mg l!1, equal to that of the control salinity in the dry-down experiment. The water level was maintained at the rim of the pot, approximately 5 cm above the soil level, for the duration of the experiment. All plants were harvested after all of the T. ramosissima replicates were no longer able to remain upright (lodged) due to deterioration of the root-shoot connection. The experiment was initially designed to conclude once each species had died. However, the four native species continued grow well, without significant signs of stress, for 58 days, and although the Tamarix species lodged they maintained some green leaf tissue. Oxidation-reduction measurements were conducted weekly with additional measurement made two and four days after the initial flooding. Redox measurements were made using a platinum electrode (VWR platinum electrode and Orion model 290A display) and leachate collected from the drainhose at the bottom of the pot. At harvest, plant parts were separated into leaves, stem, and roots and oven dried at 603C to constant weight. Calculations and statistical analysis Relative growth rates (RGR) were calculated from the initial and final dry biomass weights of leaves, stems, and roots using the formula, RGR [g g!1 day!1]"[ln(final weight)!ln(initial weight)]/days of growth. Initial plant weights were estimated using measurements made from three additional plants harvested at the time of transplanting for each species. Initial mean plant dry weights ranged from 0)10 to 0)21 g for leaves and stems and from 0)05 to 0)07 g for roots for all plants except willow which was 0)42 g for the leaves, 0)44 g for the stems, and 0)11 g for the roots. The total water transpired by each plant over the drought experiment was calculated by subtracting the mean water loss of the control pots from the water loss by pots with plants. Water-use efficiency (WUE) was calculated by dividing the total dry biomass production by the cumulative transpiration over the growing period (Glenn et al., 1998). The final soil solution salinity was found using the formula, [((EC * 640)/1000)/moisture content]"[mg salt/g water], assuming complete dissolution of the salts. In order to compare the accuracy of our calculated final soil solution salinities we converted them to pressure potential values (bars) using the relationship y (bars)"!0)8276;(g NaCl/1000g H2O)#1)1143 from Robinson and Stokes (1959) and plotted them against measured osmotic potentials. The measured osmotic potential values were found using a pyschrometer and our sample soil wetted to various moisture contents. Although the pyschrometer measures both matric and osmotic potentials, we did not subtract out the matric potential because of the minimal effect it has on the total water potential of such a sandy soil. Relationships between dependant variables (days to harvest, soil moisture, WUE, and final soil solution salinity for the drought experiment) and treatments were tested using a two-way (drought experiment) and one-way (inundation experiment) ANOVA analysis with the independent variables species and salinity. A Tukey post hoc test was used to separate means for root and shoot biomass for the inundation experiment. The General Linear Model (GLM) was used to test the same independent variables for block effects.
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Table 1. Effects of drought stress and increasing salinity on the survival and growth characteristics of five riparian species from the Colorado River delta, Mexico
Species Tamarix ramosissima Baccharis salicifolia Salix gooddingii Populus fremontii Pluchea sericea
Days to harvest
Height (cm)
Net root growth (g)
Net shoot growth (g)
OP (bars)
56 59 72 71 63
48)3 40)5 55)7 41)6 35)9
1)52 2)72 2)51 2)59 1)71
4)84 4)79 4)09 4)11 6)36
!46)5 !27)1 !22)5 !27)1 !52)8
Shown are mean number of days to harvest for all salinity treatments; mean plant height for all treatments at harvest; net root and shoot growth (dry weight basis) at harvest; and lowest osmotic potential (OP) reached at time of harvest.
Results Drought experiment The mean days to harvest, over all salinity treatments, was calculated for each species (Table 1). Tamarix ramosissima had the shortest growing period at 56 days and S. gooddingii had the longest at 72 days. Mean plant height, for all salinity treatments, ranged from 40)5 cm for Baccharis to 48)3 cm for Tamarix. Net root growth (final root mass minus mean sample initial root mass) ranged from 1)52 g for Tamarix to 2)72 g for Baccharis. Net shoot growth (final shoot mass minus mean sample initial shoot mass) ranged from 4)09 g for Salix to 6)36 g for Pluchea. The lowest osmotic potential reached at time of harvest, ranged from !22)5 bars for Salix to !52)8 bars for Pluchea. Table 2 summarizes the statistical results for a two-way ANOVA with species and salinity as the independent variables. Days to harvest, WUE, and above-ground biomass RGR were all significantly different for species and salinity ( p(0)05), but not significant for the interaction term. Final soil moisture and final soil solution salinity were significantly ( p(0)05) different for species, salinity, and the interaction term was also significant. Table 2. Two-way ANOVA results (F-ratio and p) that tested the variance between means for species, salinity, and the interaction of species and salinity on the following dependent variable: days to harvest, final soil moisture content, maximum soil solution concentration (salt point); water use efficiency (WUE); and relative growth rates (RGR) of plants over the experiment
Source
Days to harvest
Soil moisture
Salt point
WUE
RGR
Species F P
4)37 0)004
27)08 0)001
29)84 0)001
22)66 0)001
9)78 0)001
Salinity F P
3)98 0)012
62)29 0)001
3)16 0)033
Spp;salinity F P
0)70 0)749
7)81 0)001
3)81 0)001
23)57 200)85 0)001 0)001 0)001 0)103
1)64 0)105
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Figure 1. Mean cumulative transpiration of riparian plants (Tamarix, ; Baccharis, ; Salix, ; Populus, ; Pluchea, ) grown on four salinity treatments. (a) 500 mg l!1; (b) 1000 mg l!1; (c) 2000 mg l!1; (d) 4000 mg l!1.
