Table Of ContentThe Great Basin Naturalist
Plblishkd atPhono, Utah, by
Bricham Young Um\ersi'it
ISSN 0017-3614
Volume52 September 1992 No. 3
GreatBasin Naturalist52(3),pp. 195-215
PLANTADAPTATION IN THE GREAT BASIN AND COLORADO PLATEAU
Jonathan P. Comstock antlJames R. Elileringer
—
Al5STlU(X Adapti\efeaturesofplantsoftlieGreat Basinarereviewed. Thecombinationofcoldwintersandanarid
tosemiaridprecipitation regime resultsinthedistinguishingfeaturesofthevegetationinthe Great B;isinandGolorado
Plateau. Thepriniaiyeffectsoftheseclimaticfeaturesluisefromhowthe\ structurethehvdrologicregime.Waterisdie
most limitingfactortoplantgrowth, andwateris most reliabK axailahle in theearl\-springafterwinter recharge ofsoil
moisture. This factor determines main characteristics ofroot moipholog\, growth phenolog\- ofroots and slKX)ts, and
photos\ndieticphysiolog): Sincewintersarehpicallvcoldenoughtosuppressgrowth, anddroughtlimitsgrowthduring
thesummer,thecooltemperaturescharacteristicofthepeakgrowing.seasonarethesecondmostimportiuitclimaticfactor
influencingplant habit luidperform;uice. The combination ofseveral distinct stress periods, including low-temperature
stressinwinterand springandhigh-temperaturestresscombinedwithdroughtinsummer,appearstohavelimitedplant
habittoagreaterdegreethiui foundin thewarmde.serts tothesouth. Nonetheless,coolgrowingconditionsandamore
reliable springgrowingseason resultin higherwater-u.seeiiiciencvand productivih"in thevegetation ofthecoldde.sert
thaninwarmdesertswithequiv;ilenttotalrainfallamounts. Edapliicfactorsarealsoimportimtinstructuringcommunities
intheseregions,andhalophvticconnnunitiesdominatemain landscapes.Thesehaloph\-ticcommunitiesofthecolddesert
sharemoresj^eciesincommonwithwarmdesertsthandothenonsdinecommunities.TheGoloradoPlateaudiffersfrom
theGreat Basinin havinggreateramountsofsmninerrainfall,in some regionslesspredictableriiinfall,sandiersoils,and
streamswhich drain intoriver.systemsratherthanclosedbasinsandsaltplavas. One resultofthe.seclimaticandedapliic
differencesisamore importantsummergrov\ingseasf)n on the (Colorado j'lati'auandasonu-wliat <ii"eaterdi\c'rsilication
ofplanthabit,phenolog),andphysiolog)'.
Keyicords:colddesert,plantadaptation, waterstress,phenalo'^ij.salinitt/. Great Basin.ColoradoPlateau.
Several features arising from climate and Nevadaandincreaseboth tothenorth andea,st,
geolog)' impose severe limitations on plant and to the southeast moving into the Colorado
gro\\i:h andactivit)^-inthe Great Basin andCol- Plateau (Fig. 1,Table 1).The fractionofannual
orado Plateau. The climate is distinctlv conti- precipitation during the hot sununer months
nental with cold winters and warm, often dn (|une-Se[)tember) varies considerabh; from
sununers. Annualprecipitation levelsare lowin l()-2()9f in northern Nevada to 30-40% along
mm
the basins, ranging from 100 to 300 (4-12 theboundaiAoftheColdand .\Iojavedesertsin
inches), and t)piciilly increasingwith elevation southwestern Nevada and southern Utah, and
to 500 mm (20inches) or more inthe montane 35-50%throughout much oftheColorado Pla-
zones. Precipitation levels are lowest along tlu^ t(^au. Winter jirecipitation falls primariK' as
southwestern boundan' of the Great Basin in snow in the Great Basin and liii£her elevations
DepartmentofBiologv-.Universit)-oft'tali.SaltLakeCit^.Utiih84112.
195
196 Great Basin Naturalist [\blume 52
TaHI.K 1. Srlec'tec! climatic data tor l(m-elc'\ation sites in different regions ofthe Great B;Lsin, Moja\e Desert, and
ColoradoPlatean.Viiluesarebasedona\eragesforthe U.S.WeatherBureaustationsindicated.Thetlireedixisionsofthe
yearpresented herereflectecologicallyrelevant units, butareunequalin length.The fixe months ofOctober-Februan
represent aperiod oftemperature-imposedplantdorm;uicyandwinteri-echarge ofsoil moisture. The spring mondisof
March-Mav represent the potential growing period at cool temperatures immediately follcnving winter recharge. The
summerandei\r\\ fall fromJiuiethrough Septemberrepresentapotentialwarmgrowingseason in areaswith sufficient
summerrainoraccesstoothermoisturesources.
19921 PlantAdaptation 197
in the southern (Ircat liasin. hut suiiiiiici' pre-
cipitation is suhstantially greater on the Colo-
rado I'lateau (Tahk" \). Soils and drainage
patterns also differ in crucial wa\s. The high-
lands of the Colorado Plateau generally drain
into the Colorado Hixer sv'stem. In manv areas
extensive exposure of marine shales from the
Chinl(\ \hmcos, and Morrison Brnshv-Basin
formations wc^ithcr into soils that restrict plant
diversitvandtotal coverdue to high concentra-
tions of NaSOa, and the formation of clavs that
do not allow water infiltration (Potter et al.
