Download Tibetan Alpine Tundra Responses to Simulated Changes in Climate

Tibetan Alpine Tundra Responses to Simulated Changes in Climate: Aboveground Biomass and
Community Responses
Author(s): Yanqing Zhang and Jeffrey M. Welker
Source: Arctic and Alpine Research, Vol. 28, No. 2 (May, 1996), pp. 203-209
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Arctic and Alpine Research, Vol. 28, No. 2, 1996, pp. 203-209
Tundra
to Simulated
inClimate:
Tibetan
Responses
Changes
Alpine
Biomass
andCommunity
Responses
Aboveground
Abstract
Yanqing Zhang
Departmentof Ecology, Northwest
PlateauInstituteof Biology, The
Chinese Academyof Sciences,
Xining, 810001, P. R. China.Present
address:Departmentof Rangeland
Resources,Utah State University,
Logan, Utah 84322-5230, U.S.A.
Jeffrey M. Welker
NaturalResourceEcology Laboratory
and the Departmentof Rangeland
EcosystemScience, ColoradoState
University,Fort Collins, Colorado
80523, U.S.A. Presentaddressand
reprintrequests:Departmentof
RangelandEcology, Universityof
Wyoming,Laramie,Wyoming82071,
U.S.A.
High-elevation ecosystems are predicted to be some of the terrestrial habitats most
sensitive to changing climates. The ecological consequences of changes in alpine
tundra environmental conditions are still unclear especially for habitats in Asia.
In this study we report findings from a field experiment where an alpine tundra
grassland on the Tibetan plateau (37'N, 101oE) was exposed to experimental
warming, irradiance was lowered, and wind speed reduced to simulate a suite of
potential changes in environmental conditions. Our warming treatment increased
air temperatures by 5oC on average and soil temperatures were elevated by 3'C
at 5 cm depth. Aboveground biomass of grasses responded rapidly to the warmer
conditions whereby biomass was 25% greater than that of controls after only 5
wk of experimental warming. This increase was accompanied by a simultaneous
decrease in forb biomass, resulting in almost no net change in community biomass
after 5 wk. Lower irradiance reduced grass biomass during the same period. Under
ambient conditions total aboveground community biomass increased seasonally
from 161 g m-2 in July to a maximum of 351 g m-2 in September, declining to
285 g m-2 in October. However, under warmed conditions, peak community biomass was extended into October due in part to continued growth of grasses and
the postponement of senescence. Our findings indicate that while alpine grasses
respond favorably to altered conditions, others may not. And, while peak community biomass may actually change very little under warmer summers, the duration of peak biomass may be extended having feedback effects on net ecosystem
CO2 balances, nutrient cycling, and forage availability.
Introduction
High-elevation and high-latitude ecosystems are predicted
to be some of the terrestrialhabitats most sensitive and vulnerable to changing climates (Chapin et al., 1992; Korner, 1992;
Grabherret al., 1994). These ecosystems are composed of slowgrowing plants and are dominatedby soils which can be high in
organic matter(Billings, 1987; Bowman et al., 1993). Both plant
growth and possibly organic matter decomposition are expected
to increase under warmer climates, though the relative balance
between the two and whether these habitats are carbon sources
or sinks for CO2are still unclear (Oechel et al., 1993; Brooks et
al., 1995).
A suite of abiotic conditions may be modified as weather
patterns and regional climates change altering biospheric and
atmospheric processes in tundra ecosystems (Maxwell, 1992;
Shaver et al., 1992; Jonasson et al., 1993; Grabherret al., 1994;
Larigauderie and K6rner, 1995). For instance, warmer air temperatures will likely alter the flux of water from these ecosystems to the atmosphere drying soils and contributing to increased cloud formation. Simultaneously, warmer conditions
may increase plant growth, primary production and carbon sequestration, so long as cloud cover is not affected and other
factors such as water or nutrients do not limit photosynthesis
and growth (Haag, 1974; Bowman et al., 1993; Wookey et al.,
1995).
The ecological consequences of changes in tundra environmental conditions will be manifested in a host of processes
? 1996 Regents of the Universityof Colorado
0004-0851/96 $7.00
including shifts in primary production (Bowman et al., 1993;
Walkeret al., 1994), trace gas fluxes (Brooks et al., 1995), plant
and soil mineral nutrition (Nadelhoffer et al., 1991; Shaver and
Chapin 1991), reproductiveplant biology (Wookey et al., 1993,
1994), leaf carbon isotope discrimination (Welker et al., 1993),
as well as changes in species dominance (Walker et al., 1994).
However, it is unclear whether all these processes are sensitive
to short-termchanges in environmental conditions in all tundra
habitats or whether multiple years of climate change are necessary to elicit detectable alterations in plant performance and
species abundance. To date, most studies of alpine tundra responses to in situ changes in climate, using field manipulations,
have been confined to sites in North America and in Western
Europe (Kirner, 1992; Chapin et al., 1995; Kennedy, 1995;
Welker et al., 1995) without the consideration of the extensive
alpine tundra in Asia, and in particular,western China.
