Microclimate, freezing tolerance, and cold acclimation along an elevation gradient for seedlings of the Great Basin Desert shrub, Artemisia tridentata

Journal of Arid Environments (2003) 54: 769–782doi:10.1006/jare.2002.1106, available online at http://www.sciencedirect.com

Microclimate, freezing tolerance, and cold acclimationalong an elevation gradient for seedlings of the Great

Basin Desert shrub, Artemisia tridentata

Michael E. Loik%* & Sean P. Redarw

%Department of Environmental Studies, University of California,Santa Cruz, CA, 95064, U.S.A.

wUnited States Forest Service, Fawnskin, CA, 92333, U.S.A.

Vegetation, microclimate, seedling frequency, freezing tolerance, and coldacclimation were compared for seedlings of Artemisia tridentata collectedfrom 1775, 2175, and 2575 m elevation in the eastern Sierra Nevada,California. Data were used to test the hypothesis that ecotypic differences instress physiology are important for seedling survival along gradients fromdesert to montane ecosystems. The vegetation canopy cover and A. tridentataseedling frequency were greatest at 2575 m, compared to 1775 and 2175 m.Snow cover ameliorated temperatures near the soil surface for part of thewinter and depth varied across elevations. Freezing tolerance was comparedfor seedlings maintained in growth chambers at day/night air temperatures of251C/151C. The temperature at which electrolyte leakage and Photosystem IIfunction (FV/FM) from leaves were half-maximum was approximately�13?51C for leaves of seedlings from all three elevations. Shifting day/nightair temperatures from 251C/151C to 151C/51C initiated about 1?51 ofacclimation by plants from all three altitudes, with seedlings from the highestelevation exhibiting the greatest acclimation change. Measurements ofambient air and canopy temperatures at the three elevations indicated thatwintertime average low temperatures were consistent with the measureddegree of freezing tolerance. At small spatial scales used in this study, pollenand seed dispersal between study sites may have precluded resolution ofecotypic differences. Patterns of freezing tolerance and cold acclimation maydepend on a combination of mesoclimate and microclimate temperatures,canopy cover, snow depth, and snow melt patterns.

# 2003 Elsevier Science Ltd.

Keywords: air temperature; electrolyte leakage; frost; sagebrush; stress

Introduction

Cold is considered one of the most important factors limiting the productivity ofplants (Levitt, 1980; Larcher, 1995). Many species are geographically limited by sub-zero temperatures that affect key physiological processes (Levitt, 1980; Nobel, 1988;Loik & Nobel, 1993a). Episodic sub-zero air temperatures (i.e. ‘cold snaps’) are

*Corresponding author. Fax: +1 831 459-4015. E-mail: [email protected]

0140-1963/03/040769 + 14 $30.00/0 # 2003 Elsevier Science Ltd.

770 M. E. LOIK & S. P. REDAR

common features of the abiotic environment; it is estimated that over half of theEarth’s land mass has a mean minimum temperature below 01C for some time of theyear (Sakai & Larcher, 1987). In cold desert ecosystems sub-zero temperatures canoccur near the soil surface based on unstirred boundary layers immediately above thesoil surface, radiation of infrared radiation from the surface to a cold nighttime sky,and catabatic cold air drainage (Nobel, 1997). For example, air temperatures 1 cmabove the soil surface can be 71 lower than those at 1 m, but dependent on canopycharacteristics (Geiger, 1965; Nobel, 1997). Additionally, the temperature near thesoil surface exhibits much greater diurnal changes than the mixed air above (Nobel,1997).

Extreme temperatures can be particularly important for seedling survival andestablishment in arid environments. For many ecosystems in the western UnitedStates, plant recruitment is limited by the ability of seedlings to survive in the abioticenvironment near the soil surface (Franco & Nobel, 1988; Smith & Nowak, 1990).Episodic events of extreme sub-zero air temperatures can contribute to high rates ofseedling mortality (Jordan & Nobel, 1979; Fenner, 1985; Sakai & Larcher, 1987).Because seedlings of many species are less tolerant of extreme environmentalconditions compared to adults (Smith & Nowak, 1990; Davis & Zabinski, 1992), thedegree of abiotic stress tolerance can determine successful establishment and therebylimit species distributions to certain regions or microsites. For example, theelevational and latitudinal distribution of saguaro (Carnegiea gigantea) is determinedby periods of episodic sub-zero air temperatures (Shreve, 1911; Turnage & Hinckley,1938; Steenburgh & Lowe, 1983; Nobel, 1988). Indeed, geographic distribution formany desert species is limited by the occurrence of freezing temperatures (Nobel,1980a, b; Loik & Nobel, 1993a; Pockman & Sperry, 1997; Smith et al., 1997). Franco& Nobel (1989) showed that the microclimate under certain shrubs and grassesreduced the intensity of environmental stress experienced by the desert succulentsFerocactus acanthodes and Agave deserti seedlings. They found strong similaritiesbetween the seedling’s temperature tolerance and temperatures experienced withinthe protective shrub canopy microclimate.

