Journal of Arid Environments (2003) 54: 769–782
doi:10.1006/jare.2002.1106, available online at http://www.sciencedirect.com
Microclimate, freezing tolerance, and cold acclimation
along 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 cold
acclimation were compared for seedlings of Artemisia tridentata collected
from 1775, 2175, and 2575 m elevation in the eastern Sierra Nevada,
California. Data were used to test the hypothesis that ecotypic differences in
stress physiology are important for seedling survival along gradients from
desert to montane ecosystems. The vegetation canopy cover and A. tridentata
seedling frequency were greatest at 2575 m, compared to 1775 and 2175 m.
Snow cover ameliorated temperatures near the soil surface for part of the
winter and depth varied across elevations. Freezing tolerance was compared
for seedlings maintained in growth chambers at day/night air temperatures of
251C/151C. The temperature at which electrolyte leakage and Photosystem II
function (FV/FM) from leaves were half-maximum was approximately
13?51C for leaves of seedlings from all three elevations. Shifting day/night
air temperatures from 251C/151C to 151C/51C initiated about 1?51 of
acclimation by plants from all three altitudes, with seedlings from the highest
elevation exhibiting the greatest acclimation change. Measurements of
ambient air and canopy temperatures at the three elevations indicated that
wintertime average low temperatures were consistent with the measured
degree of freezing tolerance. At small spatial scales used in this study, pollen
and seed dispersal between study sites may have precluded resolution of
ecotypic differences. Patterns of freezing tolerance and cold acclimation may
depend 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 of
plants (Levitt, 1980; Larcher, 1995). Many species are geographically limited by subzero 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 the
Earth’s land mass has a mean minimum temperature below 01C for some time of the
year (Sakai & Larcher, 1987). In cold desert ecosystems sub-zero temperatures can
occur near the soil surface based on unstirred boundary layers immediately above the
soil 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 cm
above the soil surface can be 71 lower than those at 1 m, but dependent on canopy
characteristics (Geiger, 1965; Nobel, 1997). Additionally, the temperature near the
soil surface exhibits much greater diurnal changes than the mixed air above (Nobel,
1997).
Extreme temperatures can be particularly important for seedling survival and
establishment in arid environments. For many ecosystems in the western United
States, plant recruitment is limited by the ability of seedlings to survive in the abiotic
environment near the soil surface (Franco & Nobel, 1988; Smith & Nowak, 1990).
Episodic events of extreme sub-zero air temperatures can contribute to high rates of
seedling mortality (Jordan & Nobel, 1979; Fenner, 1985; Sakai & Larcher, 1987).
Because seedlings of many species are less tolerant of extreme environmental
conditions compared to adults (Smith & Nowak, 1990; Davis & Zabinski, 1992), the
degree of abiotic stress tolerance can determine successful establishment and thereby
limit species distributions to certain regions or microsites. For example, the
elevational and latitudinal distribution of saguaro (Carnegiea gigantea) is determined
by periods of episodic sub-zero air temperatures (Shreve, 1911; Turnage & Hinckley,
1938; Steenburgh & Lowe, 1983; Nobel, 1988). Indeed, geographic distribution for
many 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 grasses
reduced the intensity of environmental stress experienced by the desert succulents
Ferocactus acanthodes and Agave deserti seedlings. They found strong similarities
between the seedling’s temperature tolerance and temperatures experienced within
the protective shrub canopy microclimate.
The western limit of the Great Basin Desert is delineated by inland drainage from
the Sierra Nevada and Cascade Mountains. Vegetation borders are determined by
dispersal ability (Davis & Zabinski, 1992; Clark et al., 1998; Chambers et al., 1999),
and recruitment success (Smith & Nowak, 1990; Hoffman & Blows, 1994), which in
turn may be restricted by the ability to survive episodic freezing temperatures for
certain species (Loik & Nobel, 1993a, b). The purpose of this research was to examine
the ecotypic variation in freezing tolerance over an elevation gradient for seedlings of
Artemisia tridentata Nutt. (Asteraceae). This species is a widespread evergreen shrub
occurring over 7 106 ha of the Great Basin Desert, it is economically important
because cattle will not eat it, and it has exhibited spatial migrations in response to
historic climate change (Whitlock & Bartlein, 1993; Nowak et al., 1994). We
examined seedlings from the eastern Sierra Nevada of California, where A. tridentata
occurs between 1000 and 3500 m (Young et al., 1995). We focused on this species and
location because of the potential for dramatic changes in elevational distribution for
sagebrush across a wide range of elevations in response to climate change (Nowak
et al., 1994). Specifically, we compared membrane and photosynthetic responses
to freezing for leaves of A. tridentata seedlings in relation to the thermal
microenvironment measured in the field, as well as the tendency for these responses
to acclimate following a change in day/night air temperatures in a common garden. We
tested the specific hypothesis that A. tridentata seedlings from different elevations
exhibit ecotypic differentiation in freezing tolerance and ability to undergo cold
acclimation.
