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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, G03S08, doi:10.1029/2007JG000477, 2008
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Energy and water additions give rise to simple responses in plant
canopy and soil microclimates of a high arctic ecosystem
Patrick F. Sullivan,1 Jeffrey M. Welker,1 Heidi Steltzer,2 Ronald S. Sletten,3
Birgit Hagedorn,1 Seth J. T. Arens,1 and Jennifer L. Horwath4
Received 30 April 2007; revised 13 February 2008; accepted 4 March 2008; published 17 May 2008.
[1] Energy and water inputs were increased during the snow-free season to test the
sensitivity of a cold, dry ecosystem to climate change. Infrared radiators were used to
provide two levels of supplemental radiation (T1 and T2) to prostrate dwarf-shrub, herb
tundra in northwest Greenland. The higher radiation addition was combined with
supplemental water in a factorial design. Radiation additions increased midday canopy
temperatures by up to 4.0!C and 6.0!C and growing season mean shallow soil
temperatures by 1.3!C and 2.4!C in T1 and T2 plots, respectively. Soil warming was
measured at and probably exceeded 10 cm in depth. There was no evidence of soil drying
in plots that received additional radiation, in contrast with other studies, nor was there
evidence that supplemental water interacted with radiation additions to affect soil
temperatures. Water additions were generally undetectable against a background of large
seasonal changes in soil water content. We suggest that well-drained soils and strong
seasonal controls on soil water contents (e.g., soil thaw and evapotranspiration) limit
the system’s sensitivity to changes in precipitation during the brief growing season. In
general, multifactor changes in climate gave rise to simple changes in the vegetation
microclimate of this cold, dry ecosystem.
Citation: Sullivan, P. F., J. M. Welker, H. Steltzer, R. S. Sletten, B. Hagedorn, S. J. T. Arens, and J. L. Horwath (2008), Energy and
water additions give rise to simple responses in plant canopy and soil microclimates of a high arctic ecosystem, J. Geophys. Res., 113,
G03S08, doi:10.1029/2007JG000477.
1. Introduction
[2] General circulation models predict that arctic regions
will experience changes in climate that are amplified relative to
temperate and tropical regions [Manabe and Stouffer, 1980,
1996; Manabe et al., 1991; Serreze et al., 2000; Moritz et al.,
2002; Holland and Bitz, 2003; ACIA, 2004; Serreze and
Francis, 2006]. Changes in the Arctic climate are already
underway. Observations made during the 20th century
revealed rising air temperatures [Chapman and Walsh, 1993;
Overpeck et al., 1997], melting glaciers [Dyurgerov and Meier,
1997], reductions in perennial sea ice [e.g., Comiso, 2002] and
longer growing seasons [Stone et al., 2002; Dye, 2002]. Longterm records from western Greenland (1873–2001) and more
contemporary records from northwestern Greenland (1961–
1990) show strong warming trends during summer [Box,
2002]. Conservative estimates predict a !4!C increase in
1
Environment and Natural Resources Institute and Department of
Biological Sciences, University of Alaska, Anchorage, Alaska, USA.
2
Natural Resource Ecology Laboratory, Colorado State University, Fort
Collins, Colorado, USA.
3
Quaternary Research Center, University of Washington, Seattle,
Washington, USA.
4
Geography Department, Augustana College, Rock Island, Illinois,
USA.
Copyright 2008 by the American Geophysical Union.
0148-0227/08/2007JG000477$09.00
arctic air temperatures and a !20% increase in arctic precipitation by 2100 [ACIA, 2004].
[3] High latitude ecosystems are important components of
the global climate system because they occupy a substantial
proportion of the terrestrial surface, exhibit high spatial and
temporal variability in their surface energy budgets and, in
some cases, hold large stores of soil carbon [e.g., Chapin et
al., 2000]. In the High Arctic, soil moisture appears to be the
most important determinant of the surface energy balance
[Eugster et al., 2000]. If changes in climate lead to soil
drying, increases in sensible heat flux, which directly feed
back to increase air temperatures, may accelerate regional
warming. If soil moisture increases with climate change,
reductions in sensible heat flux may feedback to dampen
regional warming. Because changes in temperature and
precipitation are expected to coincide [ACIA, 2004], the net
effect of these changes on soil moisture is uncertain. This
uncertainty is compounded by a paucity of reliable precipitation and soil moisture data from high latitude weather
stations and by the limited capacity of atmospheric models
to simulate precipitation [Serreze et al., 2003].
