the effects of management on soil and plant carbon sequestration in slash pine plantations

Journal of Applied Ecology

2001

38

, 932–941

© 2001 British Ecological Society

Blackwell Science, Ltd

The effects of management on soil and plant carbon sequestration in slash pine plantations

JIANPING SHAN*, LAWRENCE A. MORRIS and RONALD L. HENDRICK

D.B. Warnell School of Forest Resources, The University of Georgia, Athens, GA 30602, USA

Summary

1.

Intensively managed pine plantations in the south-eastern United States can play animportant role in global carbon sequestration both through accumulation of carbon inwood used in long-lasting products as well as through increased soil carbon storage.Fertilization and understorey-elimination are two commonly used intensive manage-ment practices in the south-eastern United States that have the potential to increasecarbon storage in vegetation and affect soil carbon.

2.

In this study, we assessed the effects of these practices on carbon accumulation invegetation biomass and in the soil of 17-year-old slash pine

Pinus elliottii

plantations inthe flatwoods of northern Florida, USA.

3.

Three treatments, fertilization, understorey-elimination, and fertilization plusunderstorey-elimination, were evaluated and compared with an untreated control.

4.

All three treatments increased above-ground biomass accumulation compared withthe untreated control; understorey-elimination also increased biomass of the forestfloor litter, with or without fertilization.

5.

Although understorey-elimination increased above-ground production, as a resultof reduced below-ground production total net primary production was decreased inplots from which the understorey was eliminated.

6.

Soil carbon storage was lower in plots where the understorey was eliminated, with orwithout fertilization. This appeared to be the result of reduced fine root growth andmortality but also may have reflected reduced litterfall inputs early in the rotation.

7.

Our results indicate that intensive management of pine plantations on sandy flat-woods soils can increase carbon sequestration, but these increases will be the result ofincreased carbon accumulation in biomass and its long-term uses rather than throughincreased soil carbon.

Key-words

: below-ground carbon allocation, fertilization,

Pinus elliottii

, soil carbon,spodosols, understorey-elimination.


(2001)

38

, 932–941

Introduction

Several authorities have suggested that pine planta-tions of the south-eastern United States can play animportant role in global carbon sequestration boththrough accumulation of carbon (C) in wood used inlong-lasting products as well as through increased soilC storage (Huntington 1995; Van Lear, Kapeluck &Parker 1995). Meanwhile, C cycling in forests plays akey role in long-term forest productivity. The import-ance of soil C in determining soil physical, chemical

and biological properties has been well documented(Jurgensen

et al

. 1997). It is particularly important forsandy soils, which lack inorganic colloidal materials(Carlyle 1993). Currently, intensive forest managementactivities, including mechanical site preparation, ferti-lization and competition control, are practised in pineplantations throughout the Coastal Plain of the south-eastern United States (Harding & Jokela 1994). Slashpine

Pinus elliottii

Engelm. is one of the major planta-tion species in the South-east because it grows fast andis easily established in the relatively infertile soils of theCoastal Plain (Gholz, Fisher & Pritchett 1985; Brown& Thompson 1988). These pine plantations are usuallytreated with herbicide once or twice to control competingvegetation and fertilized two or more times during arotation (Pritchett & Comerford 1982; Allen 1994).

*

Present address and correspondence: Jianping Shan,Weyerhaeuser Company, PO Box 238, Old StagecoachRoad, Oglethorpe, GA 31068, USA (fax 912 4725314; [email protected]).

JPE648.fm Page 932 Thursday, September 20, 2001 2:11 PM

933

Management of soil and plant carbon sequestration

© 2001 British Ecological Society,


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38

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The benefits of these intensive management activ-ities on pine seedling survival, growth and yield, andnutrient cycling, have been well documented in suchplantations. It is not clear, however, how intensive man-agement will affect the accumulation, allocation anddynamics of soil C in the southern pine plantations(Jokela, McFee & Stone 1991; Harding & Jokela 1994).Thus, it is difficult to predict how intensive manage-ment will affect long-term productivity and, further-more, global carbon sequestration by pine plantations.For instance, fertilization can increase plant productiv-ity and hence increase litterfall inputs, thereby increas-ing soil C, as suggested by Johnson (1992). On theother hand, fertilization might reduce root mortalityrates (Gower, Vogt & Grier 1992), which will in turndecrease the input into the soil C pool. Thus, we do notknow whether fertilization will increase soil C storage.Understorey-elimination will certainly reduce theabove-ground litterfall from competing vegetationearly in the rotation of the plantation and below-ground root litter input throughout the rotation. Forexample, Carlyle (1993) reported that in a 3-year-oldmonterey pine

