IMPACT OF SAVANNA FIRE SCARS ON HEAT AND MOISTURE …tropical savannas and subject to frequent yet...

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IMPACT OF SAVANNA FIRE SCARS ON HEAT AND MOISTURE FLUXES TOTHE ATMOSPHERE AND FEEDBACKS TO THE LOCAL BOUNDARY LAYER.

C.K. Wendt1*, J. Beringer1, N.J. Tapper1 and L.B. Hutley2

1School of Geography and Environmental ScienceMonash University, Victoria 3800, Australia

2Faculty of Education, Health and ScienceCharles Darwin University, Darwin, Northern Territory 0909, Australia

1. INTRODUCTION

The far north of Australia is dominated bytropical savannas and subject to frequentyet non-destructive burning of the grassyunderstorey. Each year within northernAustralia, huge tracts of savanna are burnt(~250,000 km2), with this area likely toincrease under predicted climate change(Williams et al., 2001). Caused mainly bypastoralists, Aboriginal landholders andconservation managers, as well as bylightning strikes (Russell-Smith et al., 2000;Williams et al., 2002), this frequent burningis thought to be the greatest natural andanthropogenic environmental disturbancewithin this extensive ecosystem (Russlel-Smith et al., 2000). Fire can influenceclimate both directly and indirectly. Directeffects occur through the emission of smokeand trace gases from the burning andcombustion of fuels. Indirect effects occurvia fire and subsequent fire scars and arelikely to radically alter surface properties andthe surface energy budgets, which formimportant feedbacks to the atmosphere(Beringer et al., 2003).

Fire scars, which refer to the charred andblackened landscape after a fire event, canradically alter the surface energy budgets byreducing surface albedo, increasingavailable energy for partitioning intoconvective fluxes and increasing substrate

* Corresponding author address: Chris K. Wendt,Monash University, School of Geography &Environmental Science, Victoria 3800, Australia.Email: [email protected] Ph. +61-3-9905-2953

heat flux. These changes affect surface-atmosphere coupling and have the potentialto influence atmospheric motion and moistconvection at a range of scales. This studyis an initial investigation into the effect of firescars on the boundary layer and forms thebasis for further study into the larger scaleeffects of tropical savanna fire scars on localto regional climate.

2. SITE DESCRIPTION

The project field sites were located on theGunn Point Peninsula (Burnt Site12°29.712S, 131°09.003E and Unburnt Site12°18.113S, 131°06.002E) approximately35km south east of Darwin near thetownship of Howard Springs, in the coastal,humid zone of the Northern Territory,Australia.

Howard Springs has a mean annual rainfallof 1750mm, with approximately 95% fallingduring the pronounced wet season fromDecember to March (Hutley et al., 2000).This is followed by an essentially rainlessdry season, lasting from May to September(Taylor & Tulloch, 1985). A transitionalperiod occurs during the months of Octoberand November which is characterised byincreases in humidity and temperature,occasional thunderstorms and the onset ofcanopy flushing by many of the tree andshrub species (Williams et al., 1997).

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Daily maximum temperatures at DarwinAirport range from 30.4oC (July) to 33.1oC(October and November) (Hutley et al.,2000). The maximum and minimumtemperatures have a range of 7oC in the wetseason and 11oC in the dry season(McDonald & McAlpine, 1991).

The vegetation of Gunn Point Peninsulaconsists of Eucalypt-dominated woodlands,open forests, closed forests and seasonallyflooded swamps and wetlands (Hutley et al.,2000). The burnt and unburnt sites consistprimarily of Eucalypt open forest vegetationand are typical of the tropical savannas ofNorthern Australia. A previous study byHutley et al., (2000) at the same site showedthere are approximately 600-750 stems perhectare, with Eucalyptus tetrodonta andEucalyptus miniata the dominant species,accounting for approximately 80% of totalbasal area and 50% of the total canopycover in the region (Hutley et al., 2000).O’Grady et al., (1999) also recorded thatoverstorey LAI varies seasonally and rangesbetween 0.6 in the dry season to 0.95 in thewet season for the same region.