In this study we report findings from a field experiment
where alpine tundra on the north eastern side of the Tibetan
plateau, Qinghai Province, China was exposed to experimental
warming using polyethylene greenhouses (Havstromet al., 1993;
Wookey et al., 1993), irradiancewas lowered using shade cloth
(Chapin and Shaver, 1985) and wind speed at the ground was
lowered with short side fences. Our initial studies have been
directed towards identifying short-term responses to simulated
environmentalchange focusing on abovegroundbiomass of the
three dominant life forms and community compositional attributes.
Y. ZHANGANDJ. M. WELKER/ 203
TABLEI
Methods
andMaterials
FIELD SITE
Xia (1989) and Cincottaet al. (1992) give complete details
of the vegetation and geography of our study site, so here we
present only a brief overview. Our researchsite is located near
the Haibei Alpine Meadow Ecosystem Research Station (37?N,
101oE) at an elevation of 3250 m. The field site is located in the
foothills of Mt. Lenglongling in the eastern part of the Qilian
Mountains, 160 km northwestof Xining. The vegetation of our
site is typical of a Kobresia humilis meadow (Zhou et al., 1987;
Yang et al., 1989; Zhang and Zhou, 1992) with associated species such as Elymus nutans intermixed with herbaceous forbs.
Snow covers these tundrasites for 8 mo between October and
late May and permanent snow cover occurs at 4200 m with
treeline occurringat approximately2950 m.
SETUP
EXPERIMENTAL
Our study was initiated in June 1991 and the first season
was completed in October 1991. Four treatmentswere implemented in June:(1) Minigreenhouses(G) were used to warm air
temperatureand subsequentlysoil temperaturesusing a design
similar to Havstromet al. (1993) and Wookey et al. (1993). The
greenhouses covered an area 2 X 5 m with a center height of
80 cm. The frame was constructedof narrow (8.0 mm) steel
tubing and clear polyethylene plastic film (0.1 mm) was used as
a cover. A 400-cm2 hole was cut into the top of the tent to
facilitate rainfall entry into these plots. (2) Solar irradiance(S)
was reducedby 30% using shade cloth to simulate increases in
cloudiness (Chapin and Shaver, 1985). Shade cloth was placed
across a frame similar to the one used for the minigreenhouses.
(3) Wind speed at the ground level was reduced by placing 20
cm high side fences (SF) aroundthe entire plot. (4) Control (C)
plots had only the steel frame across the top of the 2 X 5 m
area. A completely randomizeddesign was used to establish the
16 treatmentplots consisting of four treatments(G, S, SF, C)
replicatedfour times.
TE MONITORING
MICROCLIMA
Air temperature,relative humidity, and wind speed were
measuredat 20 cm above the soil surface in the center of the
experimentalplots using a thermometer,humidity sensor and an
anemometer.Soil temperaturewas measuredat 5, 10, and 15 cm
depths using soil thermometersand soil moisturewas measured
using tensiometers(Soil Moisture Corp., Calif., U.S.A.). Absolute vapor density was calculated using the average air temperaturesand averagerelativehumidities.All parameterswere measured over the entire growing season in the four differenttreatments. Measurementswere taken to assess both daily changes
in abiotic conditions and taken periodically over the course of
the summerto examine seasonal patterns.Daily patternsof abiotic conditions were measured in the treatmentplots being recorded every 2 h between 0800 and 2000 h from 3-5 August.
Seasonal patterns were based on measurementstaken at 0800,
1400, and 2000 every 10 d between July and October.No observations were made at night and thus our estimates of greenhouse effects on temperatureare likely an over estimationof the
daily averages as at night when there is no irradiance,temperaturedifferencesbetween ambientand warmedconditionswould
be minimal as reportedby Wookey et al. (1993) and Welker et
al. (1995).
204 / ARCTIC AND ALPINE RESEARCH
Abiotic conditions from the four treatments between July and
October 1991
GreenControl house
Treatments
Mean air temperature (QC)
Mean soil temperature (?C)
Side
fence
12.38
17.78
14.33
12.91
5 cm
10 cm
15 cm
12.79
12.29
12.19
16.07
15.14
13.95
13.85
12.76
12.74
12.44
12.06
12.10
4.00
12.00
6.00
5.80
10 cm
14.80
21.07
30.39
14.94
Vapor density (g m-3)
Soil suction (Kpa)
Shade
ATTRIBUTES
VEGETATION
We quantifiedplant height, species frequency,and species
cover along with abovegroundplant biomass. Abovegroundbiomass was measuredusing double sampling whereby ocular estimates were coupled to destructive harvest in 0.1 m2 plots
(Mueller-Dombois and Ellenberg, 1974; Cook and Bonham,
1977). Double sampling was used because of the limited treatment applicationarea and this techniquepreserved the area for
sampling in later years. Correlationsbetween estimates and actual abovegroundharvests were highly significant(r2 = 0.92, p
< 0.01) as measuredeight differenttimes over the course of the
growing season. Clipped vegetation was sorted into live and
dead and then pooled by life form into grasses, sedges, and
forbs. Green tissue was then oven dried at 600C for 48 h and
weighed. Aboveground biomass was tested for significant harvest date and treatmenteffects using analysis of variance techniques for a completely randomizeddesign (SAS Inc., 1988).