Water-use characteristics Figure 1 compares the mean cumulative water transpired by each species and each salinity treatment over the experiment. As the level of salinity increased, Salix and Populus transpired less but survived longer. On the other hand, Pluchea and Tamarix showed only a minor decrease in transpiration with increasing salinity, but also survived longer. Baccharis transpired an intermediate amount between the two groups. Figure 2 is the mean final percent moisture of the soil at time of harvest for each species at each salinity treatment. At the control salinity of 500 mg l!1 the mean final percent moisture was nearly the same among plant species ( p'0)05). As the salinity increased, Salix, Populus, and Baccharis were able to extract progressively less water from the soil, while, Tamarix and Pluchea showed no signs of diminishing water uptake in response to salinity. WUE increased slightly for each species with increasing salinity (Fig. 3) but there were no significant species differences ( p'0)05) for WUE at the control salinity. Also noticeable is the slightly lower WUE for Tamarix for the duration of the experiment. Salt effects and growth rates The lowest mean calculated osmotic potential reached by each species at each salinity is shown in Fig. 4. The osmotic potential represents the maximum level of salt tolerance in terms of a limit to which they can extract water from the soil. Tamarix and Pluchea exhibited a much higher salt point and hence a better ability to extract water from the saline soil than Salix, Populus, or Baccharis. When plotted against the measured osmotic potentials obtained from the pyschrometer, a clear linear relationship is observed (linear
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Figure 2. Mean final percent soil moisture (w : w) in root zone of riparian plants grown on four salinity treatments. Tamarix ( ); Baccharis ( ); Salix ( ); Populus ( ); Pluchea, ( ).
regression: y"0)88x#0)70; r 2"0)84) (Fig. 5). The calculated osmotic potentials are approximately 12% higher than the measured potentials. The effects of drought stress and salinity on the relative growth rates (RGR) are displayed in Fig. 6. Tamarix and Pluchea showed the highest RGR’s with only a minor decrease in growth above the control salinity level. Salix had the lowest RGR with Populus being the second lowest and Baccharis again being the intermediate species. Inundation experiment Anoxic conditions in the soil were reached by the second week after flooding as determined by the redox potentials (Fig. 7). The overall performance of each species under inundated conditions is summarized in Table 3. Root and shoot masses were significantly ( p(0)05) different between species. Tamarix had the lowest final root and shoot mass (Table 3) and was the only species to lodge. All other species grew well and showed no significant signs of stress. Discussion Because these experiments were conducted under controlled and simplified greenhouse conditions, direct extrapolations cannot be made to performance of the same species under field conditions. However, by holding constant environmental variables that cannot be controlled in the field we can obtain specific direct comparisons among species. Results for final soil moisture content in the drought experiment indicate a clear difference in the ability of each species to use water at increasing salinity levels. Tamarix, a facultative halophyte, and Pluchea, a ruderal phraetophyte shrub, showed the highest salt tolerances, similar to results reported by Glenn et al. (1998). By contrast, Salix, Populus, and Baccharis displayed a low level of salt tolerance. At the control salinity treatment however, all species showed a nearly identical ability to use available water,
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Figure 3. Mean water use efficiency (WUE) for riparian plants grown on four salinity treatments. Tamarix ( ); Baccharis ( ); Salix ( ); Populus ( ); Pluchea, ( ).
extracting water to a level of less than 2)5% moisture. While Populus and Salix have been shown to be obligate phreatophytes along the Colorado River (Busch et al., 1998) our results indicate that they are capable of extracting water from soil with very low moisture contents. As the salinity level increased, Baccharis, Salix, and Populus all showed an impaired ability to extract water. These results suggest that Tamarix possess a competitive advantage in terms of salt tolerance at high salinities, but not drought tolerance at low levels of salinity. This is important because at the present time the salinity of
Figure 4. Mean final osmotic potential (bars) in soils from riparian plants grown on four salinity treatments. Tamarix ( ); Baccharis ( ); Salix ( ); Populus ( ); Pluchea, ( ).
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Figure 5. Regression analysis of calculated osmotic potentials vs. measured osmotic potentials using the psychrometer method (y"0)70#0)88x, r 2"0)84). 500 TDS ( ); 1000 TDS ( ); 2000 TDS ( ); 4000 TDS ( ); linear regression ( ).
Colorado River water at the Southern International Boundary is only slightly higher, 723 mg l!1, than the control salinity 500 mg l!1 (Paul McAleese, USBR, pers. comm.). By examining the rates of transpiration between species, in terms of cumulative transpiration (Fig. 1), differences in water use become more evident. At the highest salinity treatment, 4000 mg l!1 TDS, the glycophytes exhibited reduced rates of transpiration by day 28. This corresponds to a cumulative transpiration of one quarter (0)75 l) of the total added water. In other words, the glycophytes began experiencing salt stress after the soil solution had been concentrated to approximately 5333 mg l!1. At harvest, the glycophytes transpired a total of 1)5 l of water, half of the added water, corresponding to a salinity of roughly 8000 mg l!1. This is consistent with earlier findings by Glenn et al. (1998) that showed the maximum salt tolerance, under non-water stressed conditions, was 8 g l!1 for Salix, Populus, and Baccharis. In comparison, Tamarix and Pluchea continued to use water at about the same rate until day 50, at which time their water use slowed dramatically until their death. The first signs of salt stress corresponded to a salinity of about 20,000 mg l!1 for Tamarix and 12,000 mg l!1 for Pluchea. Both continued to extract water (2)5 l total) to a salinity of about 24,000 mg l!1. The osmotic potentials reached for each species (Fig. 4) further illustrates the difference among species. In a non-saline soil, all species were equal in their ability to extract water until the day of harvest. At a treatment salinity of 1000 mg l!1 TDS, the species began to diverge, with Tamarix and Pluchea reaching potentials as low as !46 and !54 bars, respectively. Compared with the measured osmotic potentials (Fig. 5), the calculated osmotic potentials slightly overestimated the soil’s osmotic potential. This is probably because when calculating the osmotic potential a complete dissolution of salts in an ideal solution is assumed. However, salts do not dissolve completely nor is their activity coefficient 1)0 in a soil solution. Nevertheless, the calculated osmotic potential method offers an accurate and consistent estimate of osmotic potential.
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Figure 6. Mean above ground relative growth rates (RGR, g g!1 d!1) of riparian plants on four salinity treatments. Tamarix ( ); Baccharis ( ); Salix ( ); Populus ( ); Pluchea, ( ).
The significantly lower water use efficiency we found for T. ramosissima compared with the other plants counters some earlier findings. Using carbon isotope discrimination, Busch & Smith (1995) found Tamarix to have a significantly higher WUE than native Populus and Salix trees along the Colorado River. This, they hypothesized, would provide an advantage in drier environments with a stochastic water supply. Measuring stomatal conductance and photosynthetic rates, Cleverly et al.