[4] Field manipulations, intended to simulate climate
change, have become more common in recent years [Shen
and Harte, 2000]. These studies have provided valuable
insights into both the pattern and process of climate changeinduced ecosystem change. Climate variables are highly
interactive and are not expected to change in isolation
[ACIA, 2004; IPCC, 2007]. Thus, factorial manipulations
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al., 1995], an agricultural system [Kimball, 2005] and high
arctic wet tundra [Nijs et al., 2000]. Our experiment expands
on previous work in the sense that supplemental energy and
water were applied to a cold, dry ecosystem with welldrained soils, where previous investigators have suggested
that both water and temperature may limit ecosystem function [Teeri, 1973; Bliss, 2000]. We expected to observe strong
interactions between energy supplements, water additions
and vegetation responses, as shown in previous studies
[Harte et al., 1995; Zavaleta et al., 2003].
2. Materials and Methods
Figure 1. Annual, April –September and October – March
air temperature anomalies calculated using 1-year and 5-year
moving windows. Data are from the Thule OP Site (WBAN:
17605) for 1952 through 2006.
of multiple climate variables have helped to identify underlying mechanisms, while providing a more realistic depiction of ecosystem change in the High Arctic [Wookey et al.,
1993; Welker et al., 1993; Havstrom et al., 1993; Baddeley
et al., 1994; Dormann, 2003; Illeris et al., 2003].
[5] This paper describes plant canopy and soil microclimates of an experiment established in high arctic Greenland to test for complex ecosystem responses to multivariate
changes in climate forcing. Air, canopy and soil temperatures, as well as soil water contents, were measured in
response to long-wave radiation and water supplements,
applied singularly and in combination. Previous studies using
infrared (IR) radiators have examined grasslands [Wan et al.,
2002; Zavaleta et al., 2003], a subalpine meadow [Harte et
2.1. Site Description
[6] The experiment was established in prostrate dwarfshrub, herb tundra within a 7 km2 catchment on the Pituffik
Peninsula, Greenland (76! 330N, 68! 340W; elevation 180 m
asl). Data from the Thule Operations site (United States Air
Force) in Pituffik show a mean annual air temperature of
"11.6!C and mean annual precipitation of 12.2 cm between
1971 and 2004. Over the same period, growing season
(June, July, and August) air temperatures averaged 3.5!C
and approximately 50% of precipitation fell between
October and April as snow. Examination of the Pituffik
air temperature record during the last climate normal period,
from 1971 to 2000, shows a warming trend of 0.5!C/decade
in annual air temperatures, a warming trend of 0.8!C/decade
between April and September and no evidence of a trend in
temperatures between October and March (Figure 1).
[7] Prostrate dwarf-shrub, herb tundra occupies approximately 8% of the ice-free Arctic land surface [CAVM Team,
2003]. Soils at the site are subject to intensive frost action.
Vascular plant cover is approximately 50% and the patterned ground is a mixture of nonsorted nets, weakly formed
stripes and frost boils. The vascular plant community, which
maintains an open canopy less than 5 cm in height, is
dominated by three species: the deciduous dwarf-shrub
Salix arctica Pall., the graminoid Carex rupestris All. and
the wintergreen dwarf-shrub Dryas integrifolia M. Vahl..
The live biomass and litter of these three species account for
approximately 70% of vascular plant cover. The soil is a
Typic Haploturbel [Soil Survey Staff, 1998] with a maximum thaw depth slightly greater than 1 m. The particle size
distribution of the near surface soil (0– 12 cm) in vegetated
areas is 67– 74% sand, 20– 34% silt and 5 – 8% clay. The
bulk density of soils in vegetated areas (0 – 12 cm) is
approximately 1.70 g cm"3 and the soil organic carbon
(SOC) content is between 1.4 and 2.5 kg m"2. The particle
size distribution of the near surface soil (0 – 12 cm) in
unvegetated areas is 54– 64% sand, 33– 38% silt and 3 –
7% clay, with a bulk density of approximately 1.75 g cm"3
and SOC content (0 – 12 cm) between 0.3 and 0.4 kg m"2.