Differences between treatments were considered significant
when p < 0.05 (SAS Inc., 1988). Mean separationsfor aboveground biomass within each harvest date were conducted using
Duncan's multiple range test. Species importancevalues, an index of community attributes,were calculated using the cover,
composition and frequencydata collected from for each species
in the four treatmentsusing permanent0.5 x 0.5 m quadrants
(Mueller-Domboisand Ellenberg, 1974).
Results
The greenhousetreatmentincreased mean air temperatures
by 20% from 12.4 to 17.80C over the course of the growing
season (Table 1). Warmerair temperaturessubsequentlyresulted
in higher soil temperaturesat 5, 10, and 15 cm with the largest
increase in soil temperatureat 5 depth cm. where the average
temperaturewas 3.3?Chigher undergreenhouse(G) as opposed
to ambient (C) conditions (Table 1). The mean vapor density
was also increasedunderwarmertemperaturesof the greenhouse
from 4 to 12 g m-3 while soil suction was essentially the same
between all treatmentplots, except for under shaded (S) conditions, were soil suction was consistently higher indicatinglower
soil water content. The shade treatment,while reducing irradiance, also resultedin a slight increasein temperaturesand a IVC
increase in soil temperatureat 5 cm. The shade treatmenthad
no effect on soil temperaturesat 10 cm or 15 cm nor did the
shade treatmentalter the vapor densities. Side fences had no
effect on ambient air temperaturesand subsequentlyno effect
on soil temperatures.
Daily air temperaturesof all four treatmentsreached their
maxima at 1400 h, though soil temperaturesat 5 cm lagged
-
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oa
,30
40 -
.
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20-
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E
S10-
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10
8
10
14
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20
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24-
10
20
31
10
20
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10
20
30
10
20
31
10
20
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10
20
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20
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E
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16
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Time of day
D-O Control
Shading
20
31
July
0-0
-
10
20
30
10
August
20
30
September
Control
A-A Shading
Greenhouse V-V Side fence
"- GreenhouseV--v Sidefence
FIGURE 1. Mean daily air temperature (?C), 5 cm soil temperature (?C), and 10 cm soil suction (kPa) between 3 and 5
August 1991 at Haibei Alpine Meadow Ecosystem Research Station.
behind reaching their peak at 1600 h or at 1800 h as measured
in early August 1991 (Fig. la, lb). Soil water was generally
constant over the course of the day with a slight decrease in the
late afternoon under ambient conditions but rose progressively
during the afternoonunder warmedconditions (Fig. Ic). Overall,
soil water suctions were significantly (p < 0.05) higher in the
shaded plots than in the other three treatments.
Seasonal patterns of abiotic conditions indicate that the air
temperatures were consistently higher in the greenhouses (G)
and that after 10 August, temperaturesgradually declined until
late September(Fig. 2a). These trends in air temperatureare also
manifested in the seasonal pattern of soil temperatures. The
greenhouses had the largest effect on soil temperaturesin late
July, and continued to result in elevated temperaturesthrough
the end of September (Fig. 2c). Seasonal soil temperaturesin
control and treatment plots tracked air temperatureswith soil
temperaturesin control plots varying from a high of 150C in
July to a low of 50C in late Septemberwhile temperaturesin the
greenhouses varied from a high of 190C in mid-July and early
August to a low of 80C in late September.
Aboveground biomass was initially similar among all treatments for forbs, sedges and grasses (Fig. 3a). Within 5 wk after
the warming treatments were implemented, grass biomass was
significantly higher in the warmed as compared to control conditions (Fig. 3b). Conversely, grass biomass was significantly
reduced during this same period under shaded conditions (Fig.
3b). Reductions of wind using side fences (SF) had no significant
effect on grass, sedge or forb biomass (Fig. 3b).
By September, grass biomass differences between control
and warmed plots were nonsignificantthough forb biomass was
FIGURE 2. Seasonal progression of the mean air temperature
('C), 5 cm soil temperature(?C), and 10 cm soil suction (kPa)
from July to Septemberin 1991 at Haibei Alpine Meadow Ecosystem Research Station.
significantly (p < 0.05) lower in the greenhouses (G) as opposed
to control conditions (C) (Fig. 3c). Lower irradiancehad a significant effect on grass growth and in September,grass biomass
was 36% less in shaded (S) as opposed to control conditions.