The understorey at both sites consisted of amosaic of small trees and shrubs with aseasonally continuous cover of annualgrasses, with some perennial grasses alsopresent (Hutley et al., 2000). Probably ofmost significance to this study is thepresence of the C4 grass Sorghum duringthe wet season. Hutley et al., (2000)showed that at ground level Sorghum spp. isresponsible for a dramatic change in LAI,from a wet season value of 2-3 to a dryseason value of 0.2, when the grasssenesces. It is the abundance and annualdrying out of the Sorghum, along with thepresence of coarse woody debris thatcreates the available fuel to drive many ofthe wildfires in the area.

The soils of the region are generallyextensively weathered and laterised, weaklyacidic and low in nutrients. A detaileddescription of the soils can be found in(Cole, 1986; Pidsley et al., 1994; Cook et al.,1998 & Hutley et al., 2000).

3. FIRE SCARS

Fire scars throughout the Northern Territorycan range from a few square meters tohundreds of square kilometers. The firescar investigated in this study was centeredat approximately 12°29S, 131°09E and wasapproximately 100km2 in area (Figure 1).Burning occurred courtesy of a wildfire onday 241, with the fire being patchy andranging between low (<1000 kWm-2) tomoderate intensity (3500 kWm-2). Figure 1shows the location and timing of fire scars inthe Gunn Point region and the location ofthe climate/flux tower and the two tetheredballoon sites.

Figure 1: Location and timing of fire scars in the Gunn Point region, including the locationof the experimental sites. Burnt and Unburnt sites are indicated as a blue dot. (modified

from Tropical Savannas CRC map maker – North Australian Fire Informationhttp://savanna.ntu.edu.au/information/savanna_mapmaker.html).

4. METHODS

4.1 Meteorological & Flux Measurement

An eddy covariance system was used tocollect meteorological and flux data before,during and after the fire event. It has beenpreviously described by Beringer et al.,(2003) and will be briefly described here. Aflux tower was installed approximately 10 kmoff Gunn Point Road (12°29.712S,131°09.003E). The tower was 25m tallallowing measurements to be made abovethe canopy (canopy height of 12-14m).Sensors were positioned as close to the topof the tower as possible to maximise thesurface area within the effective sensorfootprint (Schmid, 1997). Three dimensionalwind velocities were measured using a 3-Dultrasonic anemometer (Campbell ScientificInc., model CSAT3) at a height of 25m andwere co-ordinate rotated (McMillen, 1988).Turbulent fluctuations of CO2 and H2O weremeasured using an open path infrared gasanalyser (Licor, model LI-7500). Incoming

and reflected short-wave radiation weremeasured using a pair of pyranometers(Middleton Instruments, model EP-07) whileincoming and emitted long-wave radiationwere measured using a pair ofpyrgeometers (Eppley Labs Inc., modelPIR). Net radiation above the surface wasalso measured using a Frischen type netradiometer (REBS, model Q*7.1) with awind-speed-dependant dome coolingcorrection to applied to the results.

Soil heat flux plates (REBS, model HFT3)and soil temperature probes (CampbellScientific Inc., model TCAV) were used tomeasure ground heat flux via thecombination method (Fuchs & Tanner,1968) at four representative locations. Allclimatic variables were measured every 10-seconds and 30-minute averages recordedto a data logger (Campbell Scientific Inc.,model CR5000).

Throughout this paper all times are given aslocal Central Standard Time (CST = UTC

http://savanna.ntu.edu.au/information/savanna_mapmaker.html

+9.5 Hours), while the term “daily” refers tothe 24-h period from midnight to midnightand the term “daytime” refers to the periodwhen net radiation is positive (10:00 –18:00). Solar noon at Howard Springs wasapproximately 13:00 CST during the periodof measurements. The tower data wererecorded during 2003 from day 214 – 299and split into three data blocks, pre-fire (214240), fire (241-261) and post-fire (262-299).