Pinus radiata

plantation, weeds alonehad an average of 1·3 tons C ha

–1

year

–1

of above-groundbiomass. This C source, along with related weed rootlitter, resulted in a 25% higher C concentration in thesurface soil (0–15 cm) than in weed-free plots. Thus,understorey-elimination will probably reduce the Cstorage, except in stands where extremely large inputsfrom increased tree growth offset this loss.

The objective of this study was to quantify the effectsof fertilization and understorey-elimination on vegeta-tion and soil C storage and dynamics near the end ofthe rotation for slash pine growing on coarse-texturedsoils. This information is essential for understandingthe effects of intensive forest management on global Cbudgets and for estimating potential C sequestration.

Materials and methods

The study was conducted at three research installationsof the University of Georgia Plantation ManagementResearch Cooperative (PMRC) Site Preparation/Sec-ond Rotation Study, Florida, USA (Shiver, Rheney &Oppenheimer 1990). The sites were located on poorlydrained and somewhat poorly drained spodosols (TypicHaplauods) on Champion International Co. lands atBryceville (30°23

′

N, 81°56

′

W), and Rayonier Inc.lands at Yulee (30°38

′

N, 81°36

′

W) and Callahan(30°34

′

N, 81°50

′

W).Each of these three study sites consisted of nine

0·2-ha plots of slash pine planted in 1979 that had beensubjected to varying levels of site preparation, controlof competing vegetation, and fertilization. In eachplot, the measurement plot area was 800 m

2

with a 6·1-m wide buffer zone along each edge. Trees were plantedat 1·8

×

2·4 m, 1·8

×

3·1 m and 2·1

×

3·1 m spacing at

Bryceville, Yulee and Callahan, respectively. In thisstudy, we chose four plots (each receiving one of threetreatments and one control) at each site, which repres-ented the extreme differences in vegetation conditionand fertilization: (i) control, chop + burn; (ii) fertilized(F), chop + burn + fertilize; (iii) understorey-eliminated(H), chop + burn + understorey-elimination; (iv) fertil-ized plus understorey-eliminated (FH), chop + burn +understorey-elimination + fertilize.

Fertilized plots (treatment F) received an initial applic-ation of di-ammonium phosphate at a rate of 224 kgha

–1

, applied in a 1·2-m band over the rows 1 year afterplanting in 1980. In 1992, at the age of 12 years, 280 kgha

–1

di-ammonium phosphate, 280 kg ha

–1

urea and228 kg ha

–1

KCl were broadcast in the fertilized plots.In the understorey-eliminated plots (treatment H),

vegetation was sprayed with a 3% solution of Roundup®(glyphosate; Monsanto Corporation, St Louis, MO)prior to site preparation. After pines were planted,applications of Roundup®, Garlon® (triclopyr; DowAgrosciences Inc., Indianapolis, IN) and occasionallydiesel fuel on buds were used to eliminate all competingvegetation (Shiver, Rheney & Oppenheimer 1990).In plots without understorey-elimination treatments,understorey vegetation was mainly composed of gall-berry

Ilex glabra

[L.] A. Gray, wiregrass

Aristida strictaMichx

. and sawtooth palmetto

Serenoa repens

[Bartr.]Small. Additional descriptions of these sites can befound in Shiver, Rheney & Oppenheimer (1990).

The experimental design was a factorial block design,with two levels of fertilization (no fertilization vs. repeatedfertilization) and two levels of competition control(no understorey-elimination vs. complete understorey-elimination), replicated in three complete blocks (sites).