Figure 7. Mean redox potential and standard deviations of soils from riparian plants under inundated conditions.
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Table 3. Effects of saturated soil conditions on the mean root and shoot mass (dry weight) and lodging of five riparian species from the Colorado River delta, Mexico
Species Tamarix ramosissima Baccharis salicifolia Salix gooddingii Populus fremontii Pluchea sericea
Root mass (g) 9)8 17)8 26)9 25)5 27)1
a ab b b b
Shoot mass (g) 24)7 70)8 51)2 56)5 75)9
a b b b b
Lodged (Y/N) Y N N N N
Means were separated using a Tukey post hoc test; different letter denote means that are different at p(0)05.
(1997) found Tamarix to have similar a WUE as Salix but a higher WUE than Pluchea. Our results show that the WUE for Tamarix was noticeably lower than the other plants and was also less affected by an increase in salinity. Although all of these findings differ, it must be noted that calculations of WUE can be an artifact of the measurement procedure. Regardless, Anderson (1982), Busch & Smith (1995), Sala et al. (1996), and Cleverly et al. (1997) agree that Tamarix is capable of desiccating a floodplain because of a higher whole plant transpiration rate which was not supported by our data. The relative growth rates of each species were significantly different from one another (Table 1). However, no species showed any noticeable decrease in RGR with increasing salinity (Fig. 6). In a similar study, Shafroth et al. (1995) also noticed no significant effects on growth for T. ramosissima and P. fremontii at water salinities as high as five times that of the Rio Grande. However, they did find a significantly negative effect of elevated salinity level on the germination rates of P. fremontii. The response of each species to saturated soil conditions was clearly evident. All Tamarix replicates lodged after 58 days, while the other species continued to grow. The shoot to root connection of the T. ramosissima plants had deteriorated to such a point that the plant was no longer able to support itself. Tamarix was the only species not to develop a prolific adventitious rooting system in the water ponded above the soil in the pots. It has been shown in previous studies (Blom et al. 1994, Krasny et al. 1988) that an adventitious rooting system is vital to surviving anoxic conditions. The Salix plants produced the most and largest adventitious roots of all species. This correlated well with their impressive mean stem length, root and shoot biomass, and overall vigor. The Populus plants also grew exceptionally well under saturated soil conditions. However, they did show signs of stress, with leaf burn at their growing tips. The Baccharis and Pluchea also both grew well under flooded conditions. Most surprising was the biomass production of Pluchea. In the dry-down experiment, Pluchea also produced the most shoot biomass relative to the other native riparian species. The highly adaptive and resilient nature of Pluchea can probably explain why it is often the first species to colonization disturbed habitats and one of the last to be removed. Conclusion The decline of natural riparian communities in the arid south-west has been linked to the disruption of natural stream flows (Stromberg et al. 1996) and the invasion of the exotic species Tamarix (Cleverly et al., 1997; Busch & Smith 1995). Our results show that although T. ramosissima had superior salt tolerance it was equivalent to native species in
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growth and water use characteristics, and inferior in flood tolerance. This suggests that under a more natural hydrologic regime in the Colorado River, Tamarix would possess less of a competitive advantage over the natural plant communities. If seasonal floods were allowed to flow freely over the banks a natural desalinization of the river banks and deposition of fresh alluvium would take place. This would encourage the germination and growth of native species while slowing the spread of Tamarix. Even if a natural flow regime could not be restored, periodic releases as a management practice would help slow, or even reverse, the deterioration of the riparian communities. References Anderson, J. (1982). Factors controlling transpiration and photosynthesis in Tamarix chinensis Lour. Ecology, 63: 48–56. Artiola, J. (1999). Manual of operating procedures for the analysis of selected soil, water, plant tissue and wastes, chemical and physical parameters. Arizona: Soil, Water, and Plant Analysis Laboratory, University of Arizona. 111 pp. Blom, C., Voesenek, L., Banga, M., Engelaar, W., Rijnders, J., Van De Steeg, H. & Visser, E. (1994). Physiological ecology of riverside species: adaptive responses of plants to submergence. Annals of Botany, 74: 253–263. Brock, J. (1994). Tamarix spp. (salt cedar), an invasive exotic woody plant in arid and semi-arid riparian habitats of Western USA. In: de Waal, L.C., Child, L.E., Wade, P.M. & Brock, J.H. (Eds), Ecology and Management of Invasive Riverside Plants, pp. 27–44. West Sussex, England: John Wiley and Sons Ltd. 217 pp. Busch, D. & Smith, S. (1995). Mechanisms associated with decline of woody species in riparian ecosystems of the southwestern U.S. Ecological Monographs, 65: 347–370. Cleverly, J., Smith, S., Sala, A. & Devitt, D. (1997). Invasive capacity of Tamarix ramosissima in a Mojave Desert floodplain: the role of drought. Oecologia, 111: 12–18. DeLoach, C., Pitcairn, M. & Woods, D. (1996). Biological control of saltcedar in southern California. In: Proceedings of Saltcedar Management Workshop, Rancho Mirage, CA. California Exotic Pest Plants Council, pp. 30–31. Di Tomaso, J. (1998). Impact, biology, and ecology of saltcedar (Tamarix spp. ) in the southwestern United States. Weed Technology, 12: 326–336. Ellis, L. (1995). Bird use of saltcedar and cottonwood vegetation in the Middle Rio Grande Valley of New Mexico, USA. Journal of Arid Environments, 30: 339–349. Fornasari, L. (1997). Host specificity of Coniatus tamarisci (Coleptera: Curculionidae) from France: potential biological control agent of Tamarix spp. in the United States.Environmental Entomology, 26: 349–356. Gladwin, D. & Roelle, J. (1998). Survival of plains cottonwood (Populus deltoides subsp. monilifera) and saltcedar (Tamarix ramosissima) seedlings in response to flooding. Wetlands, 18: 669–674. Glenn, E. & Brown J. (1998). Effects of soil salt levels on the growth and water use efficiency of Atriplex canescens (Chenopodiaceae) varieties in drying soil. American Journal of Botany, 85: 10–16. Glenn, E., Tanner R., Mendez, S., Kehret, T., Moore, D., Garcia, J. & Valdes C. (1998). Growth rates, salt tolerance and water use characteristics of native and invasive riparian plants from the delta of the Colorado River, Mexico. Journal of Arid Environments, 40: 281–294. Krasny, M., Zasada, J. & Vogt, K. (1998). Adventitious rooting of four Salicaceae species in response to a flooding event. Canadian Journal of Botany, 66: 2597–2598. Ohmart, R., Anderson, B. & Hunter, W. (1988). Ecology of the Lower Colorado River from Davis Dam to the Mexico-United States Boundry: a Community Profile. Springfield, VA: National Technical Information Service. 296 pp. Robinson, R. & Stokes, R. (1959). Electolyte Solutions. New York, NY: Academic Press, 559 pp. Sala, A., Smith, S. & Devitt, D. (1996). Water use by Tamarix ramosissima and associated phreatophytes in a Mojave Desert floodplain. Ecological Applications, 6: 888–898. Shafroth, P., Friedman J. & Ischinger L. (1995). Effects of salinity on establishment of Populus fremontii (cottonwood) and Tamarix ramosissima (saltcedar) in southwestern United States. Great Basin Naturalist, 55: 58–65.