The western limit of the Great Basin Desert is delineated by inland drainage fromthe Sierra Nevada and Cascade Mountains. Vegetation borders are determined bydispersal ability (Davis & Zabinski, 1992; Clark et al., 1998; Chambers et al., 1999),and recruitment success (Smith & Nowak, 1990; Hoffman & Blows, 1994), which inturn may be restricted by the ability to survive episodic freezing temperatures forcertain species (Loik & Nobel, 1993a, b). The purpose of this research was to examinethe ecotypic variation in freezing tolerance over an elevation gradient for seedlings ofArtemisia tridentata Nutt. (Asteraceae). This species is a widespread evergreen shruboccurring over 7� 106 ha of the Great Basin Desert, it is economically importantbecause cattle will not eat it, and it has exhibited spatial migrations in response tohistoric climate change (Whitlock & Bartlein, 1993; Nowak et al., 1994). Weexamined seedlings from the eastern Sierra Nevada of California, where A. tridentataoccurs between 1000 and 3500 m (Young et al., 1995). We focused on this species andlocation because of the potential for dramatic changes in elevational distribution forsagebrush across a wide range of elevations in response to climate change (Nowaket al., 1994). Specifically, we compared membrane and photosynthetic responsesto freezing for leaves of A. tridentata seedlings in relation to the thermalmicroenvironment measured in the field, as well as the tendency for these responsesto acclimate following a change in day/night air temperatures in a common garden. Wetested the specific hypothesis that A. tridentata seedlings from different elevationsexhibit ecotypic differentiation in freezing tolerance and ability to undergo coldacclimation.

SAGEBRUSH SEEDLING FREEZING TOLERANCE 771

Materials and methods

Study sites

Three study sites were located along Bishop Creek, located south-west of Bishop,California (371220 N latitude, 1181220 W longitude). The bottom of the canyon opensto the north-east and the canyon spans an elevation gradient of 1513 m, from Bishop,California at 1270 m to Lake Sabrina at 2783 m. The canyon is about 19 km long byroad, and the study sites were approximately 5 km apart in a north-east to south-westdirection. Three research sites were established in or near the canyon spanning anelevation gradient of 800 m. The highest site was located slightly west of Aspendell at2575 m (371120 000N, 1181350 2300W), the middle site was located near the SouthernCalifornia Edison power plant south of California highway 168 at 2175 m (371 160

4800N, 1181 3201800W), and the lowest site was located south-west of Buttermilk Roadat 1775 m (371200 2400N, 1181310 1400W). These sites were chosen for elevation andaccessibility during winter months.

Climate and microclimate

Weather records from Bishop Municipal Airport, the nearest weather station, werecollected from the National Climate Data Center (www.ncdc.noaa.gov), and theWestern Regional Climate Center (www.wrcc.sage.dri.edu). Precipitation data werecollected for the highest elevation site from the California Department of WaterResources Data Exchange Center (cdec.water.ca.gov).

Air temperatures in the seedling microhabitat were recorded 1 cm above the soilsurface using Onset HOBO XT II temperature dataloggers at each site. Measure-ments were made at 1 h intervals from the fall of 1997 to the spring/summer of 1998.The wire sensors were shielded from direct sunlight and snow by placing them intoventilated opaque plastic vials. The dataloggers were placed in thick plastic bags andburied several centimeters to protect from snow, rodents, and theft. Snow depth wasrecorded for the three sites on various dates from November 1997 through March1998.

Species composition and seedling establishment

Three 30 m line transects were randomly located at each site. At 0?3 m intervals, theidentity and height of species contacting a 0?25 cm diameter pole were recorded. Therelative frequency and coverage was determined using software developed for westernNorth American ecosystems (National Park Service, 1992). The frequency ofseedlings of A. tridentata occurring at each elevation was determined using detailedsearches beneath shrubs within 3� 30 m belt transects placed adjacent to each linetransect.