Forb biomass was slightly higher in side-fenced areas as compared to control conditions.
Between Septemberand Octobergrass in control plots started to senesce and biomass began to decline (Fig. 3c, 3d). However, under warmed (G) conditions, grass biomass was significantly (p < 0.01) higher in warmed as opposed to control conditions in October which postponed community senescence (Fig.
3d). This prolonged growth, or postponed senescence during the
fall in warmed plots occurred as the greenhouses maintained
warmer air and soil temperaturesthan ambient conditions (Fig.
2). Biomass of grasses and forbs were slightly lower under shaded (S) conditions in October, while sedge biomass was significantly (p < 0.05) higher under these same reduced irradiance
conditions (Fig. 3d).
Total community aboveground biomass in all four treatments was not significantly different in July and total aboveground biomass in warmed conditions, as compared to ambient
conditions, was not significantly higher until October (Table 2).
However, lowered irradiance(S) resulted in a 23% decrease in
total community biomass within 5 wk of treatmentapplications.
Total biomass under reduced irradiance(S) continued to be the
lowest over the course of the season reaching a maximum of
only 80% of the peak biomass under ambient conditions (Table
2).
Species importancevalues as a measure of community level
responses are presented in Table 3. Under reduced radiation(S)
Y. ZHANGANDJ. M. WELKER/ 205
200 ,
E 200
150 -
150-
U
E
o
S100
b
a
0E
o
aaaa
a
bb
a
E
100
a aa
a aaa
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Grasses Sedges
a
Forbs
Grasses Sedges
July
"
-
August
200 -
0-
50
10
150
o 200
50
b
aI150"-
o
Forbs
a
Control
E
0
FIGURE 3. The aboveground
biomass of grasses, sedges, and
forbs in control, greenhouses,
shaded, and side fenced treatment plots sampled in July, August, September, and October
1991. Superscripts of the different letters denote biomasses
which were significantly different (p < 0.05) for each individual sampling date.
Shading
reductions in Elymus and Festuca were associated with increases
in Stipa and Scirpus which dramaticallyaltered the composition
and structureof these plant communities. Changes in community
composition and structure under warmer conditions (G) were
manifested by lower importance values for Poa and Kobresia
with correspondingincreases in importance values for Stipa and
Oxytropis (Table 3).
Discussion
Total maximum abovegroundbiomass at our Tibetan alpine
tundra site ranged from 161 to 351 g m-2 under ambient conditions (Table 2). These ranges in biomass are similar to the peak
aboveground biomass at other alpine tundra sites such as on
Niwot Ridge, Colorado, U.S.A., where the intercommunity
aboveground biomass in different vegetation types ranges from
71 to 309 g m-2 (Walker et al., 1994). Our environmental manipulations simulating climate warming resulted in warmer air
and soil temperaturesbetween 1 and 5'C, which is within the
ranges of increase reportedfor higher elevations in Western Eu-
rope over the past 15 yr (Rozanski et al., 1992; Grabherret al.,
1994) and is within the ranges predictedfor tundrahabitatsunder
a doubling of CO2 over the next 50 yr (Maxwell et al., 1992).
The season long average increases are also similar to those accomplished in other tundra experimental warming treatments
though our lack of nighttime measurementsmeans our averages
are slightly higher than those actually experienced by plants and
soil in these treatmentplots (Chapin and Shaver, 1985; Wookey
et al., 1993; Parsons et al., 1994; Kennedy, 1995; Welker et al.,
1995). However, most importantly, higher temperatures were
maintained in our warmed plots into October and may partially
explain the extended growing season observed for grasses.
Grass and forb biomass productionwas especially sensitive
to warmer conditions (Fig. 3). Grass abovegroundbiomass was
25% greater under warmer conditions after only 5 wk of warming while forb biomass decreased by 30% (Fig. 3b). Differences
in abovegroundgrass biomass between warmer and control conditions were diminished by September when grass biomasses
were not significantly different (Fig. 3c).
However, it appearsthat community senescence, which usu-
TABLE 2
Total aboveground biomass (g m-2) from the four treatments in July, August, September, and October 1991
Date
Control
Greenhouse
Shade
Side fence
3 Jul.
161.16
157.52
145.86
164.00
+
+
+
?
10.23a
7.13a
7.51a
9.25a
1 Aug.
269.36
252.37
206.69
247.30
+ 17.57a
+ 16.57a
+ 17.63b
+ 10.80a
Differencesbetweenthe treatmentswithineach monthat p < 0.05 are noted by differentletters.