1985). In other areas massive sandstone out-
cropsoftendominatethelandscape. Shrubsand
trees mav root through ven shallow rock"v soils
intonatural jointsandcracks in tliesub.stratum.
Deeper soils are generallv aeolian deposits
forniinti sands orsandvloams. In contrast, high
elevations of the Cireat Basin drain into closed
\alleys and evaporative sinks. This results in
Great Basin greater average salinitA' in the lowland soils of
the Great Basin, with NaCd being the most
Mojave common salt (Flovxers 1934). andamoreexten-
sive development of haU)ph\tic or salt-tolerant
vegetation. Soils tend to be deep, especialK at
Colorado Plateau
lower elevations, and van' in texture from clav
to sandv loams. Summer-active species with
Fig. 1. Distributionofthemajordesertvegetationzones different photosvnithetic pathwavs, such as C4
illtlieGreatBitsinandColoradoPlateau.Numbersindicate grasses and CAM succulents, are poorlv repre-
l(K'atioiisofclimatestationsforwhichdataarepresentedin
Table1. MostoftheMojaveDesertindicatedisgeologically sented in nuich of the Crreat Basin, but the
partoftheGreat Basin,but,duetoitslowerelevationand combination of increased summerrain, sandier
warmertemperatures,itisclimaticallvdistinctfromtherest soils, and milderwinters at the lowerelevations
oftheregion.
oftheColoradoPlateauhasresultedinagreater
expression ofphenological diversit\.
antl drier Mojax'e De.sert portion of the Cjreat The interactions ofedaphic factors and cli-
Basin will he considered primariK as a pointof mate are complex and often subtle in their
e()inj)arison, andformoretlioronii;hcoxerageof effects on plant performance. Furthermore,
that region we recommend the reviews h\ [)Iant distributions are rarelv determined bv a
Ehleringer (1985), MacMahon (1988), and singlefactorthroughoutth(irgeographicrange.
Smith and Nowak (1990). Forthe higher mon- e\en though a single factor mav appear to con-
tane and alpine zones of the desert mountain trol distribution in thecontext of alocal ecosvs-
ranges, the reader is referred to rexiews l)\ tcMii. Species-spcxific characteristics generally
\'asekandThome (1977) and Smith and Knapp do not inqxirt a narrow re(|uirement for a spe-
( 1990).Weareindebtedin onrownc()\erag(^of cific environment, but rather a unique set of
the cold desert to other rec-ent rexic^ws. includ- "rangesoftolerance"toalargearrav ofenxiron-
ing Caldwell (1974, 19S5). West (19SS). mental j)arameters. In different enviromncntal
Dobrowolski et al. (1990), and Smith and contexts, differenttolerances mavbe more lim-
Nowak (1990). iting,bothabioticandbioticinteractionsmaybe
The Great Basin and the Colorado Plateau altered, andthesamesetofspeciesmay.sortout
shareimportantclimaticfeaturessuchasoverall in different spacial ])attenis. A further conse-
ariditv; frequent summer droughts, and conti- (juence of this is that a local combination of
nental winters; yet the\^ differ in other ecjualK species, whicli we might refer to as a Great
importantfeatures. Temperatures on the Colo- Basin plant communitv. represents a region of
radoPlateauaresimilartoecjuixalentelexatioiis oxerlap in the geograpln'calK more extensive
198 Great Basin Naturalist [N'oluiiie 52
andtreiieralK miicjuetlistrihutioiisoteachconi- (Allciirolfia occich'ittdlis and Salicontia spp.),
ponenfspecies. In fact, thedistributionsofspe- and greasewood (Sarcobatiis vcniiicitkitiis),
cies commonly associated in the same Great may themselves show zonation due to degrees
Basin connnunitv' may be strongly contrasting oftolerance. They tend to occur in close prox-
outside the Great Basin. This is an essential imitv,however,ontheedgesofsaltplavas,saline
point in attemptingtodiscuss plantadaptations flats with shallow water tables, and near saline
with the implication oi cause and effect, seeps overawide range ofelevations, tempera-
because few species discussedwill have a strict tures, and seasonal rainfall patterns in both the
andexclusixerelationshipwiththeenvironment Great Basin and southern warm deserts
ofinterest. Unless we can show local, ecot\pic (MacMahon 198S). This relativeindependence
differentiation in the traits discussed, we need of distribution from prevailingclimate is a spe-
totakeabroadviewoftherelationshipbetween cial characteristic of strongly halophytic plant
environment and plant characters. In a few communities throughout theworldand reflects
instances,asmallnumberofedaphicfactorsand the overriding importance of such extreme
plant characters, such as tolerance ofveiyhigh edaphic conditions. Species found on better-
salinity in soil wdth shallow groundwater, seem drained, moderatelysaline soils,where groimd-
tobeofoverridingimportance.Inmostcaseswe water is not found within the rooting zone,
need to ask, what are the common elements of include winterfat {Ccratoidcs laiuita) and
climate overthediverse ranges of all these spe- shadscale {Atiiplcxconfeififolia). Thesespecies