Above-ground tree biomass of stems, branches andneedles was estimated from allometric equations relat-ing biomass to diameter at breast height (d.b.h.) usingdiameter inventory data at age 17 years from PMRCand the equations of Swindel

et al

. (1979).Above-ground biomass of shrubs and herbs was

determined by destructively harvesting four 2

×

2-msubplots in the buffer zone of each plot in August 1997during the biomass peak for shrubs and herbs. Shrubswere separated into leaves and stems, dried at 70

°

C,and weighed. Herbs were dried at 70

°

C and weighed.The mass of tree coarse roots (> 10 mm in diameter)

was estimated using a regression equation developedby Santantonio, Hermann & Overton (1977). Gholz &Fisher (1982) demonstrated that this equation is suit-able for slash pine and used it successfully in a chron-osequence of slash pine plantations.

Small and fine root biomass was determined fromsoil cores collected in May 1997. In each plot, five cylin-drical soil cores, 5·0 cm in diameter and 30 cm in depth,were collected at random locations in the buffer zonesaround the plot. Roots were recovered from the soil


934

J. Shan, L.A. Morris & R.L. Hendrick



,

38

,932–941

using an automated root washer–hydropneumatic rootelutriator (Gillison’s Variety Fabrication Inc., Benzo-nia, MI) (Smucker, McBurney & Srivanstava 1982).After washing, roots were sorted into three sizeclasses: < 0·5 mm, 0·5–2·0 mm (these two categorieswere collectively termed fine roots) and 2·0–10·0 mm(termed small roots). The last two size classes weredried at 70

°

C and weighed.The biomass of roots < 0·5 mm was determined

indirectly. First, the length of roots was determined bythe line intercept method, after all roots > 0·5 mm hadbeen removed. The grid array used for length deter-mination was calibrated with small cut segments ofmonofilament fishing line cut with lengths rangingfrom 0·2 to 5·0 cm, and in amounts proportional tothose in actual root samples. We developed a regressionequation of the length vs. intercept counts (

y

= 0·158 +0·101

x

, where

y

is the length in metres and

x

is theintercept count,

r

2

= 0·99) by counting the interceptsof different known length of monofilament cut with0·2–5·0-cm segments. Then we used the regressionequation to estimate root length based on the interceptcounts. Next, random segments of roots < 0·5 mm totall-ing about 50 cm in length were collected, dried at 70

°

Cand weighed, and an average root-specific length density(cm g

–1

) ratio for roots < 0·5 mm was calculated. Finally,the biomass of roots < 0·5 mm in diameter was calcu-lated by multiplying the total root lengths by the averageroot-specific length density. A detailed description of thismethod can be found in Hendrick & Pregitzer (1993).

In November 1996, four minirhizotron tubes (AnnArbor Plastics, Ann Arbor, MI) with inner diametersof 5·08 cm were permanently installed in each plot at a45

°

degree angle. In each plot, one minirhizotron tubewas 2 m long and the other three were 1 m long. All ofthe above-ground portion of the tubes was paintedblack, then white to exclude light and minimize tem-perature differences. The bottom end was sealed with arubber stopper and the top end was capped to preventsoil, water, light or rainfall from entering the tubes.From April 1997 to March 1998, images were recordedon VHS videotape with a Circon Microvideo 9011Color Agricultural Camera (Circon Co., Santa Bar-bara, CA). The image sampling dates were on 25 April,25 June, 5 August, 25 September and 12 December in1997 and on 5 March in 1998. The video images weredigitized sequentially using the software programme

developed by Hendrick & Pregitzer (1992).Additional information about installation of tubes,digitizing the video images and analysis of the digitizeddata can be found in Hendrick & Pregitzer (1992).

Stem and branch production was calculated using theaverage annual biomass increment from age 14 to age

17. Needle and shrub leaf production was assumedto be equal to their annual litterfalls. Shrub stemproduction was calculated by assuming it had thesame ratio of production to biomass as slash pine treesfor the same treatment. Palmetto leaf production wasassumed to equal its biomass. Herb biomass wasused as annual production. The coarse root (> 10 mmin diameter) production was calculated assuming ithad the same production to biomass ratio as the slashpine stems for the same treatment. Small root (2–10 mm in diameter) production was calculated assum-ing it had the same production to biomass ratio as treebranch.

Fine root (< 2·0 mm in diameter) production andmortality (turnover) from April 1997 to April 1998 werecalculated by multiplying the initial biomass by theratio of annual length production and mortality toinitial length density along with minirhizotron tubes.

