temperature responses to infrared-loading and water table manipulations in peatland mesocosms

Journal of Integrative Plant Biology 2008, 50 (11): 1484–1496

Temperature Responses to Infrared-Loading and WaterTable Manipulations in Peatland Mesocosms

Jiquan Chen1∗, Scott Bridgham2, Jason Keller3, John Pastor4, Asko Noormets1

and Jake F. Weltzin5

(1Department of Environmental Sciences, University of Toledo, Toledo, Ohio 43606, USA;2Center for Ecology and Evolutionary Biology, University of Oregon, Eugene, Oregon 97403, USA;

3Smithsonian Environmental Research Center , Edgewater, Maryland 21037, USA;4Department of Biological Sciences, University of Minnesota, Duluth, Minnesota 55811, USA;

5National Phenology Network, Tucson, Arizona 85719, USA)

Abstract

We initiated a multi-factor global change experiment to explore the effects of infrared heat loading (HT) and water tablelevel (WL) treatment on soil temperature (T) in bog and fen peatland mesocosms. We found that the temperature variedhighly by year, month, peatland type, soil depth, HT and WL manipulations. The highest effect of HT on the temperatureat 25 cm depth was found in June for the bog mesocosms (3.34–4.27 ◦C) but in May for the fen mesocosms (2.32–4.33 ◦C)over the 2-year study period. The effects of WL in the bog mesocosms were only found between August and January, withthe wet mesocosms warmer than the dry mesocosms by 0.48–2.03 ◦C over the 2-year study period. In contrast, wetter fenmesocosms were generally cooler by 0.16–3.87 ◦C. Seasonal changes of temperatures elevated by the HT also varied bydepth and ecosystem type, with temperature differences at 5 cm and 10 cm depth showing smaller seasonal fluctuationsthan those at 25 cm and 40 cm in the bog mesocosms. However, increased HT did not always lead to warmer soil, especiallyin the fen mesocosms. Both HT and WL manipulations have also changed the length of the non-frozen season.

Key words: global change; infrared; mesocosm; peatlands; warming experiment; water table.

Chen J, Bridgham S, Keller J, Pastor J, Noormets A, Weltzin JF (2008). Temperature responses to infrared-loading and water table manipulationsin peatland mesocosms. J. Integr. Plant Biol. 50(11), 1484–1496.

Available online at www.jipb.net

Predicting the impacts of future climate on terrestrial ecosys-tems and developing sound adaptation strategies requires asolid understanding of the feedbacks between global warmingand ecosystem function (Shaver et al. 2000). Facing this chal-lenge, the ecological community in recent years has acceleratedtheir effort by installing manipulative experiments mimicking var-ious future conditions, including treatments of elevated CO2/O3

in controlled chambers or free-air CO2 enrichment (FACE,

Received 13 May 2008 Accepted 14 Jul. 2008

Supported by the National Science Foundation (DEB9707426).∗Author for correspondence.

Tel: +1 419 530 2664;

Fax: +1 419 530 4421;

E-mail: <[email protected]>.

C© 2008 Institute of Botany, the Chinese Academy of Sciences

doi: 10.1111/j.1744-7909.2008.00757.x

Norby et al. 2005), alteration of precipitation (e.g. RaMPs; Fayet al. 2000) and soil warming (Harte et al. 1995).

Soil microclimate, especially temperature, will be the vari-able most directly altered with climate change because ofincreased radiative forcing (IPCC 2007) and affects virtuallyall other ecosystem processes (Shaver et al. 2000). However,manipulative warming experiments have not produced a clearanswer as to the quantitative effects of increased radiativeinput on soil temperature. For example, 15 W/m2 augmen-tation of downward infrared flux in a montane meadow inColorado, USA increased summer soil temperature by up to3 ◦C (Harte et al. 1995). Similarly, a 1.6–4.1 ◦C increase at15 cm depth was observed in peatland mesocosms in north-ern Minnesota, USA in response to 78–191 W/m2 downwardinfrared flux during the growing season (Bridgham et al. 1999).In contrast, no change in soil temperature was seen in agreenhouse experiment in a tussock tundra near Toolik Lake,Alaska, USA (Hobbie et al. 1999), although air temperature

Temperature Responses to Heat and Water Manipulations 1485

increased by 4–5 ◦C. From the above studies, it is clear thatthe effect of warming on the soil temperature increase ismediated by ecosystem type. Further, ecosystems will respondnot only to changes in mean soil temperature, but, perhapsmore importantly, to changes in soil temperature depth profilesand soil temperature dynamics at various temporal scales(e.g., hourly to intra-annually). Understanding these changesin temperature within time and space is critical for predictingthe biological and physical consequences of ongoing globalwarming.

The response of northern peatlands to changes in soil tem-perature is particularly important because: (i) they are commonin northern latitudes where future warming is projected to begreater than average (IPCC 2007); (ii) they are responsible forabout 8% of the global methane flux (Bartlett and Harriss 1993);and (iii) they contain approximately one-third of the world’ssoil carbon reserve (Gorham 1991; Bridgham et al. 2006).Should soil emissions of carbon dioxide and methane frompeatlands increase as the result of increased soil temperature,there is a significant potential for these ecosystems to augmentanthropogenic production of these two important greenhousegases (Bridgham et al. 1995).