cies?Onesuchcommonelement,whichwillbe are,inturn,replacedathigherelevationsandon
emphasized throughout this re\iew, is the moister, nonsaline soils bvbig sagebnish iAiic-
importance of reliable winter recharge of soil inisia tridcntatd), rabbitbrush [Chnjsoiluntinus
moistureinanaridtosemiaridclimate. B\-iden- sp.), bitterbnish {Piirsliia sp.), and spinv hop-
tifying such common elements and focusingon sage {GiYHfiaspinosa). Shadscale is often fcnmd
them,wedonotfullydescribetheautecologvof in areas where soils are notably saline in the
an\'species, butwe attempt acogenttreatment lower half of the rooting zone, but not in the
ofplant adaptations tothe Great Basin environ- upper soil lavers. Such a tolerance of mt)der-
ment, andanexplanationoftheunicjuefeatures atelysalinesoilsseemstocontrolitsdistribution
ofitsplantconnnunities. around playas, especially in the wetter, eastern
portion of the Great Basin (western Utah) and
Climate, Edaphic Factors, and Plant lowerelevationsinthewarm Mojave Desert. In
Distribution Patterns the more arid regions of western and central
Nevada, however, the shadscale connnunitv
Typical zonation patterns observed in spe- occurs widely on nonsaline slopes lower in ele-
cies distributions aroundplayas (the saline flats vation,warmer, anddrierthan thosedominated
at the bottom of closed-drainage basins) are by big sagebrush. These higher bands of
quitedramatic, refl(^ctingano\erridingeffectof shadscale have been variously inteipreted in
salinit)'onplantdistribution in theCireat Basin. terms of ariditv tolerance and climate (Billings
Moving out from the basin center is a gradient 1949) or an association with limestone-derived
ofdecreasing soil salinityoften correlated with calcareous soils (Beatlev 1975). The latter
progressively coarser-textured soils. Along this author points out that even on nonsaline soils
gradientthereareconspicuousspecies replace- percent cover in the shadscale connnunitv is
ments, often resulting in concentric zones of lower than expected for the level of precipita-
contrasting vegetation (Flowers 1934, Vest tion, and argut^s that this indicates stress from
1962). In the lowest topographic zone, saline ecUiphic factors. Thus, shadscale distribution is
groundwatermaybeveryneavthesurface. Soils most correlated with salinitv tolerance in some
are ven' saline, fine textured, and subject to regions andothereckiphicandclimatic tolcMan-
occasional flooding and anoxic conditions, in ces in other regions.
this enxiromnent the combination of available Where the higher elevations of thc> Cyreat
moisture with other poteutiallv stressful soil Basin conu^ in contactwith the lower-elevation,
characteristicsseemstobemoreimportantthan generallv drier, and warmer Mojave Desert
climaticfactorsoftemperatiu'eorseasonal rain- region, comminn'ties ck)minated by creosote
fallpatterns. Speci(>s found here, such asdesert (Larrca tridfufafa) replace sagebrush commu-
saltgrass {Distichlis spic<il(i), pickleweeds nities on nonsaline slopes and bajadas.
19921 PlantAdaitxtion 199
Shadscak' can ht' toiiiul liotli on saline soils at cal. and plieiiological traits lonnd in llie (k)mi-
\en lowt'k'\ations in tlu^ Mojaw and asatran- nant shrubs rell(^ct the [)riman importance of
sitional band at liigli eknations l)et\\een creo- such acool springgrowing.season.
soteandsagebmsh. Elementsofthecolddesert
shnibconimnnities,adaptedtocontinentalwin-
PlI()T(lSY\'THKSIS
ter's and a cool s[)ring growing season, can be
tonnd throughout the winter-rain-doniinated —
Piiotosyxtiiktk; I'ATIIWAVS. The pro-
\h)ja\"e Desert region as a high-elexation band
onaridmountainranges.Theyalsoextendtothe cess of photosvnthesis in plants relies on the
acquisitionofCO2fromtheatmo.sphere,which,
bsoiumtohdeaalstpraetcihpiigthatieolne\arteigoinmseionftothteheCosltroornagdKo- when coupledwith solarenerg\',is transformed
intoorganic molecules to make sugars andpro-
i'latean, and northward at low elexations into
vide for plant growth. In moist climates plant
Idaho. Washington, andexen British (-oluinbia.
communitiesoften achieveclo.sedcanopiesand
Nhning up from bajadas of the southern warm
1(){)% coverof thegroundsurface. Underthese
deserts, there appears to be no suitable combi- conditions competition forlight maybe among
nation of temperature and precipitation at an\'
the most important plant-plant interactions. In
elevation to support floristic elements of the the deserts total plant cover is much less than
colddesert. Asprecipitation increaseswithalti- 100%, and in the Great Basin closer to 259f.
tude, zones with equivalent total precipitation
Photosviithesisisnotgreatlvlimitcxlbv available
do not \et ha\e cold winters and are occupied
light, but rather bv water, mineral nutrients
In warmdesertshnibconnnunitiesgradinginto needed to .synthesize enzAines and maintain
chaparral composed of unrelated ta.xa. Higher metabolism, and maximum canopv leaf-area
ele\ationswithcoldwintershavesufficientpre- development.