2.2. Experimental Design
[8] Three relatively homogenous 70 # 16 m blocks of
tundra were delineated within a 70 # 60 m area and five
treatments were assigned to 2.0 # 0.8 m plots in a
randomized complete block (RCB) design: ambient (A),
irrigation (W), infrared level I (T1), infrared level II (T2)
and infrared level II + irrigation (T2W). Blocking was not
used to address perceived differences in soils or vegetation
across the blocks, but was utilized because AC power was
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Figure 2. Daily maximum and minimum air temperature at 2 m and soil temperature at 5 cm (!C)
measured between 8 June and 19 August of 2004 and 2005.
delivered separately to each block. Each treatment regime
was replicated twice in each block, such that n = 6 at the
site-level. At the ecosystem-scale, vascular plants and bare
soil/cryptogamic crust each cover 50% of the ground
surface. Plots were oriented to span the transition between
vascular plants and bare soil/cryptogamic crust, such that
each comprised approximately 50%, to facilitate scaling
from the plot- to ecosystem-level.
[9] IR radiators (Kalglo Electronics Co. Inc., Bethlehem,
PA), 1.6 m in length and 12 cm wide, were suspended 125 cm
above the soil surface from rebar tripods installed at the end of
each IR plot (1 radiator/plot). The spatial distribution of the
experimental radiation flux was measured on one day with
light winds and consistent cloudy conditions using a thermopile probe with a spectral coverage of 0.19 to 6 mm (Oriel
Instruments, Stratford, CT). In 2004 and 2005 the IR treatments were initiated during the first week of June, when the
plots became 50% snow-free, and suspended during the final
week of August, before snowpack development. The IR
radiators were not run during the winter months because funds
were not available to support the electrical power costs and
because the research team was not available to monitor the site.
[10] Precipitation records for the last climate normal
period (1971 – 2000) were used to design an irrigation
experiment that maintained seasonal patterns, but increased
the magnitude of growing season precipitation by approximately 50%. Analysis of the historical record showed that
precipitation during July was nearly double precipitation
during June and August. Consequently, plots were irrigated
weekly with 2 mm of supplemental water in June and
August and with 4 mm of supplemental water in July, such
that 32 mm of supplemental water was added between early
June and late August of 2003, 2004, and 2005. Irrigation
water was passed in series through a 1 mm filter (General
Electric, Co., Fairfield, CT), an activated carbon mixed-bed
prefilter and a mixed-bed ultrapure filter (Barnstead International, Dubuque, IA). To reduce soil temperature artifacts
and evaporative loss during irrigation, the water was equil-
Table 1. Results of a Multiple Linear Regression That Employed Stepwise Model Selection to Examine the Controls on Canopy
Temperature, Which Was Measured Directly Using an Infrared Thermometer on Six Dates During the 2004 Growing Season (n = 162)
Source of Variation
Parameter Estimate
Standard Error
Partial r2
F
P
Intercept
Air temperature at 2 m, !C
Supplemental radiation, W/m2
Solar radiation, W/m2
Wind speed, m/s
"2.561
1.367
0.032
0.018
"1.229
1.830
0.082
0.005
0.001
0.240
NA
0.09
0.04
0.19
0.54
1.96
277.69
43.22
207.17
26.21
0.16
<0.01
<0.01
<0.01
<0.01
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Table 2. Soil Warming by Depth and Treatment Between 8 June
and 19 August of 2004 and 2005a
2004
2005
Treatment
2 cm
5 cm
10 cm
2 cm
5 cm
10 cm
T1
T2
T2W
W
+1.2
+2.5
+1.8
+0.0
+1.1
+2.1
+2.0
"0.6
+0.4
+1.7
+2.0
"0.2
+1.3
+2.2
+2.5
"0.5
+2.0
+2.8
+2.7
"0.2
+1.6
+2.5
+3.2
"0.3
Treatment minus control, !C.
a
ibrated with ambient temperatures for at least 24 hours and
applied to plots in the evening.