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 opens
to 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 by
road, and the study sites were approximately 5 km apart in a north-east to south-west
direction. Three research sites were established in or near the canyon spanning an
elevation gradient of 800 m. The highest site was located slightly west of Aspendell at
2575 m (371120 000 N, 1181350 2300 W), the middle site was located near the Southern
California Edison power plant south of California highway 168 at 2175 m (371 160
4800 N, 1181 320 1800 W), and the lowest site was located south-west of Buttermilk Road
at 1775 m (371200 2400 N, 1181310 1400 W). These sites were chosen for elevation and
accessibility during winter months.
Climate and microclimate
Weather records from Bishop Municipal Airport, the nearest weather station, were
collected from the National Climate Data Center (www.ncdc.noaa.gov), and the
Western Regional Climate Center (www.wrcc.sage.dri.edu). Precipitation data were
collected for the highest elevation site from the California Department of Water
Resources Data Exchange Center (cdec.water.ca.gov).
Air temperatures in the seedling microhabitat were recorded 1 cm above the soil
surface using Onset HOBO XT II temperature dataloggers at each site. Measurements 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 into
ventilated opaque plastic vials. The dataloggers were placed in thick plastic bags and
buried several centimeters to protect from snow, rodents, and theft. Snow depth was
recorded for the three sites on various dates from November 1997 through March
1998.
Species composition and seedling establishment
Three 30 m line transects were randomly located at each site. At 0?3 m intervals, the
identity and height of species contacting a 0?25 cm diameter pole were recorded. The
relative frequency and coverage was determined using software developed for western
North American ecosystems (National Park Service, 1992). The frequency of
seedlings of A. tridentata occurring at each elevation was determined using detailed
searches beneath shrubs within 3 30 m belt transects placed adjacent to each line
transect.
Plant material
Freezing tolerance and cold acclimation were compared for seedlings grown in a
common garden. On November 11, 1997 twenty plants from each elevation were
collected for laboratory studies. Small (p 10 cm tall) non-reproductive plants were
randomly selected and carefully transplanted into conical pots (4 cm diameter 21 cm
tall) 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 thoroughly
to facilitate transplant survival during transport to the laboratory.
772
M. E. LOIK & S. P. REDAR
Seedlings were transported to the laboratory within 24 h and placed in a Percival
controlled environment chamber at day/night air temperatures of 251C/151C, 30%
relative humidity, a 12 h/12 h day/night light cycle, and photosynthetically active
radiation (PAR; 400–700 nm) of 450 mmol m2s1 on a horizontal surface at the top of
the seedlings. Plants were maintained in the growth chamber for a minimum of 30
days before use in experiments to ensure transplant success and acclimation to
chamber conditions. To measure the acclimatory change in membrane and
photosynthetic susceptibility to freezing temperatures, seedlings were shifted to
average 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 were
monitored with Onset HOBO XT II temperature dataloggers. Plants were watered
twice 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 maximum
quantum yield (as FV/FM) and plasmalemma membrane damage (as electrolyte
leakage) were measured. Leaf temperature treatments were performed as described by
Loik & 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 heat
exchanger connected to a circulating waterbath within the cooler. The waterbath used
60% (v/v) ethylene glycol as the circulated fluid. A small laboratory freezer was used
for treatments below 51C. Temperature in the freezer was regulated by a heat
exchanger within the freezer through which warm water was circulated.
