Great Basin Naturalist
Volume 52 Number 3 Article 1
12-18-1992
Plant adaptation in the Great Basin and Colorado Plateau
Jonathon P. Comstock University of Utah
James R. Ehleringer University of Utah
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Recommended Citation Comstock, Jonathon P. and Ehleringer, James R. (1992) "Plant adaptation in the Great Basin and Colorado Plateau," Great Basin Naturalist: Vol. 52 : No. 3 , Article 1. Available at: https://scholarsarchive.byu.edu/gbn/vol52/iss3/1
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]. The Great Basin Naturalist PUBLISHED AT PROVO, UTAH, BY BRIGHAM YOUNG UNIVERSITY
ISSN 0017-3614
VOLUME 52 SEPTEMBER 1992 o. 3
Great Basin Naturalist 52(3). pp. 195-215
PLANT ADAPTATION IN THE GREAT BASIN AND COLORADO PLATEAU
Jonathan P. Comstock' and James R. EhleringerI
ABS1"RACT.-Adaptive features ofplants ofthe Great Basin are reviewed. The combination of cold winters and an arid to semiarid precipitation regime results in the distinguishing features of the vegetltion in the Great BlIsin and Colorado Pluteau. The primuly effects of these climatic features arise from how they structure the hydrologic regime. Water is the most limiting fuetor to plant growth, and water is most reliably avaibble in the early spring after winter recharge of soil moisture. This factor determines many charactelistics of root morphology, growth phenology of roots und shoots, und photosynthetic phYSiology. Since winters are typically cold enough to suppress growth. and drought limits growth during the summer, the cool temperatures characteristic ofthe peak growing season are the second most important climatic factor influencing plant habit and performance. The combination of several distinct stress periods, including low-temperature stress in winter and spring and high-temperature stress combined with drought in summer, appears to have limited pluot . habit to a greater degree than found in the warm deserts to the south. Nonetheless, cool growing conditions and a more reliable spring growing season result in higher water-use effiCiency and productivity in the vegetation of the cold desert than in warm deserb" with equivalent total rainfall amounts. Edaphic factors are also important in structuring communities in these regions, and halophytic communities dominate many landscapes. These halophytic communitie... ofthe cold desert share more species jn common with warm deserts than do the nonsaline communities. The Colorado Plateau differs from the Great Basin in having greater amounts of summer rainfall, in some regions less predictable rainfall. sandier soils. and streams which drain into r.iver systems rather than closed basins and salt playas. One result of these climatic and edaphic differences is a more important summer growing season on the Colorado Plateau and II somewhat greater diversification ofplant habit, phenology. and phYSiology.
Key uxm:J., ccld de8ert, plant adaptafun, water stress, phencwgy, salinity, Great Basin, Cowrado Ploteau.
Several features arising from climate and Nevada and increase both to the north and east, geology impose severe limitations on plant and to the southeast moving into the Colorado growth and activity in the Great Basin and Col Plateau (Fig. 1, Table 1). The fraction ofannual orado Plateau. The climate is distinctly conti precipitation dnring the hot summer months nental with cold winter.; and warm, often dry (june-8eptember) varies considerably, from summers. Annual precipitation levels are low in 10--20% in northern Nevada to 3Q...40% along the basins, ranging from 100 to 300 mm (4-12 the boundary oftheCold and Mojave deserts in inches), and typically increasing with elevation southwestern Nevada and southern Utah, and to 500 mm (20 inches) or more in the montane 35-50% throughout much ofthe Colorado Pla zones. Precipitation levels are lowest along tlle teau. Winter precipitation falls primarily as southwestern boundary of the Great Basin in snow in the Great Basin and higher elevations
J Depmt1l'lent of6'ology, UDivoe'3ityofU'ah. Salt LaJreCity. Utah 841l.2..
195 196 GREAT BASIN NATURALIST [Volume 52
TAIll..F. 1. Selected climatic data for low-elevation sites in different regions of the Great Basin, Mojave Desert, nud Colomdo Plateau. Values are based. on averages for the U.S. Weather Bureau stations indicated. The three divisions of the year presented here reflect ecolOgically relevant units, but are unequal in length. The five months of October-February represent a period of temperature-imposed plant dormancy and winter recharge of soil moisture. The spring months of March--May represent the potential growing period at cool temperatures irrllTIediately follmving winter recharge. The summer and early fall from June through September represent a potential warm growing season in areas ",,,th sufficient summer rain or access to other moisture sources.
Total precipitation Mean temperature
Region Map # Weather Elevation Annual Oct·Feb Mar-May Jun-5ep Annual Oct-Feb Mar~May Jun.Sep (Fig I) sl.tion (m) (mm) (%) (%) (%) ('C) ('C) ('G) ('C)
Northern 1 Fort Bidwell 1370 402 63 24 13 9.0 3.0 8.0 17.3 Gre;.lt Basin 2 Reno 1340 182 61 24 15 9.5 3.3 8.4 1M 3 Elko 1547 230 52 29 19 7.6 0.1 7.1 17.5 4 Snowville 1390 300 43 33 24 7.4 0.7 6.2 18.4
Southern 5 Sarcobatus 1225 85 45 22 33 13.5 6.4 12.5 23.1 Great BilSin 6 Caliente 1342 226 47 24 29 11.7 4.1 11.2 21.5 7 Fillmore 1573 369 44 34 22 11.0 3.0 10.0 21.7
Mojave Desert 8 Trona 517 102 70 19 II 19.0 11.3 18.4 29.0 9 Beaverdam 570 169 50 23 2$ 18.3 11.0 16.9 2$.6
Colorado 10 Hunksville 1313 132 36 19 45 11.4 2.1 11.5 22.8 Plateau 11 Grand Junction 1478 211 39 25 36 11.3 2.4 10.9 229 12 Blanding 1S41 336 48 19 33 9.7 2.1 8.7 19.9 13 Tuba City 1504 157 36 21 41 12.6 4.8 12.0 22.8 14 Chaco Canyon 1867 220 35 20 45 10.3 2.6 9.4 20.6 of the Colorado Plateau, which is thought to be internal drainage typical ofthe Great Basin. In a critical feature ensuringsoiJ moisture recharge this paperwe address the salient morphological, and a reliable spring growing season (West physiological, and phenological specializations 1983, Caldwell 1985, Dobrowolski et aI. 1990). ofnative plant species as they relate to sUivivai DUring the winter period, precipitation is gen and growth under the constraints of these erally in excess ofpotential evaporation, but low potentially stressful limitations. We emphasize temperatures do not permit growth or photo (1) edaphic factors, particularly soil salinity and synthesis, and exposed plants may experience texture, and (2) the climatic regime ensuring a shoot desiccation due to dry winds and frozen fairly dependable, deep spring recharge of soil soils (Nelson and Tieman 1983). Strong winds moisture despite the overall aridity, as factors can also cause major redistributions ofthe snow molding plant adaptations and producing the pack, sometimes reversing the expected unique aspects ofthe regional plants and vege increase in precipitation with elevation and tation. The majority of the Great Basin lies at having important consequences to plant distri moderately high elevations (4000 ft and above), butions (Bransonet aI. 1976, Sturges 1977, West and it is occupied by cold desert plant commu and Caldwell 1983). The important growing nities. Reference to "the Great Basin" and its season is the cool spring when the soil profile is . environment in this paperwill refer to this high recharged from winter precipitation; growth is elevation region as distinct from that comer of usually curtailed by drying soils coupled with the MOjave Desert that occupies the southwest high temperatures in early or mid-summer. A ern comer of the Great Basin geographiC unit clear picture ofthis climatic regime is essential (Fig. 1). Our emphaSiS will be placed on these to any discussion of plant adaptations in the cold desert shrub communities in both the region. Great Basin and the Colorado Plateau ranging A second major feature affecting plant per from the topographiC low points of the saline fonnance is the prevalence ofsaline soils caused playas or canyon bottoms up to the pinyon-juni by the combination oflow precipitation and the per woodland. The lower-elevation, warmer, 1992] PLANT ADAPTATION 197
