Forest Harvesting Effects on Soil Temperature, Moisture, and Respirationin a Bottomland Hardwood Forest

A. J. Londo, M. G. Messina,* and S. H. Schoenholtz

ABSTRACTThe effect of forest disturbance on C cycling has become an issue,

given concerns about escalating atmospheric C content. We examinedthe effects of harvest intensity on in situ and laboratory mineral soilrespiration in an East Texas bottomland hardwood forest between 6and 22 mo after harvesting. Treatments included a clearcut, a partialcut wherein approximately 58% of the basal area was removed, andan unharvested control. The soda-lime absorption technique was usedfor in situ respiration (CO, efflux) and the wet alkali method (NaOH)was used for laboratory mineral soil respiration. Soil temperature andmoisture content were also measured. Harvesting significantly (P =0.05) increased in situ respiration during most sampling periods. Thiseffect was attributed to an increase in live root and microflora activityassociated with postharvesting revegetation. In situ respiration in-creased exponentially (Qu relationship) as treatment soil tempera-tures increased (mean range 8.3-29.1°C), but followed a parabolic-type pattern through the range of soil moisture measured (meanrange 10.4-31.5%). Mean rates of laboratory mineral soil respirationmeasured during the study were unaffected by cutting treatment formost sampling sessions. Overall, the mean rate of CO; efflux in theclearcuts (7.15 g CO, m 2 d~') was significantly higher than that inthe partial cuts (5.95 g CO2 m 2 d~'), which in turn was significantlyhigher than that in the controls (4.95 g CO, m 2 d '). Mass balanceestimates indicate that these treatment differences will have little orno long-term effect on C sequestration of these managed forests.

CARBON POOLS AND CYCLING in forest ecosystems arebeing increasingly studied. A recent international

conference was devoted entirely to C forms and func-tions in forest soils (McFee and Kelly, 1995). The focusof much of the attention on soil C concerns the effects

A.J. Londo, School of Forestry and Wood Products, Michigan Tech.Univ., Houghton, MI 49931; M.G. Messina, Dep. of Forest Science,Texas A&M Univ., College Station, TX 77843-2135; S.H. Schoenholtz,Dep. of Forestry, Missississippi State Univ., Box 9681, MississippiState, MS 39762. Received 3 Nov. 1996. *Corresponding author(m-messina@tamu.edu).

Published in Soil Sci. Soc. Am. J. 63:637-644 (1999).

of forest management on the ability of soil to performas either a C sink or source, particularly in light ofgrowing concern over escalating atmospheric C content.Unfortunately, C cycling studies in forest ecosystemshave traditionally emphasized aboveground portions ofthe system, with belowground segments generallytreated as a single system component (Edwards andHarris, 1977). Furthermore, management effects on soilC pools and fluxes have been poorly studied comparedwith other soil characteristics such as inorganic elementsand physical factors.

Many of the studies on management impacts on soilC concern effects on C contents in major ecosystempools and are retrospective in nature (Richter et al.,1995; Van Lear et al., 1995). While such studies providevaluable insight into the long-term effects of forest man-agement, they do not offer detailed analyses of conse-quences on important C fluxes.

One C flux that is particularly responsive to ecosystemperturbations is CO2 efflux from the soil surface, com-monly termed soil respiration and usually expressed asthe combined production of CO2 from heterotrophicdecomposition and root respiration. Soil respiration isa major pathway of ecosystem C flow (Redmann, 1978;Holland et al., 1995) and is expected to respond to forestharvesting. Although clearcutting may augment soil Cefflux due mostly to increased soil temperatures (Ed-wards, 1975; Edwards and Ross-Todd, 1983; Mattsonand Smith, 1993), a recent literature review by Johnson(1992) of the effects of forest harvesting on soil Cchanges showed either no effects, or changes of <10%of preharvest content. However, of the thirteen studiesreviewed, none involved sites with extensive wetlandcharacteristics. Detrital soil C transformations, includ-ing soil respiration, have not been studied extensivelyin floodplain forests, even though soil respiration may

Abbreviations: OM, organic matter.

638 SOIL SCI. SOC. AM. J., VOL. 63, MAY-JUNE 1999

be a significant flux in these C-rich ecosystems. Pulliam(1993), in one of the few studies done in floodplainforests (Georgia), showed that overall floodplain detri-tal processing was dominated by aerobic respiration andgaseous CO2 export. However, Pulliam's study did notinvolve the influence of harvest disturbances.