Beginning in 1994, we initiated a multi-factor global changeexperiment to explore the effects of infrared heat loading (i.e.warming) and the changes in water table level on bog andfen peatland mesocosms (Figure 1) (see details in Bridghamet al. 1999). Mesocosms offer several advantages that would bedifficult or even impossible to achieve otherwise, regardless ofcertain trade-offs of experimental control versus ‘ecological re-ality’. For example, mesocosms allow for precise determinationof complete input/output budgets for carbon, water, nutrientsand energy because each mesocosm is physically separatedfrom its surrounding environment. This method has been suc-cessfully used for gaining a more mechanistic understandingof how changing temperature and water table levels affectplant community composition and productivity (Weltzin et al.2000, 2001, 2003), dissolved organic carbon export (Pastoret al. 2003), porewater chemistry (JR White et al., unpubl. data,2008), dynamics of decomposition (Updegraff et al. 2001; Kelleret al. 2004; JR White et al., unpubl. data, 2008), overall soilcarbon gain or loss (SD Bridgham et al., unpubl. data, 2008),and thermal and radiative budgets (Noormets et al. 2004). Theprimary drawback of the mesocosm approach is that manybiophysical processes operate at spatial scales larger thana few square meters (e.g. hydrological dynamics of northernpeatlands), suggesting a deficiency of this approach for theunderstanding and modeling of these processes (Rouse 2000).

In this paper, we are particularly interested in the magnitudeand change of soil temperature at multiple temporal scalesand at various depths in these bog and fen mesocosms.Specifically, our objectives are to: (i) quantify the effects of theenhanced infrared inputs and changes in water table levels onsoil temperature at multiple temporal scales (days to years); (ii)

examine the changes in temperature depth profiles in responseto these treatments; and (iii) explore how these changes differbetween bog and fen ecosystems.

Results

Although we present depth-specific patterns in temperatureresponse, for clarity, we focus primarily on the temperaturesmeasured at 25 cm depth (T25), which is close to the center ofthe mesocosms (Figure 1). Here we present temperature acrossthe 2-year study period as well as monthly and daily means.

The temperature of the experimental mesocosms variedhighly by peatland type, heat loading treatment (HT) and watertable manipulation (WL) (Figure 2). The overall mean T25 of thefen mesocosms of all of the treatments was 6.85 ◦C, whereasthat of the bog mesocosms was 6.52 ◦C throughout the 2-yearstudy period. It was significantly affected (ANOVA with year andmonth as nested variable, results not shown) by year, month,ecosystem type, water and heat level treatments, and the inter-action between HT and WL (P < 0.001 2 in all cases). However,there was no other significant interaction between ecosystemtype and WL, ecosystem type and HT, or the three termsconsidered together. Despite the significant interaction betweenHT and WL, the overall effects of the HT were relatively clear(Figure 2). On average, medium infrared loading resulted in a0.14 ◦C and 0.69 ◦C increase for the fen and bog mesocosms,respectively, during the 2-year study period, while high infraredloading elevated T25 by 1.31 ◦C and 1.14 ◦C.

We were concerned that the large seasonal temperaturedifferences, likely responsible for the large error bars in Fig-ure 2, were swamping out important interactions among ourtreatments and between the two peatland types, so we consid-ered seasonal and monthly changes in T25 individually withinthe bog and fen mesocosms. The monthly T25 reached itslowest in February (−0.67 ± 0.30 ◦C) and highest in August(18.59 ± 0.64 ◦C) (Table 1). When averaged across all treat-ments, the soils remained below 0 ◦C from January to April. Thehigh infrared loading treatment brought the monthly averagetemperature of the bog and fen mesocosms above zero inApril and January, respectively. When we considered individualmonths individually in the bog and fen mesocosms (Table 2),there was surprisingly no main effect of infrared heat loading(HT) on T25 in the fen mesocosms (P = 0.181 2), althoughthere was a significant interaction between the water table(WL) and HT treatments. The water table treatment also hada significant main effect (P = 0.001 3) that varied monthly(Month × WL) (P = 0.018 9). The wettest treatment (WL0)in the fen mesocosms was significantly cooler than the drymesocosms (P = 0.000 1 and 0.05, T-test, for wet < mediumand medium < dry, respectively) between December and April.

