Great Basin Naturalist
Volume 57 Number 1 Article 7
3-7-1997
Ecophysiology of the temperate desert halophytes: Allenrolfea occidentalis and Sarcobatus vermiculatus
James D. Trent U.S. Department of Agriculture, Agricultural Research Service, Conservation Biology Rangelands Unit, Reno, Nevada
Robert R. Blank U.S. Department of Agriculture, Agricultural Research Service, Conservation Biology Rangelands Unit, Reno, Nevada
James A. Young U.S. Department of Agriculture, Agricultural Research Service, Conservation Biology Rangelands Unit, Reno, Nevada
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Recommended Citation Trent, James D.; Blank, Robert R.; and Young, James A. (1997) "Ecophysiology of the temperate desert halophytes: Allenrolfea occidentalis and Sarcobatus vermiculatus," Great Basin Naturalist: Vol. 57 : No. 1 , Article 7. Available at: https://scholarsarchive.byu.edu/gbn/vol57/iss1/7
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 57(1), © 1997, pp. 57--65
ECOPHYSIOLOGY OF THE TEMPERATE DESERT HALOPHYTES: ALLENROLFEA OCCLDENTALIS AND SARCOBATUS VERMICULATUS
James D. morl , Robert R. Blank 1,2, and James A. Young l
ABSTRACI:-NuOierous basins of the intermountain area often have extensive playa surfaccs that arc nearly (lt~void of vegetation. Margins of these playas support sparse communities dominated by the chcnopod shrubs Allem'Vlfea ncciden talis (iodine bush) and Sarcobr;.tu.s veoniculatus (black gt'e.asewuud). These plants estahlish and persist in an environment where halomorphic soils induce extreme osmotic stress and atmospheric precipitation is very low and erratic and occurs largely during the winter when temperatures are too low for growth. We measured net CO2 assimilation rates, leaf con ductances, transpiration rates, water-use efficiencies, and stem xylem JXltentials for these two C3 species. Data were col lected in above-average (lOOl) and bclow-avemge (1992) precipitation years. Net CO2 assimilation rates for AUenrolfea were statistically simi.lar in 1991 and 1992 but in general declined for Sarcobatus in 1992. For both species, leaf conduc tances and leaf lranspin\tion nltes declined signilicantly from 1991 to 1992. with the decline significantly greater for Snr· cobatus. Water-usc efficicncie.<; doubled from 19m to 1992 for hath plant species. Prcdawn xylem waler potentials were -2.2 and -3.3 MPa for Alle7lrol/ea and -1.8 and -2.6 MPa Jor SarcobaLus beginning in May 1991 and 1992, re.<;pectively. and dropped to --3.8 and -4.2 MPa for Allenrolfea and -1.8 and -2.8 MPa for Sarcobatus by September 1991 and 1992, 1"eSpt'.ctively. Afternoon xylem water potentials were --3,1 and -2.0 MPa for Allenrolfea and -2.6 and -2.2 MPa for Sarco 1x~ beginning in May 1991 and 1.992, respectively. Xylem water IXltentials dropped to -5.0 MPa for Allewulfca and-3.4 Mfa for Sarcobatus by September of both 1991 and 1992. }obr AIlenmlfea, in general, the total soil water potential within tbtl :r.one of maximum root activity is more negative than the plant's predawn xylem potential, which suAAesls that the plant is partially phr~toplJyticand/or has a large capacitalU.:e due to its extensive woody root syslcm.
Key words: iodine bush, hku:k grea.~ewoQd, rhotmynthesis. conductance, transpiratuJt1., water potential, t()(lter~u.sc e!JU;iency, salt desl:f1.