in the southern Great Basin, but summer pre cipitation is substantially greater on the Colo rado Plateau (Table 1), Soils and drainage patterns also differ in crucial ways. The high lands of the Colorado Plateau generally drain into the Colorado River system. In many areas extensive exposure of marine shales from the Chinle, Mancos, and Morrison Brushy-Basin formations weather into soils that restrict plant diversity and total cover due to high concentra tions of NaSO" and the formation of clays that do not allow water infiltration (Potter et ai, 1985). In qther areas massive sandstone out crops often dominate the landscape. Shruhs and trees may root through very shallow rocky soils into natural joints and cracks in the substratum. Deeper soils are generally aeolian deposits forming sands or sandy loams. In contrast, high elevations of the Great Basin drain into closed valleys and evaporative sinks. Tbis results in Great Basin greater average salinity in the lowland soils of the Great Basin, with NaCl being the most common salt (Flowers 1934), and a more exten sive development of halophytic or salt-tolerant vegetation. Soils tend to be deep, especially at Colorado Plateau o lower elevations, and vary in texture from clay to sandy loams. Summer-active species with Fig. L Distribution ofthe major desert vegetation zones different photosynthetic pathways, such as C, in the Great Basin and Colorado Plateau. Numbers indicate locations ofclimate stations for which data are presented in grasses and CAM succulents, are poorly repre Table 1. Most ofthe Mojave Desert indicated is geologically sented in much of the Great Basin, hut the part ofthe Great Basin, but, due to its lower elevation and combination ofincreased summer rain, sandier warmer temperatures, it is climatically distinct from the rest soils, and milder winters at the lower elevations ofthe region. ofthe Colorado Plateau has resulted in a greater expression ofphenological diversity. and drier Mojave Desert portion of the Great The interactions of edaphic factors and cli Basin will be considered primarily as a point of mate are complex and often subtle in their comparison, and for more thorough coverage of effects on plant performance. Furthermore, that region we recommend the reviews by plant distributions are rarely determined by a Ehleringer (1985), MacMahon (1988), and single factor throughout theirgeographic range, Smith and Nowak (1990). For the higher mon even though a single factor may appear to con tane and alpine zones of the desert mountain trol distribution in the context ofa local ecosys ranges, the reader is referred to reviews by tem. Species-specific characteristics generally Vasek and Thome (1977) and Smith and Knapp do not impart a narrow requirement for a spe (1990). We are indehted in onrown coverage of cific environment, but rather a unique set of the cold desert to other recent reviews, includ «ranges oftolerance" to a large array ofenviron ing Caldwell (1914, 1985), West (1988), mental parameters. In different environmental Dobrowolski et a1. (1990), and Smith and contexts, different tolerances may be more lim Nowak (1990). iting, both abiotic and biotic interactions may he The Great Basin and the Colorado Plateau altered, and the same set ofspecies may sort out share important climatic features such as overall in different spacial patterns. A further conse aridity, frequent summer droughts, and conti quence of this is that a local combination of nental winters; yet they differ in other equally species, which we might refer to as a Great important features. Temperatures on the Colo Basin plant community, represents a region of rado Plateau are similar to equivalent elevations over1ap in the geographically more extensive 198 GHEAT BASIN NATUHALIST [Volume 52 and generally unique distributions ofeach com (Allenrolfia occidentalis and Salicornia spp,), ponent species. In fact, the distrihutions ofspe and greasewood (Sarcobatus vennicu1atus), cies commonly associated in the same Great may themselves show zonation due to degrees Basin community may be strongly contrasting of tolerance. They tend to occur in close prox outside the Great Basin. This is an essential imity, however, on the edges ofsalt playas, saline point in attempting to discnss plant adaptations flats vvith shallow water tables, and near saline with the implication of cause and effect, seeps over a wide range ofelevations, tempera because few species discussed will have a strict tures, and seasonal rainfall patterns in both the and exclusive relationship with the environment Great Basin and southern warm deserts of interest. Unless we can show local, ecotypic (MacMallOn 1988), This relative independence differentiation in the traits discussed, we need of distribution from prevailing climate is a spe to take a broadview ofthe relationship between cial characteristic of strongly halophytic plant environment and plant characters. In a few communities throughout the world and reflects instances, a small numberofedaphic factors and the overriding importance of such extreme plant characters, such as tolerance ofvery high edaphic conditions. Species found on better salinity in soil with shallow groundwater, seem drained, moderately saline soils, where ground to be ofoverriding importance. In most cases we water is not found \vithin the rooting zone, need to ask, what are the common elements of include wiuterfat (Ceratoides lanata) and climate over the diverse ranges ofall these spe shadseale (Atriplex corifertifolia), These species cies? One such common element, which \!liB be are, in turn, replaced at higherelevations and on emphasized throughout this review, is the moister, nonsaline soils by big sagebrush (Arte importance of reliable \vinter recharge of soil misia tridentata), rabbitbrusb (Chrysothamnus moisture in an alid to semiarid climate. Byiden sp,), bitterbrush (Purshia sp,), and spiny hop tif);ing such common elements and focusing on sage (Grayia spinosa), Shadscale is often found them, we do not fully describe the autecology of in areas where soils are notably saline in the any species, but we attempt a cogent treatment lower half of the rooting zone, but not in the ofplant adaptations to the Great Basin environ upper soil layers. Such a tolerance of moder ment, and an explanation ofthe unique features ately saline soils seems to control its distribution ofits plant communities. around playas, especially in the \vetter, eastern portion of the Great Basin (western Utah) and CLIMATE, EDAPHIC FACTOHS, AND PLANT lower elevations in the warm Mojave Desert. In DISTHIBUTION PATTEHNS the more arid regions of \vestern and central Nevada, hmvever, the shadscale community Typical zonation patterns observed in spe occurs 'widely on nonsaline slopes lower in ele cies distributions around playas (the saline flats vation, warmer, and drier than those dominated at the bottom of closed-drainage basins) are by big sagebrush, These higher bands of quite dramatic, reflecting an ovenidingeffectof shadscale have been variously interpreted in salinity on plant distribution in the Great Basin. terms of aridity tolerance and climate (Billings Moving out from the basin center is a gradient 1949) or an association with limestone-derived of decreasing soil salinity often correlated \vith calcareous soils (Beatley 1975), The latter progressively coarser-textured soils. Along this author points out that even on nonsaline soils gradient there are conspicuous species replace percent cover in the shadscale community is ments, often resulting_in concentric zones of lower than expected for the level of precipita contrasting vegetation (Flowers 1934, Vest tion, and argues that this indicates stress from 1962), In the lowest topographic zone, saline edaphic factors. Thus, shadscale disbibution is groundwater may be very near the surface. Soils most correlated with salinity tolerance in some are very saline, fine textured, and subject to regions and other edaphic and climatic toleran occasional flooding and anoxic conditions. In ces in other regions. this environment the combination of available Where the higher elevations of the Great moisture with other potentially stressful soil Basin come in contact with the lower-elevation, characteristics seems to be more important than generally drier, and warmer Mojave Desert climatic factors oftemperature or seasonal rain region, communities dominated by creosote faU pattems, Species found here, such as desert (Larrea tridentata) replace sagebrush commu saltgrass (Distichlis spicata), pickleweeds nities on nonsaline slopes and bajadas. 1992] PL\NT ADAPTATION 199