In contrast in the bog mesocosms, HT did affect T25

(P = 0.008 1) but WL did not (P = 0.473 6), and again

1486 Journal of Integrative Plant Biology Vol. 50 No. 11 2008

Figure 1. (A) Schematic illustration of continuous measurements of physical variables in 54 peatland mesocosms, including net radiation (Rn), soil

temperature (Ts) at 5, 10, 25 and 40 cm depth, volumetric soil moisture (%), surface soil heat flux (G), bottom heat flux (Gv) and horizontal heat flux (Gh)

at 15, 25 and 45 cm. Other variables recorded at a nearby permanent climatic station include precipitation (P), water temperature (Tp), temperature

of added water (Twater) and photosynthetically-active radiation (PAR). (B) The network of nine stations that were connected to the central desktop

computer via coax cables and simultaneously recorded all physical variables at the 54 mesocosms at 20 s intervals. Each station was independently

controlled by a CR10 datalogger, two AM614 Multiplexers, and one AM2T multiplexer to record and store the 30-min means. The data from each of

the nine stations was scheduled to download to the PC twice per day.

there was a significant interaction between the two treatments(P = 0.025 8). However, within individual months there werestrong main HT effects in both the bog and fen mesocosms.The largest effect of infrared loading (HT) on T25 was found inJune for the bog mesocosms (3.34–4.27 ◦C) but in May for the

fen mesocosms (2.32–4.33 ◦C) (Figure 3A,B) over the 2-yearstudy period. Between January and April, medium and highinfrared loading caused an average T25 increase of 0.35 ◦Cin the bog mesocosms and 0.05 ◦C at the fen mesocosms,with significant differences between medium and high infrared


Figure 2. The mean and standard error of the mean temperature at 25 cm (T25) at the peatland mesocosms with three levels of heat loading treatment

(HT) and water table manipulation (WL) in bog ( ) and fen ( ) mesocosms in 1999 and 2000.

The large error bars reflect significant variability among seasons and between the 2 years.

Table 1. Monthly mean soil temperature at 25 cm (T25, ◦C) of bog and fen mesocosms microcosms with the heat loading treatment (HT) and the

water table manipulation (WL) from January 1999 through December 2000

Bogs FensMonth

Ambient Medium High Ambient Medium High

Wet Int. Dry Wet Int. Dry Wet Int. Dry Wet Int. Dry Wet Int. Dry Wet Int. Dry

1 −0.00 −0.84 −1.11 −0.07 0.27 −0.68 −0.16 −0.41 −0.37 0.27 −0.10 −0.09 −0.04 0.27 0.05 0.11 0.29 0.20

2 −0.41 −1.10 −1.16 −0.26 −0.52 −0.59 −0.48 −0.52 −0.34 −0.23 −0.43 −0.36 −0.75 −0.12 −0.34 −0.64 −0.21 −0.27

3 −0.45 −0.86 −0.59 −0.20 −0.34 −0.20 −0.20 −0.20 −0.10 −0.22 −0.46 −0.24 −0.50 −0.15 −0.31 −0.38 −0.12 −0.16

4 −0.14 −0.14 −0.06 −0.03 −0.11 0.02 0.02 0.05 0.01 −0.08 −0.19 −0.04 −0.17 −0.10 −0.15 −0.02 0.01 0.41

5 0.29 0.80 1.70 3.17 1.50 4.65 3.94 3.45 4.10 0.72 2.04 6.44 3.00 5.43 4.31 4.89 5.60 8.75

6 9.83 8.21 9.54 12.80 11.88 12.92 13.80 13.66 12.94 11.88 12.81 15.83 13.35 14.46 14.08 14.97 13.82 15.83

7 17.48 17.37 17.24 18.13 17.85 17.89 18.75 18.69 17.99 18.06 18.37 19.99 18.28 18.53 18.74 19.10 18.35 19.87

8 18.30 17.98 17.59 18.74 18.45 18.24 19.32 19.15 18.59 18.24 18.40 20.03 18.30 18.70 18.62 19.18 18.55 19.86

9 15.30 14.46 13.85 15.74 15.38 14.75 16.23 15.69 15.40 14.71 14.59 16.62 14.51 15.14 14.68 15.59 14.97 16.32

10 9.28 7.95 7.48 9.70 9.15 8.45 10.17 9.52 8.88 7.89 7.68 10.45 7.89 8.71 7.87 9.26 8.37 10.06

11 5.20 3.98 3.93 5.61 5.14 4.87 5.90 5.58 5.30 3.99 3.65 6.15 4.29 4.79 3.67 5.45 4.52 5.74

12 1.15 0.17 −0.29 1.27 0.68 0.93 1.51 0.47 1.55 0.81 0.70 1.63 0.66 1.03 0.66 1.26 1.35 2.10

Ambient, Medium and High represent mesocosms with infrared heat loading of 0, 40 and 85 W/m2, respectively. Wet, Intermediate (“Int.”) and Dry

represent mesocosms with water levels of +1 cm, −10 cm and −20 cm, respectively, manipulated between May 15 and October 15 (shaded rows).

See Figure 2 for the overall means (SE) by treatment over the entire study period.

loading treatments (P < 0.0001). In May and June, the mediumwarming treatment caused an average temperature increaseof 2.76 ◦C and 1.64 ◦C for the bog and fen mesocosms, re-spectively, while the high warming treatment elevated T25 by3.58 ◦C and 3.49 ◦C. From July to the later months, the T25 inthe medium and high warming treatments was elevated abovethe ambient mesocosms by 0.70 ◦C and 1.13 ◦C in the bog

mesocosms and 0.31 ◦C and 1.52 ◦C in the fen mesocosms,respectively (Figure 3A,B).