4.2 Boundary Layer Measurements

Two identical tethered balloon systems wereused to take vertical profiles of the boundarylayer. Simultaneous soundings were takenat the burnt and unburnt site, with eachsystem comprising a Vaisala tetheredballoon, winch, radiosonde (produced by theUniversity of Canterbury Physics &Astronomy Department) and receivingsystem (SALCOM, model 15-12-0000). Theradiosondes recorded temperature,pressure, humidity, wind speed anddirection, with the data being logged directlyto a laptop using a Windows based softwarepackage (University of Canterbury). Due tothe aviation operating restrictions of the areameasurements were made to a maximumheight of approximately 530 m above groundlevel. The systems were placed in themiddle to downwind side of the burnt andunburnt patches in order to minimize edgeeffects. Each sounding took approximately1 hour to complete. Soundings wereconducted simultaneously at 01:00, 06:00,09:00, 12:00, 15:00, 18:00 and 21:00 ateach site for days 247-249, 254-255, 261-262, 267 and 274-276. In total 172soundings were conducted between theburnt and unburnt sites. An intensiveobservational day was conducted on day267 (24/09/03) with soundings beingconducted every hour between 06:00 and21:00 and this is presented as the diurnaldata set. All other data were screened,processed and combined to form singleprofiles for each time period. Profiles arepresented here as layered height whichrefers to mean data in a 25 m block; forexample 25 m data is the mean of all databetween 0 m and 25 m, while 50 m data isall data between 25 m and 50 m.

5. RESULTS & DISCUSSION

The wildfire on day 241 of 2003 immediatelyaffected the land surface by significantlychanging the albedo. The consumption ofthe dry grassy understorey, coarse woodydebris and the blackening of the now bareground surface resulted in a decrease inalbedo. Due to some technical difficultiesalbedo values for 2003 are currentlyunavailable. However previous studies from2002 at the same site showed albedo almosthalved from 0.12 pre-fire to 0.07 post-fireduring a similar intensity late dry-seasonfires (Beringer et al., 2003).

The heat from the understorey fire causedsubstantial canopy scorch and, as a result,significant leaf-drop occurred in the weeksfollowing fire. This canopy damage wasfound to reduce evapotranspiration and alterthe partitioning of energy from latent heating(LE) into sensible heating (H). This is bestrepresented by the Bowen ratio (β = H/LE,Bowen 1926), with values of β near 1indicating equal partitioning of energy as Hand LE, values less than 1 indicating themajority of energy is being used inevapotranspiration and values greater than1 indicating the majority of energy is beingused for sensible heating of the surroundingatmosphere. Mean daytime β for the pre-fireperiod was 2.5, indicating that the majority ofavailable energy was being dissipated viasensible heating. These values are typicalof tropical savannas during the mid to latedry season, when the understorey grasseshave senesced, the canopy LAI hasdecreased and there are low soil moisturelevels (Hutley et al., 2000).

Fire resulted in dramatically increaseddaytime values of β to an average of 6.1 forthe 3 weeks following the event (fire period)as the canopy was scorched andtranspiration reduced to near zero. Thisincrease in β indicated nearly 2.5 timesmore energy was being dissipated assensible heating than was being dissipatedby evapotranspiration (latent heating).There was also a sharp increase in β values2 weeks after the fire (Figure 2), which isindicative of leaf fall from the dominant treesfollowing the fire.

Bowen Ratio and Total Daily Rainf allFire occuring on day 241

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Figure 2: Pre- and post-fire Bowen ratios (� = H/LE) and daily total rainfall (mm) for theburnt site. Fire occurred on day 241 (29th August 2003). Leaf drop is evident on day 256,

while the rain events show a clear decrease in � as evaporation is increases.

Interestingly, post-fire mean daytime βvalues returned to 2.5 (similar pre-firevalues) more quickly than previous studiesin the same area (Hutley et al., 2000;Beringer et al., 2003). Although the fire wasof similar intensity to those of previousstudies it was found that this recovery wasdue primarily to the amount of rainfall post-burn. Rainfall after the fire increased theamount of water available for evaporationand subsequently increased the amount ofenergy that was dissipated as latent heat,reducing the Bowen ratio. Rainfall alsoresulted in faster flushing of the canopy(post-fire) foliage and germination ofunderstorey grasses, which was veryobvious in the week following the fire. Soil

water content, which averaged 5.1% pre-fire, increased to 6.5% during the fire periodand 6.9% post-fire due to these sporadic,pre-monsoon rainfall events typical of thistime of the year. Figure 3 shows a clearincrease in latent heating following theincreases in soil moisture due to the rainfallevents.