The objectives of this study were to determine theeffects of different levels of forest harvesting on soiltemperature and moisture, and to determine howchanges in these two dynamic soil properties influencedC efflux in an east Texas bottomland hardwood ecosys-tem. This study was part of an overall research projectexamining the effects of harvest intensity on a varietyof ecosystem components, including water quality, wild-life, regeneration, and soil chemical and physical prop-erties.

MATERIALS AND METHODSThe study was located in the Neches River floodplain in

Tyler County, Texas, (30°39'N, 94°5'W) on land owned byTemple-Inland Forest Products Corporation, Diboll, TX. TheNeches River originates within the Coastal Plain of northeastTexas and flows south to the Sabine River immediately inlandof the Gulf of Mexico near Beaumont, TX. Overbank floodingin the study area occurs only in years of extreme winter precipi-tation since the river is controlled by two upstream dams.Other research in the same general vicinity showed overbankflooding occurs less than twice per decade (Streng et al., 1989).Other than filled sloughs and swamps, no extensive floodingoccurred during the study period. The general study area is abroad, level flat within the first river bottom with all treatmentplots situated within 2 km of the river. Elevation ranges from= 17 to 19 m above sea level with microsite variation sufficientto influence plant species occurrence. The climate is warmand humid with an average annual temperature of =19.4°Cand a range in monthly means of 10.0°C in January to 27.2°Cin July. The frost-free season is 241 d. Annual precipitationaverages 132 cm and is generally well distributed (Griffithsand Bryan, 1987). Soils are varied but are predominantly AerieDystraquerts (Ozias series: fine, smectitic, thermic Aerie Dys-traquerts), Fluvaquentic Dystrochrepts (lulus series: coarse-loamy, siliceous, active, thermic Fluvaquentic Dystrochrepts),and Fluvaquentic Eutrochrepts (Laneville series: fine-silty,siliceous, thermic Fluvaquentic Eutrochrepts) (R. Dolezel,1992, personal communication). Soils vary in texture from clayto loam, but are predominantly clay loams and loams. Not allsoils on the study site are hydric, and therefore not all ofthe study area is jurisdictional wetland. The area was heavilylogged in the early 1920s, but has been largely undisturbedsince (Norman Davis, 1992, personal communication). Theoverstory of the existing 65-yr-old stand was principallysweetgum (Liquidambar styraciflua L.) and water oak (Quer-cus nigra L.) with a midstory heavily dominated by ironwood(Carpinus caroliniana Walt.) (Messina et al., 1997). Un-derstory composition varied spatially due to occurrence ofnatural tree-fall gaps.

Three harvesting intensities (clearcut, partial cut, and anuncut control) were tested on approximately square 8.1-haplots. Clearcuts had all standing woody vegetation (>5-cmdiameter at breast height) severed with a mechanical har-vester. Basal area of the partially cut plots was reduced by=58% following marking by Temple-Inland foresters to im-prove stand composition and condition, and to promote ad-vanced reproduction of desirable species.

Treatments were arranged in three blocks, each containingthree contiguous plots with one treatment per plot. Blockswere located along separate first-order, intermittent headwa-ter streams which approximately bisected the blocks. Streamchannel and treatment plot slopes were <1%. Treatmentswere arranged downstream in the order: control, partial cut,and clearcut to avoid confounding effects of stream position ontreatments. In other words, the control was always upstream ofthe partial cut, which in turn was always upstream of theclearcut. A preharvest characterization of the soil total C,forest composition, and basal area in all plots showed thatthere was no significant correlation between these soil andstand properties and upstream-downstream orientation. There-fore, we believe that the systematic treatment assignmentwithin blocks did not compromise our treatment effect inter-pretations. A constant assignment of treatments to plots wasdone to accommodate a companion study on streamwaterchemistry. Streamside management zones extending =20 mfrom each stream bank were left largely undisturbed withonly occasional selective harvesting. Harvesting occurred inSeptember 1992, during dry conditions (i.e., no tire trackingor rutting occurred). No postharvest site preparation was per-formed.