Plant material

Freezing tolerance and cold acclimation were compared for seedlings grown in acommon garden. On November 11, 1997 twenty plants from each elevation werecollected for laboratory studies. Small (p 10 cm tall) non-reproductive plants wererandomly selected and carefully transplanted into conical pots (4 cm diameter� 21 cmtall) filled with soil composed of 1 part potting soil (Supersoil Inc., San Mateo, CA,U.S.A.), 1 part native soil, and 1 part Perlite. The seedlings were watered thoroughlyto facilitate transplant survival during transport to the laboratory.


Seedlings were transported to the laboratory within 24 h and placed in a Percivalcontrolled environment chamber at day/night air temperatures of 251C/151C, 30%relative humidity, a 12 h/12 h day/night light cycle, and photosynthetically activeradiation (PAR; 400–700 nm) of 450 mmol m�2s�1 on a horizontal surface at the top ofthe seedlings. Plants were maintained in the growth chamber for a minimum of 30days before use in experiments to ensure transplant success and acclimation tochamber conditions. To measure the acclimatory change in membrane andphotosynthetic susceptibility to freezing temperatures, seedlings were shifted toaverage day/night air temperatures of 151C/51C for 2 weeks before use in experiments(Nobel, 1988; Loik & Nobel, 1993a,b). Air temperatures in the chamber weremonitored with Onset HOBO XT II temperature dataloggers. Plants were wateredtwice per week and fertilized once per week with a 0?1 strength Hoagland’s solution(Hoagland & Arnon, 1950).

Temperature treatments

In order to examine tolerance of episodic, low-temperature events (i.e. ‘‘cold snaps’’),leaves were treated to sub-zero air temperatures after which Photosystem II maximumquantum yield (as FV/FM) and plasmalemma membrane damage (as electrolyteleakage) were measured. Leaf temperature treatments were performed as described byLoik & Harte (1996) with minor modifications. Low-temperature treatments reaching�51C were conducted within a modified 0?18� 0?26� 0?18 m picnic cooler.Temperatures within the cooler were maintained by placing a 0?18� 0?18 m heatexchanger connected to a circulating waterbath within the cooler. The waterbath used60% (v/v) ethylene glycol as the circulated fluid. A small laboratory freezer was usedfor treatments below �51C. Temperature in the freezer was regulated by a heatexchanger within the freezer through which warm water was circulated.

Ten leaves per individual were excised and pooled for each of five randomly selectedplants per elevation. They were placed in small glass vials for the low-temperaturetreatment. Temperature treatments started at room air temperature (average 221C)and decreased at a rate of 31h�1 to the desired temperature (a rate similar to thatmeasured in the field; see also Nobel, 1988). Air temperatures within the chamber orfreezer were monitored using a datalogger and 0?5-mm-diameter 30 gauge copper–constantan thermocouples. For leaf temperatures, the thermocouple was woventhrough the mid-section of the leaf.

Leaves were treated for 1 h at a particular temperature, then warmed at the rate of31h�1 in the dark until reaching 201C before being used for chlorophyll a fluorescence(FV/FM) and electrolyte leakage measurements (described below).

Electrolyte leakage

Membrane damage was assessed by electrolyte leakage techniques adapted from Chenet al. (1982) and Loik & Harte (1996). Following low-temperature treatment, a leafdisc was punched from each leaf using a number one cork borer (3 mm diameter).Discs from the same plant, but different leaves, were pooled and placed in separateglass vials with 1?5 ml of distilled water. The discs were then vacuum infiltrated for5 min to ensure that surface tension would not prevent the exchange of ions fromdamaged cells to the bathing solution. The vials were then placed on a rotary shaker at60 cycles min�1 for 18–24 h, after which the discs were removed from the bathingsolution. The conductivity of the bathing solution was then measured using a ColeParmer conductivity meter. Between each measurement the probe was rinsed withdistilled water and carefully dried with a Kimwipe. Measurements were made for fiveindividuals per site, treatment, and day/night air temperature combination.