206 / ARCTIC AND ALPINE RESEARCH
2 Sept.
351.36 1 15.55a
334.61 1 13.97a
278.93 ? 13.78b
370.08 1 6.45a
2 Oct.
285.68 +
346.19+
266.21 1
300.80 -
5.49b
11.81a
10.63b
5.07b
TABLE3
The importance value of dominant plant species between four
treatmentplotsa
Plant species
Elymus nutans
Festuca ovina
Poa pratensis
Koeleria cristata
Stipa aliena
Kobresia humilis
Carex scabriostris
Scirpus distigmaticus
Saussurea superba
Gentiana aristata
Oxytropis ochrocephala
Trigonella ruthenica
Taraxacum mongolicum
Potentilla bifurca
Aster flaccidus
Oxytropis caerulea
Potentilla anserina
Gentiana straminea
aC-control;
G-greenhouse;
C
G
S
SF
52.05
35.05
21.12
26.32
22.29
19.65
12.62
15.10
24.16
8.15
16.17
8.69
8.67
10.50
15.34
11.87
8.83
14.65
54.07
39.08
14.53
17.63
29.01
6.55
10.14
10.72
30.61
10.11
34.95
16.33
10.93
10.20
9.25
13.55
8.74
8.86
23.80
23.64
27.99
18.22
31.74
18.66
14.04
35.22
27.18
22.13
15.82
8.01
9.48
9.02
8.62
11.23
8.67
21.62
56.09
33.42
20.04
23.86
21.21
23.85
13.32
16.54
21.13
10.01
26.33
7.75
9.19
13.94
5.71
8.40
16.91
14.88
S-shade;
SF-side
fence.
ally starts in September, was postponed until sometime in October under warmer (G) conditions as evidenced by no decline
in abovegroundcommunity biomass between Septemberand October (Table 2). This postponing of senescence and subsequently
an extension of the growing season under warmed conditions,
resulted in part because peak grass biomass was not realized
until early October amounting to 177 g m-2 (Fig. 3d).
The ability of the grass life form at our site to exhibit a
rapid, positive response to warmer conditions and to extend the
season of growth is likely the result of (1) the existence of a
large leaf area at the time of treatment application, (2) the inherent physiological capacity of grasses to alter patterns of resource allocation (Welker et al., 1985, 1987; Welker and Briske,
1992), (3) their morphological and demographic capacity to
elongate fall tillers (Briske and Butler, 1989), and (4) the ability
to grow when environmentalconstraintsare temporallyremoved
(Sala et al., 1992).Grasses at other tundrasites have also exhibited an ability to respond rapidly to simulatedchanges in climate
as exemplified by Calamagrostis biomass increases in the subarctic at Abisko, Sweden under warmer conditions (Parsons et
al., 1995). The grass growth response reportedby Parsons et al.
(1995), in what is typically a dwarf shrub dominatedecosystem,
was due in large part to an extensive, preexisting network of
underground Calamagrostis meristems, capable of rapid shoot
extension and leaf development up through the dwarf shrub understory.
The shift in alpine tundracommunity biomass characteristics whereby maximum biomass is maintained into the autumn
is different from what might be observed in arctic tundradominated by deciduous dwarf shrubs. Prolonged growth of many
arctic plants in autumn is unlikely due to photoperiodic cues
which control senescence (Murry and Miller, 1982). Thus, even
if conditions in arctic tundrawere warmer in fall, the ability of
many dominant life forms to either produce new fall foliage or
continue expansion of existing leaf and shoot biomass is limited
by life history traits.And while graminoids, such as Eriophorum
may constitute a large fraction of the biomass in these systems
(Shaver et al., 1992), extended growth in fall under warmertemperaturesmay be unlikely due to the low solar angles in autumn.
The ability of grasses to utilize favorable conditions at the
end of the season is a trait similar to that observed for other
tundra lifeforms such as evergreen shrub species (Karlsson,
1985; Welker et al., 1995). For instance, Welker et al. (1995)
have found evidence that Dryas octopetala, a wintergreen species, has the capacity to exhibit net carbon assimilation at the
end of the season under warmer,wetter,and fertilized conditions
when plants in control conditions have ceased gaining carbon,
which is made possible in part by its evergreen nature. In addition, Karlsson (1985) found that 20% of the carbon acquired
by the evergreen dwarf shrub, Vacciniumvitis-idaea occurredin
spring and in autumn,before leaf emergence or after leaf senescence in the deciduous species, Vaccinium uliginosum. Thus,
evergreen dwarf shrubs are also a tundralife form which due to
their inherent life history characteristicscan respond to changes
in environmental conditions which occur in spring, and fall
(Wookey et al., 1993; Welker et al., 1995).
The opportunisticbehavior of grasses we observed was not
evident for forbs. During the initial 5-wk period forb biomass
was reduced under warmer conditions while grass biomass was
increasing (Fig. 4b). The opposite response for forbs may have
been due in part to the grasses out-competing forbs for water,
nutrients and or light. However, the overall community level
response was that total biomass was not different between
warmed (G) and control (C) conditions after 5 wk of experimental applications (Table 2). This observation of similar community biomass undermodified environmentalconditions is consistent with the observationsof Chapin and Shaver (1985). These
authors found that arctic tundra total community production
(currentyears growth) in perturbedand in control plots remained
the same. This inherent buffering was achieved because some
species or life forms increased growth while others exhibited
reduced growth.They concluded that conditions favorable for
one species or life form are less favorable for others, though the
total community or ecosystem productionchanges annuallyvery
little (Chapin et al., 1995). This attributeof tundra ecosystems
may be the result of the inherently low nutrientlevels available
to plants in tundra which constrains system level primary production (Shaver et al., 1992).