Yearly litterfall was assessed by a litter trap collectionmethod with four 0·5-m

2

traps placed in each plot (atotal of 48 traps), which were installed in May 1997.Each trap was a wooden frame measuring 71

×

70 cmwith a nylon screen of 2-mm mesh on the bottom of theframe supported by four legs, about 30 cm high. Litter-fall was collected six times from July 1997 to June 1998.After collection, litter was oven-dried at 65

°

C to con-stant mass and weighed. Forest floor litter was sampledfrom a 0·0625-m

2

area at four points in each plot.

At each plot, soil horizon differentiation and thicknessto 1-m deep were determined using a bucket auger andaveraging eight soil core samples. Soil bulk densities ofA, E, Bh and E

′

horizons were estimated using the ringsample method (Blake 1965). Rock in soils was negli-gible for all the plots. Twelve 5·0-cm diameter soil coresfor each soil horizon were collected on each plot andcombined into three subsamples for each plot. Thesubsamples of each soil horizon were analysed for totalC by using a LECO CNS analyser (LECO Corpora-tion, St Joseph, MI).

CO

2

evolution rates in the field were measured approx-imately bi-monthly from May 1997 to April 1998 usinga portable infrared gas analyser system (IRGA) (LI-6200; LI-COR Inc., Lincoln, NB) (Cropper, Ewel &Raich 1985). In May 1997, eight randomly selectedlocations marked by small PVC sticks were perman-ently installed at each experimental plot. These eightmeasurements were treated as subsamples for eachplot. On every measuring date, soil respiration rate, soiltemperature and soil moisture were measured at a15-cm depth. The TDR (Time Domain Reflectometry)


935




,

38

,932–941

methods (Topp, Davis & Annan 1982) were used forsoil moisture measurement.

All data were analysed using the Statistical AnalysisSystem (SAS Institute Inc., 1987). Two-way analysis ofvariance for the appropriate factorial block design wasused to test for significant differences by using Duncan’smultiple range test at the 0·05 level of significance. Nointeraction of fertilization and understorey-eliminationwas found throughout all the variables investigated.

Results

Above-ground components of tree biomass weregreater in all three treatments (F, H, FH) than in thecontrol plots. Above-ground tree biomasses of the twounderstorey-eliminated treatments (H and FH) werealmost identical (Table 1). For the below-ground com-ponents, estimated coarse root biomass in control plotswas significantly less than in the other three treatments,whereas fine root biomass in the control plots wasgreater than in the two understorey-eliminated treat-ments (H and FH). Understorey-elimination reducedthe below-ground proportion of total biomass. Theproportions of below-ground biomass to total biomasswere 21·9%, 20·3%, 18·1%, and 17·9% in control,

fertilized, understorey-eliminated and fertilized plusunderstorey-eliminated plots, respectively. This trendwas more apparent when the proportion of fine rootbiomass to total biomass was considered, which was3·8%, 2·5%, 1·4% and 0·9% for control, fertilized,understorey-eliminated and fertilized plus understorey-eliminated, respectively (Table 2). The main effectof understorey-elimination was significant for all treebiomass components, while fertilization only decreasedthe very fine root (< 0·5 mm) biomass significantly.

The two understorey-eliminated plots had greaterneedle production than the control plots. The effect of

Table 1. Biomass distribution in the late rotation (age 17) slash pine plantations under different management intensity onflatwoods sites in northern Florida, USA

TreatmentsControl (Mgha−1)

F* (Mg ha–1)

H* (Mg ha–1)

FH* (Mg ha–1)

Main effects (P-value)

FertilizationUnderstorey-elimination Interaction

Tree (biomass)Stems 75·6a† 106·0b 124·9b 125·6b 0·10 0·01 0·11Branches 5·7a 8·5b 10·1b 10·2b 0·11 0·01 0·13Needles 4·2a 5·8b 6·8b 6·8b 0·10 0·01 0·11Tree total 85·5a 120·3b 141·7b 142·6b 0·10 0·01 0·11

