Journal of Geophysical Research: Biogeosciences

Energy and water additions give rise to simple responses in plant canopy and soil microclimates of a high arctic ecosystem

Authors


Abstract

[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.

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]. Long-term 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 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 change-induced ecosystem change. Climate variables are highly interactive and are not expected to change in isolation [ACIA, 2004; IPCC, 2007]. Thus, factorial manipulations 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 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 well-drained 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

2.1. Site Description

[6] The experiment was established in prostrate dwarf-shrub, herb tundra within a 7 km2 catchment on the Pituffik Peninsula, Greenland (76° 33′N, 68° 34′W; 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).

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.

[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 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 μm (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 μm 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 equilibrated 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 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 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 plot-level, reflecting subtle differences in aspect, from north- to the south-facing sides of D. integrifolia clones. There was an 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 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.

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 VariationParameter EstimateStandard ErrorPartial r2FP
Intercept−2.5611.830NA1.960.16
Air temperature at 2 m, °C1.3670.0820.09277.69<0.01
Supplemental radiation, W/m20.0320.0050.0443.22<0.01
Solar radiation, W/m20.0180.0010.19207.17<0.01
Wind speed, m/s−1.2290.2400.5426.21<0.01

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).

Table 2. Soil Warming by Depth and Treatment Between 8 June and 19 August of 2004 and 2005a
Treatment20042005
2 cm5 cm10 cm2 cm5 cm10 cm
  • a

    Treatment minus control, °C.

T1+1.2+1.1+0.4+1.3+2.0+1.6
T2+2.5+2.1+1.7+2.2+2.8+2.5
T2W+1.8+2.0+2.0+2.5+2.7+3.2
W+0.0−0.6−0.2−0.5−0.2−0.3

[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.

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.

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.

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.

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).

[17] Experimental water additions were dwarfed by large seasonal changes in soil water contents at our site. Water 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).

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.

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.

[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, well-drained 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 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].

Acknowledgments

[30] 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 records. This paper was substantially improved by the comments of an anonymous associate editor and two anonymous reviewers.