The one life form in our study which appeared to be the
least responsive to simulated climate warming were the sedges,
consisting primarilyof Kobresia humillis. The lack of significant
increases in biomass until the end of the first season underwarmer or shaded conditions indicates that this life form has a relatively low sensitivity to temperatureand irradiance. However,
other sedges, such as Kobresia myosuroides on Niwot Ridge,
Colorado, exhibits an increase in biomass underelevated nutrient
availability (Bowman et al., 1993). This would suggest that
while the warmerconditions in soils under our minigreenhouses
may have elevated soil mineralization and increased nutrient
pools available to plants (Jonasson et al., 1993; Robinson et al.,
1995) the increases were either not sufficient to alter Kobresia
growth, or that Kobresia root uptake rates are low, and its ability
to compete for soil nutrients with grasses is low (Black et al.,
1994; Falkengren-Grerup,1995). Even though soil nutritionmay
have been alteredunder warmed conditions, the ability of sedges
at our site to acquire these resources in a competitive setting
appears to be limited, in part due possibly to resource capture
by soil microbes (Jackson et al., 1989; Schimel et al., 1989).
However, in future years changes in rooting patternsmay enable
this species to capitalize on changes in soil resources.
In conclusion, our findings suggest that Tibetan alpine
grasses are predisposed to rapid increases in biomass under simulated climate warming due in part to their inherent life history
Y. ZHANGAND J. M. WELKER/ 207
traits.In addition,the ability of grasses to produce tillers late in
the season underwarmerconditionsextends the periodof carbon
gain and extends the period in which the community exhibits
maximumabovegroundbiomass. We find that sedges at our site
are insensitive in the short term to changes in environmental
conditions, while forbs may decrease at the expense of grass
biomass. Increases in cloudiness over the Tibetan alpine tundra
would likely result in lower abovegroundbiomass, but if accompanied by higher rainfallthe effects may be counter-acting.The
extension of peak community biomass into the autumnmay in
the long term have cascading effects on net ecosystem CO2fluxes, nutrientcycling, and forage availabilityto grazers.
Acknowledgments
We thank Dr. PremindaJacob and Chen Bo for assistance
in the field and Dr. Andy Parsons for assistance with graphics
and for reviewing an earlierversion of the manuscript.Research
was supportedby Haibei Alpine Meadow Ecosystem Station900318, the Biosphere Program, U. S. State DepartmentGrant
1753-900561 and in part by U.S. InternationalTundraExperiment (USITEX)(NSF/OPP-9321730)awardedto JMW.We also
wish to thanktwo reviewers for their helpful comments and critique of an earlier version of the manuscript.
References
Cited
Billings, W. D., 1987: Constraintsto plant growth, reproduction
and establishmentin arctic environments.Arctic and Alpine
Research, 19: 357-365.
Black, R. A., Richards,J. R., and Manwaring,J. H., 1994: Nutrient uptake from enriched microsites by three Great Basin
perennials.Ecology, 75: 110-122.
Bowman, W. D., Theodose, T. A., Schardt,J. C., and Conant,R.
T., 1993: Constraintsof nutrientavailability on primaryproduction in two alpine tundracommunity.Ecology, 74: 20852097.
Briske, D. D. and Butler, J. L., 1989: Density-dependentregulation of rametpopulationswithin the bunchgrassSchizachyrium scoparium:interclonalversus intraclonalinterence.Journal of Ecology, 77: 963-974.
Brooks, P. D., Williams, M. W., Walker,D. A., and Schmidt, S.
K., 1995: The Niwot Ridge snow fence experiment:Biogeochemical responses to changes in the seasonal snowpack. In
Tonnessen, K., Williams, M. W., and Tanter,M. (eds.), Biogeochemistry of Seasonally Snow-CoveredCatchments(Proceedings of a Boulder Symposium, July 1995). International
Association of Hydrological Sciences Publication 228, 293302.
Chapin,E S., Shaver,G. R., Giblin, A. E., Nadeloffer,K. J., and
Laundre,J. A. 1995. Responses of arctictundrato experimental and observed changes in climate. Ecology, 76: 694-711.
Chapin, E S., Jefferies, R. L., Reynolds, J. E, and Svoboda, J.,
1992: Arctic plant physiological ecology in an ecosystem context. In Chapin,E S., Jefferies, R. L., Reynolds, J. E, Shaver,
G. R., and Svoboda, J. (eds), Arctic Ecosystemsin a Changing
Climate: An Ecophysiological Perspective. San Diego: Academic Press, 441-452.