lne vast pluvial lakes that occupied the basin Sarcobatus, found in numerous plant commu of the intermountain area during the Pleisto nities in temperate deserts (e.g., Billings 1945), cene (Russell 1885) exposed extensive lake is moderately salt tolerant and can survive at plains to colonization by plants as the waters the low end of the moisture gradient in salt evaporated during the late Pleistocene. lIalo desert communities (Skougard and Brotherson morphic soils, wind erosion, and atmospheric 1979). Sarcobab..., which employs the C3 photo aridity hindered colonization of these environ synthetic carbon reduction pathway, can be ments (Billings 1945, 1949, West 1983). Lower phreatophytic if the groundwater table is high portions of these basins remain nearly free of enough (Robinson 1958, Rickard 1965, Groen vegetation as barren playa surfaces. Margins of eveld 1990). Allenrolfea, also a C3 (scanning tbese playas currently limit plant colonization. electron microscopy ofleafsection did not sbow Communities of shrubs dominated by Cheno sbuctures indicative of C4 pathways), is much podiaceae characterize mucb ofthe pluvial lake more restricted in ecological amplitude, being plain environment (Billings 1949). Apparently, limited to a few communities directly at the these shrubs am! half-shrubs bave undergone margin of playas where soils are often poorly explosive evolution in successfully exploiting drained, have high surface soil salinity, but are ti,e lake plain environments (Stutz 1978). The non-sodic (Shantz 1940, Skougard and Broth North American endemic chenopods S"rcoba ersou 1979). tus vermiculatu. (Hook.) Torr. (black grcase The purpose of this study was to determine wood) and the monospecific Allenrolfea occi plant strategies that allow Allenrolfea and dentalis (S. Wats) Kuntz (iodine bush) have Sarcobatu.s to sUlVive, indeed flourish, in this colonized extremely saline habitats of these harsh, extremely saline playa margin environ temperate desert basins (Young et al. 1995). ment. Our working hypothesis postulated that
Ius. Dqurtmcnt"rAgriculture, AgrictJtur.l1 Research s.~. Omsctv:LllolllJi 57 58 GREAT BASIN NATURALIST [Volume 57 roots of Sarcobatus tap shallow, lower osmotic plant-water relations and carbon exchange potential groundwater, while roots of Allen were measured in above-average and helow rolfea primarily seek moisture from the eolian average precipitation years. In the 1st study mounds on which they are found, This hypothe diurnal measurements (6--8 measurements from sis was tested by studying plant-soil relation sunrise to sunset) of net photosynthesis, leaf ships.and measuring plant ecophysiological conductance, transpiration, relative humidity, attributes for Allenrolfea and Sarcobatus over leaftemperature, and photosynthetically active 2 yr, Fortuitously, the years differed markedly radiation (PAR) were taken with an Ll-6200 in precipitation, which provided insight into portable photosynthesis system (LI-COR, Inc" water acquisition. Lincoln, NE) equipped with a 0.25-L leafcham ber. Plots (4 blocks) were randomly selected that METHODS contained both Allenrolfea and Sarcobatus, and ecophysiology measurements were taken The study site is in Eagle Valley, an embay by block on 10 July, 2 August, and 25 Septem ment of pluvial Lake Lahontan, near the Hot ber 1991 and on 10 July, 8 August, and 23 Sep Springs Mountain Range about 80 km north tember 1992. To facilitate measurements, stems east of Reno, Nevada (119° 15'W, 39°45'N, containing several leaves were inserted into the 1234 m), Tbe landscape consists ofa lake plain chamber. Mter measurement stems were har rising in a series of small (>1 m) escarpments vested and returned on ice to the laboratory bordering a large playa. A band oflarge (>10 where leaf areas were measured. Allenrolfea ill ) sand dunes forms a disjunct arc across the eeophysiologieal measurements were based on lake plain. Allenrolfea forms a very sparse com cylindrical leaf area, as stomata appeared to munity, with plants located on low mounds cover the entire leaf Sarcobatus measurements (0.1-0,5 m high) on a former playa surface. were based on a I-sided flat leaf because in Occasional Sarcobatus and/or Atriplex lenti formis ssp. tOrt'eyi (Torrey saltbush) share the samples we examined stomata occurred mounds with the smaller Allenrolfea, Discontin