Shadscale can be found both on saline soils at cal, and phenological traits found in the domi very low elevations in the Mojave and as a tran nant shrubs reflect the primary importance of sitional band at high elevations between creo such a cool spring growing season. sote and sagebmsh. Elements ofthe cold desert shrub communities, adapted to continental\vin PHOTOSYNTHESlS tel'S and a cool spling growing season, can be found throughout the \vin~er-raiI1-dominated PHOTOSYNTHETIC PATHWAYS.-The pro Mojave Desert region as a high-elevation hand cess of photosynthesis in plants relies on the on arid mountain ranges. Theyalso extendto the acquisition ofCO from the atmosphere, which, southeast at high elevations into the strongly 2 when coupled \\'lth solar energy, is transformed bimodal precipitation regime of the C~)lorado into organic molecules to make sugars and pro Plateau, and northward at low elevations into vide for plant growth. In moist climates plant Idaho, Washington, and even British Columbia. communities often achieve closed canopies and Moving up from bajadas of the southern warm 100% cover ofthe ground surface. Under these deserts, there appears to be no suitable combi conchtions competition for light may be among nation of temperature and precipitation at any the most important plant-plant interactions. In elevation to support HOlistic elements of the the deserts total plant cover is much less than cold desert. As precipitation increases with alti 100%, and in the Great Basin closer to 25%. tude, zones with equivalent total precipitation Photosynthesis is not greatlylimited byavailable do not yet have cold \\rinters and are occupied light, but rather by water, mineral nutrients by warm desert shrub communities grading into needed to synthesize enzymes and maintain chaparral composed of unrelated taxa. Higher metabolism, and maximum canopy leaf-area elevations with cold winters have sufficient pre development. cipitation to support forests. Other elements Three biochemical pathways of photosyn common in shadscale and mixed-shrub commu thesis have been described in plants that differ nities ofthe Great Basin, such as winterfat and in the first chemical reactions associated with budsage (Artem.isia spinosa), are often found the capture of CO, from the atmosphere. The outside the Great Basin in cold-winter but most common and most fundamental of these largely summer-rainfall grasslands. pathways is referred to as the C3 pathway From these patterns ofcommunity distribu because the first product ofphotosynthesis is a tion within the Great Basin and Colorado Pla 3-carbon molecule. The other two pathways, C4 teau, and also from a consideration ofthe more and CAM, are basically modifications of the extensive ranges andaffinities ofthe component primary C 3 pathway (Osmond et al. 1982). The species, we can isolate a few key features ofthe C4 pathway (first product is a 4-carbon mole environment that are largely responsible for the 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 a process that results in reduced photosynthetic patterns ofsoil salinity and texture generated by rates in C3 plants. The C 4 pathway can confer a the overall aridity combinedwith eitherinternal much higber temperature optimum for photo drainage basins or the in situ weathering of synthesis and a greater water-use efficiency. As specific rock types. The most importantclimatic water-use efficiency is the ratio of photosyn variables are slightly more subtle. There is thetic carbon gain to transpirational water loss, clearly not a requirement for exclusively winter C4 plants have a metabolic advantage in hot, dry rainfall, but there is a requirement for at least a environmentswhen soil moisture is available. In substantial portion ofthe annual rainfall to come grasslands C4 grasses become dominant at low dming a continental v.linter. This permits winter elevations and low latitudes where annual tem accumulation ofprecipitation to a greater depth peratures are warmest. In interpreting plant in the soil profile than will occur in response to distrihution in deserts, the seasonal pattern of less predictable summer replenishment of rainfall usually restticts the periods of plant drying soil moisture reselVes. Under an overall growth, and the temperature during the rainy arid climate, winter recharge maintains a pre season is thus more importantthan mean annual dictably favorable and dominant spling growing temperature. The C4 pathwayis often associated season even in many areas of strongly bimodal with summer-active species and also with plants rainfall. Most of the physiological, morphologi- ofsaline soils. \Vhile C 3 grasses predominate in 200 GREAT BASIN NATURAUST [Volume 52
most of the Great Basin, C, grasses become temperatw'es (Ehleringer and Bjorkman 1978, increasingly important as summerrain increases Mooneyet al. 1978, Comstock and Ehleringer to the south, and especially on the Colorado 1984,1988, Eh!eringer 1985). This presumably Plateau. Halophytic plants are often C" such as reflects the specialization of these Great Basin saltbush (Atriplex spp.) and saltgrass (Distichlis shrubs towards utilization of the cool spring spp.), and this may give the plants a competitive growing season. Positive photosynthetic rates advantage from increased water-use efficiency are maintained even when leaf temperatures on saline soils. are nearfreezing, which permits photosynthetic The third photosynthetic pathway is CAM activity to begin in the very early spring (DePuit photosynthesis (Crassulacean Acid Metabolism), and Caldwell 1973, Caldwell 1985), CAM plants open theirstomata at night, capture Unusually high maximum photosynthetic CO, and store it as malate in the cell vacuole, rates of46ILmoi CO, m",-I have been reported and keep their stomata closed during the day for oneirrigated Great Basin shrub, rabbitbrush (Osmond et al. 1982). The CO, is then released (Chrysothamnus nauseosus) (Davis et al. 1985). from the vacuole and used for photosynthesis This species is also unusual in having a deep tap the follOwing day. Because thestomataare open root that oftentaps groundwater, unusually high only at night when it is cobl, water loss associ levels of summer leaf retention (Branson et al. ated with CAM photosynthesis is greatly 1976), and continued photosynthetic activity reduced. This pathway is often found in succu throughout the summer drought (Donovan and lents such as cacti and agave, and, although Ehleringer 1991). All of these characters sug CAM plantsare present io theGreat Basin, they gest greater photosynthetic activity during the are a relatively minor component ofthe vegetation. warm summer months than is found in most Photosynthetic rates ofplants, like most met Great Basin shrub species. abolic processes, show a strong temperature SHOOT ACTMTY AND PHENOLOGY.