Chapin, E S. and Shaver, G. R., 1985: Individualisticgrowth
response of tundraplant species to environmentalmanipulations in the field. Ecology, 66: 564-576.
Cincotta, R. P., Zhang, Y. Q., and Zhou, X. M., 1992: Transhumant alpine pastoralismin northwesternQinghaiprovince:An
evaluation of livestock population response during China's
agrarianeconomic reform.Nomadic People, 30: 3-25.
Cook, C. W. and Bonham, C. D., 1977: Techniquesfor vegetation measurementsand analysis for pre- and post-mining inventory. Science Series No.28, Range Science Department,
Colorado State University.
208 / ARCTIC AND ALPINE RESEARCH
Falkengren-Grerup,U., 1995: Interspecies differences in the
preferenceof ammoniumand nitratein vascularplants. Oecologia, 102: 305-311.
Grabherr.G., Gottfried,M., and Pauli, H., 1994: Climate effects
on mountainplants. Nature, 369: 448-450.
Haag, R. C., 1974. Nutrient limitations to plant production in
two tundra communities. Canadian Journal of Botany, 52:
103-116.
Havstrom,M., Callaghan,T. V., and Jonasson, S., 1993: Differential growth responses of Cassiope tetragona, an arctic
dwarf-shrub,to environmentalperturbationsamong threecontrastinghigh and subarcticsites. Oikos, 66: 389-402.
Jackson,L. E., Schimel, J. P., and Firestone,M. K., 1989. Shorttermpartitioningof ammoniumand nitratebetween plantsand
microbes in an annual grassland.Soil Biology and Biochemistry, 21: 409-415.
Jonasson, S., Havstrom,M., Jensen, M., and Callaghan, T. V.,
1993: In situ mineralizationof nitrogen and phosphorus of
arctic soils afterperturbationssimulatingclimate change. Oecologia, 95: 179-186.
Karlsson,P. S., 1985: Effect of waterand mineralnutrientsupply
on a deciduous and evergreen dwarf shrub: Vacciniumuliginosum L. and V. vitis-idaea L. Holarctic. Ecology, 8: 1-8.
Kennedy, A. D., 1995: Simulated climate change: are passive
greenhousea valid microcosnfor testing the biological effects
of environmental perturbation?Global Change Biology, 1:
29-42.
Korner,Ch., 1992. Response of alpine vegetation to global climate change. In: InternationalConferenceon LandscapeEcological Impactof ClimateChange. Lunteren,The Netherlands,
Catena Verlag, Supplement,22: 85-96.
Larigauderie,A., and KSrner,Ch., 1995. Acclimation of leaf
dark respirationto temperaturein alpine and lowland plant
species. Annals of Botany, 76: 245-252.
Maxwell, B., 1992: Arctic climate: Potential for change under
global warming. In Chapin, E S., Jefferies, R. L., Reynolds,
J. E, Shaver,G. R., and Svoboda, J. (eds), Arctic Ecosystems
in a Changing Climate:An EcophysiologicalPerspective. San
Diego: Academic Press, 11-34.
Mueller-Dombois,D. and Ellenberg,H., 1974: Aims and Methods of VegetationEcology. New York:John Wiley. 547 pp.
Murry,C. and Miller, P. C., 1982. Phenological observationsof
majorplant growth forms and species in montaneand Eriophorum vaginatum tussock tundrain central Alaska. Holarctic
Ecology, 5: 109-116.
Nadelhoffer,K. J., Giblin, A. E., Shaver,G. R., and Laundre,J.
A., 1991: Effects of temperatureand substratequality on element mineralizationin six arctic soils. Ecology, 72: 242-253.
Oechel, W. C., Hastings, S. J., Vourlitis,G., Jenkins,M., Riechers, G. and Grulke,N., 1993: Recent change of Arctic tundra
ecosystem from a net carbondioxide sink to a source. Nature,
361: 520-523.
Parsons, A. N., Welker, J. M., Wookey, P. A., Press, M. C.,
Callaghan,T. V., and Lee, J. A., 1994: Growth responses of
four sub-arctic dwarf shrubs to simulated climate change.
Journal of Ecology, 82: 307-318.
Parsons, A. N., Press, M. C., Wookey, P. A., Welker, J. M.,
Robinson,C. H., CallaghanT. V., and Lee, J. A., 1995: Growth
and reproductiveoutput of Calamagrostis lapponica in response to simulated environmentalchange in the subarctic.
Oikos, 72:61-66.
Robinson, C. H., Wookey, P. A., Parsons, A. N., Potter,J. A.,
Callaghan,T. V., Lee, J. A., Press, M. C., and Welker,J. M.,
1995: Responses of plant litter decomposition and nitrogen
mineralisationto simulated environmentalchange in a high
arctic polar semi-desert and a subarctic dwarf shrub heath.