Latent Heat Flux and Soil Moisture Content(Fire occuring on day 241)

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Figure 3: Daily mean soil moisture (%) and latent heating (MJ.m2.day-1) for the burnt site.Increases in soil moisture content correspond to increases in latent heating. Noting the

increase in rainfall frequency results in higher soil moisture and subsequently higherlatent heating of the atmosphere.

In terms of overall magnitude of the heatfluxes (Table 1), the pre-fire daily meansensible heating was 5.94 MJ m-2 day-1

which is a moderate value due to the drysoils, low LAI, high canopy resistance andrelatively high levels of incoming radiationduring the day at this time of the year(Beringer et al., 2003). Sensible heatingincreased to 6.49 MJ m-2 day-1 during the fireperiod and again during the post-fire periodto 6.92 MJ m-2 day-1), which may indicate aseasonal trend. Latent heating was seen to

decrease from 3.10 MJ m-2 day-1 pre-fire to2.10 MJ m-2d ay-1 during the fire period,before recovering to 3.58 MJ m-2 day-1 post-fire. The ground heat flux showed a nearthree-fold increase from 0.12 MJ m-2 day-1

pre-fire to 0.35 MJ m-2 day-1 during the fireperiod. This is a result of increased radiantload on the soil due to the significantlyreduced overstorey cover because of leafdrop and because of removal of vegetationand blackening at the soil surface.

Table 1: Daily mean heat flux values for pre-fire, fire and post-fire periods.Data Set (day) Sensible Heat (H)

(MJ m-2 day-1)Latent Heat (LE)

(MJ m-2 day-1)Ground Heat (G)

(MJ m-2 day-1)Pre-Fire (214-240) 5.90 3.10 0.12Fire (241-261) 6.49 2.10 0.35Post-Fire (262-299) 6.92 3.58 0.36

Although not as large as previous changesreported at the same site by Beringer et al.,(2003) (a moderate fire during 2001 resultedin a change in daily sensible heat flux of 4.9MJ m-2 day-1 pre burn to 7.6 MJ m-2 day-1

post-burn), the changes during the 2003 fireare still evident. Even accounting for theeffect of rain shortly after the fire event, the

impact of fire in tropical savannas on thepartitioning of energy from latent heating tosensible heating can clearly be seen toimpact the overlying atmosphere.

Boundary layer profiles taken in the weeksfollowing the fire show that the atmosphereover the fire scar is warmer and more

effectively mixed. Simultaneous hourlyvertical temperature profiles for the morningof day 267 (24/09/03) show that the burntsite is warmer throughout each profile to themaximum height measured (350 m to 400

m) in every case except at 08:00. At thistime the two sites are almost at equilibrium(i.e. no significant difference in temperature)(Figure 4).

Morning Temperature Soundings

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Figure 4: Morning temperature soundings for burnt and unburnt sites on day 267(24/09/03). Soundings were conducted every hour on the hour from 06:00 to 21:00,however only every second hour is shown. Unburnt soundings are shown with and

dashed line and burnt sounds with a solid line.

Interestingly, at 06:00 the near-surface (0-75m) air temperature at the burnt site waswarmer by 1.8oC when compared to theunburnt site (we might have expected thedifferences to disappear overnight). Atheights greater than 75m, the two profilesconverge. Reasons for this are not entirelyclear, but may indicate enhanced heating oflower zones of the profile over the relativelycleared burnt land surface as the sun rises.However this is inconsistent with thesimilarity of profiles at 08:00. Converselythe fire scar may still heating the lowerlevels of the boundary layers as a result ofincreased heat retention of the blackenedland surface over night, although this isunlikely and discussed later in this section.