Ten leaves per individual were excised and pooled for each of five randomly selected
plants per elevation. They were placed in small glass vials for the low-temperature
treatment. Temperature treatments started at room air temperature (average 221C)
and decreased at a rate of 31h1 to the desired temperature (a rate similar to that
measured in the field; see also Nobel, 1988). Air temperatures within the chamber or
freezer were monitored using a datalogger and 0?5-mm-diameter 30 gauge copper–
constantan thermocouples. For leaf temperatures, the thermocouple was woven
through the mid-section of the leaf.
Leaves were treated for 1 h at a particular temperature, then warmed at the rate of
31h1 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 Chen
et al. (1982) and Loik & Harte (1996). Following low-temperature treatment, a leaf
disc 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 separate
glass vials with 1?5 ml of distilled water. The discs were then vacuum infiltrated for
5 min to ensure that surface tension would not prevent the exchange of ions from
damaged cells to the bathing solution. The vials were then placed on a rotary shaker at
60 cycles min1 for 18–24 h, after which the discs were removed from the bathing
solution. The conductivity of the bathing solution was then measured using a Cole
Parmer conductivity meter. Between each measurement the probe was rinsed with
distilled water and carefully dried with a Kimwipe. Measurements were made for five
individuals per site, treatment, and day/night air temperature combination.
SAGEBRUSH SEEDLING FREEZING TOLERANCE
773
Photosystem II responses to freezing
Chlorophyll a fluorescence from Photosystem II (PSII) is a sensitive indicator of
biochemical and biophysical susceptibility to low- and high-temperature stresses
within chloroplasts (Larcher, 1995). In particular, a decrease in the maximum
quantum yield of PSII, FV/FM, marks the destabilization of chloroplast membranes
due to temperature stress (Larcher, 1995; Loik & Harte, 1996; Loik et al., 2000b).
After temperature treatments, individual leaves were placed in the sensor head of a
Hansatech FMS1 Fluorescence Monitoring System with the following settings:
modulated light source set at 3, the pulse light source set at 100, and the actinic light
source set at 50. FV/FM, the maximum quantum yield of Photosystem II, was recorded
for one leaf for each of five individuals per elevation, treatment, and day/night air
temperature combination.
Statistical analysis
Paired sample t-tests were used to determine if air temperatures in the seedling
microhabitat were colder for the highest vs. the lowest elevation sites. A single test was
performed for a 3?61 and 5?21 expected rate of adiabatic cooling between the high- and
low-elevation sites because humidity can vary the adiabatic cooling rate (Ahrens,
1991). No comparisons were made between the middle elevation site and the other
two sites because of insufficient microclimate data.
Electrolyte leakage and FV/FM were plotted against temperature in order to
determine LT50. LT50 is the temperature at which FV/FM or electrolyte leakage was
50% of that measured at 201C and is correlated with whole plant survival for a variety
of 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 tested
for 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 was
considered significant. Statistica (StatSoft, Tulsa, OK) was used to compute all twoway 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 recording
locations to our study sites (Fig. 1). Maximum air temperatures averaged 381C in July
for Bishop, and 251C in August for Aspendell. Minimum air temperatures averaged
61C in January for Bishop, and 101C in February for Aspendell. Extreme
minimum record air temperatures, indicative of episodic cold spells, averaged about
181C for both Bishop and Aspendell in January. Average annual precipitation for
1961–1990 was 135 mm for the 1775 m site, 313 mm for the 2175 m site, and 422 mm
for the 2575 m site.
Snow depth varied by site and date (Fig. 2(A)). All three sites were covered with
snow from December into February. Average snow depth was nine-fold greater at the
2575 m site compared to the 2175 m in mid-February, and snow was absent at the
1775 m site by this time. Temperature at 1 cm height above the soil surface was
recorded hourly at the upper and lower elevation sites (Fig. 2(B,C)). The lowest
recorded temperature of all sites, 13?01C, occurred at the highest elevation on 16
November, and 9?21C at the lowest elevation on 3 December.