Because sampling began in March 1993, we evaluated respi-ration during the period of vigorous vegetation recovery inthe first growing season following harvesting. This intervalwas characterized by rapid invasion of herbaceous speciesand copious sprouting of woody vegetation. Some herbaceousspecies, particularly Rubus spp., exceeded 2 m in height.Sprouting vegetation occasionally exceeded 3 m during thisperiod. Regrowth was most profuse in the clearcut and af-forded almost complete ground cover, but areas in the partialcut also had considerable herbaceous vegetation offering totalground cover under canopy gaps. Understory vegetative char-acteristics in the clearcut remained largely unchanged duringthe study. Several sampling attempts early in the growingseason were suspended because of the water table risingaboveground, which did not pose excessive problems duringmost of the study.

Within each treatment plot, eight sampling points for soilrespiration were established on the Ozias soil, a fine smectiticthermic Aerie Dystraquert. The Ozias is the most commonseries in all three blocks. Sampling points were randomly lo-cated in areas with similar surface characteristics (at least 3 mfrom the nearest tree, stump, or slash pile). Total soil C variedinsignificantly by treatment and mean plot values ranged from=20 to 24 Mg ha'1 in the 0- to 15-cm depth (Wang, 1996,unpublished data).

In situ soil respiration was determined under a static cham-ber using soda-lime [NaOH + Ca(OH2)] as the alkali absor-bent (Edwards, 1982). Seventy-two 2.9-L tin chambers, eachcovering a soil surface area of 0.019 m2, were used as thestatic chambers. Chambers were coated with silver paint andcovered with small plywood shelters to reduce unnatural heat-ing of the soil surface. Soda-lime was predried for 24 h at100°C and 30 g were weighed into a jar that was then placedunder a chamber. The chambers were set approximately 1 cminto the soil to ensure a good seal between the soil and canbut also minimize root severing. Any CO2 flush that occurredfrom positioning the can was considered minimal or at leastconstant among treatments. Surface litter was left undisturbed.After a 24-h exposure period, the soda-lime was dried andreweighed in the lab. Carbon dioxide respired was determinedby the increase in weight multiplied by a correction factor of1.41 to account for water produced by chemical reactionsand driven off by the drying process (Edwards, 1982). Soiltemperatures were measured at a depth of 10 cm at =1000 h

LONDO ET AL.: FOREST HARVESTING EFFECTS ON SOIL TRAITS IN A BOTTOMLAND HARDWOOD FOREST 639

each time chambers were placed in the field. Two blanks weremeasured for each treatment to account for ambient CO2levels inside the chambers. Blanks, consisting of sealed cham-bers containing soda-lime, were placed on the soil surface.Sampling began =6 mo after harvesting with 15 collectionsmade approximately monthly during the length of the study(18 mo).

Near the incubation chambers, soil samples (without Olayers) were collected to a depth of 15 cm with a standardbucket auger to determine harvesting effects on mineral soilrespiration rates. Temperature of the top 15 cm was deter-mined for each sample at the time of collection. In the lab,soil was passed through a 2-mm sieve to remove large rootsand soil organisms. A 50-g subsample was oven dried at 100°Cfor 24 h for gravimetric moisture content. Soil samples wereincubated by treatment group using the wet alkali absorptionprocedure at field soil temperature measured at time of collec-tion and averaged by treatment (Alef, 1995). Fifty grams ofsoil were placed in a 946-mL airtight canning jar, along with=7 mL of water, and placed in an environmental chamber for13 d. After the third day, a vial containing 30 mL of 0.5 MNaOH was placed in each jar. The initial 3 d of incubationwere to allow the soils to stabilize after handling. Carbondioxide respired by the soil was determined by titration (An-derson, 1982). Blank determinations were made by incubatinga jar containing only water and a vial of NaOH. Since treat-ment temperatures sometimes differed substantially, and sinceincubator space was limited, any sample backlogs were storedat 2°C until incubation.

As stated above, preharvest analysis of total soil C andseveral stand variables indicated no upstream-downstreamgradient in these variables. However, as an added precaution,a covariance analysis was used with preharvest average totalsoil C content as the covariate (C. Gates, 1992, personal com-munication) because treatments were not randomly assignedto plots. This analysis was run on the final raw data set withall data points individually represented. There were nine aver-age C values from the treatment plots associated with the CO2data from the 1079 samples (months X chambers). Carbondioxide values were regressed against average total soil Cand residual CO2 values (actual minus predicted) calculated.Residual CO2 values were then added to the overall meanCO2 to produce new CO2 values for use in the analysis ofvariance for determining treatment effect. All statistical calcu-lations were made using the Statistical Analysis System soft-ware (SAS Institute, 1987).