-Gener dependence, Usually, photosynthetic rates are ally speaking, there is a distinct drought in early reduced at low temperatures because of the summer (June-July) in the Great Basin Cold temperature dependence of enzyme-catalyzed Desert, the Mojave Desert, and the Sonoran reaction rates, increase with temperature until Desert, All of these deserts have a substantial some maximum value (which defines the "tem winter precipitation season, but they differ in peratureoptimum"), and then decrease again at the importance of the summer and early fall higher temperatures. The temperature optima rainy season (July-October) in supportinga dis and maximum photosynthetic rates of plants tinctive period of plant growth and activity show considerable variation, and they generally (MacMahon 1988). The relationship between reflect the growing conditions of their natural climate andplantgl'Owing season is complex and environments. includes total rainfall, seasonal distribution of PHOTOSYNTHETIC ADAPTATION,-ln the rainfall, and predictability ofrainfall in different spring the photosynthetic temperature optima seasons as important variables. Furthermore. in of the dominant shrub species are typically as very arid areas the seasonality of temperatures low as 15 C (40 F) (Caldwell 1985), correspond may be as impoltant as that of rainfall. In the ing to theprevailing environmental tempera Great Basin, cold winters allow the moisture tures (midday maxima generally less than 20 C). from low levels of precipitation to accumulate As ambient temperatures rise 10-15 C in the in the soil for spring use, while hot summer summer, there is an upward shift ofonly 5-10 C temperatures cause rapid evaporation from in the photosynthetic temperature optima of plants and soil. High temperatures can prevent most shruhs, coupled with a slower decline of wetting ofthesoil profile beyond a few centime photosynthesis at high temperatures. The max ters depth in response to summer rain, even imum photosynthetic rates measured in most when summer rain accounts for a large fraction Great Basin shrubs under either natural or irri oftheannual total (Caldwellet a1. 1977). As total gatedconditions range from 14 to 19 ILmol CO, annual rainfall decreases, the relative impor n1" S-1 (DePuit·and Caldwell 1975, Caldwell et tance of the cool spring growing season al. 1977, Evans 1990). These rates are quite increases as theonlypotential growing period in modest compared to the high maxima of 25 to which available soil moisture approaches the 45 ILmo! CO. m" ,-1 observed in many warm evaporative demand (Thomthwaite 1948, Com desert species adapted to rapid growth at higher stock and Ehleringer 1992). Finally, reliability 1992] PLANT ADAPTATION 201 of moisture is important to perennials, and as expansion or contraction of vegetative phases average total precipitation decreases, the vari and even the omission of reproductive phases. ance between years increases (EWeringer Most species initiate growth in early spring 1985); variability of annual precipitation is dis (March) when maximum daytime temperatures cussed in more detail later in the section on are 5-15 C and while nighttime temperatures annuals and life-history diversity. Summer rain are still freezing. Delayed initiation of spring is more likely to be concentrated in a few high growth is generally associated with greater intensity storms that may not happen every year summer activity and may be related to an ever at a given site and may cause more runoffwhen green habit, a phreatophytic habit, or occupa they do occur. The ability of moisture from tion of habitats with greater summer moisture winter rain to accumulate over several months availability from regional rainfall patterns, greatly enhances its reliability as a moisture runoff, or groundwater. Higher-than-average resource. Thus, most plants in the Great Basin winter and spring precipitation tends to prolong have their primary growing season in the spring vegetative growth and delay reproductive and early summer. Some species achieve an growth till laterin thesummer(Sauerand Uresk evergreen canopy by rooting deeply, but few 1976, Cambell and Harris 1977). Strongvegeta species occur that specialize on growth in the tive dormatlCY may be displayed in mid-summer hot summer season (Branson et aI. 1976, Cald (Everett et aI. 1980, Evans 1990), although root well et aJ. 1977, Everett et aI. 1980). Ehleringer growth (Hodgkinson et aI. 1978) and increased et aI. (1991) measured the ability of common reproduction (West and Gastro 1978, Evans, perennial species in theColorado Plateau to use Black, atld Link 1991) may still occur in moisture from summer convection storms. response to rain at that time. in years with They obselved that less than half of the water below-average spring and summer precipita uptake by woody perennial species was from tion, leaf senescence is accelerated and flower surface soil layers saturated by summer rains. ing may not occur in many species. Prevalence of summer-active species increases The time taken to complete the full annual along the border between higher basins and the growth cycle including both vegetative and southeast Mojave Desert where summer rain is reproductive stages is strongly related to rooting more extensive, and especially on the Colorado depth in most ofthese communities, with deep Plateau where summer rain is greatest. Summer rooted species prolonging activity further into temperatures are also lower on the Colorado the summer drought (Pitt and Wikeem 1990). Plateau than in the eastern Mojave (Table 1), Theexact timingoffloweringand fruit setshows allowing more effective use ofthe moisture. considerable variation among different shrub Most phenology studies, especially from the species. Some, especially those that are more northern areas, emphasize the directionaL drought-deciduous, Ilower in late spring and sequential nature of the phenological phases early summer ju~t prior to or concurrent \vith (Branson et aI. 1976, Sauer and Uresk 1976, the onset of summer drought. Many evergreen Cambell and Harris 1977, West and Gastro shrub species begin Ilowering in midsummer 1978, Pitt and Wikeem 1990). A single periodof (Artemisia) or in the fall (Gutierrezia and spring vegetative growth is usually followed by Chrysothamnus). These late-Ilowering species a single period of flowering and reproductive generally do not appear to utilize stored reserves growth. Many species produce a distinct cohort for Ilowering, but rely on current photosynthe ofephemeral spring leaves and a later cohort of sis during this least favorable period. In the case evergreenleaves (Daubenmire 1975, Millerand of Artemisia tridentata, it has been shown that Schultz 1987). For most species, multiple carbohydrates used to fill fruits are derived growth episodes associated with intermittent exclusively from the inflorescences themselves, spring and summer rainfall events do not occur. while photosynthate from vegetative branches In yeat~ with unusually heavy August and Sep presumably continues to support root growth. tember rains, a distinct second periodofvegeta Summer rain during this time period does not tive growth may occur in some species (West promote vegetative shoot growth but does and Gastro 1978). Climatic variations from year increase water use by and the ultimate size of to year do not change the primary importance inllorescences (Evans 1990). Evans, Black, and of spring growth or the order of phenological Link (1991) have argued that this pattern of events. In particular years, they may cause Ilowering, based on residual deep soil moisture 202 CHEAT BASIN NATUHALISr [Volume 52