2.3. Microclimate Monitoring
[11] Climate at the study site was monitored using a meteorological tower, equipped with sensors for air temperature and
relative humidity (HMP50, Vaisala Inc., Helsinki, Finland),
wind speed (05103, R. M. Young, Traverse City, MI), solar
radiation (LI-200X, Licor Biosciences, Lincoln, NE), precipitation (12’’ heated rain gauge, Met One Instruments. Inc., Grants
Pass, OR) and volumetric soil water content (CS616, Campbell
Scientific, Logan, UT). Sensors were read every 15 min and
hourly means were logged to a CR23X data logger (Campbell
Scientific, Logan, UT). Hourly air temperatures at 20 cm were
monitored in 3 replicates of each treatment during the 2003
growing season with Hobo Pro Series loggers housed in
radiation shields (Onset Computer Corp., Bourne, MA). Hourly
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soil temperatures at 2 cm beneath a closed D. integrifolia canopy
were measured using Thermochron iButton temperature
loggers in 6 replicates of each treatment throughout the
2004 and 2005 growing seasons (Maxim/Dallas Semiconductor Corp., Dallas, TX). Hourly soil temperatures at 5 and
10 cm beneath a closed D. integrifolia canopy were
measured using Hobo outdoor 4-channel external temperature loggers in two replicates of each treatment throughout
the 2004 and 2005 growing seasons (Onset Computer
Corp., Bourne, MA). Midday D. integrifolia canopy
temperatures were measured in each plot (10 measurements
per plot) with a handheld infrared temperature probe (Fluke
Corporation, Everett, WA). Canopy temperatures were
measured on six dates under a variety of weather conditions
in 2004, from 5 cm above the canopy, where the probe has a
1.5 cm diameter field of view. Volumetric soil water content
in the upper 12 cm was measured on weekly intervals
beneath a closed D. integrifolia canopy in each plot using a
handheld HydroSense time domain reflectometer (TDR)
probe (Campbell Scientific, Logan, UT). Three measurements were made and averaged at the plot-level to capture
fine-scale variability in soil water contents.
2.4. Statistical Analyses
[12] To estimate the infrared radiation addition to each
plot, spatial trends in the radiation data were fit with a
second-order polynomial and universal kriging was used to
Figure 3. Daily mean soil warming at 2 cm in T1 and T2 plots (treatment- control), along with daily
mean wind speed at 2 m (m/s) during the 2004 and 2005 growing seasons. A: control; T1: low level
radiation addition; T2: high level radiation addition.
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Figure 4. Average hourly wind speed at 2 m (m/s) and soil temperature at 2, 5 and 10 cm (!C) beneath a
closed D. integrifolia canopy in each of the experimental treatments between 8 June and 19 August of
2004 and 2005. A: control; T1: low level radiation addition; T2: high level radiation addition; T2W: high
level radiation and water addition; W: water addition.
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average plot-level range of 8.8!C on 17 June 2004, when solar
radiation was high (!900 W/m2) and wind speeds were light
(!1.5 m/s). Canopy temperatures in ambient plots were up to
13.6!C warmer than air temperatures, although increasing
wind speeds rapidly reduced the difference, such that canopy
and air temperatures were similar when measurements were
made with wind speeds of !5 m/s. Air temperature, wind
speed, solar radiation and supplemental radiation explained
86% of the variation in canopy temperature, over the range of
observed values (F = 234.1, P < 0.01; Table 1). Wind speed
was the most important determinant of canopy temperatures,
alone explaining 54% of the variation, across all treatments.
Identification of supplemental radiation as a significant term
indicates that experimental energy additions had a significant,
albeit nonuniform, canopy warming effect. Midday canopy
temperatures on the six measurement dates were between 0.4
and 3.8!C warmer in T1 plots and between 0.2 and 6.2!C
warmer in T2 plots when compared with ambient plots.
Figure 5. Daily average soil water content (v/v) at 7.5 cm
beneath a closed D. integrifolia canopy in a plot that
received supplemental water, along with daily total
precipitation (mm) during the 2004 and 2005 growing
seasons. Arrows indicate days when plots were irrigated
(June and August: 2.0 mm, July: 4.0 mm).
interpolate the residuals in S-Plus 4.0 (MathSoft Engineering and Education Inc., Cambridge, MA). Variation in soil
temperatures and weekly measurements of volumetric soil
water contents were examined across treatments using
analysis of variance (ANOVA) in the general linear model
(GLM) procedure of SAS 9.1 (SAS Institute, Cary, NC).
Comparisons of interest were made using Tukey’s Honest
Significant Difference (HSD). Plant canopy temperatures
were modeled using air temperature, wind speed, solar
radiation and experimental radiation additions as independent variables using multiple linear regression and stepwise
model selection in the REG procedure of SAS 9.1.
3. Results
3.1. Air and Canopy Temperatures
[13] Daily maximum and minimum air temperatures at 2 m
were higher, on average, in 2005 than in 2004 (Figure 2).