ShrubLeaves 1·0 0·8 0·66Stems 2·4 2·0 0·81Shrub total 3·4 2·8 0·78Palmetto 1·0 0·2 0·18Herb 0·1 0·2 0·47Total 89·0 123·5 141·7 142·6 0·09 0·01 0·10

Below-groundCoarse (>10 mm) 17·4a 24·0b 27·8b 27·6b 0·12 0·01 0·10Small (2–10 mm) 3·1ab 3·6a 1·1b 1·9ab 0·37 0·03 0·85Fine (0·5–2 mm) 1·5a 1·2ab 0·8ab 0·4b 0·20 0·03 0·83Very fine (< 0·5 mm) 2·9a 2·7ab 1·7bc 1·2c 0·01 0·01 0·73Fine roots (< 2 mm) 4·4a 3·9ab 2·5bc 1·6c 0·15 0·01 0·71Roots total 24·9 31·5 31·4 31·1 0·24 0·39 0·11

Vegetation total 113·9 155·0 173·1 173·7 0·07 0·01 0·06Forest floor 21·2 26·0 30·0 31·3 0·320 0·04 0·55

*F, fertilized; H, understorey-eliminated; FH, fertilized plus understorey-eliminated.†Means followed by a different letter at the same row are significantly different (α = 0·05).

Table 2. Standing biomass and net primary production of allfine roots (total) and of pine fine roots (pine) expressed as apercentage of total vegetation biomass and production orpine biomass and production in late rotation (age 17) slashpine plantations under different management intensity onflatwoods sites in northern Florida, USA

Control (%)

F* (%)

H* (%)

FH* (%)

Biomass Total 3·8 2·5 1·4 0·9Pine 2·3 1·3 1·4 0·9

Production Total 33·2 30·0 12·8 10·4Pine 27·3 20·8 12·8 10·4

*F, fertilized; H, understorey-eliminated; FH, fertilized plus understorey-eliminated.


936




,

38

,932–941

Table 3.

Allocation of net primary production among vegetation components of late rotation (age 17) slash pine plantations under different managementintensity on flatwoods sites in northern Florida, USA

TreatmentsControl (Mg ha

−

1

year

−

1

)F* (Mg ha

–1

year

–1

)H* (Mg ha

–1

year

–1

)FH* (Mg ha

–1

year

–1

)

Main effects (

P

-value)


TreeStems 5·7 7·7 5·4 6·2 0·11 0·27 0·44Branches 0·5 0·7 0·5 0·6 0·09 0·44 0·37Needles 4·2a† 4·7ab 5·1b 5·5b 0·07 0·04 0·98Tree total 10·4 13·1 11·0 12·3 0·06 0·75 0·75

ShrubLeaves 0·6 0·3 0·66Stems 0·2 0·2 0·81Shrub total 0·8 0·5 0·78Palmetto 1·0 0·2 0·18Herb 0·1 0·2 0·47Total 12·3 14·0 11·0 12·3 0·18 0·11 0·62

Below-groundCoarse (> 10 mm) 1·3 1·8 1·2 1·4 0·19 0·38 0·46Medium (2–10 mm) 0·3 0·3 0·1 0·1 0·62 0·02 0·99Fine (< 2 mm) 7·6a 6·9a 1·8b 1·6b 0·76 0·01 0·46Roots total 9·2 9·0 3·1 3·1 0·66 0·05 0·77Vegetation total 21·5 23·0 14·1 15·4 0·26 0·01 0·94

*F, fertilized; H, understorey-eliminated; FH, fertilized plus understorey-eliminated.†Means followed by a different letter at the same row are significantly different (

α =

0·05).

fertilization (main effect) on branches, needles andtotal tree net primary production (NPP) was close tostatistically significance (

P

= 0·09, 0·07, 0·06 forbranches, needles and tree total, respectively) (Table 3).Understorey-elimination (main effect) significantlydecreased the total below-ground NPP allocation.Understorey-elimination reduced total NPP of treesand competing vegetation combined by 33·6%, whilefertilization increased total NPP by 8·2%. The reduc-tion of NPP caused by understorey-elimination mainlyoccurred in fine roots, while the increase of NPP caused

by fertilization was mainly due to increases in pinecomponents.