RESULTS AND DISCUSSIONSoil Surface Carbon Dioxide Efflux, Moisture,

and TemperatureAverage in situ respiration rates varied significantly

(P = 0.05) among treatments in the order: clearcut >partial cut > control (Table 1), although this rankingvaried somewhat among sampling periods (Fig. la).Rates in all treatments were highest in late spring anddecreased throughout the growing season to a low inthe winter (Fig. la). Differences among treatments weregenerally greater in the warmer months than in thecolder months.

Mean soil temperature varied significantly (P = 0.05)in the order: clearcut > partial cut > control (Table 1)and varied seasonally as expected, with one exception(Fig. Ib). An elevated soil temperature was recordedin February 1994, when day and night air temperatures

Table 1. Effects of harvesting treatment on several soil variablesin a Texas bottomland hardwood ecosystem._________

Treatment

VariableEfflux (g CO2 m~2 d-')Soil moisture (%)Soil temperature (°C)Mineral soil respiration

(,jig C02 g-1 d->)

Control4.95 (0.25)8121.9 (0.30)a20.3 (0.29)a

61.2 (8.40)a

Partial cut5.95 (0.36)b22.1 (0.27)a21.6 (0.34)b

61.2 (S.OO)a

Clearcut7.15 (0.38)c19.3 (033)b22.7 (0.44)c

56.0 (4.80)a

t Values are 16-month averages; parenthetical values are one standarderror.

t Values within rows followed by different letters vary significantly(P = 0.05).

were unseasonably high (26 and 11°C, respectively) andbarren tree crowns allowed solar radiation to penetrateto ground level. Clearcut soil respiration in that periodappeared to respond to the high temperature (Fig. la).Soil moisture was highest in the winter and spring andlowest in late summer (Fig. Ic). The clearcut was signifi-cantly (P = 0.05) drier than the other treatments (Table1), particularly during the growing season (Fig. Ic). Thiswas probably due to greater rates of evaporation fromthe soil surface in the first growing season followingcutting, and the observed large amounts of herbaceousgrowth in the clearcuts thereafter.

The nature of the effect of soil temperature and mois-ture on soil respiration (i.e., the shape of the responsecurve) did not differ among treatments. Soil respiration

F M A M J J A S O N D J F M A M J J A

1993-94 MonthFig. 1. In situ (a) soil respiration, (b) soil temperature at 10 cm, and

(c) soil moisture content (0-15 cm) in an East Texas bottomlandhardwood forest following clearcutting and partial cutting. Verticallines are one standard error.

640 SOIL SCI. SOC. AM. J., VOL. 63, MAY-JUNE 1999

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Soil Temperature (°C)Fig. 2. Relationship between in situ soil respiration and soil tempera-

ture in an East Texas bottomland hardwood forest for the (a)control, (b) partial cut, and (c) clearcut treatments through a 16-mo period. Each point represents means among three blocks foreach sampling session, or 24 individual field collections.

rates increased exponentially with soil temperature;however, the magnitude of response varied among treat-ments (Fig. 2). A Q10 relationship between rates of CO2efflux and soil temperature showed that for every 10°Cincrease in soil temperature, respiration rates increasedby a factor of about 2.09 in the control, 2.16 in theclearcut, and 2.51 in the partial cut. Temperature ex-plained 31 to 58% of the variation in the respirationrates. Because the exponential function fit to these datais multiplicative, the efflux for a given temperature is aproduct of the intercept and the slope. Intercepts forthese functions decreased in the order: clearcut > con-trol > partial cut; therefore, for any given soil tempera-ture, the clearcuts had the greatest amount of predictedsoil surface CO2 efflux, particularly at higher tempera-tures (Table 2). Soil temperatures at the lower end of

Table 2. Effects of soil temperature on predicted soil surface CO2efflux for each harvesting treatment. ___ ___

Soil surface effluxt

Treatment

ControlPartial cutClearcutRange

10°C

1.91.72.30.6

15°C

2.72.63.40.8

20°C- g CO2 m 2 d~l

4.04.25.01.0

25°C

5.86.67.31.5

Range

3.94.95.0

t Efflux values were predicted from the equations in Fig. 2.

the range were similar among treatments, but effluxdiverged as temperatures increased (Fig. 2). High-endtemperatures decreased among treatments in the order:clearcut > partial cut > control. Furthermore, the rangeamong CO2 efflux rates increased with soil temperature,and the rates of increase of CO2 efflux with temperaturevaried, with clearcut ~ partial cut > control.