Interannual differences in temperature are highlighted by the
frequency of freezing air temperatures. Growing season air
temperatures were below 0!C for 293 hours during the cool
2004 season, while air temperatures fell below 0!C for only
48 hours during the warm 2005 season. There was no evidence
that air temperatures at 20 cm were significantly greater in T1
(P = 0.47) or T2 (P = 0.18) plots when compared with ambient
plots. Canopy temperatures varied considerably at the plotlevel, reflecting subtle differences in aspect, from north- to the
south-facing sides of D. integrifolia clones. There was an
3.2. Soil Temperatures
[14] In contrast with air temperatures, soil temperatures in
ambient plots rarely fell below 0!C during the growing
season. Radiation additions during 2004 (means of kriged
surfaces) in T1, T2 and T2W plots were approximately 30,
60 and 50 W/m2, respectively. In 2005, radiation inputs
were increased in T2W plots to more closely match those in
T2 plots. Statistical comparisons were made using soil
temperatures measured at 2 cm, as sensors were installed
in all of the experimental plots. During the 2004 growing
season, soil temperatures at 2 cm were significantly warmer
than ambient plots in T1 (+1.2!C, P < 0.01), T2 (+2.5!C,
P < 0.01) and T2W plots (+1.8!C, P < 0.01) (Table 2).
Soil temperatures at 2 cm in W plots were not significantly
different than ambient plots in 2004 (+0.0!C, P = 0.99). In
2005, soil temperatures at 2 cm were significantly warmer
than ambient plots in T1 (+1.3!C, P < 0.01), T2 (+2.2!C,
P < 0.01) and T2W plots (+2.5!C, P < 0.01). Soil temperatures at 2 cm in W plots were, again, not significantly
different than ambient plots ("0.5!C, P = 0.58).
[15] There was not a consistent seasonal pattern in the soil
warming effect at 2 cm during the two years of observation
(Figure 3). Periods of reduced soil warming in T1 and T2
plots generally occurred during periods with relatively high
wind speeds. There was not a clear diurnal pattern in the soil
warming effect at 5 or 10 cm depth (Figure 4). At 2 cm
depth, however, there was a trend toward reduced soil
warming during the midafternoon in all of the plots receiving supplemental IR radiation. This diurnal pattern of
shallow soil warming corresponds closely with the diurnal
wind speed pattern, with the highest wind speeds typically
observed during the midafternoon.
3.3. Soil Water Contents
[16] Precipitation amounts were similar during the growing seasons of 2004 (7.3 cm) and 2005 (6.6 cm), but the
pattern of precipitation was different (Figure 5). In 2004,
precipitation was recorded on 31 d, with an average of
2.3 mm/event, between 8 June and 19 August. Over the
same period in 2005, precipitation was recorded on 13 d,
with an average magnitude of 5.1 mm/event.
[17] Experimental water additions were dwarfed by large
seasonal changes in soil water contents at our site. Water
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Figure 6. Effects of IR warming and irrigation (treatment- control) on midday volumetric soil water
contents (0 – 12 cm, %) beneath a closed D. integrifolia canopy in all of the experimental plots during the
2004 and 2005 growing seasons. Bars are 1.0 SE. A: control; T1: low level radiation addition; T2: high
level radiation addition; T2W: high level radiation and water addition; W: water addition.
additions were most often not detectable in continuous
measurements of soil water contents at 7.5 cm in one plot
that received supplemental water (Figure 5). When water
additions were detectable, the effect on soil water contents
was both subtle (<1.0%) and ephemeral (<2 d). Experimental
water additions were never apparent in weekly point measurements made in all of the experimental plots (0 – 12 cm,
Figure 6).
4. Discussion
4.1. IR Radiation Additions and Air Warming
[18] IR radiation additions in our study (30 and 60 W/m2)
were intermediate among those in previous experiments (22
W/m2 [Harte et al., 1995]; 80 W/m2 [Shaw et al., 2002]).
Energy additions increased temperatures in the soil and
plant canopy, but not in the atmosphere, above the plant
canopy. This observation is consistent with results in a
montane meadow [Saleska et al., 1999], but contrasts with
reports from high arctic graminoid tundra [Nijs et al., 2000]
and tallgrass prairie [Wan et al., 2002]. Open air warming is
not expected to occur because there are too few molecules
of radiatively active gases between an IR radiator and the
plant canopy. Different reports of IR radiator effects on air
temperatures may reflect differences in canopy structure and
measurement height. In both cases where air warming was
reported, air temperature measurements were taken within
the canopy of graminoid-dominated ecosystems, which tend
to be decoupled from conditions in the overlying atmosphere. Our measurements were made above the canopy,
while those in the montane meadow were made in a
comparatively rough canopy that is probably well coupled
with the atmosphere.