There were also significant water table effects within individ-ual months in both the bog and fen mesocosms. Significant ef-fects of the water table treatments on T25 in the bog mesocosmswere only found between August and January, with the wetmesocosms warmer than the dry mesocosms by 0.48–2.03 ◦C


Table 2. The P-values based on nested (Month) ANOVA models for

effects of water table level (WL), heat loading (HT), year (1999–2000),

and ecosystem type (fen vs. bog) on monthly mean temperature at 25-

cm depth in the soil

Source Fen Bog

Yeara <0.000 1 0.007 6

Month <0.000 1 <0.000 1

WL 0.001 3 0.473 6

HT 0.181 2 0.008 1

Month × WL 0.018 9 0.953 8

Month × HT 1.000 0 0.995 0

WL × HT 0.003 6 0.025 8

Month × WL × HT 0.940 6 0.926 9

P-values greater than 0.05 are presented in bold.

on average (Figure 3C). In contrast, the T25 of the wet fenmesocosms was consistently lower (0.16–3.87 ◦C) than in thedry mesocosms, except in January and November (Figure 3D).In the fen mesocosms, the high warming treatment (HT2) andwet water table (WL0) levels did not significantly increaseT25, except in May (Figure 3B,D). The T25 differences in wetmesocosms appeared higher than those of dry mesocosms,with this difference being positive in the bog mesocosms and

Figure 3. Monthly mean difference of temperature (�T25, ◦C) of mesocosms with infrared heat loading treatment (HT) and water table manipulation

(WL) from T25 of the ambient bog (A, C) and fen (B, D) mesocosms in 1999 and 2000.

Temperature differences of all WL levels were averaged in (A) and (B), while temperature differences of all HT treatments were averaged in

(C) and (D).

negative in the fen mesocosms (Figure 3C,D), supporting thesignificant WL × HT interaction in the ANOVAs (Table 2). Thus,water played a significant role in peatland thermal conditions.

The change in the daily average temperatures at 5 and25 cm depth (T5 and T25) showed complex responses to thewarming and water table treatments (Figure 4). Daily meanT25 significantly differed by ecosystem type, year, water tabletreatment, infrared loading treatment (except May), and inter-actions between WL and HT (Table 3), but the (Year × WL)interaction was only significant in May. Other than February,the interactive effects of (Year × WL × HT) did not show anysignificant influence on T25. The combined interactive effectfrom all variables (i.e. Type × Year × WL × HT) was notsignificant in the winter months. Near the surface, T5 variedseasonally between (−13.87 ◦C to 25.88 ◦C) and (−4.19 ◦C to22.85 ◦C) for the bog and fen mesocosms, respectively, andthe T25 varied between (−3.48 to 22.80 ◦C) and (−1.27 ◦C to22.38 ◦C) (Figure 4). The variation of T5 was much higherin the bog than in the fen mesocosms. On many occasions,between JD1 and JD150, both the T5 and T25 daily meanswere lower in the heated mesocosms than those of the am-bient mesocosms (i.e. the heated mesocosms were cooler).Careful examinations of the differences between the bog andfen mesocosms indicated that the fen mesocosms were not


Figure 4. Changes of daily mean temperatures at 5 and 25 cm in the bog and fen mesocosms over the course of 1999.

Three levels of heat loading treatment (HT) and water table manipulation (WL) had been maintained throughout the study period since 1994 (see

Bridgham et al. 1999).

Table 3. The P-values based on nested (Year) ANOVA models for effects

of water table (WL), heat loading (HT), year (1999–2000), and ecosystem

type (fen vs. bog) on daily mean temperature at 25-cm soil depth in four

selected months over the 2-year study period

Source February May August November

Type <0.000 1 <0.000 1 <0.000 1 <0.000 1

WL <0.000 1 <0.000 1 0.022 7 <0.000 1

Year <0.000 1 <0.000 1 <0.000 1 <0.000 1

HT <0.000 1 0.161 2 <0.000 1 <0.000 1

Year × WL 0.828 7 0.039 4 0.056 5 0.115 0

Year × HT <0.000 1 <0.000 1 0.003 2 0.008 1

WL × HT <0.000 1 <0.000 1 <0.000 1 <0.000 1

Year × WL × HT <0.000 1 0.489 8 0.080 6 0.971 9

Type × WL <0.000 1 <0.000 1 <0.000 1 0.997 0

Type × Year × WL 0.003 0 0.000 4 <0.000 1 <0.000 1

Type × HT <0.000 1 0.003 1 0.000 9 0.689 0

Type × Year × HT 0.000 6 0.022 8 0.002 6 0.027 0

Type × WL × HT 0.000 2 <0.000 1 0.638 3 0.007 6

Type × Year × WL × HT 0.140 4 0.012 5 0.000 6 0.477 1

P-values greater than 0.05 are presented in bold.

always warmer than the bog mesocosms. Before JD70, theT5 of the fen mesocosms was warmer than that of the bogmesocosms but lower between JD70 and JD150. After JD150,the fen mesocosms were generally warmer. Additionally, the T25

in the fen mesocosms was not significantly different from thatof the bog mesocosms before JD120 but was higher betweenJD120 and JD190 (3–6 ◦C). The bog mesocosms under ambientand medium warming treatments were slightly cooler than thefen mesocosms after JD240.