and the unreliable summer rains, may contrib 1980). Fall and winter precipitation is the most ute to competitive dominance within these important in promoting good spring growth of communities. The more favorable and much perennials (Beatley 1974). Comstock et al. more reliable spring growing sea'iOn is used for (1988), looking at one year's growth in 19 vegetative growth and competitive exploitation Mojave species, described an important cohort ofthe soil volume, while reproductive growth is of twigs initiated during the winter period that delayed until the less favorable season, and may accounted for most vegetative growth during be successful only in years with adequate the following spring. Although new leaves were summer precipitation. produced in response to summer rain, summer Most grasses in the northern part of the growth in many of the species was largely a
Creat Basin utilize the C3 pathway and begin further ramification of spring-initiated floral growth very early in the spring. These species branches. In most species summergrowth made complete fruit maturation by early or mid little contribution to perennial stems. Despite summer, often becoming at least partially dor the preferredwinter-spring orientation ofmany mant thereafter. On the Colorado Plateau, with shrubs, winter recharge is much less effective much greater amounts ofsummerprecipitation, and reliable in the Mojave Desert than in the there is a large increase in species number and Great Basin. Due to warmer temperatures, cover by C4 grasses such as bluestern winter dormancy may be less complete, but (Andropogon) and grama (Boutelima), espe vigorous growth rarely occurs until tempera cially at warmer, lower elevations and on deep tures rise further in the early spring. Rapid sandy soils. Many of these species occur in growth may be triggered by rising spring tem
mixed stands with the C3 species but delay ini peratures or may be delayed until major spring tiation ofgrowth until Mayor June; they can be rains provide sufficient moisture (Beatley 1974, considered summer-active rather than spring Ackerman et aI. 1980). Furthermore, a shal lower soil moisture recharge often results in active. In contrast, some C4 grasses such as sand dropseed (Sporoholus cryptaoorus), galleta fluctuating plant water status and multiple grass (Hilmiajamesiii), and three-awn (Aristirla episodes of growth and flowering during the purpurea) are widespread in the Great Basin spring and early fall. Some Great Basin species where summer rain is only moderate in long that also occur in the Mojave, such as winterfat and shadscale, commonlyshow multiple growth term averages and VC1Y inconsistent year to year. Spring growth of these widespread species and reproductive episodes per year under that tends to be slightly or moderately delayed com climate (Ackerman et aI. 1980) but not in the pared to co-occurring G, grasses, but they are Creat Basin (West and Castro 1978). The still able to complete all phenological stages degree to which this difference is due entirely based on the spring moisture recharge. They to environmental differences as opposed to eco typic differentiation does not appear to have show a greater ability than the C3 species to respond to late spring and summer rain with been studied. renewed growth (Everett et al. 1980), however, which compensates in some years for their latcr WATER RELATIONS development. Other C4 grasses in the Great Basin may be associated with seeps, ADAPTATION TO LIMITED WATER.-Stoma streamsides, or salt-marshes, and show a tal pores provide the pathway by which atmo summer activity pattern. C4 shnths such as salt spheric CO, diffnses into the leafreplacing the bush (Atriplex) show similar, spIing-active CO2 incorporated into sugar molecules during growth patterns to the C3 shrubs, but may show photosynthesis. Because water vapor is present slightly greater tolerance of summer drought at very high concentrations inside the leaf, (Caldwell et al. 1977). opening stomata to capture COz inevitably Phenology in the Mojave Desert shows both results in transpirational water loss from the similarities and strong contrasts to the Great plant; thus, leaf water content is decreased, Basin. Although rainfall is largelybimodal in the resulting in a decrease in plant water potential eastern Mojave, absolute amounts are very low. (\V). Thus, plant water status, transpiration, and The summer is so hot that little growth normally acquisition of water from the soils have a tre occurs at that time unless more than 25 mm (J mendous impact on photosynthetic rates and inch) comes in a single storm (Ackerman et al. overall plant growth. 1992] PLAI\iT AOAPTATIOI\i 203
Many soils in the Great Basin are flne tex much of the evidence for this is quite indirect. tured, which has both advantages and disadvan Nonetheless, the osmotic potential of the cyto tages for plant growth. Infiltration of water is plasm mnst also be balanced or suffer dehydra slower in fine-textured soils, increasing the like tion. The cytoplasmic solutes must have the lihood of funoff and reduced spring recharge, special property oflowering the osmotic poten especially on steeper slopes. They are also more tial of tbe cell sap without disrupting enzyme prone to water-logging and anoxic conditions. function or cellular metabolism, and are hence The deep root systems ofGreat Basin shrubs are termed "compatible" solutes (Wyn Jones and very sensitive to anoxia, and this can be a strong Gorham 1986), The use of specific molecules determining factor in species distributions shows considerable variation across species (Lunt et 'II. 1973, Groeneveld and Crowley apparently due to both speCies-specific varia 1988). Unusually wet years may even cause root tions in cell metabolism and taxonomic relation dieback, especially at depth. Once water enters ships. Some frequently encountered molecules the soil profile, the extremely high surface areas thought to function in this manner include of flne-textured soils with high clay and silt amino acids such as proline and also special content give them a much higherwater-holding sugar-alcohols. Some plants appear to generate capacity than that found in sandy, coarse-tex low osmotic potentials by actively manufactur tured soils, Much ofthis water is tightly bound ing larger quantities ofdissolved organic mole to the enormous surface area of the small cules per cell in response to water stress, a pmticles, however, and is released only at very process referred to as "osmotic adjustment." negative matric potentials. Plants must be able This process may be costly, however, requiring to tolerate low tissue water pote~tials to make the investment of large amounts of materials use ofit. (new solutes) at a time when water stress is As soil water is depleted during summer, largely inhibiting photosynthetic activity, An plants reduce water consumption byclosing sto alternative method seems to involve dramatic mata (DePuit and Caldwell 1975, Cambell and changes in cell water volume. Several deselt Harris 1977, Caldwell 1985, Miller 1988) and species have been observed to reduce cell water reducing total canopy leaf area to a minimum volume by as much as 80% withoutwilting when (Branson et aI. 1976). Evergreen species shed subjected to extremely low soil water potentials only a portion of the total canopy, however, (Moore et aL 1972, Meinzer et aL 1988, Evans maintaining the youngest leafcohorts through eta1. 