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Smith, S, Devitt D., Sala A., Cleverly J. & Busch D. (1998). Water relations of riparian plants from warm desert regions. Wetlands, 18: 687–696. Stromberg, J. (1998). Dynamics of Fremont cottonwood (Populus fremontii) and saltcedar (Tamarix chinensis) populations along the San Pedro River, Arizona. Journal of Arid Environments, 40: 133–155. Stromberg, J. & Patten, D. (1991). Flood flows and dynamics of Sonoran riparian forests. Rivers, 2: 221–235. Stromberg, J. Tiller, R. & Richter, B. (1996). Effects of groundwater decline on riparian vegetation of semiarid regions: the San Pedro, Arizona. Ecological Applications, 6: 113–131. Szaro, R. (1989). Riparian forest and shrubland community types of Arizona and New Mexico. Desert Plants, 9: 1–138.
Tolerance of five riparian plants from the lower Colorado River to salinity drought and inundation
Matthew W. Vandersande*-, Edward P. Glenn*-? & James L. Walworth*A *The University of Arizona, Department of Soil, Water, and Environmental Science, College of Agriculture, Arizona, U.S.A. -Environmental Research Laboratory, 2601 E. Airport Drive, Tucson, Arizona 85706, U.S.A. A 429 Shantz Building, Tucson, Arizona 85721, U.S.A.
Two greenhouse experiments were conducted to compare the effects of salt stress and water stress on four native riparian species and one invasive species collected from the lower Colorado River, Mexico. Within a drying soil at the control salinity level, Populus fremontii, Salix gooddingii and Baccharis salicifolia were able to extract water from the soil equal to that of Tamarix ramosissima and Pluchea sericea. Yet, at elevated salinity levels T. ramosissima and P. sericea exhibited a superior water-use ability. Under flooded conditions all native riparian species outperformed T. ramosissima. The results show that the invasive species T. ramosissima has a competitive advantage over native species mainly with respect to salt tolerance. This suggests that pulse flooding along the river could reduce Tamarix’s competitive advantage by flushing out accumulated salts from the bankside and subjecting T. ramosissima to prolonged inundation. 2001 Academic Press Keywords: Tamarix; saltcedar; Riparian wetlands; Colorado River; Cottonwood-Willow association; salt stress; inundation; drought.
Introduction Riparian ecosystems of the south-western United States and north-west Mexico have undergone a remarkable change in plant community composition over the past 50 years (Busch & Smith, 1995; Stromberg & Patten, 1991). The perennial rivers were originally dominated by gallery forests of native trees such as Populus fremontii and Salix gooddingii and mesophytic understory species such as Baccharis salicifolia (Szaro, 1989). However, the invasive, salt-tolerant Eurasian shrub, saltcedar (Tamarix ramosissima), has replaced much of the native vegetation and has negatively modified the riparian environment (Cleverly et al., 1997). Often growing in monoculture or with arrowweed (Pluchea sericea), the Tamarix–Pluchea association has replaced more than 90% of the original ?Corresponding author. E-mail: [email protected] 0140}1963/01/090147#13 $35.00/0
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gallery forest vegetation in the riparian zone of the lower Colorado River, Mexico (Szaro, 1989). This replacement vegetation reportedly degrades the habitat value of the riparian zone for birds and mammals (Ellis, 1995; Ohmart et al., 1988), while also reducing plant species diversity (Di Tomaso, 1998; Shafroth et al., 1995). The highly managed and altered flow regime of the lower Colorado River has disrupted the conditions needed for growth and survival of the native species (Stromberg, 1998). The lack of overbank flooding along regulated rivers has contributed to the combined stresses of a saline bankside with low available soil moisture (Busch & Smith 1995). These conditions favor the establishment of salt-tolerant species (e.g. T. ramosissima) over the native mesophytic species (Shafroth et al., 1995). Tamarix ramosissima is also thought to have other physiological traits that provide a competitive advantage over native tree species. These include: rapid growth, high water use—which allows it to extract water from the soil faster than native species during establishment (Smith et al., 1998; Anderson, 1982), superior drought tolerance (Cleverly et al., 1997), prolific seed production, and the ability to survive long periods of inundation (Gladwin & Roelle, 1998; Brock, 1994). The competitive advantage of Tamarix over native species is thought to be so great that biological control programs have been recommended as the best means of control (Fornasari, 1997; DeLoach et al., 1996). However, most of the comparative studies between Tamarix and native species have been conducted in the field, and few studies have made direct comparisons among species under controlled growing conditions. In contrast to field results, Glenn et al. (1998) found superior salt tolerance of T. ramosissima and P. sericea compared to Populus, Salix and Baccharis, but little difference in growth rates or water-use characteristics among these species. The purpose of this controlled greenhouse study was to determine the interaction of salt and water stress on the survival, salt tolerance, water-use characteristics, and growth rates of plant species currently found along the Colorado River. The study also aimed to determine the tolerance of the same species to long-term inundation of the root zone. These experiments were intended to simulate two contrasting but common conditions found along western riverbanks in arid environments during times of seedling establishment. The drought experiment was designed to mimic conditions found on sandbars and banksides following a flood event. As floodwaters recede, seedlings must cope with a rapid reduction of available water and elevated salinity levels resulting from capillary rise and evaporation. The inundation experiment, using the same five species, was designed to mimic saturated soil conditions found during a flood event. By conducting the experiments in a greenhouse, we could effectively monitor growth and survival under controlled conditions. By determining the factors that control the distribution of species along the river, we can better manage and help restore these deteriorating ecosystems. Materials and methods Two greenhouse experiments were conducted to compare the responses of five riparian plants to stresses commonly found along the Colorado River in Mexico. The first experiment, using modified designs of Glenn & Brown (1998) and Glenn et al. (1998), compared the responses of S. gooddingii, P. fremontii, B. salicifolia ("B. glutinosa), P. sericea, and T. ramosissima in a drying soil, and with varying levels of salinity. The second experiment compared the responses of the same five species to saturated soil conditions. Collection and propagation of plant materials and greenhouse conditions Branch tip cuttings of each species were collected from plants growing along the Colorado River in Mexico (32315 N; 11535 W). The cuttings were propagated in a greenhouse in Tucson, Arizona, U.S.A. in germinating trays containing a mixture of