The temperature differences (�T25) due to the warmingtreatments also showed a clear seasonal pattern, with smalldifferences between 1999 and 2000 (Figure 5). The largest�T25 was measured between JD120 and JD180. During thisperiod, the daily average T25 was increased by 7.15 (5.27)and 8.46 (6.46) ◦C in the bog mesocosms and 5.02 (3.77) and6.84 (8.39) ◦C in the fen mesocosms (data in parentheses arefor 2000) with medium and high infrared loading, respectively.The water level also had a significant effect on the seasonalchanges of �T25, with the highest �T25 between JD120 andJD180 generally in the wetter mesocosms. The wet mesocosmswith heat loading appeared cooler than the intermediate wet


Figure 5. Changes in difference of daily mean temperature with heat loadings (HT) from the ambient temperature (�T, ◦C at 25 cm depth) of the

peatland mesocosms for each water level (WL) treatment in 1999 and 2000.

mesocosms (i.e. WL1) on most days between JD1 and JD120.Interestingly, the driest bog and fen mesocosms (WL2), withmedium infrared loading in the summer of 2000, had lowertemperatures than the ambient mesocosms (Figure 5).

Seasonal changes of temperature increases (�T) elevated bythe infrared loading treatments varied by depth and ecosystemtype (Figure 6). In the bog mesocosms, temperature differencesat 5 and 10 cm depth showed smaller seasonal fluctuations thanthose at 25 and 40 cm. Other than the period between JD120and JD180, warming treatments produced negligible effects ontemperature at 40 cm depth. In the fen mesocosms, �T hadsignificantly lower variability throughout the year than in the bogmesocosms. A second peak in �T after JD240 was found in thefen mesocosms. The effects of enhanced infrared loading onsoil temperature was stronger in 1999 than in 2000 (i.e. clearyear effect, Tables 2,3).

Both warming and water table manipulations altered the ver-tical temperature gradients in both the bog and fen mesocosmsand the alterations varied by season (Figure 7). For the bogmesocosms, the differences in vertical temperature profiles ofall three of the water table treatments for non-growing seasons(JD20–50 and 320–350) were generally reduced by increasinginfrared-loading, while in the fen mesocosms, these differenceswere magnified, but not for JD20–50 and JD220–250. Onaverage, the temperatures at 5 and 10 cm depth in the fenmesocosms were lower in the heated mesocosms than in theambient mesocosms. The effect of the water table manipulationon the vertical temperature profile seemed complicated but withclear differences between the bog and fen mesocosms andamong the warming treatments.

Perhaps one of the most ecologically relevant effects of theclimate manipulations was an increase in the length of the non-frozen season (Table 4). Near the surface (0 cm), the ambientbog mesocosms reached above 0 ◦C on JD76, on average,yet the date was advanced to JD69 and JD65 in the mediumand high warming treatments, respectively. In late fall, the bogsurfaces began to freeze on JD325, JD337 and JD344 at theambient, medium and high warming treatments, respectively.Together, the non-frozen season was 20 and 30 d longer withmedium and high infrared loading treatments, respectively, thanin ambient mesocosms (i.e. an 8%–12% increase). Similarly inthe fen mesocosms (Table 5), the surface temperature reached0 ◦C on JD94 – about 28 d later than the bog mesocosms –and the total number of non-frozen days was also shorter (237compared with 249 d in bog mesocosms). The medium and highwarming treatments also elongated the non-frozen season by 17and 36 d (a 7%–15% increase), respectively. However at 40-cmdepth, the total number of non-frozen days in the reference bogsmesocosms was 222 d, but 296 in the fen mesocosms when thedata were combined across all water table treatments.

Discussion

The warming treatments and water table manipulations in boththe bog and fen mesocosms resulted in very different verticalsoil temperatures and at multiple temporal scales. Our resultsagree with previous reports that the warming effects on soiltemperature are dependent on ecosystem type, temporal scaleof concern and vertical position in the soil (Harte and Shaw 1995;


Figure 6. Changes in differences of daily mean soil temperature between heated mesocosms and ambient mesocosms at 5, 10, 25 and 40 cm depth.

The differences were averaged for the 2-year study period of 1999 and 2000 by water level (WL) treatment.

Shaver et al. 2000). Previous studies (Harte et al. 1995; Wanet al. 2002) used similar approaches to quantify the climaticchanges related to infrared radiation addition but they lackedcontinuous records for the exploration of the warming effectsat multiple temporal scales. This study is among the very firstto examine the soil microclimate and energy fluxes in soilwarming experiments (Noormets et al. 2004; Kimball 2005). Thedata collected through these experiments is extremely valuablefor not only understanding the temperature dynamics of thewarming, but also for validating ecosystem models that needtemperature profiles in the deep soil layers (Rouse 2000; Hilbertet al. 2000). However, soil microclimate is not only controlled bythe regional climate, but also indirectly regulated by vegetationand soil physical properties (Chen et al. 1999) through surfaceenergy exchange with the atmosphere.