1991), This allowed the leaf cells to have out the drought (Miller and Schulz 1987), sufBciently low osmotic potentials for water Although physiological activity is greatly uptake even though solute content per cell was reduced by water stress, evergreens such as actually reduced, Although total solntes perleaf sagebrush can still have positive photosynthetic (and presumably per cell) decreased, the rela rates at leafwater potentials as low as -50 bars tive concentrations of specific solutes changed (Evans 1990) and may snrvive even greater (Evans et aL 1991) such that "compatible" levels of stress. In contrast, crop plants can solutes made larger contributions to the osmotic rarely survive prolonged'I'ofless than -15 bars. potential under stress. Thus, tolerance of low Remaining functional at low water potentials leaf water potentials was achieved by a combi requires the concentration ofsolutes in the cell nation of anatomical and physiological special sap, and this appears to be accomplished by izations. The anatomical mechanisms involved several mechanisms. In many mesic species, in this magnitude ofvolume reduction and the increases in organic solutes may account for implied cell elasticity have not been studied, but most ofthechange in osmotic potential. In other the process has been shown to be fully reversible, species, and espeCially those that experience Soil texture is also an important factor in very low leafwater potentials, a large fraction of determining the ability of plant communities in the solutes is acquired by the uptake of inor a cold-winter climate to respond to summer ganic ions such as K+ (Wyn Jones and Gorham rain. In areas with moderate levels ofprecipita 1986). High concentrations of inorganic ions tion, sandy soils, because of their low water may be toxic to enzyn1e metabolism, however, holding capacity, usually hold a sparser, more and they are Widely thought to be sequestered drought-adapted vegetation than Hner-textured largely in the central vacuole, which accounts ones. In warm, arid areas, however, what has for 90% of the total cell volume, even though been called the "reverse texture" effect results 204 GREAT BASIN NATURALIST [Volume 52 in the more lush vegetation occurring in the well et aI. 1977, Sturges 1977) of the total root cuarse-textured soils. This occurs because the biomass. The vel)' large annual root invest high water-holding capacity of fine-textured ments, therefore, are not primarily related to soils allows them to hold all the moisture large storage compartments, but to the turnover derived from a single rainfall event in the upper of fine roots and root respiration necessary for most layers ofthe soil profile, where it is highly the acquisition ofwater and mineral nutrients. subject to direct evaporation from the soil. The The fine root network thoroughly permeates same amount of rainfall entering a sandy soil, the soil volume. Root densities are generally precisely because of its low wateIcholding quite high throughout the upper 0.5 meters of capacity, will penetrate to a much greaterdepth. the profile, but depth of maximum root devel It is also less likely to return to the dl)'ing surface opment varies with depth of spIing soil-mois by capillaly action. Less of the rain will evapo ture recharge, species, and lateral distance from rate from the soil surface, and a greater fraction the trunk or crown. A particularly high density ofit will await extmction and use loy plants. This in the first 0.1 m may often occur, especially inverse-texture effect fUlther restricts the effec immediately under the shrub canopy (Branson tiveness ofsummer rains in the fine soils ofthe 1976, Caldwell et al. 1977, Dohrowolski et al. Great Basin. The effect is less operative in 1990). Alternatively, maximal density may occur respect to winter precipitation in the Great at depths between 0.2 m and 0.5 m (Sturges Basin, however, hecause ofthe gmdual recharge 1980), andsometimes a secondwne ofhigh root of the soil profile to considerahle depth under density is reported at depths of 0.8 m conditions where surface evaporation is mini (Dauhenmire 1975) to 1.5 m (Reynolds and mized by cold temperatures. The comhination Fraley 1989). Regardless ofthe preci,edepth of of much sandier soils and greater amounts of maximum rooting, shrub root density is usually summer rainfall in the region of the Coloradn high throughout the upper 0.5 m and then Plateau is largely responsible for the major flo tapers off, hut may continue at reduced densi listie and ecological differences between the ties to much greater depth. The radius onateral two regions. At lower elevations on the sonth spread is usually much greater lor roots (1-2 m) east edge of the plateau, shrub-dominated than for canopies (0.1-0.5 m). Percent plant desert scrub may be replaced by grassland dom cover aboveground is often in the neighborhood inated by a mix ofspring-active C3 and summer of25% with 75% bareground, butbelowground active C4 species. the interspaces are IUled with roots throughout ROOTING DI>:PTH. MORPHOLOGY. AND PHE the profile, and root systems of adjacent plants NOLOGY.-One of the onique and ecolo;,
their tissues when water is available in the sur OPPORTUNISTIC DHOUGHT-DECIDUOUS I face soil layers, and (4) use their stored water in MULTIPLE LEAF-FLUSHING SPECIEs.-This photosynthesis with unparalleled water-use effi habit, like that of the suc'Culents, is favored by ciency by opening their stomata only at night (1) intermittent rainfall wetting only shallower when temperatures are cool (Nobel 1988). They soH layers, and (2) warm temperatures allowing are favored by (1) very warm days (30-40 C), for rapid leaf expansion in response to renewed which allow them to have higher photosynthetic soil moisture. Again, these requirements are not rates and cause competing species to have very well met in the Great Basin. The primary mois low water-use efficiencies; (2) large diurnal tem ture resource is a single, deep recharge in the perature fluctuations allowing for cool nights winter. Most shrub species are deep rooted, and (10-20 C) which allow them to have high rates rather than experiencing vacillating water avail of CO2 uptake with high water-use efficiency; ability, they have active root growth shifting to and (3) intermittent rainfall, which onlywets the deeperand deeper soil layers during the season, upper soil layers so that the limitations oftheir thus producing a gradual and continuous shallow roots and water-hoarding strategy are change in plant water status. This allows many compensatedfor by the ephemeral nature ofthe spring-active shrubs to remain partially ever soil water resource. These conditions arc some green throughout the summer, and, in regions what poorly met in the cold desert. The impor where it occurs, they are able to make rapid use tant water resource is one ofdeep soil recharge ofany moisture available from summer precip that favors deep-rooted species and confers itation without the need for renewed leaf pro muchless advantage oninternalwater hoarding. duction. Tbe ouly shrub reported to have Freezing tolerance in CAM succulents appears multiple leaf flushes in response to late spring to be associated with, low tissue water conterits, or summer rain in the Great Basin is the dimin utive Artemisia spinescens and this may inhibit uptake ofwater when it is and shallow-rooted (Everett et al..] 980). Some species found in the plentiful in the surface layers in the thermally Great Basin are reported to have multiple vacillating early spring (Littlejohn and Williams growth cycles/year where they occur i.n the 1983). Furthermore, water-use effIciencies of Mojave (Ackerman et aJ. 1980). C and C species are quite high in the cool 3 4 ANNUALS AND LIFE-HISTORY DIVERSITY. spnng. The spectacular wildflower shows (lisplayed in Nonetheless, even moderate amounts of favorable years in the Mojave Desert do not summer rain in the southern and eastenl por occur in the cold desert of the Great Basin tions of the Great Basin result in numerous (Ludwig et aJ. 1988). Anmwl species are few in species of cacti. Due to the open nature of the number, and, except in early succession after understory, many of these species have a large fire in woodlands oronvery disturbedsites, they elevational range, and they are often more rarely constitute a major fraction of total eom common in the