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washed river sand and potting medium (3 : 1 v/v). Mean temperatures during the two experiments ranged from 21)73C in February to 27)33C in May. Light transmission through the clear Plexiglas greenhouse was approximately 74% of ambient level and averaged 100,000 lux. The relative humidity in this evaporatively cooled greenhouse was about 60% for the duration of the experiment. Drought experiment Sixteen rooted cuttings of each species were selected for uniformity from the stock for the experiment. An additional three cuttings of each species were harvested to determine the approximate initial biomass. Each cutting was transplanted, root ball intact, into 10)9-l pots (26 cm diameter, 23 cm deep), without drain holes, and containing 10 kg of soil mixture. Pots were irrigated only once with 3 l of treatment solution. Each pot was then covered with a vapor barrier to minimize evaporative losses. The vapor barrier consisted of a plastic film pressed onto the soil surface, styrofoam inserts, and perlite. The plastic film had a small slit cut at the center through which the plant emerged. Each film was tied tightly around the pot to prevent moisture loss. Styrofoam semicircles (2)5 cm thick) were fitted over the plastic liner and inside the rim of the pot for insulation. Finally, perlite was poured between the plant stem and gaps in the Styrofoam to further reduce solar heating. Pots were placed in a randomized block design on a greenhouse bench with one replicate of each species, for each treatment, per block. The four blocks were situated along the bench to run along the temperature gradient of the greenhouse. Four control pots, prepared in the same manner as each treatment pots but without plants, were used to determine the evaporative loss in the absence of a plant (four repetitions;five species;four salt treatments#four controls"84 pots). Treatment solutions were prepared using Tucson municipal water and NaCl. The municipal water supply contained 423 mg l!1 of dissolved solids, of which the principal cations were Na# (45 mg l!1) and Ca2# (64 mg l!1), and the principal anions were Cl! (12 mg l!1), SO2! (114 mg l!1), and carbonates were unreported. Nutrients were 4 added to each treatment as 0)12 g l!1 (Peter’s Professional 20-20-20—water soluble fertilizer). Salinity treatments contained 0, 0)68, 1)68, and 3)68 g l!1 added NaCl. Therefore, the four treatments contained a total dissolved solids of approximately 500, 1000, 2000, and 4000 mg l!1 plus the nutrient solution. NaCl was added as a single-salt addition to solution treatments to avoid confounding the results with multiple salts, even though natural river water contains a mixture of salts. Plant height was measured weekly up until the plant was harvested. Each pot was weighed weekly, on a temperature compensated spring scale, until the plant exhibited visible signs of water stress, at which time the pot was weighed every day. Signs of water stress were first seen as yellowing or a loss of lower leaves and eventually wilting of most leaves. Since these endpoints are subjective and differed among species, plants were harvested individually when transpiration was zero (no change in pot weight) for a 2-day period. Each plant was sorted into roots, leaves, and stems at harvest. These plant parts were then oven-dried to constant weight at 603C to obtain dry biomass weights. A 30-g sub-sample of soil was oven-dried at 1053 for 24 h in order to calculate the gravimetric soil moisture content. Air dried soil subsamples were used to measure soil electrical conductivity in a fixed ratio (1: 5 soil : water) dilution (YSI Model 32 Conductance Meter) (Artiola 1997). Osmotic and matric potentials were measured using a pyschrometer equipped with Peltier and Richards thermocouples (Decagon True Psi model SC10X). Inundation experiment The inundation experiment used the same five species as the drought experiment, with four replicates, and at the control (unamended) salinity level of the drought
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experiment (five species;four replicates;one salinity"20 pots). Four cuttings of each species were selected for uniformity from the same stock and transplanted into pots 15 cm diameter by 17 cm deep. The pots were placed in a randomized block design along the temperature gradient of the same greenhouse. After 4 weeks of growth, each plant was transplanted into a 20-l pot (28 cm in diameter, 36 cm deep) with a drain hole and valve. Once in the 20-l pots, the plants were grown under unsaturated soil conditions for four weeks to allow for root establishment, then the drains were closed and the pots were flooded with Tucson municipal water having a TDS of 500 mg l!1, equal to that of the control salinity in the dry-down experiment. The water level was maintained at the rim of the pot, approximately 5 cm above the soil level, for the duration of the experiment. All plants were harvested after all of the T. ramosissima replicates were no longer able to remain upright (lodged) due to deterioration of the root-shoot connection. The experiment was initially designed to conclude once each species had died. However, the four native species continued grow well, without significant signs of stress, for 58 days, and although the Tamarix species lodged they maintained some green leaf tissue. Oxidation-reduction measurements were conducted weekly with additional measurement made two and four days after the initial flooding. Redox measurements were made using a platinum electrode (VWR platinum electrode and Orion model 290A display) and leachate collected from the drainhose at the bottom of the pot. At harvest, plant parts were separated into leaves, stem, and roots and oven dried at 603C to constant weight. Calculations and statistical analysis Relative growth rates (RGR) were calculated from the initial and final dry biomass weights of leaves, stems, and roots using the formula, RGR [g g!1 day!1]"[ln(final weight)!ln(initial weight)]/days of growth. Initial plant weights were estimated using measurements made from three additional plants harvested at the time of transplanting for each species. Initial mean plant dry weights ranged from 0)10 to 0)21 g for leaves and stems and from 0)05 to 0)07 g for roots for all plants except willow which was 0)42 g for the leaves, 0)44 g for the stems, and 0)11 g for the roots. The total water transpired by each plant over the drought experiment was calculated by subtracting the mean water loss of the control pots from the water loss by pots with plants. Water-use efficiency (WUE) was calculated by dividing the total dry biomass production by the cumulative transpiration over the growing period (Glenn et al., 1998). The final soil solution salinity was found using the formula, [((EC * 640)/1000)/moisture content]"[mg salt/g water], assuming complete dissolution of the salts. In order to compare the accuracy of our calculated final soil solution salinities we converted them to pressure potential values (bars) using the relationship y (bars)"!0)8276;(g NaCl/1000g H2O)#1)1143 from Robinson and Stokes (1959) and plotted them against measured osmotic potentials. The measured osmotic potential values were found using a pyschrometer and our sample soil wetted to various moisture contents. Although the pyschrometer measures both matric and osmotic potentials, we did not subtract out the matric potential because of the minimal effect it has on the total water potential of such a sandy soil. Relationships between dependant variables (days to harvest, soil moisture, WUE, and final soil solution salinity for the drought experiment) and treatments were tested using a two-way (drought experiment) and one-way (inundation experiment) ANOVA analysis with the independent variables species and salinity. A Tukey post hoc test was used to separate means for root and shoot biomass for the inundation experiment. The General Linear Model (GLM) was used to test the same independent variables for block effects.