The Qio relationship has been commonly used to de-scribe the effects of soil temperature on respirationrates, although not universally. Lloyd and Taylor (1994)state that exponential (Qio) and linear relationships be-tween soil respiration and temperature are inferior toArrhenius-type equations wherein the activation energyfor respiration varies inversely with temperature. Inother words, the relationship between respiration andtemperature is not a simple exponential across the nor-mal range of physiological temperatures. Nevertheless,the Q10 relationship is favored by many researchers (Ed-wards, 1975) and values have been shown to vary be-tween 1.3 and 3.3 (Reich and Schlesinger, 1992). OurQio values of 2.09 to 2.51 were centered in this range.Other researchers have preferred a linear equation todescribe the relationship between soil respiration andtemperature (Mathes and Schriefer, 1985; Pulliam,1993), but we found an exponential relationship to besuperior to a linear equation for our data.

Respiration rates in all treatments varied paraboli-cally with soil moisture, which explained 40 to 43% ofthe variation in respiration (Fig. 3). A parabolic functionhas been used by others to describe the relationshipbetween soil respiration and soil moisture (Douglas andTedrow, 1959; Paul and Clark, 1989). This reflects thegeneral observation that soil respiration rates declinein both saturated soils (Kucera and Kirkham, 1971) andvery dry soils (Kucera and Kirkham, 1971; de Boois,1974; de Jong et al., 1974).

The clearcut had a narrower soil moisture contentrange (15.4%) than either the control (19.1%) or thepartial cut (18.1%) (Fig. 3), which was unexpected. Thismay have been due to the very rapid regrowth of woodyand herbaceous vegetation in the clearcuts, resulting ingreater canopy interception of precipitation as well asa transpirational drying influence. This also occurred inthe dormant season, which is brief and "incomplete" inthis region of Texas. As with soil temperature, the great-est difference among treatments in soil moisture was atthe high end of the range.

Although respiration rates varied parabolically withsoil moisture content in all treatments, the magnitudeof response varied among treatments, as reflected bythe different intercepts and slopes for the functions (Fig.3). The moisture content at which predicted respirationrate peaked was 16.6, 18.2 and 20.0% in the clearcut,partial cut, and control, respectively. Predicted respira-tion rates at those moisture contents were 10.2, 7.6 and6.6 g CO2 m'2 d~J in the clearcut, partial cut, and control,respectively. The peak predicted respiration rate oc-curred at a lower soil moisture content in the clearcutthan in the control. The partial cut was intermediate inboth regards. Figure 3 shows that the clearcut had ninedata points at or above the grand mean efflux rate of

LONDO ET AL.: FOREST HARVESTING EFFECTS ON SOIL TRAITS IN A BOTTOMLAND HARDWOOD FOREST 641

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P>F = 0.0446

Partial CutCO2 = -6.698 + 1.569 MC - 0.043 (MC)3

R2 = 0.40

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Clearcut-0.113 (MC)2

GrandMean'

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Table 3. Effects of soil moisture on predicted soil surface CO2efflux for each harvesting treatment.

10 15 20 25 30 35

Soil Moisture (%)Fig. 3. Relationship between in situ soil respiration and soil moisture

content in an East Texas bottomland hardwood forest for the (a)control, (b) partial cut, and (c) clearcut treatments through a 16-mo period. Each point represents means among three blocks foreach sampling session, or 24 individual field collections.

6.0 g CO2 m~2 d"1, the partial cut had seven, and thecontrol had six. However, the influence of soil moisturecontent on soil surface CO2 efflux varied among thetreatments such that at low and intermediate moisturecontents, the clearcut had the highest efflux rates andthe control had the lowest. Both treatments had lowerrates than the partial cut at higher soil moisture contents(Table 3).