Photosystem II responses to freezing


Chlorophyll a fluorescence from Photosystem II (PSII) is a sensitive indicator ofbiochemical and biophysical susceptibility to low- and high-temperature stresseswithin chloroplasts (Larcher, 1995). In particular, a decrease in the maximumquantum yield of PSII, FV/FM, marks the destabilization of chloroplast membranesdue to temperature stress (Larcher, 1995; Loik & Harte, 1996; Loik et al., 2000b).After temperature treatments, individual leaves were placed in the sensor head of aHansatech FMS1 Fluorescence Monitoring System with the following settings:modulated light source set at 3, the pulse light source set at 100, and the actinic lightsource set at 50. FV/FM, the maximum quantum yield of Photosystem II, was recordedfor one leaf for each of five individuals per elevation, treatment, and day/night airtemperature combination.

Statistical analysis

Paired sample t-tests were used to determine if air temperatures in the seedlingmicrohabitat were colder for the highest vs. the lowest elevation sites. A single test wasperformed for a 3?61 and 5?21 expected rate of adiabatic cooling between the high- andlow-elevation sites because humidity can vary the adiabatic cooling rate (Ahrens,1991). No comparisons were made between the middle elevation site and the othertwo sites because of insufficient microclimate data.

Electrolyte leakage and FV/FM were plotted against temperature in order todetermine LT50. LT50 is the temperature at which FV/FM or electrolyte leakage was50% of that measured at 201C and is correlated with whole plant survival for a varietyof species (Sakai & Larcher, 1987; Nobel, 1988; Loik et al., 2000b). The average LT50

for electrolyte leakage and FV/FM for each set of day/night air temperatures were testedfor differences between elevation and day/night air temperatures by two-way ANOVA(Zar, 1996). In all statistical tests a sample size of n = 5 was used, and pp0?05 wasconsidered significant. Statistica (StatSoft, Tulsa, OK) was used to compute all two-way ANOVAs and Excel was used to compute all other statistics.

Results

Climate and microclimate

Monthly maximum and minimum air temperatures, averaged over the period 1961–1990, differed for Bishop (1270 m) and Aspendell (2783 m), the closest recordinglocations to our study sites (Fig. 1). Maximum air temperatures averaged 381C in Julyfor Bishop, and 251C in August for Aspendell. Minimum air temperatures averaged�61C in January for Bishop, and �101C in February for Aspendell. Extrememinimum record air temperatures, indicative of episodic cold spells, averaged about�181C for both Bishop and Aspendell in January. Average annual precipitation for1961–1990 was 135 mm for the 1775 m site, 313 mm for the 2175 m site, and 422 mmfor the 2575 m site.

Snow depth varied by site and date (Fig. 2(A)). All three sites were covered withsnow from December into February. Average snow depth was nine-fold greater at the2575 m site compared to the 2175 m in mid-February, and snow was absent at the1775 m site by this time. Temperature at 1 cm height above the soil surface wasrecorded hourly at the upper and lower elevation sites (Fig. 2(B,C)). The lowestrecorded temperature of all sites, �13?01C, occurred at the highest elevation on 16November, and �9?21C at the lowest elevation on 3 December.

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Figure 1. Mean monthly maximum (mean T max), minimum (mean T min), and extrememinimum (extreme T min) air temperatures for Bishop and Aspendell, California, the two closestlong-term weather recording stations to our study sites. Data are means for the period 1961–1990.


Species composition and seedling establishment

Four perennial shrub species including A. tridentata, Purshia tridentata (Pursh) DC(Rosaceae), Ephedra viridis Cov. (Ephedraceae), and Chrysothamnus nauseosus (Pallas)Britton (Asteraceae) were common at the sites (nomenclature of Hickman, 1993).Herbaceous species included Lupinus nevadensis A.A. Heller (Fabaceae), Eriogonumsp. (Polygonaceae), and Erigeron linearis (Hook.) Piper (Asteraceae). The only treespecies on any transect was Prunus andersonii Gray (Rosaceae) at the 2175 m site;Pinus monophylla Torrey & Fremont (Pinaceae) also occurred near this site, but off thestudy transects.

Percent cover of plants, litter, rock, and soil varied along the elevation gradient(Table 1). At 1775 m, E. viridis was dominant based on cover, whereas at 2175 mPurshia tridentata dominated, and at 2575 m, A. tridentata was the dominant species.Herbaceous species were more frequent and had greater cover at the highest elevationsite compared to the lower sites, and bare ground was more common at the lower twosites. At the high- and middle-elevation sites, ground was covered with more organiclitter than at the lowest site. The canopy occupied 44% of samples at 1775 m, 62% at2175 m, and 76% at 2575 m. Based on three belt transects adjacent to the linetransects at each elevation, there were an average of 0?25 A. tridentata seedlings m�2 at2575 m, and 0?03 seedlings m-2 at both 1775 and 2175 m.