Fine root production and mortality

Fine root production in the control and fertilizedplots was greater than in fertilized plus understorey-eliminated plots (Fig. 1). Understorey-elimination signi-ficantly decreased fine root production and mortality,whereas fertilization did not affect the production andannual mortality of fine roots.

Seasonal fine root length production and mortality

Fine roots grew throughout the year, with majorgrowth occurring from May to September (Fig. 2a). Inthe two understorey-eliminated treatments, productionoccurred more evenly than in plots with understoreyvegetation (control and fertilized). The peak mortal-ity of fine roots occurred at different times amongtreatments. In control plots, most mortality of fineroots occurred in late autumn, whereas in other plotsmost mortality occurred in summer and winter(Fig. 2b).

Pine proportion of fine root biomass and production

All three treatments (F, H, FH) decreased the fineroot biomass percentage of total biomass (Table 2).

0·0

2·0

4·0

6·0

8·0

10·0

12·0

14·0

Control F H FH

Treatments

(Mg

ha–1

)

Production

Mortality

b

a

a

ab

Fig. 1. Annual fine root production and mortality calculatedbased on minirhizontron observation of change in root lengthand root length density of late rotation (age 17 years) slashpine plantations in the flatwoods of northern Florida, USA(F, fertilized; H, understorey-eliminated; FH, fertilized plusunderstorey-eliminated). Error bars indicate standard errorsof means. Different letters above the bars indicate significantdifference between treatments (α = 0·05). n = 3.


937




,

38

,932–941

Although fine roots only averaged 2·2% of the totalbiomass, they averaged 21·6% of the total annualproduction, with the highest percentage in the con-trol plots and the lowest percentage in fertilized plusunderstorey-eliminated plots. The difference in the fineroot proportion of total production between fertilizedand control plots was small; however, in fertilized plotsfine root production by pines represented a lower pro-portion of total pine production than in control plots.

The seasonal respiration rates of these spodosolsin slash pine plantations were variable, with thehighest rate in early summer and lowest in the spring(March) (Fig. 3), mainly corresponding to the change

of soil temperature. For the main effects, understorey-elimination significantly decreased the soil respirationrates during May, August and September in 1997 andApril in 1998. There was no detected effect of fertiliza-tion on soil respiration at any time during the year.

Fertilization did not change total soil C storage or soilC storage in any horizon, while understorey-eliminationsignificantly decreased Bh horizon C content (Table 4).Overall, combining soil and biomass C storage, understorey-elimination increased C storage in the system by 4%,while fertilization increased C storage in the system by6% (Fig. 4). These increases were the result of greaterstorage of C in woody biomass and forest floor.

5·0

1·0

0·0

0·0

2·0

3·0

4·0

5·0

6·0

7·0

May/Jun Mar/AprJan/FebOct/Nov/DecAug/SepJul

May/Jun Mar/AprJan/FebOct/Nov/Dec

Measuring time

Aug/SepJul

Leng

th p

rodu

ctio

n(1

0–2 m

mcm

–1 d

ay–1

)Le

ngth

mor

talit

y(1

0–2 m

mcm

–1 d

ay–1

)

(a)

(b)

1·0

1·5

2·0

2·5

3·0

3·5

4·0

4·5

0·5

Control

F

H

FH

Fig. 2. Seasonal fine root length (a) production and (b) mortality of late rotation (age 17 years) slash pine plantation underdifferent management intensity (F, fertilized; H, understorey-eliminated; FH, fertilized plus understorey-eliminated) in theflatwoods of northern Florida, USA. Error bars indicate standard errors of means. n = 3.

0

2

4

6

8

10

12

14

May/97 Aug/97 Sep/97 Mar/98 Apr/98

Measuring time

Soi

l CO

2 ev

olut

ion

rate

(µm

ol m

–2 s

–1)

Control

F

H

FH

Fig. 3. Soil CO2 evolution rates under late rotation (age 17 years) slash pine plantations under different management intensity onflatwood sites in northern Florida, USA (F, fertilized; H, understorey-eliminated; FH, fertilized plus understorey-eliminated).Error bars indicate standard errors of means. n = 3.