The fact that maximum predicted rates of efflux oc-curred at lower soil moisture contents in the clearcutthan in the partial cut and control probably reflectedthe influence of soil temperature. Our soils were wetterduring cooler temperatures and drier during warmertemperatures, as is characteristic of southern bot-tomland hardwood ecosystems. Therefore, efflux ratestoward the upper end of the soil moisture range in theclearcut probably occurred during the cooler tempera-tures, which offers a viable explanation for why theresponse curve decreased so rapidly at the higher mois-ture contents. The partial cut and control soils also expe-rienced similar cycles of moisture content and tempera-ture, but at a somewhat reduced level due to themoderating influence of the canopy. Efflux rates at themid range of soil moisture content in the order: clearcut> partial cut > control were probably due to the largeramounts of light-fraction organic matter and fine rootsfrom herbaceous plants in the harvested plots.

Soil surface effluxfTreatment 15% 20% 25% Range

ControlPartial cutClearcutRange

5.17.29.90.6

———— gC026.67.58.90.8

m~2 d-> ———5.05.72.31.0

1.61.87.6

t Efflux values were predicted from the equations in Fig. 3.

It appears that temperature exerted greater controlthan did soil moisture over respiration on our site, eventhough some of the coefficients of determination (R2)were higher for the soil moisture regressions (Fig. 3)than for the soil temperature regressions (Fig. 2). Itshould be noted that the soil moisture equations weremultiple regressions and therefore will have a higher R2

since adding independent variables to a model can onlyincrease R2 (Neter and Wasserman, 1974). It is some-times recommended that the R2 be adjusted for thenumber of independent variables in the model to betterreflect the model's true predictive capabilities (Neterand Wasserman, 1974). The adjusted R2s were 0.38,0.35,and 0.35 for the control, partial cut, and clearcut, respec-tively, and better reflected the greater probabilities ofhigher F values for the moisture content regressions(Fig. 3) than those for the soil temperature regressions(Fig. 2).

Correlations between soil respiration and environ-mental variables explained a considerable amount ofthe variation in respiration rates despite the fact thatour sampling occurred across such a large area withsubstantial microsite variation, and despite the fact thatsoil temperatures were measured only when the sodalime was placed in the field. Effects of temperature andmoisture on soil respiration have been well documented;however, the importance of each varies among ecosys-tems. Edwards and Ross-Todd (1983) found that respi-ration rates were significantly affected by soil tempera-ture (^?2 = 0.90), whereas moisture content showed nosignificant (P = 0.05) effect in a Tennessee mixed hard-wood forest. Gordon et al. (1987) showed that bothtemperature and moisture significantly (P = 0.01) af-fected postharvesting soil respiration rates in an Alas-kan white spruce [Picea glauca (Moench) Voss] forest.In the Chihuahuan Desert, respiration rates were moresignificantly affected by soil moisture (R2 = 0.45) thansoil temperature (R2 = 0.16) because of limited avail-ability of water in that ecosystem (Parker et al., 1983).

The stronger relationship between soil respirationand temperature generally measured by most research-ers is not unexpected since soil respiration reflects het-erotrophic and autotrophic activities that are highlytemperature dependent. Other researchers have beenunable to demonstrate a significant influence of soilmoisture on respiration (Reiners, 1968; Anderson, 1973;Edwards, 1975). Furthermore, Lloyd and Taylor (1994)found less sensitivity of soil respiration to fluctuationsin soil temperature in biomes with relatively higher soiltemperatures, such as ours. The weaker relationship

642 SOIL SCI. SOC. AM. J., VOL. 63, MAY-JUNE 1999

between soil respiration and soil moisture content onmicrobial activity may be due to the influence of mois-ture through time. The sudden addition of water to arelatively dry soil normally causes increased soil respira-tion (de Jong et al, 1974) due to the well-establishedinfluence of a wetting-drying cycle on microbial activity(Orchard and Cook, 1983; Cook et al., 1985); however,long-term waterlogging usually decreases soil respira-tion. Therefore, unless soil water potential or contentis accounted for prior to measurement of CO2 efflux,the relationship between soil water and respiration willusually be weaker than for soil temperature. We mea-sured soil water content only when we placed our sodalime in the field and without knowledge of prior soilmoisture conditions, which probably contributed to ourcalculated weaker relationship between water contentand soil respiration.