cipitation to support forests. Other elements Three biochemical pathwavs of photosvii-
coimnoninshadscaleandmixed-shrubconnnu- thesis have been described in plants that differ
nities of the Great Basin, such as winterfat and in the first chemical reactions associated with
budsage (Ai-tcmisia spiiiosa), are often found the capture ofCO2 from the atmosphere. The
outside the Great Basin in cold-winter but most common and most fundamental ofthese
largel\- summer-rainfall grasslands. pathways is referred to as the C3 pathway
f^rom thesepatternsofcommunitv-distribu- because the first product ofphotosynthesis is a
tion within the Great Basin and Colorado Pla- 3-carbou molecule.Theothertwopathways,C4
teau, andalsofrom aconsideration ofthe more and CAM, are basically modifications of the
extensiverangesandaffinitiesofthecomponent primaiyC3pathway (Osmondet al. 1982). The
species, wecan isolate a fewke\- features ofthe C4 pathwav (first product is a 4-carbon mole-
environmentthatarelargelyresponsibleforthe cule) is a morphological and biochemical
unique plant features seen in the Great Basin. arrangement that overcomes photorespiration,
The most obvious features are related to the aprocessthat resultsin reducedphotosviithetic
patterns ofsoil salinitv andtexturegeneratedbv rates in C3plants. The C.ipathwaycan confera
the(Aerallariditv combinedwitheitherinternal much higher temperature optimum for photo-
drainasie basins or tlie in situ weathering of .synthesis and a greaterwater-use efficiency. As
specificrocktvpes.Themostimportantclimatic water-use efficiencv is the ratio of photosvn-
variables are slightlv more subtle. There is thetic carbon gain to transpirational water loss,
cknulv not arequirement forexclusivelvwinter C4plantshaveametabolicadvantageinhot,dn^
rainfall, but there is a re(|uirement forat least a environmentsw4iensoilmoistureisavailable. In
substantialportionol theannualrainfalltocome grasslands C4 grasses become dominant at low
dniingacontinentalwinterThispermitsv\inter elevations and low latitudes where animal tem-
(iccitninlatioit ofprecipitation iod'greaterdepth ]x^ratur(\s are warmest. In interpreting })lant
in the soil profile than will occurin response to distribution in deserts, the .seasonal pattern of
less predictable sunnner replenishment of rainfall usuallv restricts the periods of plant
dningsoil moisture reserves. Unck'r an overall growth, and the temperature during the rainy
arid climate, winter n^charge maintains a pre- seasonisthusmoreimportantthan m(\uiannual
ilictablv favorableandck)minantspringgrowing temperature.TheC4pathwav isofti'uassociated
season even in manv areas ofstrongly bimodal with smnmei-activespeciesandalsowith plants
rainfiill. Most of the phvsiological. moqihologi- ofsaline soils. While C3grasses pre(k)minate in
200 GreatBasin Naturalist [\'olunie 52
most of the Cireat Basin, C4 grasses beeonie temperatures (Ehleringer and Bjorkman 1978,
iiicreasinglvimportantassummerrainincreases Mooney et al. 1978, Comstock and Ehleringer
to the south, and especiaHv on the Colorado 1984, 1988, Ehleringer 1985). Thispresumably
Plateau. Halophvticplants areoften C4, suchas reflects the specialization of these Great Basin
saltbush iAfrij)Icx spp.) and saltgrass (Disticlilis shiiibs towards utilization of the cool spring
spp.),andtliis mavgixe theplantsacompetitixe growing season. Positive photosynthetic rates
advantage from increased water-use efficienc\- are maintained even when leal temperatures
on saline soils. arenearfreezing,whichpermitsphotosvnthetic
CAM
The third photo.sMithetic pathway is activitytobegin intheveryearlyspring(DePuit
photosMithesis (Crassulacean Acid Metabolism). and Caldwell 1973, Caldwell 1985).
CAMplantsopentheirstomataatnight,capture Unusuallv high maximmn photosvnthetic
COo and store it as malate in the cell \acuole, ratesof46 ixmol CO2 m ~s ' havebeen reported
and keep theii stomata closed dining the dav foroneirrigatedGreatBasinshnib, rabbitbrush
(Osmondet al. 1982). The CO2 is then released {Chnjsothamnus nauseosus) (Da\isetal. 1985).
from the vacuole and used for photos)aithesis Thisspeciesisalsounusualinhavingadeeptap
thefolloxxingda^'. Becausethestomataareopen rootthatoftentapsgroundwater,unusuallvhigh
onl\ at night when it is c()t)l, water loss associ- levels ofsummer leafretention (Branson et al.
ated with CAM photosNuthesis is greatlv 1976), and continued photo.sxnthetic activitx*
reduced. This pathwa\' is often found in succu- throughoutthesummerdrought (Donoxanand
lents such as cacti and agaxe, and, although Ehleringer 1991). All of these characters sug-
C^AM plantsarepresentintheGreatBasin,they gest greater photosvnthetic activity during the
areai-elati\eK-minorcomponentoftlievegetation. warm summer months than is found in most
Photosxntheticratesofplants,likemostmet- Great Basin shrub species. —
abolic processes, sho\\' a strong temperatm-e Shoot ACTTIMTY' and phenology. Gener-
dependence. UsualK, photosvnthetic rates are allyspeaking,thereis adistinctdroughtinearly
reduced at low temperatures because of the summer (June-|ulv) in the Great Basin Cold
temperature dependence ofenz^'uie-catah'zed Desert, the Mojave Desert, and the Sonoran