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Table 1. Effects of drought stress and increasing salinity on the survival and growth characteristics of five riparian species from the Colorado River delta, Mexico
Species Tamarix ramosissima Baccharis salicifolia Salix gooddingii Populus fremontii Pluchea sericea
Days to harvest
Height (cm)
Net root growth (g)
Net shoot growth (g)
OP (bars)
56 59 72 71 63
48)3 40)5 55)7 41)6 35)9
1)52 2)72 2)51 2)59 1)71
4)84 4)79 4)09 4)11 6)36
!46)5 !27)1 !22)5 !27)1 !52)8
Shown are mean number of days to harvest for all salinity treatments; mean plant height for all treatments at harvest; net root and shoot growth (dry weight basis) at harvest; and lowest osmotic potential (OP) reached at time of harvest.
Results Drought experiment The mean days to harvest, over all salinity treatments, was calculated for each species (Table 1). Tamarix ramosissima had the shortest growing period at 56 days and S. gooddingii had the longest at 72 days. Mean plant height, for all salinity treatments, ranged from 40)5 cm for Baccharis to 48)3 cm for Tamarix. Net root growth (final root mass minus mean sample initial root mass) ranged from 1)52 g for Tamarix to 2)72 g for Baccharis. Net shoot growth (final shoot mass minus mean sample initial shoot mass) ranged from 4)09 g for Salix to 6)36 g for Pluchea. The lowest osmotic potential reached at time of harvest, ranged from !22)5 bars for Salix to !52)8 bars for Pluchea. Table 2 summarizes the statistical results for a two-way ANOVA with species and salinity as the independent variables. Days to harvest, WUE, and above-ground biomass RGR were all significantly different for species and salinity ( p(0)05), but not significant for the interaction term. Final soil moisture and final soil solution salinity were significantly ( p(0)05) different for species, salinity, and the interaction term was also significant. Table 2. Two-way ANOVA results (F-ratio and p) that tested the variance between means for species, salinity, and the interaction of species and salinity on the following dependent variable: days to harvest, final soil moisture content, maximum soil solution concentration (salt point); water use efficiency (WUE); and relative growth rates (RGR) of plants over the experiment
Source
Days to harvest
Soil moisture
Salt point
WUE
RGR
Species F P
4)37 0)004
27)08 0)001
29)84 0)001
22)66 0)001
9)78 0)001
Salinity F P
3)98 0)012
62)29 0)001
3)16 0)033
Spp;salinity F P
0)70 0)749
7)81 0)001
3)81 0)001
23)57 200)85 0)001 0)001 0)001 0)103
1)64 0)105
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Figure 1. Mean cumulative transpiration of riparian plants (Tamarix, ; Baccharis, ; Salix, ; Populus, ; Pluchea, ) grown on four salinity treatments. (a) 500 mg l!1; (b) 1000 mg l!1; (c) 2000 mg l!1; (d) 4000 mg l!1.
Water-use characteristics Figure 1 compares the mean cumulative water transpired by each species and each salinity treatment over the experiment. As the level of salinity increased, Salix and Populus transpired less but survived longer. On the other hand, Pluchea and Tamarix showed only a minor decrease in transpiration with increasing salinity, but also survived longer. Baccharis transpired an intermediate amount between the two groups. Figure 2 is the mean final percent moisture of the soil at time of harvest for each species at each salinity treatment. At the control salinity of 500 mg l!1 the mean final percent moisture was nearly the same among plant species ( p'0)05). As the salinity increased, Salix, Populus, and Baccharis were able to extract progressively less water from the soil, while, Tamarix and Pluchea showed no signs of diminishing water uptake in response to salinity. WUE increased slightly for each species with increasing salinity (Fig. 3) but there were no significant species differences ( p'0)05) for WUE at the control salinity. Also noticeable is the slightly lower WUE for Tamarix for the duration of the experiment. Salt effects and growth rates The lowest mean calculated osmotic potential reached by each species at each salinity is shown in Fig. 4. The osmotic potential represents the maximum level of salt tolerance in terms of a limit to which they can extract water from the soil. Tamarix and Pluchea exhibited a much higher salt point and hence a better ability to extract water from the saline soil than Salix, Populus, or Baccharis. When plotted against the measured osmotic potentials obtained from the pyschrometer, a clear linear relationship is observed (linear
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Figure 2. Mean final percent soil moisture (w : w) in root zone of riparian plants grown on four salinity treatments. Tamarix ( ); Baccharis ( ); Salix ( ); Populus ( ); Pluchea, ( ).