4.2. Plant Canopy Temperatures
[19] Plant canopy temperatures varied by as much as
8.8!C in ambient plots, reflecting differences between the
north- and south-facing sides of D. integrifolia clones.
Canopy temperatures were as much as 13.6!C warmer than
air temperatures in ambient plots when wind speeds were
light. The magnitude of this temperature difference is
consistent with reports for other prostrate arctic plant
species [Mølgaard, 1982; Hart and Svoboda, 1994; Gold,
1998]. Wind speed was the most important determinant of
midday canopy temperatures, alone explaining 55% of the
variation observed over six sampling campaigns. Our results
suggest that during an average midday period in the high
arctic growing season, when air temperatures are between 6
and 9!C, wind speeds greater than !4 m/s could be the
difference between leaf temperatures that are near optimal
for photosynthesis (!15!C [Larcher, 2003]) and those that
are well below optimal. Supplemental radiation additions
were identified as a significant term in a multiple regression
model used to predict canopy temperatures. Canopy temperatures were always warmer in T1 and T2 plots than ambient
plots when measured, but the warming effect varied from
near zero to !+4!C and !+6!C, respectively, with the
greatest increases in canopy temperatures generally observed when wind speeds were light.
4.3. Soil Warming
[20] The soil warming effect at 2 cm depth varied
seasonally and diurnally with wind speed. Dependence of
the warming effect on wind speed may reflect convection of
heat away from the plant canopy and or from the heating
element [Kimball, 2005]. Most experiments using IR radiators have held input electrical energy constant and assumed
that doing so would hold the IR flux constant [Harte et al.,
1995; Wan et al., 2002; Shaw et al., 2002]. However, recent
work has shown that the efficiency of IR radiators declines
rapidly with increasing wind speed [Kimball, 2005]. Several
investigators have suggested modifying traditional IR radiators to achieve a constant canopy warming effect by
varying the electrical energy input [Nijs et al., 2000; Kimball,
2005]. Our observations suggest that such an approach
should be considered for experiments at windy sites.
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[21] The influence of wind speed and the magnitude of
soil warming effect declined with depth. Soils at 10 cm
depth were, however, warmer in T1 and T2 plots than
ambient plots, suggesting that the warming effect probably
penetrated to depths greater than 10 cm. The magnitudes of
near surface soil warming in our study are within the range
of values reported in other experiments that have used IR
radiators [Harte et al., 1995; Bridgham et al., 1999; Nijs et
al., 2000; Wan et al., 2002]. There was no evidence that soil
temperatures in plots that received supplemental water were
significantly different than ambient plots, suggesting that
outdoor storage and evening application were sufficient to
eliminate temperature artifacts during irrigation.
4.4. Soil Water Contents
[22] There was no evidence that supplemental radiation
reduced soil water contents, nor was there evidence that
supplemental water interacted with supplemental radiation
to affect soil temperatures. These observations contrast with
the results of previous IR warming experiments, where soil
drying and strong interactions between energy supplements
and soil water contents have been observed [Harte et al.,
1995; Wan et al., 2002]. The lack of an interaction between
energy additions and soil water at our site was surprising,
given that soil drying was observed in similarly dry, welldrained soils [Harte et al., 1995]. The near surface soils of
the dry upper zone in the experiment of Harte et al. [1995]
have very low bulk density (0.46 g cm"3 [Saleska et al.,
2002]) relative to soils at our site (1.70 g cm"3). Comparison of our results with those of Harte et al. [1995] suggests
that differences in soil aeration may be an important control
on the IR radiator soil drying effect.
[23] Water additions did not lead to a detectable, sustained increase in soil water contents, in contrast with our
expectations. The absence of detectable, sustained increases
in soil water can be attributed to the well drained soils
(!70% sand, !25% silt, !5% clay), the resolution (0.1%)
and precision (0.05%) of our soil water probes and the
strongly seasonal environmental and biological controls on
soil water contents at our site. The soil water regime at our
site can be divided into three distinct time periods for the
sake of discussion: early season, midseason and late season.