Freezing tolerance and cold acclimation

Electrolyte leakage for leaves from seedlings grown at day/night air temperatures of251C/151C increased for discs from leaves exposed to treatment temperatures below�12?51C (Fig. 3). The temperature leading to half-maximum electrolyte leakage(LT50) was calculated based on these patterns. For seedlings maintained at day/nightair temperatures of 251C/151C, LT50 was �13?31C for plants from 1775 m, �13?41Cfor 2175 m, and �13?91C at 2575 m. Following a shift to day/night air temperatures of151C/51C, LT50 was �15?11C, �15?21C, and �14?21C for the 1775, 2175, and2575 m sites, respectively (Fig. 3). Based on electrolyte leakage for seedlings grown atthe two day/night air temperature regimes, and treated at �121C, a significant degreeof acclimation occurred (Table 2), but there were no differences in LT50 based onelevation differences ( p = 0?106).

For leaves from seedlings grown at day/night air temperatures of 251C/151C, FV/FM

markedly decreased following treatment at temperatures between �12?51C and�17?51C (Fig. 4). This response was similar for seedlings from all three elevations, aswell as for seedlings 2 weeks following a shift to day/night air temperatures of 151C/

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Figure 2. Snow depth (A), and air temperatures in the seedling canopy measured at 2575 m(B) and 1775 m (C) elevations for the winter 1997–1998.


51C. A 50% reduction in FV/FM for seedlings grown at 251C/151C resulted fromtreatment at temperatures from �14?11C to �15?11C across the three elevations. Theeffects of growth at different day/night temperatures were significant only for seedlingsfrom the 2175 m site, but site and growth temperature did not significantly affect FV/FM in the overall ANOVA model.

Discussion

For seedlings of A. tridentata from three sites along the 800 m elevation gradientmaintained at day/night air temperatures of 251C/151C, and then shifted to 151C/51C,

Table 1. Percent cover for plant species, litter, bare soil, and rock across elevationsalong Bishop Creek, California

Cover (%)

Identity 1775 m 2175 m 2575 m

Artemisia tridentata 4 (0?9) 6 (1?1) 52 (4?3)Chrysothamnus nauseosus 3 (0?9) 0 (0) 2 (0?4)Ephedra viridis 37 (4?7) 0 (0) 0 (0)Erigeron linearis 0 (0) 0 (0) 2 (1?9)Eriogonum saxatile 0 (0) 0 (0) 7 (2?1)Lupinus nevadensis 0 (0) 0 (0) 10 (2?1)Prunus andersonii 0 (0) 2 (0?4) 1 (0?7)Purshia tridentata 0 (0) 54 (5?2) 4 (1?2)Litter 11 (2?9) 20 (3?1) 13 (3?4)Rock 0 (0) 7 (2?2) 3 (2?5)Soil 45 (5?7) 11 (2?5) 8 (3?7)

Data are means (71 S.D.) for three 30 m line transects at each elevation. Nomenclature from Hickman(1993).


results for electrolyte leakage indicate acclimation by 1?51, but no differences acrosselevations. Moreover, there were no significant differences across sites or growthtemperatures for chlorophyll a fluorescence from PSII. Freezing tolerance, based onLT50 values for freezing tolerance measured in the laboratory, is close to the minimummicroclimate temperatures recorded at the 2575 m site during the course of this study.Moreover, the temperature leading to a 50% reduction in membrane integrity andchlorophyll a fluorescence was 111 lower than the 1961–1990 average minimum airtemperatures for Bishop in February, but 51 higher than the extreme minima for thesame period. Together, these results suggest that seedlings have a ‘‘safety buffer’’ ordifference between the temperature leading to a 50% loss in membrane structure andfunction, and the air temperatures recorded during the study. Based on the 1961–1990 record, extreme minimum temperature events as low as �181C would representimportant filters on recruitment, at least in terms of membrane damage and PSIIfunction.