reaction rates, increase with temperature mitil Desert. All of these deserts ha\e a substantial
some maximum \alue (which defines the "tem- winter precipitation season, but they differ in
peratureoptimum"),andthendecreaseagain at the importance of the summer and early fall
higher temperatures. The temperature optima rainyseas(jn (|ul\-October)insupportingadis-
and niiuimum photosxnthetic rates of plants tinctive period of plant growth and acti\itv
showconsiderablevariation, andthe\'generalK (MacMahon 1988). The relationship between
reflect the growing conditions of their natural climateandplantgrowingseasoniscomplexand
environments. — includes total rainfall, seasonal distribution of
PHOTOSYNTHETIC adaptation. In the rainfall,andpredictabilitvofrainfallindifferent
spring the photosynthetic temperature optima seasons as important\ariables. Fmthermore, in
ofthe dominant shrub species are tvpicalK' as \en arid areas the seasonalih' of temperatures
lowas 15C (40F) (Caldwell 1985),correspcMid- may be as important as that of rainfall. In the
ing to the prevailing en\ironmental tempera- Great Basin, cold winters allow the moisture
tures(mi(kla\-ma.ximagenerallylessthan20C). from low lexels of precipitation to accumulate
As ambient temperatures rise 10-15 C in the in the soil for spring use, while hot summer
summer, thereisan upwardshiftofonly,5-10C temperatiu'es cause rapid evaporation from
in the photos\iithetic temperature optima of plants and soil. High temperatures can prevent
most shrubs, coupled with a slower decline of wettingofthesoilprofilebevondafewcentime-
photosynthesis at high temperatures. The max- t(Ms depth in response to sununer rain, even
imimi ph()t()s\nithetic rates measunxl in most when sununerrain accounts foralarge fraction
Great Basin shrubs undereithernatural orirri- oftheanimal total(Caldwelletal. 1977).Astotal
gatedconditions range from 14to 19 jjluioI ClO^ annual rainfall decreases, the relative impor-
m- s' (DePuit and Caldwell 1975, Caldwell et tance of the cool spring growing season
al. 1977, Evans 1990). These rates are (|uite iIicreasesastheoiiK potentialgrowingperiodin
mode.st compared to t\ie high maxima of25 to which available soil moisture approaches the
45 jjLmol CO2 m " s ' ob.sened in man\- warm- evaporativedemand(Thornthwaite 1948,Com-
dc^settspeciesadaptedto rapidgrowthathigher stock and Ehleiintier 1992). Finally, reliabilih
19921 PlantAdaitation 201
of nioisturc is important to [XTcnnials, and as expansion or contraction of xc^gt^tatixc pluuses
axerage total precipitation decreases, the \ari- andexen theomission of reproductix-e pha.ses.
ance bet\veen \ears increases (Ehleringer Most species initiate grox\th in earlx' spring
1985); \ariabilit\' ofannuiil precipitation is dis- (March)xvhen maximumda\timetemperatures
cussed in more detail later in the section on are 5-15 C and xx'hile nighttime temperatures
annuals and life-histor\' dixersitv. Summer rain are still freezing. Delaxed initiation of spring
is more likel\-to be concentratedin afew high- groxxth is generally associated xxith greater
intensit\ stormsthatmax nothappen e\eiA'\ear summer actixit\-and max-be relatedto an exer-
atagi\ensite and ma\'cause more nmoitwhen green habit, a phreatophxtic habit, or occupa-
the\ do occur. The abilits' of moisture from tion of habitats xxith greater sununer moisture
winter rain to accumidate o\er several months axailabilitx from regional rainfall patterns,
greatly enhances its reliabilits' as a moisture nmoff, or tirovmdxx'ater. Higher-than-ax-erase
resource. Thus, most plants in the Great Basin xxinterandspringprecipitationtendstoprolong
havetheirpriniar\-growingseasoninthespring vegetatixe growth and delax- reproductive
and earl\- summer. Some species achie\e an groxx'thtilllaterinthesununer(SanerandUre.sk
e\ergreen canop\' b\' rooting deepK; but few 1976,Cambelland Harris 1977). Strongxegeta-
species occur that specialize on growth in the tivedormancyma\'bedisplayedinmid-summer
hot summerseason (Branson et al. 1976, Cald- (Everettetal'. 1980, Evans 1990), although root
well etal. 1977, Everettetal. 1980). Ehleringer groxx'th (Hodgkinson et al. 1978) and increased
et al. (1991) measured the abilitv of common reproduction (W'est and Gastro 1978, Exans,
perennialspeciesintheColoradoPlateautouse Black, and Link 1991) max' still occur in
moisture from summer convection storms. response to rain at that time. In xears with
The\- obserxed that less than halfofthe water beloxx'-axerage spring and svunmer precipita-
uptake b\- wood\' perennial species was from tion, leafsenescence is accelerated and floxx'er-
suriace soil laxers saturated b\' summer rains. ing maynot occurin man\-species.
Prexalence of summer-active species increases The time taken to complete the full annual
alongtheborderbetxveenhigherbasinsandthe groxxth cxcle including both xegetatixe and
southeast Mojaxe Desertxvheresummerrain is reproductixestagesisstronglx relatedtorooting
more extensixe, andespecialK'on the Colorado depthin mostoftheseconmumities,xxithdeep-
Plateauxx'heresummerrainisgreatest. Summer rooted species prolonging actixit\' further into
temperatures are also lower on the Colorado the summer drought (Pitt and Wikeem 1990).