938




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Discussion

A number of studies have shown that fertilization candecrease C allocated to biomass and production of fineroots (Axelsson & Axelsson 1986; Haynes & Gower1995; Albaugh

et al

. 1998). Fertilization increasesnutrient availability and hence the tree presumablyneeds relatively fewer fine roots, especially very fineroots (< 0·5 mm in diameter), to adsorb nutrients andwater. In our study, understorey-elimination reducedthe competition for resources and had a similar effecton tree biomass distribution.

Two contrasting hypotheses exist regarding theeffects of soil resources on forest NPP allocation pat-tern: differential allocation and constant allocation(Hendricks, Nadelhoffer & Aber 1993). Both hypoth-eses suggest that total NPP increases with increasingsoil resource availability. With an increase of soilresource availability, NPP allocation favours shootsover roots according to the differential allocationhypotheses (Keyes & Grier 1981; Vogt

et al

. 1987;Gower, Vogt & Grier 1992; Haynes & Gower 1995),whereas the constant allocation hypothesis states thatNPP allocation to above- and below-ground remainsrelatively constant (Aber

et al

. 1985; Nadelhoffer, Aber

& Melillo 1985; Raich & Nadelhoffer 1989). Ourresults support the differential allocation hypothesisfrom both fertilization and understorey-eliminationtreatments.

The beneficial effects of the control of competitionon early volume growth (Haywood & Tiarks 1990;Shiver, Rheney & Oppenheimer 1990; Miller

et al

. 1991)and dry matter partitioning among above-ground com-ponents and leaf area (Colbert, Jokela & Neary 1990)have been well documented for young pine planta-tions in the south-eastern United States. Our resultsdemonstrate that greater stem growth can be main-tained through late rotation age and, more import-antly, we found that this benefit in growth brought byunderstorey-elimination was achieved by shifting theNPP allocation pattern, in favour of above-groundcomponents over below-ground. This provides addi-tional evidence for the differential allocation hypothesis.Although understorey-elimination does not increasesoil resource availability, it reduces the competition forthe same available soil resources.

Early peaks of fine root mortality in plots withoutunderstorey-elimination were related to the under-storey vegetation. Annual grasses and forbs were mostcommon in the understorey where the greatest amountof light penetrated the tree canopy. Greater rootmortality in control plots during fall reflected the deathof these annual plants.

Fertilization did not change soil respiration rates meas-ured in the field. Similar results were reported for soilsunder ponderosa pine seedlings (Vose

et al

. 1995) andsoils under mature slash pine plantations (Castro

et al

.1994). However, absence of change in soil CO

2

evolu-tion rate does not mean fertilization had no effect onsoil respiration. First, it may be that the two fertiliza-tion treatments applied to these stands were insuf-ficient to generate the fundamental change in soilconditions that would be necessary for a measurablechange in respiration rates 5 years after the last fertil-ization. Alternately, two counteracting effects may have

Table 4. Soil C contents of soils under late rotation (age 17) slash pine plantations under different management intensity onflatwoods sites in northern Florida, USA

TreatmentsControl (Mg ha−1)

F* (Mg ha–1)

H* (Mg ha–1)

FH* (Mg ha–1)

Main effects (P-value)


Soil horizonsA 43·1 25·0 33·8 31·0 0·09 0·76 0·20E 9·5 8·3 11·7 12·5 0·90 0·06 0·49Bh 57·4 56·5 24·9 37·3 0·60 0·05 0·55E′ 30·8 20·5 24·7 35·2 0·98 0·38 0·06Soil total (up to 1 m) 140·8a† 110·3ab 95·1b 116·0ab 0·46 0·15 0·09

*F, fertilized; H, understorey-eliminated; FH, fertilized plus understorey-eliminated.†Means followed by a different letter at the same row are significantly different (α = 0·05).

0

50

100

150

200

250

Control F H FH

Treatments

C c

onte

nt (

mg

ha–1

)

Soil

Forest floor

Vegetation

Fig. 4. Carbon storage in vegetation, forest floor and soilof late rotation (age 17 years) slash pine plantations underdifferent management intensity in the flatwoods of northernFlorida, USA (F, fertilized; H, understorey-eliminated; FH,fertilized plus understorey-eliminated). n = 3.