Our results highlight that the response of ecosystems toongoing global change will likely depend on the interactionsbetween multiple climatic variables (Norby and Luo 2004) and

ecosystem type and are also likely to vary through time. Pastoret al. (2002) predicted that boreal bogs will likely shift to fens, orvice versa, depending on ground water level and nutrient input,suggesting that global warming will likely produce more complexconsequences across landscape wetland mosaics. Specifically,we found that time (month and year) was always a significantvariable on soil temperature, regardless of water level and heatloading treatments (Tables 2,3, also see Noormets et al. 2004).Water table level played a very critical role, especially in the fenmesocosms on an annual base (Table 2), in determining the soiltemperature depth profile as well as changes in soil temperatureover time, with wet mesocosms showing stronger temperatureresponses to increased infrared loading than the dry ones(Figure 3, Tables 1,3). This is likely due to the fact that water hasa higher specific heat capacity than dry peat and the effect ofwater table on evapotranspiration rates (Noormets et al. 2004).This is in agreement with several previous studies (Bridghamet al. 1999; Weltzin et al. 2003) that effects of climatic changes


Figure 7. Vertical changes of average soil temperature during the four selected periods that represented distinct thermal regimes (see Figures 4,5)

under treatments of heat loading treatment (HT) and water table manipulation (WL) in bog and fen mesocosms in 1999 and 2000.

on ecosystems will vary highly by ecosystem type. In anotherstudy, Wan et al. (2002) found that clipping of tallgrass prairiesproduced significantly different soil temperature and moistureregimes. However, neither our study nor other similar warmingprojects can address the combined effects of the multiplevariables associated with global climatic change (e.g. changesin greenhouse gases, disturbances, N-deposition, albedo, etc.).Further, our results support previous findings (Bridgham et al.1999; Weltzin et al. 2003) that the effects of climatic changeson peatlands will vary highly by ecosystem type. Therefore, theresults cannot be readily extrapolated to the real world, not evenspatially, as claimed by Rouse (2000).

A major discovery of this study is that the soils of theheated mesocosms can be cooler than the unheated ambientmesocosms (Figure 5), likely because of high latent heat lossduring the growing season (Noormets et al. 2004) and sensibleheat loss during the freezing season, where snow cover wassignificantly reduced (Harte et al. 1995; Groffman et al. 2001).This phenomenon was recorded mostly between December and

April but was not uncommon in other months (Figure 5). Wealso found that this cooling effect occurred more often and wasstronger in the fen mesocosms than in the bog mesocosms(Table 1, Figures 5,6). The wettest fen mesocosms (WL0) wereconsistently cooler than those of the driest mesocosms, withthe strongest effect during the growing season (Figure 3). Harteet al. (1995) reported a similar phenomenon in the sub-alpinemeadows; however, they attributed this to unusual “occasions”.We suspect that this cooling effect is a typical temperatureresponse in high-altitude regions where the snow cover andwater cycle (specifically evapotranspiration, or ET) may playmore important roles in ecosystem energy balance (Bridghamet al. 1999). We previously demonstrated that the removal ofsnow cover in winter months by the increased infrared loadingin our mesocosm experiment greatly elevated the sensible heatloss from the soil (Noormets et al. 2004). During the growingseason, a plant community with a sufficient water supply fromits soil can transpire more water into the atmosphere than theunheated mesocosms and, consequently, cause a temporarily


Table 4. The starting (S) and ending (E) Julian Day (JD) as well as the duration (D) of the non-frozen season (i.e., daily average temperature > 0 ◦C)

at different depths of the bog mesocosms with three warming treatments (Ambient, Medium and High infrared loading) and three water table levels