only on the hairy and flattened leafsurface. uous colonies of Distichlis spicata (desert salt In the 2nd study more intensive afternoon grass) are the only herbaceous vegetation found ecophysiology measurements were taken. Mea in the communities. Mounds and intermound surements were taken as mentioned above in soils are extremely saline (Tables 1,2) and con the diumal study; however, we measured pre sist of coarse-textured eolian material overly dawn and afternoon stem xylem potentials 'with ing clay- and silt-rich lacustrine sediments a Scholander-type pressure chamber. Measure remnant of pluvial periods of Lake Lahontan ments were taken on 30 April, 10 June, 8 July, (Blank et aJ. 1992), The high osmotic potential 30 July, and 23 September 1991 and 20 April, of the soil seedbed limits new plant recruit 27 May, 26 June, 16 July, 7 August, and 23 ment (Blank et al, 1994), September 1992, Prior to the studies, 8 perforated PVC In August 1991 we determined rooting tubes were installed throughout the study area depth and root length density for Allenrolfea to monitor the water table, The tubes extended by digging into 4 mounds occupied exclusively to a depth of 3 m. Measurements were taken by Allenrolfea. Roots were collected at depths monthly throughout 1991 and 1992. A sample of 0-30, 30-60, 60-90, 90-120, and 120-150 of the groundwater was returned to the labo em, The volume of soil taken was 3780 cm3, ratory and electrical couductivity (EC) deter Soil was washed from roots in tubs and organic mined using a salinity drop tester; total water debris was picked from the roots, Root length potential was measured on selected samples was determined using a Comair root length using a Decagon DC-lO thermocouple psy scanner (Hawker de Havilland, Salisbury, South chrometer (mention of trade names does not Australia). A subsample of roots were stained imply endorsement by the USDA), with a congo red solution (congo red stains Separate studies were conducted in 1991 dead roots) and examined with a light micro and 1992, The 30-yr average annual precipita scope to determine the proportion of dead tion for this region is 11.5 em (Reed 1941). In roots, which was then used to correct root 1991 and 1992 the study area received 14,6 lengtb density. em and 9.6 cm of precipitation, respectively To prepare samples of Allenrolfea for scan (based on rain gauges at the study site), Thus, ning electron microscopy, we immediately 1997] DESERT HAWPHYTE PHYSIOLOGY 59 TAl:lI,E 1. Pedon descriptions and seleeted attributes of mound occupied by Allenrolfeo ocd.dentalis. Mound. is near where the ecophysiological measurements were taken. Saturated paste Munsell 1l Ca+! Horizon Depth color pH Sand Silt Clay N,' CI- S04- Mgi-Il K+ N03- (em) ------percenl-~·· - ...... •... ------Inmol------_. Av 0-1.5 2.5Y 6/3 7.8 78.8 7.1 14.1 990 960 33.0 57.8 3.2 7.3 6.61 CI 1.5-20 2.5Y 8.512 8.1 75.6 12.6 11.7 540 550 12.7 5.9 1.3 5.9 0.05 C2 20-76 2.5Y 6.512 6.3 &5.7 5.8 8.5 310 300 43.1 27.3 2.9 4.6 0.49 il" 76-86 2.5Y 612 8.0 nd nd nd 270 2&> 41.6 18.9 2.0 4.0 0.08 C3 86-1Z2 5Y613 5.2 68.0 16.0 16.0 380 410 12.3 12.3 3.3 3.4 0.01 2C 122-157 2.5Y 7'" 8.0 48.2 28.0 23.8 160 170 16.5 4.6 0.9 2.6 0.11 ZCg 157+ 2.5Y 712 8.1 8.1 54.9 37.0 120 140 6.5 2.3 0.5 3.Z 0.02 TAULE 2. Selected mean anion and cation values &om saturation extracts talc:cn at 2-10 em from 4 microsiles (6 repli cates) and from ground\...-ater samples (4 replicates) at the Ea~lc Valley study site. Microsite Chloride Sodium Sulfate Potassium Nitrate -- ~------_... _.. mmol-··· -- -- _. ------. --_.. Bare mound 1580 1450 39 66 8.1 Allellrolfea 600 690 63 102 3.3 Sarcobatf/.~ 1460 134{} 40 24 4.4 Groundwater 220 200 12 14 0.06 placed freshly excised leaves in a solution of5% mean square. In the 2nd experiment, data were glutaraldehyde, 25% polyethylene glycol, and analyzed by year to determine the species and 10% acetone. After 1 wk we dehydrated the monthly effect on physiological parameters. leaves using a graded series ofalcohol and then Similar error terms were selected as in the 1st critical-painl-dried them prior to observation. experiment. A probability value of P < 0.05 Near the time diurnal and afternoon mea was used throughout the analyses to test sig surements were taken, we collected soil at nillcance of F values. Only significant differ depths of 20, 40, and 60 em from mounds ences are reporled in the text. When signifi (6 replicates) near where plant measurements cant interactions occurred between main were taken. Though Ihe mounds were largely effect means, only those judged to have eco occupied by Allen.