774
M. E. LOIK & S. P. REDAR
Temperature (oC)
40
30
Bishop
1270 m
mean T
Lake Sabrina
2783 m
max
mean T max
20
mean T min
10
0
-10
mean T
min
extreme T min
extreme T
min
-20
Ja Fe Ma Ap My Ju Jl Au Se Oc No De Ja Fe Ma Ap My Ju Jl Au Se Oc No De
Month
Figure 1. Mean monthly maximum (mean T max), minimum (mean T min), and extreme
minimum (extreme T min) air temperatures for Bishop and Aspendell, California, the two closest
long-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), Eriogonum
sp. (Polygonaceae), and Erigeron linearis (Hook.) Piper (Asteraceae). The only tree
species 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 the
study 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 m
Purshia tridentata dominated, and at 2575 m, A. tridentata was the dominant species.
Herbaceous species were more frequent and had greater cover at the highest elevation
site compared to the lower sites, and bare ground was more common at the lower two
sites. At the high- and middle-elevation sites, ground was covered with more organic
litter than at the lowest site. The canopy occupied 44% of samples at 1775 m, 62% at
2175 m, and 76% at 2575 m. Based on three belt transects adjacent to the line
transects at each elevation, there were an average of 0?25 A. tridentata seedlings m2 at
2575 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 of
251C/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/night
air temperatures of 251C/151C, LT50 was 13?31C for plants from 1775 m, 13?41C
for 2175 m, and 13?91C at 2575 m. Following a shift to day/night air temperatures of
151C/51C, LT50 was 15?11C, 15?21C, and 14?21C for the 1775, 2175, and
2575 m sites, respectively (Fig. 3). Based on electrolyte leakage for seedlings grown at
the two day/night air temperature regimes, and treated at 121C, a significant degree
of acclimation occurred (Table 2), but there were no differences in LT50 based on
elevation 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, as
well as for seedlings 2 weeks following a shift to day/night air temperatures of 151C/
SAGEBRUSH SEEDLING FREEZING TOLERANCE
775
70
Snow depth (cm)
60
50
40
2575 m
2175 m
1775 m
30
20
10
0
(A)
25
20
2575 m
15
10
5
Canopy air temperature (oC)
0
-5
-10
-15
(B)
25
20
1775 m
15
10
5
0
-5
-10
-15
Nov
(C)
Dec
Jan
Feb
Time (month)
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 from
treatment at temperatures from 14?11C to 15?11C across the three elevations. The
effects of growth at different day/night temperatures were significant only for seedlings
from 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 gradient
maintained at day/night air temperatures of 251C/151C, and then shifted to 151C/51C,
776
M. E. LOIK & S. P. REDAR
Table 1. Percent cover for plant species, litter, bare soil, and rock across elevations
along Bishop Creek, California
Cover (%)
Identity
1775 m
2175 m
2575 m
Artemisia tridentata
Chrysothamnus nauseosus
Ephedra viridis
Erigeron linearis
Eriogonum saxatile
Lupinus nevadensis
Prunus andersonii
Purshia tridentata
Litter
Rock
Soil
4 (0?9)
3 (0?9)
37 (4?7)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
11 (2?9)
0 (0)
45 (5?7)
6 (1?1)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
2 (0?4)
54 (5?2)
20 (3?1)
7 (2?2)
11 (2?5)
52 (4?3)
2 (0?4)
0 (0)
2 (1?9)
7 (2?1)
10 (2?1)
1 (0?7)
4 (1?2)
13 (3?4)
3 (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 across
elevations. Moreover, there were no significant differences across sites or growth
temperatures for chlorophyll a fluorescence from PSII. Freezing tolerance, based on
LT50 values for freezing tolerance measured in the laboratory, is close to the minimum
microclimate temperatures recorded at the 2575 m site during the course of this study.
Moreover, the temperature leading to a 50% reduction in membrane integrity and
chlorophyll a fluorescence was 111 lower than the 1961–1990 average minimum air
temperatures for Bishop in February, but 51 higher than the extreme minima for the
same period. Together, these results suggest that seedlings have a ‘‘safety buffer’’ or
difference between the temperature leading to a 50% loss in membrane structure and
function, and the air temperatures recorded during the study. Based on the 1961–
1990 record, extreme minimum temperature events as low as 181C would represent
important filters on recruitment, at least in terms of membrane damage and PSII
function.