[Wet, Intermediate (“Int.”) and Dry] in 1999 and 2000

Ambient Medium HighDepth WL Year

S E D S E D S E D

0 cm Wet 1999 86 332 247 72 349 278 72 349 278

2000 70 325 256 60 326 267 58 336 279

Int. 1999 86 319 234 85 354 270 69 349 281

2000 68 320 253 66 325 260 60 339 280

Dry 1999 86 331 246 72 343 272 72 349 278

2000 62 320 259 60 323 264 57 339 283

Mean 76.3 324.5 249.2 69.2 336.7 268.5 64.7 343.5 279.8

SD 12.0 6.5 12.5 12.5 15.5 9.0 7.5 6.5 2.5

10 cm Wet 1999 97 343 247 72 349 278 72 349 278

2000 82 336 255 63 335 273 61 342 282

Int. 1999 89 332 244 88 354 267 72 349 278

2000 77 324 248 62 336 275 62 335 274

Dry 1999 88 333 246 75 349 275 73 349 277

2000 74 325 252 67 335 269 65 343 279

Mean 84.5 332.2 248.7 71.2 343.0 272.8 67.5 344.5 278.0

SD 11.5 9.5 5.5 13.0 9.5 5.5 6.0 7.0 4.0

25 cm Wet 1999 148 365 218 115 363 249 111 365 255

2000 133 343 211 105 362 258 101 359 259

Int. 1999 121 349 229 122 353 232 111 354 244

2000 126 352 227 108 352 245 104 361 258

Dry 1999 117 349 233 111 355 245 109 359 251

2000 105 361 257 104 358 255 106 362 257

Mean 125.0 353.2 229.2 110.8 357.2 247.3 107.0 360.0 254.0

SD 21.5 11.0 23.0 9.0 5.5 13.0 5.0 5.5 7.5

40 cm Wet 1999 130 365 236 1 365 365 115 365 251

2000 129 365 237 106 365 260 106 365 260

Int. 1999 142 365 224 111 354 244 67 365 299

2000 146 365 220 129 365 237 126 365 240

Dry 1999 148 353 206 1 360 360 1 365 365

2000 152 365 214 128 365 238 127 365 239

Mean 141.2 363.0 222.8 79.3 362.3 284.0 90.3 365.0 275.7

SD 11.5 6.0 15.5 64.0 5.5 64.0 63.0 0.0 63.0

cooler soil temperature. This is consistent with the argumentsmade by Petrone et al. (2000) in the sub-arctic regions ofCanada. Due to this importance of latent heat, soil moisturewould also be important because this is the sole source for ET,with wet soil losing more water through ET in boreal wetlands(Guo and Schuepp 1994).

Further studies are needed to explore these mechanisms toconfirm the above speculations. One of the needs is to modelthe energy and water flows of the system to seek the regu-lating processes in order to predict temperature. Nevertheless,this cooling effect, if confirmed, may have great implications

in understanding ecosystem responses to the future climate.Logically, global warming will reduce snow cover of mid- tohigh-altitude regions (IPCC 2007). This suggests that the areasaround the snow-free region will expand, yet their soil temper-ature may be reduced during some period of the year. Giventhat warming will likely increase the length of growing season(and non-frozen period; Tables 4,5), the warming effects overtime will also alter the temporal dynamics of wetland thermalenvironment. The scientific community needs to pay particularattention to the ecological and physical consequences of thiscooling effect (Groffman et al. 2001).


Table 5. The starting (S) and ending (E) Julian Day (JD) as well as the duration (D) of the non-frozen season (i.e., daily average temperature > 0 ◦C)

at different depths of the fen mesocosms with three warming treatments (Ambient, Medium and High infrared loading) and three water table levels

[Wet, Intermediate (“Int.”) and Dry] in 1999 and 2000

Ambient Medium HighDepth WL Year

S E D S E D S E D

0 cm Wet 1999 105 339 235 89 353 265 90 349 260

2000 103 326 224 82 323 242 70 336 267

Int. 1999 96 332 237 89 344 256 73 349 277

2000 90 324 235 82 325 244 59 336 278

Dry 1999 88 332 245 88 345 258 85 350 266

2000 82 325 244 71 324 254 67 355 289

Mean 94.0 329.7 236.7 83.5 335.7 253.2 74.0 345.8 272.8

SD 11.5 7.5 10.5 9.0 15.0 11.5 15.5 9.5 14.5

10 cm Wet 1999 117 354 238 102 353 252 86 353 268

2000 105 345 241 92 331 240 74 337 264

Int. 1999 111 345 235 97 350 254 95 353 259

2000 103 336 234 89 336 248 65 346 282

Dry 1999 97 338 242 97 349 253 86 353 268

2000 84 326 243 84 330 247 61 336 276

Mean 102.8 340.7 238.8 93.5 341.5 249.0 77.8 346.3 269.5

SD 16.5 14.0 4.5 9.0 11.5 7.0 17.0 8.5 11.5

25 cm Wet 1999 130 365 236 125 350 226 114 365 252

2000 137 365 229 126 352 227 106 357 252

Int. 1999 133 357 225 120 365 246 115 365 251

2000 127 361 235 106 364 259 105 363 259

Dry 1999 121 360 240 115 365 251 104 365 262

2000 106 354 249 106 354 249 93 354 262

Mean 125.7 360.3 235.7 116.3 358.3 243.0 106.2 361.5 256.3

SD 15.5 5.5 12.0 10.0 7.5 16.5 11.0 5.5 5.5

40 cm Wet 1999 1 365 365 1 365 365 1 365 365

2000 152 365 214 139 365 227 132 365 234

Int. 1999 1 365 365 1 365 365 1 365 365

2000 129 365 237 110 365 256 105 365 261

Dry 1999 1 365 365 1 365 365 1 365 365

2000 134 365 232 129 365 237 105 365 261

Mean 69.7 365.0 296.3 63.5 365.0 302.5 57.5 365.0 308.5

SD 75.5 0.0 75.5 69.0 0.0 69.0 65.5 0.0 65.5

Materials and Methods

Our study was carried out at the Fens Research Facility (FRF)of the University of Minnesota, about 70 km north of Duluth,MN, USA, where a mesocosm facility was constructed within-tact monoliths from a bog and a fen ecosystem in 1994(Bridgham et al. 1999). The source sites for the mesocosmconsisted of a bog and a fen in the townships of Toivola andAlborn, respectively, in northern Minnesota (47◦N, 92◦W). Themonoliths (2.13 m2 surface area, 0.60 m depth) were placed inplastic tanks of similar dimension that had been sunken into a

large field and insulated with 8 cm of sprayed foam to reduceheat exchange with the surrounding soil.