pinyon-juniperoreven the mon munity biomass. This is undoubtedly related to tane zone than on the desert piedmont slopes. several complex factors, but various aspects of Almost all of these cacti arc small, usually 5-20 precipitation patterns are likely to he among the em high, with a small, globose (e.g., Pediocactus most important. To begin with, the pancity of simpsonii), prostrate (e.g., Opuntia poly summer rain in some parts of the Great Basin canthal, or low, caespitose habit (e.g., may largely eliminate an entire class of C, Echinocereus triglochidiai'us). This allows them summer annuals importantin the floras ofother to take advantage of the warmer daytime tem regions including the Colorado Plateau. Other peratures near the ground in the spring and aspects than seasonality are also crucial, how facilitates an insulating snowcover during the ever. Very low means ofannual precipitation are coldest winter periods. The numberofand total commonly associated with large annual floras, cover by cacti increase considerably with but eorrelated with low mean precipitation is increased summer rainfall on the Cc)lorado Pla high year-to-year variation in preeipitation teau, but only in the eastern Mojave with both which some authors have argued is equally summer rain and warm spring temperatures do' important. The coefficient of variation (CV) in wefind the l~rger barrel-cactus (e.g., Ferocactus precipitation shows a relationship to mean pre acanthoides) and tall, shrubby chollas (e.g., cipitation in the Great Basin and Colorado PIa Grunt;.a acanthocarpa). tecH! (Fig. 2) very similar to that found in warm 1992) PLANT ADAPTATION 211
100 ,-~--~-~--~-~-/--, ~ ~ 0 0.6 § 80 BS '":J c= iii '"0 ...... 0.5 o u '"~ 80 ~ LJ >'" ~ -O.4~,. 0 o '0 0 0 .c: ...... ~0° 0 0 '" c: 0° 08 ~G) 15 '0 ~ 0 (l)U 0.3 lb0'o~ 6'Q-6...;O-':O~"~~"'~-: • 0 ~J 0 ~ __ _ 0 0 0 ~oo 0 20 Q) 0,2 0 0 '" o 0 :J c: o '"c: 0.1 .. o'--~---u>-'----~-~-~---' o 0.1 0.2 0.3 0'< 0.5 0.6 0.0 '--~-~-~~-~-~~~---' CV for mean annual 100 200 300 400 500 precipitation Mean annual precipitation, mm Fig. 3. l1lfl relationship hetwL--C1l reliability of annual Fig. Z. The relationship hetv.'c..'Cll mean predpi!:ation and prl",cipitation nnd lift:-history strategy ofherhuceous plllnl,~. the v;lriability ofrainfall betvo,Ieen years as measured by the The site with greatest representation of anonal:-; is Death coefIicient of variation in annual preeipitation. The data Valley in the Mojave Desert, the second highest is Canyoll include points s<:attcrcd throughout the Creat Basin in Utah hUlds in the Colorado Plateau orsontheastcrn Utuh, and the and Newda and the O>lorOOo Plntf'...au in Utah and Arizona. other three sites are Great 13n...in Cold Desert Uf shruh The line !:ihown is the It;!asl squares best fit for the d:1.ta; LV steppe (dal.:.l we~'collccted hy Kim Ha~r and previuusly = 1.27 - 0.403 • log(mean annual prt'.cipitation, mill) (n = published in Schalfer and Gadg;l 1975). 69 sites, r < .001). Arizona monsoon, and the region where the deserts (Ehlminger 1985). Althou~h mean pre fraction of summer rain increases substantially cipitation has the greatest single eHect, there moving southward. This zone also has some of are, additionally, important geographic influ the most arid sites of the entire region located ences on the CV of precipitation which are along the transition to the Mojave Desert in independent of mean precipitation. A multiple southern Nevada and the canyon country of regression of the CV of precipitation on southe~L'itern Utah, and these sites can be log(mean annual precipitation), latitude, and expected to have the highest variability due to elevation in the Great Basin has an r" of .81 ,md both low mean rainfall amI geo~mphie position indicates that each vdriable in the model is correlated with regional weather patterns. highly significant (p < .001 or better), For a Because the Great Basin and Colorado Plateau given mean precipitation, the CV increases with are only semiarid, the CV of annual precipita decre.:lsing altitude in the Great Basin, but an tion is not usually a" high as in many ofthe more independent effect ofelevation wa" not signifi arid warm de.serts (Beatley 1975, Ehlerin~er cant in the Colorado Plateau, The CV also 1985), but particular sites may he both arid and increases from north to south in the Great Ba~;n highly unpredictahle. and increases from south to north in the Colo Harper (cited in SchafTer 'md Gadgil1975) rado Plateau, which results in a latitudinal band found that the prevalence of annuals was posi of gre,..,test annual variability running through tively associated with the CV in annual precipi southern Nevada and Utah. This hand is related tation for five sites located in the Great Basin, to two major cl"ipects ofregional climate. Moving Colorado Plateau, and Mojave Desert (Fig. 3). southward in the Great Basin, temperat.ures The lar~est annual populations occurred in gradually increase, favoring moister air masses Death Valley (Mojave), followed hy Canyon and more intense storms, but sites are more lands (Colorado Plateau in southeastern Utah). removed from the most common winter storm One interpretation of this relationship is that tracks, and thc number.of rainy days per year high V'ariability in total precipitation between decrease" (Hou~hton19(9), Movin~nOlthward years may be associated with high rates ofmor from Arizona and New Mexico, the southenl tality and therefore favor early reproductionand Nevada and Utah band ofhi~hestprecipitation an annual habit (Schalkr and Gadgil ] 975). variability also corresponds to the northernmost Many desert annuals are facultatively perennial extent of summer storms associated with the in better-than.average years, and some have 212 GREAT BASIN NATURALIST [Volume 52
perennial races or sister species (Ehleringer between short- and long-lived perennials may 1985). The dynamics and distributions ofthese be influenced byvery similar climatic parameters, closely related annual and perennialtaxa should sometimes operating over different time scales. receive further study in regard to theirexpected Other factors that may be important in the life span, reproductive output, and relationships ecology of Great Basin annuals include the to climatic predictability. Another perspective is effects of the very well developed cryptogam to ask how competition between very distinct soil crusts or vesicular hOl1zons on seed preda shrub and annual species is affected by precip tion (ability of seeds to find safe sites), seed itation variability. While in many respects com germination, and seedling establishment. The plementary with the optimal life history restriction of winter growth by cold tempera arguments, this approach emphasizes how large tures could also be ofcrucial importance, inhib differences in habit affect resource capture and iting the prolonged establishment period competition rather than focusing on subtler dif enjoyed bywinter annuals in warm deserts. Fall ferences in mortality and reproductive sched germination followed bylow levels ofphotosyn ules. The lower variability of precipitation in thesis throughout the mild winteris essential for much of the Great Basin compared to the vigorous spring growth ofwinter annuals in the Mojave and Sonoran deserts, as well as the more Mojave, and, while heavy spring rains may cause reliable accumulation of moisture during the germination, such late cohorts rarely reach winter-recharge season, may favor both stable maturity (Beatley 1974). Annuals are common demographic patterns and growthofperennials. in transition zone sites of the ecotone between Annuals tend to be shallow rooted (most roots Mojave Desert and Great Basin plant commu in upper 0.1 m depth), and they are poorly nities in southern Nevada, but associated with equipped to compete with shrubs for deep soil changes in perennial species composition along moisture. If shrub density is high, and years of decreasing mean temperature gradients in that unusually high mortality are rare, then shrubs region are decreases in annual abundance may largely preempt the critical water and min (Beatley 1975). eral resources and suppress growth of annuals. Thedominant shrubs ofthewarm deserts do not LITERATURE CITED have high root densities in the upper 10 em of the soil profile (Wallace et aJ. 1980), have lower ACKEHMAN, T. L., E. M. ROl.