-olfea, some samples were logical significance were interpreted. laken from mounds occupied by both species. We inunediately placed the soil in a sealed RESULTS glass vial and returned it to the laboratory. In the laboratOlY the samples were homogenized Measurements were taken on cloudless and total soil water potential was determined days, so the PAR perpendicular to the sun wiih a Decagon SC-lO ihermocouple psy reached approximately 2000 !Lmol m-2 ,I by chrometer. The extreme salinity of samples about 1030 h for all measurement dates. Val required instrument calibration using a series ues of net COz assimilation, leaf conductance, ofsaturated salt solutions. and leaf transpiration depend on leaf area, The clata were analyzed as a split-block in which for these 2 species is difficult to un time using SAS analysis of variance (ANOVA) ambiguously define. For this reason we will models. In the 1st experiment the dala were concentrate on diurnal, seasonal, and yearly analyzed by year and monih to determine changes by species in these values. species and diurnal effeels on physiological Net CO2 assimilation rates were generally parameters, The species effect was tested by highest in Ihe late morning and decreased the species x block interaction term while the slightly through the rest of the day (Fig. 1), diurnal effect was tested by the overall error In general, for equivalent diurnal seasonal 60 GREAT BASIN NATURALIST [Volume 57 I. ...~ 1 9 9 1 1 9 9 2 :S. 12 I :ON- '. [jl 'E • -;.~ 8 .. 4 I;; z JULY 10 AUG 2 JULY 10 AUG 8 0 w 28. fi j5'-\n210 ~()Ie 8fu. ~ 15• Fig. 1. Diurnal me.'\SuremenlS of net carbon dioxide assimilation, Ic::af conductance. leaf transpiration, and water-use efficiency for AUenmlfea occUlentalis and Sarcobatus venn.ictdatus in 1991 and 1992. Error bars for this and followi.ng figures are standard errors about the means. measurement dates, rates were significantly Moreover, the differences in leaf conductance higher for both plant species in 1991 (wet) between AUenrolfea and Sarcobatus were sig than in 1992 (dry); however, a similar pallern nificantly less in 1992 than in 1991 (Figs. 1,2). was not evident in the more comprehensive In 1991 leaf conductance varied significantly afternoon data set (Fig, 2). In 1991 net CO2 by time of day for SaI'cabattls, but a diurnal assimilation was highest in midsummer, pallern was more mute in 1992 (Fig. 1). Leaf whereas in 1992 there was no significant trend transpiration rates followed the same trends as with season (Fig. 2). leafconductance (Figs. 1, 2). For the first 2 dates in 1991, leaf conduc Water-use efficiency ofAUenrolfea was sig tance was generally highest in the morning nificantly greater than Sarcobatus for all mea and decreased in the afternoon, with a slight increase again by late afternoon (Fig. 1). This surement dates in 1991 and 1992, witl, the pallern was not evident in the 25 September exception of the 25 September 1991 measure 1991 diurnal measurement. Leaf conductance ment (Fig. 1). Water-use efficiencies in 1991 for both Sarcobattls and Allenralfea was signif for both plants were significantly less than in icantly less in 1992 than in 1991 (Figs. 1, 2), 1992, 1997] DESERT HALOPHYTE PHYSIOLOGY 61 z 10~---~------...., o :s~ 8 - '", :;ON CIJ' 6 CIJ E <0 '" E 4 o "- tii 2 z 1 9 9 1 1 99 2 oOJ z >=''' 150 0'" ::>'" §i E 100 0° OE E ,,-<-o~-O-- .~ ....._ ... ~ 50 tr .Q- -0 t-.-..... --' oL_~_~_~_...., __~_~,:>o-:::;:-o~:::O~::::O~::-:--::::{)~:::-{)~J z 16,.------:------...------,,-----,-.,--:-=-. o -<>- A. occidentalis I- <~ 12 --e-- S. vermiculatus a:\n r a.'"ClJE ~o 8 a:E I-E ~ O oo>-~o • 00- 4,1.,..:~;::::::;::::::::=~~:::_r-~~~~.~~.~~;.~~.