Plateau than in the eastern Mojaxe (Table 1), Theexacttimingoffloxx'eringandfniitsetshoxvs
alloxxing more effectixe use ofthe moisture. considerable xariation among different shrub
Mostphenolog)-studies, especiallx'from the species. Some, especiallx those that are
morenorthernareas,emphasizethedirectional, drought-deciduous, lloxxer in late sprin>j; and
sequential nature of the phenological phases earlx summer just prior to or concurrent xxith
(Branson et al. 1976, Saner and Uresk 1976, the onset of summer drought. Manx-exergreen
Cambell and Harris 1977, West and Gastro shRib species begin floxxering in midsummer
1978,PittandW'ikeem 1990). Asingleperiodof (Artonisia) or in the fall {Gutierrczia and
springvegetative groxvth is usually folloxved by Chn/sothainntts). These late-flowering species
a single period of floxxering and reproductix'e generallxdonotaj)peartoutilize"storedreserx'es
groxx'th. Manx-species produce adistinct cohort for floxx'ering. but relx on current photo.sxnthe-
ofephemeral springleavesandalatercohortof sisduringthisleastfax-orabk"period. Inthecase
exergreenleaxes(Daubenmire 1975, Millerand (){ Aticmisia fridoitafa. it has been shoxxn that
Schultz 1987). For most species, multiple earlx)lix-drates used to fill fruits arc dcrixcd
groxxth episod(\s associated xxith intermittent exclnsixi'lx from the inflorescences theniselxes,
springandsummerrainfall exentsdo notoccur. xxhile photosxnthate from xegetatixe l)ranches
In xears xxith unusually heavyAugust and Sep- presumablx continues to support root groxx'th.
temberrains,adistinctsecondperiodofxegeta- Summer rain during this time period does not
tixe growth may occur in some species (West promote xegetatixe shoot groxxth but does
andGastro 1978). Climaticxariations from xear increase xvater use by and the ultimate size of
to xear do not change the primaty importance inflorescences (Exans 1990). Exans, Black, and
of spring gro\xi:h or the order ofphenological Link (1991) haxe argued that this pattern of
exents. In particular \ears, thex' ma\- cause floxx'ering, ba.sedon residual deep soil moisture
.
202 Great Basin Naturalist [Volume 52
and the unreliable summer rains, ma)' contrib- 1980). Fall andwinterprecipitation is the mo.st
ute to competitixe dominance within these important in promoting good spring growth of
comnumities. The more favorable and much perennials (Beatley 1974). Comstock et al.
more reliable springgrowingseason is usedfor (1988), looking at one years growth in 19
\egetative growth and coiupetitive exploitation Mojave species, described an important cohort
ofthesoil\olume,while reproductivegro\\i:h is of twigs initiated during the winterperiod that
delayeduntilthelessfavorableseason,andmay accounted for most vegetative growth during
be successful only in years with adequate the followingspring. Although newleaveswere
summerprecipitation producedin response to summerrain, summer
Most grasses in the northern part of the growth in many of the species was largeK' a
Great Basin utilize the G,5 pathway and begin further ramification of spring-initiated floral
growth very early in the spring. These species branches.Inmostspeciessummergrowthmade
complete fruit maturation by early or mid- little contribution to perennial stems. Despite
sunnner, often becoming at least partially dor- thepreferredwinter-springorientationofmany
mantthereafter. OntheColorado Plateau, with shmbs, winter recharge is much less effective
muchgreateramountsofsummerprecipitation, and reliable in the Mojave Desert than in the
there is alarge increase in species numberand Great Basin. Due to warmer temperatures,
cover by C4 grasses such as bluestem winter dormancy may be less complete, but
(Andropogon) and grama {Bouteloua), espe- vigorous growth rarely occurs until tempera-
cialK at warmer, lower elevations and on deep tures rise further in the early spring. Rapid
sandy soils. Many of these species occur in growth luay be triggered by rising spring tem-
mixed stands with the C3 species but delay ini- peratures or maybe delayeduntil majorspring
tiation ofgrowth until MayorJime; theycanbe raiusprovidesufficient moisture (Beatley 1974,
considered suiumer-active rather than spring- Ackenuan et al. 1980). Furthermore, a shal-
actix'e. Incontrast,someC4grassessuchassand lower soil moisture recharge often results in
dropseed {Sporoholii.s cri/ptcmdnis), galleta fluctuating plant water status and multiple
grass(Hilariajainesiii),andthree-awn {Arisfkla episodes of growth and flowering during the
purpurea) are widespread in the Great Basin spring and early fall. Some Great Basin species
where sunuuer rain is only moderate in long- that also occurin the Mojave, such as winterfat
termaveragesandveiyinconsistentyeartoyear. andshadscale,commonlyshowmultiplegrowth
Spring growth of these widespread species and reproductive episodes peryear under that
tendstobe slighth'ormoderatelydelayedcom- climate (Ackennan et al. 1980) but not in the
Great Basin (West and Gastro 1978). The
paied to co-occurring C5 grasses, but they are
still able to complete all phenological stages degree to which this difference is due entirely
based on the spring moisture recliarge. The\' toenvironmentaldifferencesasopposedtoeco-
show a greater abilit)' than the G,; species to t\pic differentiation does not appear to have
been studied.
respond to late spring and simuiier rain witli
renewedgrowth (Everettet al. 1980), however,
which compensates in someyears fortheirlater Water Relations
developuKMit. Other C4 grasses in the Great
—
Basin may be associated with seeps, Ai:)APTATION TO LIMITED W.ATER. Stoma-
streamsides, or salt-marshes, and show a tal pores provide the pathvx'av by which atmo-
summeractivity'pattern. G4shrubs such as salt- spheric COo diffuses into the leaf replacing the
bush (Atriplex) show similar, spring-actixe CO2 incorporated into sugar molecules during
growthpatterns tothe (v;shrubs, but mayshow photosynthesis. Becausewatervaporis present
slightly greater tolerance of sunuuer drouglit at \eiy high concentrations inside the leaf,
(Caldwell et al. 1977). opening stomata to capture COo inevitably
Phenolog)' in the Mojave Desertshows both results in trauspirational water loss from the
similarities and strong contrasts to the Great plant; thus, leaf water content is decreased,
Basin.AlthoughrainfallislargeK biiuodalinthe resulting in a decrease in plant water potential
eastern Mojave, absolute amoimts arevvwlow. (^).Thus,plantwaterstatus, transpiration, and
Thesunuuerissohotthatlittlegrowth normally ac(juisiti()n ofwater from the soils have a tre-
occurs at that time unless more than 25 nun (1 mendous impact on photosynthetic rates and
inch) comes in a single storm (Ackerman et al. overall plant grovxth.