Oikos, 74: 503-512.
Rozanski, K., Araguis-Aragufs, L., and Gonfiantini,R., 1992:
Relation between long-termtrendsof oxygen-18 isotope composition of precipitationand climate. Science, 258: 981-985.
Sala, O. E., and Lauenroth,W. K. and Parton,W. J. 1992. Longterm soil water dynamics in the shortgrass steppe. Ecology,
73: 1175-1181.
SAS InstituteInc., 1988: SAS/STAT User's Guide, Release 6.03.
Ed. SAS Inst. Inc. Cary, NC.
Schimel, J. P., Jackson, L. E., and Firestone, M., 1989: Spatial
and temporal effects on plant-microbialcompetition for inorganic nitrogen in a California annual grassland. Soil Biology
and Biochemistry, 21: 1059-1066.
Shaver, G. R. and Chapin, E S., 1991: Production:biomassrelationships and element cycling in contrastingarctic vegetation
types. Ecological Monographs, 61: 1-31.
Shaver, G. R., Billings, W. D., Chapin, E S., Giblin, A. E., Nadelhoffer, K. J., Oechel, W. C. and Rastetter, E. B., 1992:
Global change and the carbon balance of arctic ecosystems.
Bioscience, 42: 433-441.
Walker,M. D., Webber,P. J., Arnold, E. H., and Ebert-May,D.,
1994: Effects of interannualclimate variationon aboveground
phytomass in alpine vegetation. Ecology, 75:393-408.
Welker,J. M., Rykiel, E. J., Briske, D. D., and Goeschel, J. D.,
1985: Carbon import among vegetative tillers within two
bunchgrasses:assessment with carbon-11 labelling. Oecologia,
67: 209-212.
Welker,J. M., Briske, D. D., and Weaver,R. W., 1987: Nitrogen15 partitioningwithin a three generation tiller sequence of the
bunchgrasSchizachyriumscoparium:response to selective defoliation. Oecologia, 74: 330-334.
Welker, J. M. and Briske, D. D., 1992: Clonal biology of the
temperate caespitose graminoid Schizachyriumscoparium: A
synthesis with reference to climate change. Oikos, 56:357365.
Welker,J. M., Wookey, P., Parsons, A. P, Callaghan,T. V., Press,
M. C., and Lee, J. A., 1993: Leaf carbon isotope discrimination and demographicresponses of Dryas octopetala to water
and temperaturemanipulations in a high arctic polar semidesert, Svalbard. Oeclogia, 95: 463-749.
Welker, J. M., Svoboda, J., Henry, G., Molau, U., Parsons, A.
N., and Wookey, P. A., 1995: Response of two Dryas species
to ITEX environmental manipulations:A synthesis with circumpolar comparisions. In: Proceedings from the 6th International Tundra Experiment (ITEX). Ottawa, Canada. April
1995. Abstract.
Wookey, P. A., Parsons, A. N., Welker, J. M., Potter, J., Callaghan, T. V., Lee, J. A., and Press, M. C., 1993: Comparative
responses of phenology and reproductivedevelopment to simulated environmental change in sub-arctic and high arctic
plants. Oikos, 67: 490-502.
Wookey, P. A., Welker, J. M., Parsons, A. N., Press, M. C.,
Callaghan, T. V., and Lee, J. A., 1994: Differential growth,
allocation and photosynthetic responses of Polygonum viviparum to simulated environmentalchange at a high arctic polar semi-desert. Oikos, 70: 131-139.
Wookey, P. A., Robinson, C. H., Parsons, A. N., Welker,J. M.,
Press, M. C., Callaghan, T. V., and Lee, J. A., 1995: Environmental constraints on the growth, photosynthesis and reproductive development of Dryas octopetala at a high arctic polar
semi-desert, Svalbard. Oecologia, 102: 478-489).
Xia, W. P., 1988: A brief introductionto the fundamentalcharacteristics and the work in Haibei Research Station of Alpine
Meadow Ecosystem. Proceedings of the International Symposium of an Alpine Meadow Ecosystem. Bejing: Academic
Sinica, 1-10.
Yang, E T., Wang, Q. J., and Shi, Sh. H., 1989: Seasonal and
annual biomass dynamics of Kobresia humilis meadow. Proceedings of the InternationalSymposiumof an Alpine Meadow
Ecosystem. Beijing: Academia Sinica, 71-80.
Zhang, Y. Q. and Zhou, X. M., 1992: The quantitativeclassification and ordination of Haibei Alpine Meadow. Acta Phytooecological et Geobotanica Sinica, 1:36-42.
Zhou, X. M., Wang, Zh. B., and Du, Q., 1987: Qinghai Vegetation. Qinghai People Press.
Ms submittedJune 1994
Revised ms submittedJune 1995
Y. ZHANGAND J. M. WELKER/ 209