Another factor that leads to a weaker relationshipbetween soil respiration and soil moisture is the interac-tion between soil moisture and temperature, sometimestermed the hydrothermal factor (Kononova, 1966). Wil-dung et al. (1975) showed that soil moisture and temper-ature were interdependent in their effects on soil respi-ration rates, and concluded that the mutual regulationof soil respiration rate by temperature and moisture wasbest described by a soil temperature X water interactionor multiplicative term in regression equations. We foundbetter descriptive ability for soil respiration when weregressed CO2 efflux, averaged among all harvest treat-ments, on both soil temperature and moisture (Fig. 4).However, we found no significant interaction term inour equation of best fit to our data:

Soil CO2 efflux = -14.294 + 0.274T + 1.633M- 0.043(M)2 [1]

where T is soil temperature (°C) and M is the soil watercontent (%). This equation reflects the parabolic natureof the soil moisture effect.

With few exceptions, our soil had the greatest mois-

Fig. 4. Combined effects of soil temperature and moisture contenton in situ soil respiration in an East Texas bottomland hardwoodforest for all harvest treatments combined through a 16-mo period.

ture contents during periods of low temperature, andthe lowest contents during high temperatures. Figure 4shows that the lowest efflux rates occurred during peri-ods of high moisture content and low temperatures,i.e., during the winter. The highest rates were usuallyrecorded with intermediate moisture contents (15-25%)and higher temperatures (22-28°C), i.e., during thewarmer months, particularly late spring and early sum-mer. Variation in efflux rates tended to increase withtemperature due to greater differences between ratesin harvested plots and those in the uncut control (Fig.la and 4), probably influenced by greater differencesin moisture content among treatments (Fig. Ic; Reiners,1968). Figure 4 also indicates a greater sensitivity ofefflux to soil moisture during high temperatures, whenrates decreased at both high and low moisture contents.Efflux rates varied less with soil moisture when tempera-tures were low; then rates steadily decreased with in-creasing moisture contents. Furthermore, Figure 4shows a greater sensitivity of soil respiration to tempera-ture at higher moisture contents.

The seasonally dependent response of soil respirationto soil moisture and temperature has been measured byothers. Carlyle and Than (1988) showed a much betterfit between measured and modeled soil respiration be-neath an Australian Monterey pine (Plnus radiata D.Don) stand when they included a moisture-dependentQio term in their model, leading them to conclude thatunder conditions of low moisture availability, the Q10relationship is strongly moisture dependent. Froment(1972) concluded that soil temperature chiefly regulatessoil respiration in the winter, and that soil moisture ismore influential in the summer. Our data (Fig. 4) alsoshow the stronger influence of temperature in the coolmonths (i.e., with higher moisture contents), and thestronger influence of moisture during the warm months(i.e., with higher temperatures).

Mineral Soil RespirationMineral soil respiration rates were 61.2,61.2, and 56.0

jig CO2 g"1 d"1 for the control, partial cut, and clearcut,respectively (Table 1). These rates are probably artifi-cially high due to the disturbance caused by collection,sieving, and the associated handling. Due to limitedincubator space, we allowed only 3 d for equilibrationbefore our 10-d incubation period, which may have beeninsufficient (Edwards and Ross-Todd, 1983). However,disturbance effects were equal among treatments, so weused these rates only to describe differences amongtreatments in a fashion similar to that described by Ed-wards and Ross-Todd, 1983). For this reason, we didnot attempt to partition respiration between microbialand root respiration, which has been shown to providevariable and uncertain results (Coleman, 1973; Mattsonand Smith, 1993).

Mineral soil respiration rates did not vary significantly(P = 0.05) among treatments when averaged acrossthe entire course of the study (Table 1). Rates variedseasonally with higher values recorded in the warmestmonths (July, August, and September) and lower values

LONDO ET AL.: FOREST HARVESTING EFFECTS ON SOIL TRAITS IN A BOTTOMLAND HARDWOOD FOREST 643

in the winter through early spring in all treatments (Fig.5). There appeared to be a trend in respiration ratesthrough time that may have reflected a delayed effectof harvesting. Prior to October of the first postharvestgrowing season, the control respiration rates were gen-erally higher than those in the harvested treatments,particularly in the warmer months. After October, theharvested treatments tended to have higher respirationrates than the control. The control values followed sea-sonal trends with, for instance, similar rates in Augustof successive years. Alternatively, rates in the harvestedplots appear to have increased with time, except for thelower rates recorded in the winter months, and werebecoming considerably higher than the control rateswhen the study was terminated in July 1994 (Fig. 5).