1992] PlantAnAPTYnox 203
Main soils in the (Ticat Basin arc liiu^ t(^\- much of the exick^ice for this is (|uite indirect.
tured,whichhasbotli atKantagcs anddisadxan- Nonetheless, the osmoticpotential ofthe cxto-
tages for plant growth. Infiltration of water is plasm irnistalsobebalancedorsufferdehxdra-
slowerinfine-texturedsoils,increasingthelike- tion. The cytoplasmic .solutes must haxe the
lihood ofrunoffand reduced spring recharge, special propeitx of loweringtheosmoticpoten-
especialK'onsteeperslopes.Theyarealsomore tial of the cell sap xxathout dismpting enz\nne
prone to water-logging and anoxic c-onditions. function orcellular metabolism, and are hence
ThedeeprootsystemsofCireat BasinsluMihsare termed "compatible" solutes (W'xii Jones and
ver\'sensitivetoanoxia, andthiscanbeastrong Gorham 1986). The use of specific molecules
determining factor in species distributions shows considerable^ xariation across species
(Limt et al. 1973, CiroeneN'eld and Crowley apparentlx' due to both species-specific xaria-
19S8). Unnsualh'wet\earsma\'e\encause root tionsincell metabolismandtaxonomicrelation-
dieback, especiallyat depth. Oncewaterenters ships. Some frecjuentlx encountered molecules
thesoilprofile,theextremelyhighsurfaceareas thought to function in this manner include
of fine-textured soils with high clav and silt amino acids such as proline and also special
content gi\ethemamuchhigherwater-holding sugar-alcohols. Soiueplants appearto generate
capacit\' than that foimd in sandy, coarse-tex- low osmotic potentials bx' actixeK" manufactur-
tured soils. Much of this wateris tighth'bound ing largerquantities ofdissolxedorganic mole-
to the enormous surface area of the small cules per cell in response to water st^^ss. a
particles, howe\er, and is released onl\ at \en' process referred to as "osmotic adjustment."'
negatixe matric potentials. Plants nuist be able This process ma\' be costh; hoxx'exer, recjuiring
to tolerate low tissue water potentials to make the inxestment of large amovmts of materials
use ofit. (nexv solutes) at a time xx'hen xx'ater stress is
As soil water is depleted during sunuuer, largely inhibiting photosvnthetic activitv'. An
plantsreducewaterconsumptionb\ closingsto- alternatixe method seems to inxolve dramatic
mata (DePuit and Caldwell 1975, CambeJl and changes in cell xx'ater xolume. Sexeral desert
Harris 1977, Caldwell 1985, Miller 1988) and specieshaxebeenobserx'edtoreducecellxx'ater
reducing total canop\' leaf area to a minimum xolumebx'asmuchas80%xx'ithoutxxiltingxx'hen
(Bran.son et al. 1976). Evergreen species shed subjectedtoextremelx'loxx'soilxxaterpotentials
only a portion of the total canop\, however, (Moore et al. 1972, Meinzeret al. 1988, Evans
maintainingthe youngest leaf cohorts through- et al. 1991). This alloxx'ed the leafcells to have
out the drought (Miller and Schulz 1987). sufficiently loxv osmotic potentials for xx'ater
Although plnsiological actixit)' is greatK' uptake exen though solute content })ercell xx'as
reduced b\' water stress, exergreens such as actuallyreduced. Althoughtotalsolutesperleaf
sagebnish canstillhavepositive photosviithetic (and presumablx per cell) decreased, the rela-
rates at leafwaterpotentials as lowas —50 bars tix'e concentrations of specific solutes changed
(Exans 1990) and may surxive even greater (Evans et al. 1991) such that "compatible"
](nels of stress. In contrast, crop plants can solutesmadelargercontributionstotheosmotic
rareK"sunixeprolongedM^oflessthan -15bars. potential untk'r stress. Thus, tolerance of loxv
Remaining functional at loxx' xx'ater potentials leafxxaterpotentials was achieved bv a combi-
requires the concentration ofsolutes in the cell nation ofanatonncal and phxsiological special-
sap, and this appears to be accomplished b\ izations. The anatomical mechanisms inxolxed
several mechanisms. In manx mesic species, in this magnitude of xolume reduction and the
increases in organic solutes may account for im]iliedcellelasticitx'haxenotbeenstudied,but
mostofthechangeinosmoticpotential. Inother tlieprocess hasbeen shown tobefnllx rexcrsible.