Electrolyte leakage measurements suggests a 1?51 acclimation response by theseedlings, especially for those from the 1775 and 2575 m sites. With regard to theeffects of freezing temperatures on Photosystem II function, chlorophyll fluorescenceindicates a shift in FV/FM of 0?61 only for seedlings from 2575 m. Differences in FV/FM

from PSII would indicate variation in susceptibility of the photosynthetic apparatus tolow temperatures (Berry & Bjorkman, 1980; Larcher, 1995). The lack of differencesacross sites and growth temperatures suggests a limited ability of the photosyntheticapparatus to acclimate or evolve ecotypic differences, at least at the spatial scale of sitecomparisons in the present study. Differences in membrane susceptibility following ashift to lower growth temperatures could be beneficial for seedling survival astemperature extremes, snow cover, and other aspects of the microenvironment varyover short temporal scales. Variation within populations for cold acclimation has beendemonstrated for many other species (Smithberg & Weiser, 1968; Green, 1969;Maronek & Flint, 1974; Eiga & Sakai, 1984; Loik & Nobel, 1993a). However, in theseother studies differences in cold tolerance were found in plants from regions spanningbroad elevation and latitudinal gradients whereas in the present study the greatestgeographic range between sites was about 5 km, and 800 m in elevation. Freezingtolerance for Artemisia skorniakowi and A. pamirica from the arid East PamirMountains was similar to our results for A. tridentata (Tyurina, 1957). However, these

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Figure 3. Electrolyte leakage as a function of treatment air temperature for seedlingsof Artemisia tridentata from 2575 m (A), 2175 m (B), and 1775 m (C). Data are means 7 1S.E. (n = 5) for plants maintained at day/night air temperatures of 251C/151C (*) or 14 daysafter shifting to 151C/51C (~).


values were based on measurements for summer frost hardiness which could be higherthan those determined after winter acclimation. Also, our results are based onseedlings maintained at 251C/151C and 151C/51C, and acclimation might have beengreater (and LT50 lower) for plants if the day/night temperatures of the chamberscould have been set lower than 151C/51C. A different photoperiod within growthchambers compared to natural populations may also affect the degree of coldacclimation (Howe et al., 1995; Rinne et al., 1998).

Microclimatic effects could complicate patterns of variation in freezing toleranceand cold acclimation for A. tridentata over the 800 m of our elevation gradient. Plantsat higher elevations may avoid low temperatures beneath the snow pack, while

Table 2. Summary of two-way ANOVA results for LT50 for membranes (asdetermined by electrolyte leakage) or Photosystem II function (as determined by

measurements of FV/FM) following temperature treatment in the laboratory

Independentvariable

Electrolyteleakage

FV/FM

Site 0?106 0?626Growth temperature 6?992* 0?042Site� growth temperature 1?031 2?393

Data are F values, for n = 5 plants per elevation site or growth temperature; *po0?05.


exposed plants at lower elevations experience low temperatures within their tolerancerange. Thus, selection for cold tolerance may be based on microclimate at small scales.Moreover, other stresses, such as drought and high-temperature stress, as well aswind-blown ice, snow, and dust, can also reduce seedling survival (Tranquillini, 1979;Baskin & Baskin, 1998). Also, PSII responses to freezing temperatures can becomplicated by high PFD, which has been shown to affect recruitment in seedlings ofother species (Ball et al., 1997; Egerton et al., 2000). Given the broad distribution ofA. tridentata across North America, differences among populations in the ability totolerate freezing would seem likely when compared across larger scales. In this regard,Meyer et al. (1990) found that seed germination response variables for A. tridentatawere correlated with the mean January temperatures at various seed collection sites.

For A. tridentata, pollen and seeds are dispersed by wind (Shultz, 1993), so thepotential for the mixing of pollen and seed from high and low elevations is likely at thespatial scale of our comparisons. In the eastern Sierra Nevada cold air in the highmountains descends downslope, frequently generating strong winds. During theautumn, when A. tridentata is flowering, cold fronts generate strong northerly winds,while differential surface heating in the Owens Valley and other nearby valleys cangenerate strong southerly winds (Hidy & Klieforth, 1990). Combined, high rates ofpollen and seed dispersal may reduce the potential for differentiation betweenpopulations from higher and lower elevations (Nowak et al., 1994).