During the second growing season, the harvested siteswere thickly revegetated with herbaceous species. Eventhough the soil samples were sieved, the substantiallygreater amounts of organic matter (OM) in the samplesfrom the harvested treatments than in the control mayhave been due to fine roots of herbaceous species androot exudates that passed through the sieve. Also, rootsfrom harvested trees were decomposing, thereby sup-plying OM sources that could have increased respirationin harvested plots. Furthermore, the large amount oflogging slash on the soil surface was decomposing rap-idly and supplying additional OM for soil respiration.

Other research has shown that harvesting affects min-eral soil respiration. Edwards and Ross-Todd (1983)attributed significantly (P = 0.05) higher mineral soilrespiration after clearcutting to the synergistic effectsof elevated soil temperature and moisture, especiallynear the soil surface where microbial activity is greatest;however, they incubated clearcut samples at a constant23°C, and control samples at 15°C. We incubated oursamples at field temperatures measured at sample col-lection, the range of which was always less than the 8°Crange used by Edwards and Ross-Todd (1983). Also,even though our samples were sieved after collection,light fraction OM content probably varied. Vegetativeregrowth of the harvested plots was vigorous, but un-even in cover and composition, thereby enhancing varia-tion in amounts of root material and exudates amongsamples.

CONCLUSIONSWe found that harvesting significantly increased soil

surface CO2 efflux in a Texas bottomland hardwoodforest. Similar increases in CO2 efflux were measuredby Hendrickson et al. (1989) 3 yr after cutting of mixedconifer-hardwood forests in Ontario, and by Gordonet al. (1987) in the third and fourth year after harvestingwhite spruce in interior Alaska. In contrast, soil surfaceCO2 effluxes decreased following harvesting of bothAppalachian hardwoods (Edwards and Ross-Todd,1983; Mattson and Swank, 1989) and Ontario aspen(Populus spp.) stands (Weber, 1990). These differencesare most likely due to the varying influences of soiltemperature and soil moisture in different ecosystems

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Fig. 5. Laboratory respiration of mineral soil from an East Texasbottomland hardwood forest following harvesting. Vertical linesare one standard error.

on soil microbial activity, and also due to the activityof roots following harvesting.

Based on the first 2 yr following harvest, our effluxrates represented annual C losses on the order of 4.9,5.9, and 7.1 Mg ha"1 yr"1 in the control, partial cut,and clearcut, respectively. As these sites revegetate, thevegetation will serve as a CO2 sink, balancing the in-creased soil respiration rates. On the basis of their work,Edwards and Ross-Todd (1983) concluded that there isno detectable direct impact on atmospheric CO2 concen-trations resulting from increased oxidation rates of min-eral soil organic matter when temperate forests are al-lowed to regrow immediately following harvest. Ourecosystem is even more productive than those studiedby Edwards and Ross-Todd (1983), as evidenced bysimilar southeastern bottomland hardwood forests thatachieved a standing aboveground biomass, excludingthe forest floor, of 238 Mg ha"1 by age 60 yr (Messinaet al., 1986). If we apply this biomass content to ourstands, and assume that the biomass is =50% C, and ifthe total soil C content in the 0- to 15-cm layer is =22Mg ha"1, then the annual efflux rate for the clearcutrepresents a loss of only =5% of the total ecosystem C,ignoring belowground compartments and concomitantC fixation. In light of the proven advantages clearcuttinghas for desirable regeneration of these forests (Kellisonet al., 1981; McKevlin, 1992), the differences in CO2efflux among treatments is insufficient to affect manage-ment decisions.

ACKNOWLEDGMENTSThis research was funded by the U.S. Environmental Pro-

tection Agency (Assistance Agreement no. X006977-01-0) andTemple-Inland Forest Products Corporation, Diboll, TX. Wethank James Burger for a helpful review and Marian Erikssonfor statistical assistance.

644 SOIL SCI. SOC. AM. J., VOL. 63, MAY-JUNE 1999