Electrolyte leakage measurements suggests a 1?51 acclimation response by the
seedlings, especially for those from the 1775 and 2575 m sites. With regard to the
effects of freezing temperatures on Photosystem II function, chlorophyll fluorescence
indicates 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 to
low temperatures (Berry & Bjorkman, 1980; Larcher, 1995). The lack of differences
across sites and growth temperatures suggests a limited ability of the photosynthetic
apparatus to acclimate or evolve ecotypic differences, at least at the spatial scale of site
comparisons in the present study. Differences in membrane susceptibility following a
shift to lower growth temperatures could be beneficial for seedling survival as
temperature extremes, snow cover, and other aspects of the microenvironment vary
over short temporal scales. Variation within populations for cold acclimation has been
demonstrated for many other species (Smithberg & Weiser, 1968; Green, 1969;
Maronek & Flint, 1974; Eiga & Sakai, 1984; Loik & Nobel, 1993a). However, in these
other studies differences in cold tolerance were found in plants from regions spanning
broad elevation and latitudinal gradients whereas in the present study the greatest
geographic range between sites was about 5 km, and 800 m in elevation. Freezing
tolerance for Artemisia skorniakowi and A. pamirica from the arid East Pamir
Mountains was similar to our results for A. tridentata (Tyurina, 1957). However, these
SAGEBRUSH SEEDLING FREEZING TOLERANCE
777
1.0
0.8
2575 m
0.6
0.4
0.2
0.0
(A)
Electrolyte leakage
(fraction of maximal)
1.0
0.8
2175 m
0.6
o
o
25 C/15 C
o
o
15 C/5 C
0.4
0.2
0.0
(B)
1.0
0.8
1775 m
0.6
0.4
0.2
0.0
-20
(C)
-10
0
10
Treatment temperature (oC)
20
Figure 3. Electrolyte leakage as a function of treatment 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 air temperatures of 251C/151C (*) or 14 days
after shifting to 151C/51C (~).
values were based on measurements for summer frost hardiness which could be higher
than those determined after winter acclimation. Also, our results are based on
seedlings maintained at 251C/151C and 151C/51C, and acclimation might have been
greater (and LT50 lower) for plants if the day/night temperatures of the chambers
could have been set lower than 151C/51C. A different photoperiod within growth
chambers compared to natural populations may also affect the degree of cold
acclimation (Howe et al., 1995; Rinne et al., 1998).
Microclimatic effects could complicate patterns of variation in freezing tolerance
and cold acclimation for A. tridentata over the 800 m of our elevation gradient. Plants
at higher elevations may avoid low temperatures beneath the snow pack, while
778
M. E. LOIK & S. P. REDAR
Table 2. Summary of two-way ANOVA results for LT50 for membranes (as
determined by electrolyte leakage) or Photosystem II function (as determined by
measurements of FV/FM) following temperature treatment in the laboratory
Independent
variable
Site
Growth temperature
Site growth temperature
Electrolyte
leakage
FV/FM
0?106
6?992*
1?031
0?626
0?042
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 tolerance
range. 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 as
wind-blown ice, snow, and dust, can also reduce seedling survival (Tranquillini, 1979;
Baskin & Baskin, 1998). Also, PSII responses to freezing temperatures can be
complicated by high PFD, which has been shown to affect recruitment in seedlings of
other species (Ball et al., 1997; Egerton et al., 2000). Given the broad distribution of
A. tridentata across North America, differences among populations in the ability to
tolerate freezing would seem likely when compared across larger scales. In this regard,
Meyer et al. (1990) found that seed germination response variables for A. tridentata
were correlated with the mean January temperatures at various seed collection sites.
For A. tridentata, pollen and seeds are dispersed by wind (Shultz, 1993), so the
potential for the mixing of pollen and seed from high and low elevations is likely at the
spatial scale of our comparisons. In the eastern Sierra Nevada cold air in the high
mountains descends downslope, frequently generating strong winds. During the
autumn, when A. tridentata is flowering, cold fronts generate strong northerly winds,
while differential surface heating in the Owens Valley and other nearby valleys can
generate strong southerly winds (Hidy & Klieforth, 1990). Combined, high rates of
pollen and seed dispersal may reduce the potential for differentiation between
populations from higher and lower elevations (Nowak et al., 1994).