Heating of the atmosphere by the landsurface increases fairly rapidly as more solarenergy is input into the system during themorning, with mean temperatures in the first25m increasing from 22.6oC at 06:00 to28.2oC at 10:00 and from 24.2oC at 06:00 to

29.3oC at 10:00, at the unburnt and burntsites respectively. Although the surfacetemperature at the unburnt site at 12:00pmis slightly higher than at the burnt site theprofile clearly shows that the burnt site iswarmer throughout the entire measuredprofile by up 2oC at some levels. At a heightof 375m the burnt site is seen to be 1.5oCwarmer. This increased convection from thesurface, as more energy is partitioned intosensible heating than latent heating over theburnt site (β>1, Figure 2), would produceenhanced mixing and the possible formationof a deeper boundary layer.

Heating continues throughout the day withthe burnt site temperatures considerablyhigher than the unburnt site at both 14:00and 16:00 (Figure 5). By 18:00 and 20:00differences in the profiles are substantiallyreduced.

Af ternoon Temperature Soundings

06:00 to 20:00Day 267

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Figure 5: Afternoon temperature soundings for burnt and unburnt sites on day 267(24/09/03). Soundings were conducted every hour on the hour from 06:00 to 21:00, withevery second hour is shown here. Unburnt soundings are shown with and dashed line

and burnt sounds with a solid line.

The burnt site shows a more rapid rate ofboundary layer cooling between 16:00 and18:00, indicating that once solar loaddecreases, the burnt surface loses heatmore rapidly than the unburnt surface. Thisfurther supports the contention that thedifferences observed at the surface duringthe 06:00 sounding may be a result of morerapid heating of the burnt surface.

To investigate the effect of the fire scar overa longer time scale, individual runs for aspecific time of day (e.g. all 06:00 runs)were combined and the mean taken for each25 m layer of the atmosphere, with daysbeing used as replicates. Analysis of thesedata shows that at 01:00 there is a cleartemperature inversion in the first 100m atboth sites (Figure 6). This is a typicalnocturnal profile for a system that is

characterised by clear skies, relatively highdaytime surface temperatures and rapidsurface cooling over night. Interestingly theburnt site was still warmer throughout theprofile up to a height of approximately 300mwhen the two profiles converged. Thisindicates that surface temperatures at theburnt site at 01:00 are on average, higherthan at the unburnt, and as a result there islarger sensible heating loss to the overlyingatmosphere. This may also indicate that theburnt soil is storing more heat than theunburnt soil during daylight hours. Analysisof the soil heat storage data from the fluxtower has not yet been conducted and willhopefully provide a more detailedexplanation of the mechanisms behind thisresult.

01:00 Mean Lay ered - Temperature Prof ile

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Figure 6: 01:00 mean layered temperature profiles from 13 days of simultaneoussoundings from the burnt and unburnt sites (between days 247-276). The burnt site is

represented by solid red line and the unburnt site by a dashed blue line.

By 09:00 surface heating from incomingsolar radiation is evident and a cleartemperature lapse is seen at both sites(Figure 7). Both soundings continue torecord similar temperatures throughout theprofiles with no statistically significantdifferences present..


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Figure 7: Mean layered temperature profiles from 13 days of simultaneous soundings fromthe burnt and unburnt sites (between days 247-276). Profiles show mean temperature datafor each 25m layer, (i.e. first point at 25m is equal to the mean of all data points from 0 to

25m). The burnt site is represented by solid red line and the unburnt site by a dashed blueline.

Maximum heating differences are seen at12:00 and 15:00 when the sensible heatingof the overlying atmosphere, convection andturbulence over the burnt surface is at amaximum. The burnt site was also found tohave slightly higher mean wind speedsthroughout the profiles (data not shown) withfurther analysis of data currently underway.

At 18:00 a temperature lapse is still evident,with the burnt site remaining significantlywarmer up to 200 m. At 21:00 surfacetemperatures have cooled by up to 4oC andthe burnt site still exhibits a warmer profileup to 200m after which there is a suddencrossover and the unburnt site becomeswarmer than the burnt site by similarmagnitude (Figure 7). A more in-depthanalysis of the data reveal that this was notthe result of any single run used to calculatethe mean data, rather it was a recurringevent. Further analysis of wind directionand humidity data is needed to investigatethe drivers behind this phenomenon, butinitial results suggest that it may be theproduct of a displaced air mass producingrelative cooling over the site.