~~.Jo 0--0 --00 --' o APR JUN JUL JUL SEP APR MAY JUN JUL AUG SEP 30 10 8 30 23 20 27 26 16 7 25 Day of Year Fig. 2. Mternoon measurements of net carbon dioxide assimilation leaf conductance and leaf transpiration for Allen rolfea occidentalis and Sarcobatus vermiculatus in 1991 and 1992. In 1991 and 1992, Allenrolfea had signifi Allenrolfea produces the greatest quantity cantly more negative afternoon and predawn ofits roots at 30-60 em in the hummock, with stem xylem potentials than Sarcobatus (Fig. 3). a small quantity of its roots occurring down to Soil water potentials remained between -15 120-150 em (Fig. 4). Studies of the rooting and -20 MPa at the 20-cm depth for 1991 and density of Sarcobatus indicate it exponentially until August 1992 when they dropped to -40 declines with depth (Groeneveld 1990). to -50 MPa (Fig. 3). In 1991 soil water poten Changes from an above-normal precipita tials at 40 em actually increased from about tion year (1991) to a below-normal precipita -11 MPa to -7 MPa from May to July and tion year (1992) are reflected in the depth to decreased to -5 MPa by late September. In the water table (Table 3). By summer 1992 the 1992, however, the soil water potential at 40 water table had dropped below 3 m for all em was about -6 MPa from April to July and access tubes. The EC data also show that then dropped to -20 MPa by August. There groundwater is much less saline than is the were no significant differences in leaftemper soil solution in the soil above (see Tables 1, 2). ature between plant species for 1991, the above-average precipitation year (Fig. 3). In DISCUSSION 1992, the dry year, afternoon leaf tempera tures of Sarcobatus were consistently higher Leaf conductance and transpiration of the than those ofAllenrolfea. ehenopod shrubs Sarcobatus vermiculatus and 62 GREAT BASIN NATIJRAUST [Volume 57 i. 0',-:;---;::--:;;----;------...... ,,.....--;;0---;:,.....---;0-, ~ 1991 1992 ~ ., ffi -2 ...... •• ~ -...... "'.-... ~-._~. ~ ~ '--0.. ~ 0 - ~~ "-----0~~~~-~ .. ..~'::AP:R:30=J~lJN=1:0 :J~UL;::8~JU~l:30=S~E:P:23===AP=R:20=MA=Y:-n=JUN=26=J:U:l:16=A::U=G::7 l -0- A. accidentalis ~ ., -~ ...... S. vermiculatus ~; :c&: .. ~ .. ~ ..~AP~R~3~O;J;UN;';O::;JU;l;6;;JU;l;30;SE:P;23;;;;:;_~_~_;A:P:R ;20;M;A;Y;27;JU;N~2~6 ;JU;l;';';A;U~G;7;S;E;P~24 JULY 10 AUG 2 SEPT 25 JULY 10 AUG 8 SEPT 23 I·~ "=0-c-m' p k::-.....:'r...... --:2 ~-40 . -+- 40cm :J ...... 60cm lil..., Is;,! JUN 10 JUL 8 JUl30 SEP 30 APR 20 MAY 27 JUN 26 JUL 16 AUG 7 SEP 23 ~ ...,...... ------~ Ftg. 3. Predawn and afternoon xylem potentials and dillrnalleaf temperatures by sampling date in 1991 and 1992 for AUenrolfea occidenwlis and Sarcobatus venniculatus and total soil water potentials at depths of20, 40. and 60 em below top of hummock for growing seasons in 1991 and 1992. Allenro/fea occidentalis declined to a greater may have been fundamental to maintaining extent than net assimilation of CO2 from the tissue hydration of these salt TABLE 3. \Vater table depth and electrical conductivity ~ 3.6,------, (EC) from PVC access tubes. Standard deviations pro 6 vided in parentheses. ~ 2.7 Year Depth EC ~ m-1) !- 1.8 Month (m) (dS 1991 ~ 0.9 Jan 2.1(.3) 23 (7) Feb I.S (.4) 21 (10) ~ 0 Mar 1.8 (.4) 20 (8) 0-30 30-60 60-90 90·120 120-150 Ap' 1.9 (.4) 20 (5) Soil Depth Interval (cm) May 1.7 (.3) 19 (4) Jun 1.9 (.2) 19 (5) Fig. 4. Allenrolfea occidentalis live root length density Ju! 1.9 (.2) 18 (3) by depth below top ofhummock. Aug 2.1 (.2) 21 (10) Sep 2.2 (.3) 19 (3) Oct 2.5 (.4) 25 (7) use efficiency increased from 5.7 to 8.5 mmol Nov od' nd I Dec nd od CO2 mol- H20, similar to our trend from a wet to a dry year. 1992 Jan od nd Increased soil salinity cannot fully explain Feb nd od the dramatic decreases in leaf conductance Mar nd od from a wet to a dry year. Kleinkopf and Wal Ap' nd nd lace (1974) showed that net assimilation rates May nd nd Jun >3.0 nd and transpiration were not reduced for the salt Ju! >3.0 nd tolerant species Tamarix ramosissima when Aug >3.0 nd salinity increased from 10 to 200 mmoL Pearcy Sep >3.0 nd and Ustin (1984) suggested that increased salin Oct >3.0 nd Nov >3.0 nd ity primarily reduced photosynthesis within the Dec >3.0 nd mesophyll and secondarily as a result of reduced &Data not detennined during this period. leaf conductance. Our data show a slight reduc tion in photosynthesis with a much larger reduction in leafconductance. Allenrolfea roots have tapped into the lower The effect of soil water potential on photo ing water table; it is probably a facultative synthesis, conductance, and transpiration from phreatophyte like Sarcobatus. During the course the 1st (wet) and the 2nd (dry) year is incon of this study, several Allenrolfea mounds and clusive. Based on monitor wells at the