Low-temperature tolerance changes little throughout the year for certain species
because episodic freezing can occur at high altitudes even during summer months
(Sakai & Larcher, 1987). It is not uncommon for freezing temperatures, even snow
storms, to occur in the eastern Sierra Nevada during the summer months (Anderson,
1975). However, woody temperate zone shrub species, especially those spanning
broad gradients like A. tridentata, usually have some ability to acclimate to low
temperatures. Cold acclimation is most common for species from regions with hightemperature seasonality (Sakai & Larcher, 1987; Alberdi & Corcuera, 1991). In the
present study lowering of the growth chamber’s day/night air temperature regime was
used to trigger an acclimation response. The small magnitude of response by plants to
the lowering of day/night air temperatures may indicate that A. tridentata seedlings
require greater decreases in day/night temperatures to enhance their acclimation
response. Such is the case with cacti and agaves (Nobel, 1988). Furthermore, it is
possible that lowering air temperature alone was insufficient to initiate an acclimation
response intense enough to be measured by the techniques employed. For many
species, different or additional environmental cues are needed to stimulate a robust
acclimation response. A decrease in photoperiod is important for the acclimation of
many woody shrubs, conifers, and northern tree species, but is not as important for
herbaceous species (Sakai & Larcher, 1987). In some cases, a combination of short
days and colder temperatures is required for cold acclimation (Alberdi & Corcuera,
1991). Seedlings of A. tridentata may require a greater decrease in day/night air
SAGEBRUSH SEEDLING FREEZING TOLERANCE
779
1.0
0.8
0.6
0.4
0.2
2575 m
0.0
(A)
1.0
0.9
Fv /Fm (dimensionless)
0.8
0.7
o
25 oC/15 C
o
15 C/5 oC
0.6
0.5
0.4
2175 m
0.3
0.2
(B)
1.0
0.8
0.6
0.4
0.2
1775 m
0.0
-20
-10
(C)
Treatment temperature (oC)
0
10
20
Figure 4. Photosystem II maximum quantum yield (measured as FV/FM) as a function of
treatment 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 air
temperatures of 251C/151C (*) or 14 days after shifting to 151C/51C (~).
temperatures and a decrease in photoperiod to significantly increase the acclimation
response.
The microclimate data suggest that the minimum low temperatures experienced
between the 1775 and 2575 m sites differed by several degrees. Between those two
sites a difference of 3?6–5?21 was expected based on the adiabatic lapse rate (4?5–6?51
per 1000 m depending on water content), and the actual value was 4?11. Temperature
fluctuations and low-temperature extremes were not as low when snow depths were
greater at the 2575 m compared to 1775 and 2175 m sites. During these times,
780
M. E. LOIK & S. P. REDAR
temperatures near the soil surface remained at approximately 01C. These results are
consistent with studies in the Rocky Mountains and Canada (Gieger, 1965). Despite
being 800 m lower than the highest elevation site, temperatures at the 1775 m site
were 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 the
winter, and intermittently covered at the 1775 and 2175 m sites. Seedling survival at
high altitudes may depend on sufficient snow cover to ameliorate low temperatures
near 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 the
canopy. The A. tridentata seedlings tended to occur under the canopy, whereas at the
lower elevation sites they were more exposed. This could influence seedling survival,
especially at the higher elevation because temperatures in the microclimate near the
soil 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 lowest
elevation site could lead to lower than expected temperatures near the soil surface due
to 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 from
ice encasement and snow pressure.
The mechanisms by which plants have responded to past changes in climate
depended, in part, on how physical factors influenced seedling establishment (Clark et
al., 1998). Insights into seedling tolerances to abiotic factors may help our
understanding of how future changes in climate will impact the distribution and
productivity of vegetation. The future climate of the Sierra Nevada may include
increased summer monsoon rainfall (Knox, 1991; Arritt et al., 2000), which could
enhance seedling recruitment for certain species. Experimental warming that changes
soil water content in the Rocky Mountains alters water relations, photosynthesis, and
above-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 seedling
recruitment and adult reproductive effort, require further experimentation along
gradients from high-elevation desert to montane ecosystems.
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