regression: y"0)88x#0)70; r 2"0)84) (Fig. 5). The calculated osmotic potentials are approximately 12% higher than the measured potentials. The effects of drought stress and salinity on the relative growth rates (RGR) are displayed in Fig. 6. Tamarix and Pluchea showed the highest RGR’s with only a minor decrease in growth above the control salinity level. Salix had the lowest RGR with Populus being the second lowest and Baccharis again being the intermediate species. Inundation experiment Anoxic conditions in the soil were reached by the second week after flooding as determined by the redox potentials (Fig. 7). The overall performance of each species under inundated conditions is summarized in Table 3. Root and shoot masses were significantly ( p(0)05) different between species. Tamarix had the lowest final root and shoot mass (Table 3) and was the only species to lodge. All other species grew well and showed no significant signs of stress. Discussion Because these experiments were conducted under controlled and simplified greenhouse conditions, direct extrapolations cannot be made to performance of the same species under field conditions. However, by holding constant environmental variables that cannot be controlled in the field we can obtain specific direct comparisons among species. Results for final soil moisture content in the drought experiment indicate a clear difference in the ability of each species to use water at increasing salinity levels. Tamarix, a facultative halophyte, and Pluchea, a ruderal phraetophyte shrub, showed the highest salt tolerances, similar to results reported by Glenn et al. (1998). By contrast, Salix, Populus, and Baccharis displayed a low level of salt tolerance. At the control salinity treatment however, all species showed a nearly identical ability to use available water,
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Figure 3. Mean water use efficiency (WUE) for riparian plants grown on four salinity treatments. Tamarix ( ); Baccharis ( ); Salix ( ); Populus ( ); Pluchea, ( ).
extracting water to a level of less than 2)5% moisture. While Populus and Salix have been shown to be obligate phreatophytes along the Colorado River (Busch et al., 1998) our results indicate that they are capable of extracting water from soil with very low moisture contents. As the salinity level increased, Baccharis, Salix, and Populus all showed an impaired ability to extract water. These results suggest that Tamarix possess a competitive advantage in terms of salt tolerance at high salinities, but not drought tolerance at low levels of salinity. This is important because at the present time the salinity of
Figure 4. Mean final osmotic potential (bars) in soils from riparian plants grown on four salinity treatments. Tamarix ( ); Baccharis ( ); Salix ( ); Populus ( ); Pluchea, ( ).
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Figure 5. Regression analysis of calculated osmotic potentials vs. measured osmotic potentials using the psychrometer method (y"0)70#0)88x, r 2"0)84). 500 TDS ( ); 1000 TDS ( ); 2000 TDS ( ); 4000 TDS ( ); linear regression ( ).
Colorado River water at the Southern International Boundary is only slightly higher, 723 mg l!1, than the control salinity 500 mg l!1 (Paul McAleese, USBR, pers. comm.). By examining the rates of transpiration between species, in terms of cumulative transpiration (Fig. 1), differences in water use become more evident. At the highest salinity treatment, 4000 mg l!1 TDS, the glycophytes exhibited reduced rates of transpiration by day 28. This corresponds to a cumulative transpiration of one quarter (0)75 l) of the total added water. In other words, the glycophytes began experiencing salt stress after the soil solution had been concentrated to approximately 5333 mg l!1. At harvest, the glycophytes transpired a total of 1)5 l of water, half of the added water, corresponding to a salinity of roughly 8000 mg l!1. This is consistent with earlier findings by Glenn et al. (1998) that showed the maximum salt tolerance, under non-water stressed conditions, was 8 g l!1 for Salix, Populus, and Baccharis. In comparison, Tamarix and Pluchea continued to use water at about the same rate until day 50, at which time their water use slowed dramatically until their death. The first signs of salt stress corresponded to a salinity of about 20,000 mg l!1 for Tamarix and 12,000 mg l!1 for Pluchea. Both continued to extract water (2)5 l total) to a salinity of about 24,000 mg l!1. The osmotic potentials reached for each species (Fig. 4) further illustrates the difference among species. In a non-saline soil, all species were equal in their ability to extract water until the day of harvest. At a treatment salinity of 1000 mg l!1 TDS, the species began to diverge, with Tamarix and Pluchea reaching potentials as low as !46 and !54 bars, respectively. Compared with the measured osmotic potentials (Fig. 5), the calculated osmotic potentials slightly overestimated the soil’s osmotic potential. This is probably because when calculating the osmotic potential a complete dissolution of salts in an ideal solution is assumed. However, salts do not dissolve completely nor is their activity coefficient 1)0 in a soil solution. Nevertheless, the calculated osmotic potential method offers an accurate and consistent estimate of osmotic potential.
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Figure 6. Mean above ground relative growth rates (RGR, g g!1 d!1) of riparian plants on four salinity treatments. Tamarix ( ); Baccharis ( ); Salix ( ); Populus ( ); Pluchea, ( ).
The significantly lower water use efficiency we found for T. ramosissima compared with the other plants counters some earlier findings. Using carbon isotope discrimination, Busch & Smith (1995) found Tamarix to have a significantly higher WUE than native Populus and Salix trees along the Colorado River. This, they hypothesized, would provide an advantage in drier environments with a stochastic water supply. Measuring stomatal conductance and photosynthetic rates, Cleverly et al.
Figure 7. Mean redox potential and standard deviations of soils from riparian plants under inundated conditions.
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Table 3. Effects of saturated soil conditions on the mean root and shoot mass (dry weight) and lodging of five riparian species from the Colorado River delta, Mexico
Species Tamarix ramosissima Baccharis salicifolia Salix gooddingii Populus fremontii Pluchea sericea
Root mass (g) 9)8 17)8 26)9 25)5 27)1
a ab b b b
Shoot mass (g) 24)7 70)8 51)2 56)5 75)9
a b b b b
Lodged (Y/N) Y N N N N
Means were separated using a Tukey post hoc test; different letter denote means that are different at p(0)05.