6. CONCLUSION

Once burnt, tropical savanna land typesshowed a significant decrease in albedo anda fundamental change in the partitioning ofavailable energy. Bowen ratios of 2.5 forunburnt savannas indicated that moreenergy was being partitioned into sensibleheat than into latent heating. This isexpected during the dry season, as there islow available soil moisture with low canopyLAI. However fire events increase the βthree-fold to 6.1, indicating a substantialincrease in sensible heat flux to theatmosphere, which combined withconvection and mixing, is the driver behindboundary layer heating. For comparison,deserts exhibit β values of ~7.0 (Oke.,1987).

In contrast to previous studies (Beringer etal., 2003), the effect of the fire scar in theweeks following the burn was not assignificant, which was a direct result of theinfluence of rainfall. However, taking thisinto account, the effect of the surface

changes on the overlying atmosphere wasstill clearly evident. At times of maximumheating, air above savanna fire scars are upto 2oC warmer than unburnt areas. Due tooperating restrictions it was not possible tomeasure to the top of the boundary layer(estimated from Darwin BOM data to be 1.5– 3km deep depending on daily conditions)so the depth to which these effects occurcannot be shown. However, increasedheating of the boundary layer was evidentover the burnt site to the maximum height of625 m during individual soundings. If norainfall was present after the fire or the fireintensity was to increase, it could beassumed that heating of the boundary layermay also increase.

Depending on local conditions such asaerodynamic changes to savannavegetation, the size and intensity of the fireand enhanced sensible heat fluxes overscars, it is possible that these areas couldproduce localized areas of convergence anddivergence and associated mesoscalecirculation systems (Knowles, 1993). Thepossible formation of mesoscale circulationsmay lead to an increase in convective clouddevelopment and precipitation following afire event, altering local conditions until pre-fire conditions are restored (Beringer et al.,2003). This is highly dependant on theavailability of atmospheric water vapour,which is highly seasonal. The intensity oflate dry-season fires are often an order ofmagnitude greater than early or mid dry-season fires (Williams et al., 1998) andoccur at times when there is greateratmospheric moisture available (pre-monsoonal period of August-October), so itis entirely possible that large scar fire scarscould significantly modify regionalprecipitation patterns (Beringer et al., 2003).

With a shift in fire regimes from an early-dryseason (April to May) Aboriginal patchburning, to the current regime of moredestructive late-dry season fires(Braithwaite, 1991; Williams et al., 1998),large intense landscape scale fire scars aremore likely to occur. With the ability of thesefire scars to alter the overlying atmosphereas demonstrated by this study, it is possiblethat they may, once integrated over the totalarea of Northern Australia, impact on

regional climate. Recent studies such asJohnson et al., (1999), Williams, et al.,(2001) and Beringer et al., (2003) furtheremphasise the importance of understandingvegetation-atmosphere interactions underchanging fire and weather regimes and alsopoint to possible feedbacks between thesechanges and regional climate (such as theAustralian monsoon). Future work in thisstudy is to apply a mesoscale climate model(MM5 – developed by PSU/NCARhttp://box.mmm.ucar.edu/mm5/ ) to quantifythe effect of fire scars on local and regionalscale climate. This will improveunderstanding of how atmosphericprocesses may be altered under changingfire and climate regimes.

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Cole N.M. (1986) The Savannas:biogeography and geobotany. AcademicPress, London.

Cook P.G., Hatton T.H., Pidsley D., HerczegA.L., Held A.A., O’Grady A. & Eamus D.(1998). Water balance of tropical woodlandecosystems, northern Australia: acombination of micro-meteorological, soilphysical and groundwater chemicalapproaches. Journal of Hydrology 210: 161-177.

Hutley, L. B., O'Grady, A.P., and Eamus, D.(2000). “Evapotranspiration from Eucalyptopen-forest savanna of Northern Australia.”Functional Ecology 14: 183-194.

Johnson B., Miller G., Fogel M., Magee J.,Gagan M. & Chivas A. (1999).65,000 Years

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IMPACT OF SAVANNA FIRE SCARS ON HEAT AND MOISTURE …tropical savannas and subject to frequent yet...

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