[24] During the early season period, soil water contents
rapidly decline from more than 40% by volume in the
postsnowmelt period to become relatively dry (20 – 30%).
Soil water contents during this period are largely governed
by soil thaw and are relatively impervious to precipitation.
The period begins with thawing of near surface soils in early
June and extends until soil water contents reach their
seasonal minima in early July. Net radiation tends to be
positive in this early season period and evaporative water
loss is probably important to the draw-down of soil water
contents. Leaf area increases rapidly from late June to early
July. Vascular plant transpiration may, therefore, play an
important role in the dry-down of soil water contents near
the end of this early season period.
[25] During the midseason period, soil water contents
vary between 20 and 30% by volume. During this period,
soil water contents show a small response to large precipitation events. The midseason period extends from the first
week of July until the first week of August. Air temperatures, soil temperatures and leaf area are highest during this
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period. Both evaporation and transpiration likely play
important roles in regulating soil water contents during this
period, but the proportional importance of transpiration is
probably greater than during the postsnowmelt period.
Experimental water additions during the midseason period
were detectable as subtle and ephemeral increases in soil
water content when additions were temporally isolated from
natural precipitation events. Water supplements were probably lost quickly through drainage, evaporation and transpiration during this period.
[26] During the late season period, soil water contents rise
and are most responsive to precipitation, showing relatively
large responses to small events. The late season period
extends from the first week of August until soils freezeup,
typically in mid-September. Leaf area and evaporation both
decline steeply during August and probably become only
weak regulators of soil water contents during this late
season period.
[27] Late season water additions were not detectable
during 2004, as they were superimposed upon a period of
frequent natural precipitation events and rapidly rising soil
water contents. In 2005, late season water additions were
detectable as subtle increases in soil water content when
additions were temporally isolated from natural precipitation events (e.g., 11 August 2005). In general, soil water
contents tended to increase more in response to natural
precipitation events than to our water supplements when
events were of similar size, suggesting that evaporative
water loss during or soon after irrigation may have reduced
the effect of water supplements on soil water contents.
[28] Natural precipitation events in August had greater
consequences for soil water contents than precipitation
events during July. For instance, on 16 and 17 July 2005,
the site received 12.0 mm of rain and soil water contents
increased by 2.2%. In contrast, on 4 August 2005 the site
received 8.7 mm of rain and soil water contents increased
by 5.2%. We suggest the greater response of soil water
contents to precipitation events in August reflects the
seasonal decline in evapotranspiration, which is an important regulator of soil water contents during the midsummer
period when soil water contents are relatively impervious to
natural precipitation and experimental water additions. A
similar hypothesis was presented to explain late season
increases in soil water contents of a Mediterranean annual
grassland [Zavaleta et al., 2003].
[29] Prostrate dwarf-shrub, herb tundra generally showed
relatively simple microclimate responses to energy and water
additions, in contrast with similar experiments in other ecosystems, where microclimate responses have involved strong
interactions between radiative forcing and other experimental
treatments, soil water contents and or vegetation responses
[Harte et al., 1995; Wan et al., 2002; Zavaleta et al., 2003;
Klein et al., 2005]. Our experiment provides a framework to
investigate the sensitivity of a cold, dry ecosystem to changes
in climate and provides further evidence that similar manipulations may lead to contrasting responses in different ecosystems [Harte et al., 1995; Klein et al., 2005].
[30] Acknowledgments. This project was supported by the NSF
research grant 0221606. We thank K. Olin, M. Smith, J. DeCant,
S. Cahoon, and H. Ohms for field assistance, VECO Polar Resources for
logistical support, and the United States Air Force and Greenland Contractors for both logistical support and access to long-term meteorological
8 of 9
G03S08
SULLIVAN ET AL.: ENERGY AND WATER ADDITIONS IN TUNDRA
records. This paper was substantially improved by the comments of an
anonymous associate editor and two anonymous reviewers.
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""""""""""""""""""""""
S. J. T. Arens, B. Hagedorn, P. F. Sullivan, and J. M. Welker, Environment
and Natural Resources Institute and Department of Biological Sciences,
University of Alaska, Anchorage, AK 99508, USA. ([email protected])
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IL 61201, USA.
R. S. Sletten, Quaternary Research Center, University of Washington,
Seattle, WA 98195-1360, USA.
H. Steltzer, Natural Resource Ecology Laboratory, Colorado State
University, Fort Collins, CO 80523-1499, USA.
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