939




,

38

,932–941

been caused by fertilization (Vose

et al

. 1995). Nitrogenaddition increases root respiration rates (Burton

et al

.1996; Ryan

et al

. 1996) and decreases microbial activ-ities, and hence reduces the soil respiration rates (Sod-erstrom, Baath & Lundgren 1983). These two oppositeeffects caused simultaneously by fertilization can resultin little or no overall change in below-ground CO

2

evo-lution rates. This might also be the underlying reasonfor the contrasting results of soil respiration responsesto fertilization (Van Cleve & Moore 1978; Brumme& Beese 1992; Castro

et al

. 1994; Vose

et al

. 1995;Dulohery, Morris & Lowrance 1996). Understorey-elimination reduced the below-ground biomass, whichcan account for 62% of total soil respiration underslash pine plantations (Ewel, Cropper & Gholz 1987)and, hence, reduced the soil CO

2

evolution rate.

The experimental stands were 17 years old when theresearch was conducted and the difference in soil Cstorage among treatments could have resulted fromseveral potential differences in accumulation anddecomposition during the rotation. First, earlier in therotation, large differences of both above- and below-ground inputs of organic materials could have occurred,particularly between understorey-eliminated and non-understorey-eliminated plots. Carlyle (1993) reportedthat in a 3-year-old

Pinus radiata

plantation the inputof C from weeds averaged 1·3 ton C ha

–1

year

–1

and,consequently, this input of C source resulted in 25%higher C concentration in the surface soil (0–15cm). The lack of such inputs certainly contributedto the differences in C storage between control andunderstorey-elimination treatments in our study.Secondly, understorey-elimination reduced the inputof root litter into the soil C pool throughout therotation. Ewel & Gholz (1991) pointed out that rootturnover contributed the most soil C in Florida eco-systems and the understorey roots contributed nearlyas much to soil C as tree roots for slash pine plantation.Understorey-elimination reduced fine root turnover by0·4 and 0·7 Mg C ha

–1

year

–1

in understorey-eliminatedand fertilized plus understorey-eliminated plots, re-spectively. For the 17-year duration of the plantation,these might account for 7·8 and 12·6 Mg C ha

–1

less Cin the soil C pools in plots where the understorey waseliminated. The decrease of C input as fine root turn-over resulting from understorey-elimination in the earlyyears of stand development might be even greater.

We did not find that fertilization increased soil Cstorage, as proposed by several investigators (McCarthy1983; Baker, Oliver & Hodgkiss 1986; Nohrstedt

et al

.1989; Johnson 1992). Although fertilization increasedabove-ground litterfall, it also increased the decom-position rate (J. Shan, L.A. Morris & R.L. Hendrick,unpublished data).

Fertilization and understorey-elimination did notincrease the soil C storage. Instead, understorey elim-

ination decreased the soil C. However, the decrease ofsoil C caused by understorey-elimination is still muchless than C losses caused by the conversion of forestlands into farmland (Schlesinger 1986). Both treat-ments actually increased the total C storage in the sys-tem, although the increase was mainly in the form of Csequestration in the vegetation and forest floor com-ponents. Several authorities have suggested that pineplantations of the south-eastern United States can playan important role in global carbon sequestration boththrough accumulation of C in wood used in the long-lasting products as well as through increased soil Cstorage (Huntington 1995; Van Lear, Kapeluck &Parker 1995). Our results suggest that for these sandyflatwoods soils, the role of storage in wood parts will bemore important than the role of increased soil C. Thesesoil systems generally have lower soil C than othersystems, reflecting both the favourable conditions fordecomposition that exist and the relatively low capa-city of sandy soils to protect soil C from decomposition.Our results cannot be generalized to other sites;however, it appears that intensive management of thesesouthern pine forests can contribute to global Csequestration to the extent that C assimilated intoabove-ground woody biomass is left standing or usedin long-lived products.

Acknowledgements

This research was supported by the University ofGeorgia through a university-wide assistantship, a grantfrom the USDA Forest Service, Southern Global ChangeProgram (USDA RF 246), and a US NSF grantDEB9616538. The authors appreciate the efforts ofDale Johnson in initiating this research. Thanks also toLee Ogden and Rodney Will who helped in the fielddata collection.

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Received 6 March 2000; revision received 24 April 2001


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