The experiment was a full factorial design, with three watertable level manipulations (WL), three infrared heat loadingtreatments (HT), two ecosystem types (bogs and fens) andthree replicates for each of the treatment combinations (n = 54mesocosms). The HT was accomplished above background lev-els with overhead infrared lamps. The three levels of increasedinfrared-loading, as measured at the mesocosm surface, wereambient (HT0), +78 W/m2 (HT1) (“medium”) and +191 W/m2

(HT2) (“high”). These infrared input levels were maximal values


obtained under quiescent conditions as operational heat gainsdepended on many external factors, particularly wind speed(Kimball 2005). The water table levels were initially set at ap-proximately +1 cm (WL0) (“wet”), −10 cm (WL1) (“intermediate”)and −20 cm (WL2) (“dry”), relative to the soil surface of themesocosm and maintained using a PVC-pipe manostat in asmall adjoining sump bucket. Since the initiation of the meso-cosms in 1994, the surface of the bog mesocosms has risen byover 11 cm on average due to vigorous moss growth, whereasthe surface of the fen mesocosms has remained relativelyconstant in height (SD Bridgham et al., unpubl. data, 2008).We considered this a treatment response and did not adjustwater table levels to compensate for changes in surface height.As a result, the mean water table levels in 1999 and 2000 wereWL0 = −22.0 cm, WL1 = −26.2 cm and WL2 = −34.0 cm in thebog mesocosms and were WL0 = −5.6 cm, WL1 = −13.5 cmand WL2 = −22.4 cm in the fen mesocosms.

Each of the 54 microcosms was instrumented in June of 1998to measure its microclimate and energy fluxes with calibratedsensors and dataloggers, including soil temperatures at 0, 5,25 and 40 cm below the soil surface using differentiated mea-surements of T-type thermocouples, volumetric soil moistureby a time domain reflectometry (TDR), net radiation (Rn, Q7.1Net Radiometer, REBS, Seattle, USA) at 15–20 cm above thecanopies and soil heat flux (HFT3 soil heat flux plates, REBS)of the surface (G), horizontally at 10, 25, and 45 cm (Gh) and atthe bottom (Gv, Figure 1a). Detailed changes and summaries ofenergy fluxes associated with this experiment were reported byNoormets et al. (2004). Soil moisture was recorded every 3 hand other variables were scanned every 20 s and recordedin 30-min means. For every six mesocosms, a local stationconsisting of a 40 × 45 cm enclosure, one datalogger (CR10X,Campbell Scientific, Inc. CSI, Logan, UT, USA), a AM416Multiplexer and a AM25T multiplexer were constructed to takemeasurements of four temperatures, five soil heat fluxes andnet radiation (Figure 1B). The AM416 multiplexers were usedfor heat flux plates and the AM25TC multiplexers were usedfor recording temperatures. Nine stations for all 54 mesocosms(594 variables) were networked to a central laptop computerusing a coaxial cable and a MD9 Multidrop Interface (CSI) in aseries. All of the variables were concurrently and continuouslysampled every 20 s and averaged at 30-min intervals as ofmid-August, 1998.

The soil temperatures measured from 1 January 1999 through31 December 2000 were used in this paper. Because ofinstrument failures and maintenance problems, there existedsome gaps in the data series of soil heat flux (up to 15%of the observing period). These data points were treated asmissing data in the statistical analysis. Extreme values greateror lower than expected ranges (e.g., temperature >60 ◦C)were also removed from the dataset during the quality controlprocess. In this paper, we also focused on the temperatureat 25 cm (T25) – a measurement close to the center of the

mesocosm. Temperatures at other depths were used only forprofile analyses. Means and SD at daily, monthly and annualscales were calculated by the treatment (i.e., HT and WL) andecosystem type. Temperature differences associated with theHT were calculated by subtracting from the temperatures of themesocosm under ambient conditions (i.e., the ambient).

Our initial analysis of the field data indicated that changes intemperature with Julian day (JD) exhibited distinct seasonality.Consequently, we selected a period of 30 d for each of thedistinct seasons to compare the temperature responses with thetreatment and by ecosystem type. These periods were: JD20 toJD50, JD120 to 150, JD220 to JD250 and JD320 to JD350.Because diel temperature changes are several magnitudes ofthe temperature differences among the treatments and temporalcorrelations of the data (i.e., time series), our ANOVA was appliedto all other dependent variables nested within time (i.e., monthand year). ANOVAs were carried out on daily, monthly, and yearlyscales to assess the treatment effects in the fen and bog meso-cosms, jointly or independently. Paired T-test was also used inthis study for comparisons between any two treatments (e.g.,HT0 vs. HT1). T-tests thus provide a straightforward analysisof the main effects of warming or water-table manipulationin these peatland mesocosms, but this approach ignores anyinteractions between the heating and water-table treatmentsthat are often significant in ANOVA analyses.

Acknowledgements

We thank the following individuals who helped with the instal-lation and maintenance of the mesocosm experiment: KarenUpdegraff, Mark Rudnicki, Xinli Wang, Treneice Marshall, BradDewey, Cal Harth and Mark Rudnicki. Lisa Delp edited theearlier version of the manuscript with many constructive sug-gestions.

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