(1997) found Tamarix to have similar a WUE as Salix but a higher WUE than Pluchea. Our results show that the WUE for Tamarix was noticeably lower than the other plants and was also less affected by an increase in salinity. Although all of these findings differ, it must be noted that calculations of WUE can be an artifact of the measurement procedure. Regardless, Anderson (1982), Busch & Smith (1995), Sala et al. (1996), and Cleverly et al. (1997) agree that Tamarix is capable of desiccating a floodplain because of a higher whole plant transpiration rate which was not supported by our data. The relative growth rates of each species were significantly different from one another (Table 1). However, no species showed any noticeable decrease in RGR with increasing salinity (Fig. 6). In a similar study, Shafroth et al. (1995) also noticed no significant effects on growth for T. ramosissima and P. fremontii at water salinities as high as five times that of the Rio Grande. However, they did find a significantly negative effect of elevated salinity level on the germination rates of P. fremontii. The response of each species to saturated soil conditions was clearly evident. All Tamarix replicates lodged after 58 days, while the other species continued to grow. The shoot to root connection of the T. ramosissima plants had deteriorated to such a point that the plant was no longer able to support itself. Tamarix was the only species not to develop a prolific adventitious rooting system in the water ponded above the soil in the pots. It has been shown in previous studies (Blom et al. 1994, Krasny et al. 1988) that an adventitious rooting system is vital to surviving anoxic conditions. The Salix plants produced the most and largest adventitious roots of all species. This correlated well with their impressive mean stem length, root and shoot biomass, and overall vigor. The Populus plants also grew exceptionally well under saturated soil conditions. However, they did show signs of stress, with leaf burn at their growing tips. The Baccharis and Pluchea also both grew well under flooded conditions. Most surprising was the biomass production of Pluchea. In the dry-down experiment, Pluchea also produced the most shoot biomass relative to the other native riparian species. The highly adaptive and resilient nature of Pluchea can probably explain why it is often the first species to colonization disturbed habitats and one of the last to be removed. Conclusion The decline of natural riparian communities in the arid south-west has been linked to the disruption of natural stream flows (Stromberg et al. 1996) and the invasion of the exotic species Tamarix (Cleverly et al., 1997; Busch & Smith 1995). Our results show that although T. ramosissima had superior salt tolerance it was equivalent to native species in
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growth and water use characteristics, and inferior in flood tolerance. This suggests that under a more natural hydrologic regime in the Colorado River, Tamarix would possess less of a competitive advantage over the natural plant communities. If seasonal floods were allowed to flow freely over the banks a natural desalinization of the river banks and deposition of fresh alluvium would take place. This would encourage the germination and growth of native species while slowing the spread of Tamarix. Even if a natural flow regime could not be restored, periodic releases as a management practice would help slow, or even reverse, the deterioration of the riparian communities. References Anderson, J. (1982). Factors controlling transpiration and photosynthesis in Tamarix chinensis Lour. Ecology, 63: 48–56. Artiola, J. (1999). Manual of operating procedures for the analysis of selected soil, water, plant tissue and wastes, chemical and physical parameters. Arizona: Soil, Water, and Plant Analysis Laboratory, University of Arizona. 111 pp. Blom, C., Voesenek, L., Banga, M., Engelaar, W., Rijnders, J., Van De Steeg, H. & Visser, E. (1994). Physiological ecology of riverside species: adaptive responses of plants to submergence. Annals of Botany, 74: 253–263. Brock, J. (1994). Tamarix spp. (salt cedar), an invasive exotic woody plant in arid and semi-arid riparian habitats of Western USA. In: de Waal, L.C., Child, L.E., Wade, P.M. & Brock, J.H. (Eds), Ecology and Management of Invasive Riverside Plants, pp. 27–44. West Sussex, England: John Wiley and Sons Ltd. 217 pp. Busch, D. & Smith, S. (1995). Mechanisms associated with decline of woody species in riparian ecosystems of the southwestern U.S. Ecological Monographs, 65: 347–370. Cleverly, J., Smith, S., Sala, A. & Devitt, D. (1997). Invasive capacity of Tamarix ramosissima in a Mojave Desert floodplain: the role of drought. Oecologia, 111: 12–18. DeLoach, C., Pitcairn, M. & Woods, D. (1996). Biological control of saltcedar in southern California. In: Proceedings of Saltcedar Management Workshop, Rancho Mirage, CA. California Exotic Pest Plants Council, pp. 30–31. Di Tomaso, J. (1998). Impact, biology, and ecology of saltcedar (Tamarix spp. ) in the southwestern United States. Weed Technology, 12: 326–336. Ellis, L. (1995). Bird use of saltcedar and cottonwood vegetation in the Middle Rio Grande Valley of New Mexico, USA. Journal of Arid Environments, 30: 339–349. Fornasari, L. (1997). Host specificity of Coniatus tamarisci (Coleptera: Curculionidae) from France: potential biological control agent of Tamarix spp. in the United States.Environmental Entomology, 26: 349–356. Gladwin, D. & Roelle, J. (1998). Survival of plains cottonwood (Populus deltoides subsp. monilifera) and saltcedar (Tamarix ramosissima) seedlings in response to flooding. Wetlands, 18: 669–674. Glenn, E. & Brown J. (1998). Effects of soil salt levels on the growth and water use efficiency of Atriplex canescens (Chenopodiaceae) varieties in drying soil. American Journal of Botany, 85: 10–16. Glenn, E., Tanner R., Mendez, S., Kehret, T., Moore, D., Garcia, J. & Valdes C. (1998). Growth rates, salt tolerance and water use characteristics of native and invasive riparian plants from the delta of the Colorado River, Mexico. Journal of Arid Environments, 40: 281–294. Krasny, M., Zasada, J. & Vogt, K. (1998). Adventitious rooting of four Salicaceae species in response to a flooding event. Canadian Journal of Botany, 66: 2597–2598. Ohmart, R., Anderson, B. & Hunter, W. (1988). Ecology of the Lower Colorado River from Davis Dam to the Mexico-United States Boundry: a Community Profile. Springfield, VA: National Technical Information Service. 296 pp. Robinson, R. & Stokes, R. (1959). Electolyte Solutions. New York, NY: Academic Press, 559 pp. Sala, A., Smith, S. & Devitt, D. (1996). Water use by Tamarix ramosissima and associated phreatophytes in a Mojave Desert floodplain. Ecological Applications, 6: 888–898. Shafroth, P., Friedman J. & Ischinger L. (1995). Effects of salinity on establishment of Populus fremontii (cottonwood) and Tamarix ramosissima (saltcedar) in southwestern United States. Great Basin Naturalist, 55: 58–65.
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