species, and especialK' tho.se that experience Soil texture^ is also an important factor in
xeiyloxvleafxvaterpotentials, alargefractionof determiningthe abilitx'of plantconnnunitiesin
the solutes is acquired by the uptake of inor- a coId-x\int('r climate to respond to summer
ganicions such as K+ (Wvii fones and (^orhani rain. Inareasxxith moderatelexelsofprecipita-
1986). High concentrations of inorganic ions tion, sandx' soils, because of their loxx- xxater-
may l)e toxic to enzx'me metabolism, hoxxexer. holding capacitx. nsuallx' hold a sparser, more
and they are xxidely thought to be se(juestered drought-adaptedx(^getation than finer-textured
largely in the central vacuole, xvhich accoimts ones. In xvarm, arid areas, however, what has
for 90% of the total cell xolume. exen thoush been called the "rexerse texture" effect results
204 Great Basin Naturalist [\'oliime 52
ill the more liisli xegetation oceiirrintj; in tlie well et al. 1977, Sturges 1977) ofthe total root
coarse-textured soils. This occurs because the biomass. The veiy large annual root inxest-
high water-holding capacit)' of fine-textured ments, therefore, are not primariK- related to
soils allows them to hold all the moisture largestoragecompartments,buttothetunioxer
deri\'edfromasinglerainfalleventintheupper- of fine roots and root respiration necessan- for
most layers ofthe soil profile,where it is liigliK the acquisition ofwaterand mineral nutrients.
subjectto directe\aporation from the soil. The ThefinerootnetworkthoroughK'permeates
same amount of rainfall entering a sandv soil, the soil x'olume. Root densities are grenerallv
precisely because of its low \\'ater-h()lding quite high throughout the upper 0.5 meters of
capacity',willpenetratetoamuchgreaterdepth. the profile, but depth of maximum root devel-
ItisalsolesslikeK'toreturntothedningsurface opment \aries with depth of spring soil-mois-
b\' capillaiv action. Less of the rain will exapo- turerecharge,species,andlateraldistancefrom
rate fromthesoil surface, andagreaterfraction the trunk or crowai. A particularly high densit)'
ofitwillawaitextraction andusebyplants.This in the first 0.1 m ma\' often occur, especially
inverse-textiu-eeffectfurtherrestrictstheeffec- immediateh under the shmb canopx (Branson
tiveness of summer rains in the fine soils of the 1976, Caldwell et al. 1977, Dobrowolski et al.
Great Basin. The effect is less operative in 1990). AlternatixeK', maximaldensit) mavoccur
respect to winter precipitation in the Great at depths between 0.2 m and 0.5 m (Sturges
Basin,however,becauseofthegradualrecharge 1980),andsometimesasecondzoneofhighroot
ofthe soil profile to considerable depth under densit}' is reported at depths of 0.8 m
conditions where surface e\aporation is mini- (Daubenmire 1975) to 1.5 ni (Reynolds and
mized by cold temperatures. The combination Fralev1989). Regardlessoftheprecisedepthof
of much sandier soils and greater amounts of maximum rooting, sliRib root densit\'is usualK'
summer rainfall in the region of the Colorado high throughout the upper 0.5 m and then
Plateau is largely responsible for the majorflo- tapers off, but max continue at reduced densi-
ristic and ecological differences bet\\'een the tiestomuchgreaterdepth.Theradiusoflateral
two regions. At lower elevations on the south- spreadisusuallx'muchgreaterforroots (1-2m)
east edge of the plateau, shiid^-dominated than for canopies (0.1-0.5 m). Percent plant
desertscnibmavbereplacedbygrasslanddom- coxerabox'egroundisoftenintheneighborhood
inatedbyamixofspring-activeC5andsummer- of25%xxdth75%bareground,butbeloxvground
active C4 species. the interspaces are filledxvith roots throughout
ROOTI—NC; DEPTH, MORPHOLOGY, AND PHE- the profile, and root sxstems of adjacent plants
NOLOGY. One of the unique and ecologicallv xxdll overlap. The perennial grasses that are
most important features of the Great Basin potentiallvco-dominantxxith shnibsin manx of
shmb communities is not apparent to abo\e- these communities, such as xxheatgrass
ground obseners. This is the investment ofthe {A^ropi/roii sp.), xx'iklne (Eh/nui.s sp.),
vast inajorit\- of plant resources in the growth, squirreltail {Sitaiiioii liisti-ix). Indian ricegrass
maintenance,andtunioxerofanenormousroot (On/zopsis lu/i)icii()i(h:s). and galleta grass
system.ThedominantslinibsoftheGreatBasin {Hilaiiaiainesii),generallx haxesomexxhatshal-
usuallyroottothefulldepthofthewinter-spring loxxer root .sxstems than the shrubs (Branson et
soil moisture recharge. Depending on soil tex- al. 1976, Rexiiolds and Fralex- 1989, Dobro-
ture, slope aspect, and elevation, this is gener- xvolski etal. 1990). Rootdensities ofgrasses are
allybetween 1.0 and 3.0 m (Dobrowolski et al. often as high as or higher than those of shrubs
1990). Although this represents awide rangeof in the upper 0.5 m but taper off more rapidlx
absolute ck^pths, nianv of the ([ualitatixe pat- such that shnibs usuallx haxe greaterdensitx at
terns ofrootingbehaxior are similaron most of depth andgreater maximum penetratit)n.
these sites. Ratios of rootishoot standing bio- The moistureresourcesupportingthegreat-
mass iang(^ from 4 to 7, and estimates of est amount of plant groxx'th is usuallx-the xx'ater
root:shootannualcarbon inxe.stmentareas high ston^l in the soil profile duringthexxinter. This
as 3.5. Most ofthe shrubs ha\e a flexible, gen- j)r()(ile usuallx hasapositixebalance,xxith more
eralized root system with dexelopment ofboth XXaterbeingaddedbx precipitation than isxxith-
deep taproots and laterals near the surface. draxxn bx' exapotranspiration bet\xeen October
Moreover, it is the categon offine roots < 3.0 and March. As temperatures xx-arm in March,
mm
indiameterthatconstitutes50-95% (Cald- exergreen .species nia\' begin draxxing on this