study adjacent interspaces were excavated. From site, the water table dropped significantly from these excavations it is evident that there is an 1991 to 1992 and was below 3 m for much of extensive netwurk of coarse, woody roots of the 1992 season. Since Sarcobatus is a faculta Allenrolfea. Samples oflarge roots have nearly tive phreatophyte (Romo and Haferkamp 1989), 120 rings, which may constitute annual rings. and it likely extends roots to at least this depth The moisture content of coarse, woody roots (Groeneveld 1990), a drop in the water table averaged 67% by weight when measured in could explain its reduced transpiration and the spring before appreciable plant growth. conductance in 1992. The decline in transpira These findings suggest that the water relations tion and conductance from 1991 to 1992 for of Allenrolfea may involve a large root capaci Allenrolfea is perplexing. Root distribution of tance factor. Allenrolfea suggests it obtains most of its An alternative explanation of reduced CO2 water from within the mound it grows on; yet, assimilation, conductance, and transpiration the total soil water potential within the mound rates and more efficient water use from 1991 did not change appreciably during the grow to 1992 involves nitrogen. When moisture is ing seasons from 1991 to 1992. Indeed, even available in surface soil horizons, plants can during the wet year the total soil water poten uptake sufficient, likely luxuriant, inorganic tial measured in mounds where maximum root nitrogen owing to plentiful levels in the soil length ofAllenrolfea occurs was generally more (Tables 1, 2). However, as available moisture negative than the midday xylem water poten in the upper soil profile declines as it did dur tial. One possibility is that a small number of ing the dry year of 1992, a plant obtains a 64 GREAT BASIN NATURALIST [Volume 57 greater fraction ofwater needed from lower soil and 1992. High water-use efficiency in Allen horizons closer to or within the water table. rolfea is partly a function ofan extremely rugose Inorganic nitrogen levels decline as the water epidermal layer in which the stomata are re table is approached, which suggests that plants cessed (Fig. 5). The thick boundary layer may become deficient in nitrogen (Table 2). caused by the rugose surface contributes to Khan et aI. (1994) determined that additions of lowered stomatal conductance and higher water nitrate-N to saline substrates significantly in use efficiencies (Kramer and Boyer 1995). creased carbon assimilation, transpiration, sto Allenrolfea and Sarcobatus maintain similar matal conductance, and water-use efficiency in leaf xylem potentials from a low-precipitation alfalfa. year with a water table depth remaining above 3 m to a dry year when the water table drops As compared to Sarcobatus, Allenrolfea ex below 3 m. A decrease in leafconductance best hibited signiBcantly lower stomatal conduc explains the maintenance of leaf xylem poten tance in the wet year 1991 in addition to having, tials in the dry 2nd year. Decreased water loss in general, higher water-use efficiency in 1991 as a result oflowered stomatal resistance should maintain xylem potentials. In conclusion, Allenrolfea and Sarcobatus have reduced net assimilation rates, conduc tances, and transpiration rates during a low precipitation year. A drop in water table depth, possibly in combination with reduced nitrogen uptake by the plants, explains these results. In high-precipitation years, abundant soil mois ture results in high net assimilation rates, high conductance, and high transpiration rates. These years ofhigh soil moisture result in low water use efficiencies. In contrast to this, plants have the ability to reduce leaf conductance and increase water-use efficiencies in low-precipi tation years. This allows the plants to maintain predawn and afternoon water potentials, which vary little from high- to low-precipitation years in these saline environments. LITERATURE CITED ANTLFINGER, A. K, AND E. L. DUNN. 1983. Water use and salt balance in three salt marsh succulents: Salicor nia virginica, Batis maritima, Borrichia frutescens. American Journal ofBotany 70: 561-567. 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Accepted 6 Ja"""'Y 1997 R08lNSON, T W. 1958. Phreatopbytes. U.S. Geological Survey Water-Supply Paper 1423.