-1NEY, A. WALLACE, and J. E. total root densities, and have lower total cover KINNEAll.. 1980. Phenology of desert shrubs in south when compared with Great Basin perennials. ern Nye County, Nevada. Great Basin Naturalist Mem~ airs 4: 4-23. Annuals are therefore likely to experience more BAMBEHG, S. A., A. WALLACE, E. M. Ro~1;'.JEY, and R. E. intense competition from shrubs in the Great HUNTEH. 1980. Further attributes of the perennial Basin. This conjecture is further supported by vegetation in the Rock Valley area of the northern consideling that perennials in the Great Basin Mojave Desert. Great Basin Naturalist Memoirs 4: 37-39. generally transpire 50% or more of the annual BEATLEY, J. C. 1974. Phenological events and their environ moisture input over a wide range ofyearly vari mental triggers in Mojave Desert ecosystems. Ecology ations. In the Mojave this fraction may average 55,856-863. only 27% and vary between years from 15 to ---co' 1975. Climates and vegetation patterns across the Mojave/Great Basin Desert transition of southern 50% at the same site (Lane et a1. 1984), or even Nevada. American Midland Naturalist 93: 53-70. be as low as 7% (Sammis and Gay 1979). The BENNEi'll', H., and B. ScHMIDT. 1984. On the osmoregula reduced overlap in rooting profiles and the tion in Atriplex hymenelytra (Torr.) Wats. greater availability of unused moisture (Chenopodiaceae). Oecologia 62: 80--84, BILLIKGS, W. D. 1949. The shadscale vegetation zone of resources may have favored the development of Nevada and eastern California in relation to climate annual floras in the Mojave Desert more than in and soils. American Midland Naturalist 72: 87-109. the Great Basin. With severe disturbance from BoWEHS, J. E. 1982. The plant ecology of inland dunes in grazing and other anthropogenic activities, western North America. Journal ofArid Environments 5, 199-220. exotic annual species have invaded many Great BHANSON. F. A., G. F. GIFFOHD, K. G. RENARD, and R. F. Basin communities. Once established foll0\0ng HADLEY. 1981. Rangeland hydrology. Kendall/Hunt disturbance, these annuals are not always easily Publishing, Dubuque, Iowa, displaced by short-term shrub succession. While BHAKSON, F. A., R. F. MILLEH, and I. $. McQGEEN. 1976. Moisture relationships in twelve northern desert shrub this discussion has been presented in the con communities near Grand Junction, Colorado. Ecology text of annuals versus perennials, tradeoffs 57,1104-1124. 1992] PLANT ADAYfATION 213
BRIENS. M., and F. URHER. 1982. Osmoregulation in halo and Range. Ecological Studies SO. Springer-Verlag, phytic higher plants: a oomparntive study of soluble Berlin. carbohrdrates, polyoLs, betaines and free proline. Plant DEPUIT, E. J., and M. M. CALDWELL. 1973. Seasonal pat Cell and Environment 5: 287-292. tern of net photosynthesis in Artemisia tridentata. BnQWK A. D. 1982. Halophytic prokmyotes. Pages 137 American Journal of Botany 60; 426-435. 162 in O. L. Lange, P. S. Nobel, C. B. Osmond, H. --c-' 1915. Gas exchange ofthree cool semi-desert spe Ziegler, eds.. Encyclopedia of plant physiology. New cies in relation to temperature andwater stress. Journal series. Vol.l2C. Springer-Verlag, New York. nf Ecolngy 63,835-858. CALDWELL, M. M. 1974. Physiology of desert halophytes. DF;TLING, J. K. 1969. Photosynthetic and respimtOlY Pages 35~17 in R. J. Reimold and w. H. Queen, eds., response of several halophytes to moisture stress. Ecology ofhalophytes. Academic Press, New York Unpublished doctoral dissertation., University of Utah, _~,," 1976. Root extension and water absorption. Pages Salt Lake City. 63-S5 in O. L Lange, L. Kappen, E. D. Sc&u!ze. Water DoNOVAN, L. A., andJ. R. EHLERINGER.1991. Ecophysio and plant Me. Ecological Studies. Analyses and Synthe logical differences among juvenile and reproductive sis. Vol. 19. Springer-Verlag. New York. plants ofseveralwnodyspecies. Oecolngia 86, 594-097. . 1985. Cold desert. Pages 198--212 in B. F. Chabot DOBIIOWOLSKL. J. P., M. M. CALDWELL. !U'ld J. H. RICH --and H. A. Mooney, eds., Physiologi.ca1 ecology ofNorth AliOS. 1990. Basin hydrology and plant root systems. American plant communities. Chapman and Hall Ltd., Pages 243-292 in C. B. Osmond, L. F. Pitelka, and London. G, M Hidy, eds., Plant biology ofthe Basin nnd Range. CALDWELL, M. M.> and J. H. RICHAHDS, 1989. Hydraulic Ecological Studies 80. Springer-Verlag, Berlin. lift: water efflux from upper roots improves effective EHLERINCER, J. R.l985. Annuals and perennials ofwarm ness ofwater uptake by deep roots. Oecologia 19: 1-5. deserts. Pages 162-180 in B. F. Chabot and H. A. CALDWELL. M. M., J. H. RiCHARDS> J. H. MANWARING. Mooney, eds., PhysiologicnJ ecology of North American and D. M. EISENSTAT. 1987. Rapid shifts in phosphate plant communities. Chapman and Hall Ltd., London. acqu.isition show direct competition between neigh EHLKl\INGF.R. J. R., and O. BJORKMAN. 1978. Acomparison boring plants. Nature 327; 615-616. of photosynthetic characteristics of Encelia species o..LDWELL, M. M.> R. S. WHITE, R T. MOORE, and L. B. possessing glabrous and pubescent leaves. Plant Phys G\J.4P. 1977. Carbon balance, productivity• .and water inlngy 62, 185-190. use of cold-winter desert shrub communities domi EHLEIIINCKR, J. R., S. L PHIlLIPS, W. S. F. SCllU~TF.R, .and nated by C3 and C.speeies. Oecolngia 2R 275-300. D. R. SANDQUISr. 1991. Differential utilization of CAMBELL. G. S., and G. A. HA.f\I\IS. 1977. Water relations summer rains by desert pl!U'lts. Oecologia 88: 430--434. and water use patterns for Artemisia mdentata Nutt. EISSENSTAT, D. M., and M. M. CALDWELL. 1988. Compet in wet and dry years. Ecology 58: 652-658. itive ability is linked to rates of water extraction. CLINE, J. F., D. W. UHESK, D. W. RrcHAnD, and w. H. OecoJogia 75: 1-7. RICHARD. 1971. Comparison of soil water used by a EvANS, R. D. 1990. Drought tolerance mechanisms and sagebrush-bunchgrass and a cheatgrass oommunity. resource allocation in Artemisia tridentata Nutt. ssp. Journal of Range Management 30; 199-201. tridentata. Unpublished doctoral dissertation, Wash O>MSTOCK. J. P., T. A. CoOPEIl, and J. R. EHLEIHNGER 1988. Se8.SQnal patterns of canopy development and ington State University, Pullman. carbon gain in nineteen warm desert shrub species. :EvANS. R. D., R. A. BLACK. and S. O. LINK. 1991. 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L., andK. WARD. 1984. Early plunt succession 7747-7751. on pinyon-juniper control bums. Northwest Science DAVIS, S. D., A. PAUL. and L. MALLAHE. 1990. Differential 58,57-58. resistance to water-stress induced. embolism between FF.RNANDEZ, O. A., and M. M. o..LDWF,I,L. 1975. Phenol two species of chaparral shrubs: Rhus laurina and ogy and dynamics of root growth of three cool semi~ Ceanothus megacarpm. BuUetin of the Ecological desert shrubs under field conditions. Journal of Society ofAmerica, Program and Abstracts. Ecology 63, 703-714. DAVIS, T M., N. SANKHLA, W. R. ANDERSEN. D. J. WEBER, FLOWE ItS, S. 1934. Vegetation oftbeGreat Salt Lake region. and B. N. SMlTH. 1985. High rates ofphorosyntbesisin Botanical Gazette 95: 353--418. the desert shrub ChrysothatrlJ1,1Js nauseosus ssp. FLOWEHS, T. J., P. F. n\OKE, and A. R. YRO, 1977. The alhicaulis. Great Basin Naturalist 45: 520-521. mechanism of salt tolerance in halophytes. Annual DAUBENMIHE, R. 1975. 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