Low-temperature tolerance changes little throughout the year for certain speciesbecause episodic freezing can occur at high altitudes even during summer months(Sakai & Larcher, 1987). It is not uncommon for freezing temperatures, even snowstorms, to occur in the eastern Sierra Nevada during the summer months (Anderson,1975). However, woody temperate zone shrub species, especially those spanningbroad gradients like A. tridentata, usually have some ability to acclimate to lowtemperatures. Cold acclimation is most common for species from regions with high-temperature seasonality (Sakai & Larcher, 1987; Alberdi & Corcuera, 1991). In thepresent study lowering of the growth chamber’s day/night air temperature regime wasused to trigger an acclimation response. The small magnitude of response by plants tothe lowering of day/night air temperatures may indicate that A. tridentata seedlingsrequire greater decreases in day/night temperatures to enhance their acclimationresponse. Such is the case with cacti and agaves (Nobel, 1988). Furthermore, it ispossible that lowering air temperature alone was insufficient to initiate an acclimationresponse intense enough to be measured by the techniques employed. For manyspecies, different or additional environmental cues are needed to stimulate a robustacclimation response. A decrease in photoperiod is important for the acclimation ofmany woody shrubs, conifers, and northern tree species, but is not as important forherbaceous species (Sakai & Larcher, 1987). In some cases, a combination of shortdays and colder temperatures is required for cold acclimation (Alberdi & Corcuera,1991). Seedlings of A. tridentata may require a greater decrease in day/night air

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Figure 4. Photosystem II maximum quantum yield (measured as FV/FM) as a function oftreatment air temperature for seedlings of Artemisia tridentata from 2575 m (A), 2175 m (B),and 1775 m (C). Data are means 7 1 S.E. (n = 5) for plants maintained at day/night airtemperatures of 251C/151C (*) or 14 days after shifting to 151C/51C (~).


temperatures and a decrease in photoperiod to significantly increase the acclimationresponse.

The microclimate data suggest that the minimum low temperatures experiencedbetween the 1775 and 2575 m sites differed by several degrees. Between those twosites a difference of 3?6–5?21 was expected based on the adiabatic lapse rate (4?5–6?51per 1000 m depending on water content), and the actual value was 4?11. Temperaturefluctuations and low-temperature extremes were not as low when snow depths weregreater at the 2575 m compared to 1775 and 2175 m sites. During these times,


temperatures near the soil surface remained at approximately 01C. These results areconsistent with studies in the Rocky Mountains and Canada (Gieger, 1965). Despitebeing 800 m lower than the highest elevation site, temperatures at the 1775 m sitewere lower when there was less insulating snow cover. Adults and seedlings of A.tridentata at the 2575 m site were completely snow covered for the majority of thewinter, and intermittently covered at the 1775 and 2175 m sites. Seedling survival athigh altitudes may depend on sufficient snow cover to ameliorate low temperaturesnear the soil surface.

The vegetation formed a more complete canopy at the highest elevation study site.The shrub crowns tended to touch one another, leaving only small openings in thecanopy. The A. tridentata seedlings tended to occur under the canopy, whereas at thelower elevation sites they were more exposed. This could influence seedling survival,especially at the higher elevation because temperatures in the microclimate near thesoil surface depend, in part, upon IR flux from surrounding vegetation and litter(Nobel, 1997). The large amount of bare ground without organic litter at the lowestelevation site could lead to lower than expected temperatures near the soil surface dueto a large IR flux at night, when the ground loses heat to the atmosphere (Gieger,1965; Nobel, 1997). The vegetation canopy may also serve to protect seedlings fromice encasement and snow pressure.

The mechanisms by which plants have responded to past changes in climatedepended, in part, on how physical factors influenced seedling establishment (Clark etal., 1998). Insights into seedling tolerances to abiotic factors may help ourunderstanding of how future changes in climate will impact the distribution andproductivity of vegetation. The future climate of the Sierra Nevada may includeincreased summer monsoon rainfall (Knox, 1991; Arritt et al., 2000), which couldenhance seedling recruitment for certain species. Experimental warming that changessoil water content in the Rocky Mountains alters water relations, photosynthesis, andabove-ground biomass accumulation for various species (Harte & Shaw, 1995; Loik &Harte, 1997; Loik et al., 2000a), impacting tolerance of temperature extremes (Loik &Harte, 1996), and seedling recruitment. The effects of changing climatic features (i.e.temperature and precipitation) on plant fitness parameters, such as seedlingrecruitment and adult reproductive effort, require further experimentation alonggradients from high-elevation desert to montane ecosystems.

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Documents

Microclimate, freezing tolerance, and cold acclimation along an elevation gradient for seedlings of the Great Basin Desert shrub, Artemisia tridentata