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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
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Forest Ecology and Management 260 (2010) 2148–2159
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Forest Ecology and Management
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Organic residue mass at planting is an excellent predictor of tree growth inEucalyptus plantations established on a sandy tropical soil
Jean-Paul Laclaua,∗, Joseph Levillaina,b, Philippe Deleportea, Jean de Dieu Nzilac,Jean-Pierre Bouilleta, Laurent Saint Andréa,d, Antoine Versinia,b, Louis Mareschala,b,Yann Nouvellona, Armel Thongo M’Boub, Jacques Rangerd
a CIRAD, UPR Ecosystèmes de Plantation, F-34060 Montpellier, Franceb CRDPI, BP 1291 Pointe Noire, Congoc DGRST, Brazzaville, Congod INRA, Biogéochimie des Ecosystèmes Forestiers, 54280 Seichamps, France
a r t i c l e i n f o
Article history:Received 2 June 2010Received in revised form 30 August 2010Accepted 7 September 2010
Tropical plantation forests are meeting an increasing proportion of global wood demand and compre-hensive studies assessing the impact of silvicultural practices on tree and soil functioning are requiredto achieve sustainable yields. The objectives of our study were: (1) to quantify the effects of contrastingorganic residue (OR) retention methods on tree growth and soil nutrient pools over a full Eucalyptusrotation and (2) to assess the potential of soil analyses to predict yields of fast-growing plantationsestablished on tropical sandy soils. An experiment was set up in the Congo at the harvesting of the firstrotation after afforestation of a native herbaceous savanna. Six treatments were set up in 0.26 ha plotsand replicated in 4 blocks, with OR mass at planting ranging from 0 to 46.5 Mg ha−1. Tree growth over thewhole rotation was highly dependent on OR management at planting. Over-bark trunk volume 7 yearsafter planting ranged from 96 m3 ha−1 in the treatment with forest floor and harvest residue removalat planting to 164 m3 ha−1 in the treatment with the largest amount of OR. A comparison of nutrientstocks within the ecosystem at planting and at the end of the rotation suggested that nutrient contentsin OR were largely involved in the different response observed between treatments. OR managementtreatments did not significantly modify most of the nutrient concentrations in the upper layers of themineral soil. Conventional soil analyses performed before planting and at ages 1 and 3 years were unableto detect differences between treatments despite large differences in tree growth. In contrast, linearregressions between stand aboveground biomass at harvesting and OR mass at planting (independentvariable) showed that OR mass was an excellent predictor of stand yield (R2 = 0.99). A large share of soilfertility comes from organic material above the mineral soil in highly weathered sandy soils and OR massat planting might be used in conjunction with soil analyses to assess the potential of these soils to supportforest plantations.
The area occupied by plantation forests has been expanding at anannual rate of 2.0–2.5 million ha over recent decades (FAO, 2005).Most recent plantation expansion has been based on exotic speciesmanaged in short rotations in tropical and subtropical regions.Large amounts of nutrients removed at harvesting from soils withlow fertility and under climates with high rainfall lead to manyuncertainties about the sustainable production of these tropical
∗ Corresponding author at: CIRAD, UPR Ecosystèmes de plantations, s/c UMREco&Sols, 2 place Viala-Bât. 12, 34060 Montpellier Cedex 2, France.
fast-growing plantations (Nambiar, 2008). Harvest residue man-agement is an important issue in plantation forests, both froman operational viewpoint and for maintenance of soil fertility(Mendham et al., 2003; Goncalves et al., 2004). Harvesting methodsmay lead to considerable differences in the amounts of organic mat-ter (OM) deposited on the soil surface (Jones et al., 1999; Powers etal., 2005), and soil properties can be rapidly modified under trop-ical climates (Siddique et al., 2008). Harvest residue managementduring the inter-rotation period is thus likely to greatly influencethe availability of nutrients in the soils and the sustainability offuture rotations in fast-growing plantations established on highlyweathered tropical soils (Tiarks and Ranger, 2008).
Replicated studies sharing a common experimental design wereinstalled in the late nineties at 16 sites (typical of commercial
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forestry in Australia, Brazil, Congo, China, India, Indonesia, SouthAfrica and Vietnam) to assess the effects of harvest residue man-agement on tree growth and nutrient cycling, as part of the“Site Management and Productivity in Tropical Plantation Forests”project coordinated by the Center for International ForestryResearch (Nambiar, 2008). The project set out to evaluate theeffects of soil and site management practices on the productivity ofreplanted sites and on soil fertility over successive rotations in trop-ical and subtropical regions. The greatest effects of harvest residuemanagement on tree growth and nutrient cycling were observedin soils with the lowest OM contents (Saint-André et al., 2008).Harvest residue retention at the soil surface enhanced microbialbiomass in Eucalyptus plantations in the first year after treatmentswere set up in Australia (Mendham et al., 2002), nitrogen miner-alization at several sites (Nzila et al., 2002; O’Connell et al., 2004;Goncalves et al., 2008a), and nutrient accumulation within trees(Deleporte et al., 2008; Xu et al., 2008). The effects of harvestresidue handling on soil carbon and nutrient levels were minor atall sites and soil degradation was not apparent (Tiarks and Ranger,2008).
The reserves of nitrogen and exchangeable base cations in thesoil of the experiment set up in the Congo were at the lower rangeof the soils studied in this network (Tiarks and Ranger, 2008).Afforestation of coastal savannas in the Congo with fast-growingEucalyptus clones was a suitable situation to investigate an effect ofOM management on tree growth, since the organic matter contentof coastal savannas in the Congo is extremely low (<1% in the 0–5 cmsoil layer) due to the coarse sandy texture, fast OM mineralizationunder an equatorial climate, and probably the impact of annualburnings over several centuries (Bird et al., 2000). The Congoleseexperiment was thus designed to test the hypothesis that treegrowth and nutrient stocks in the soil are largely dependent on har-vest residue conservation in Eucalyptus plantations established onnutrient-depleted tropical soils. Carbon and nutrient fluxes withinEucalyptus ecosystems have been extensively studied in this region,using eddy covariance methods (Merbold et al., 2009; Nouvellon etal., 2009), isotopic studies (Trouvé et al., 1994; D’Annunzio, 2008;Epron et al., 2009), soil respiration monitoring (Epron et al., 2006;Marsden et al., 2008; Nouvellon et al., 2008), successive biomassestimations (Saint-André et al., 2005) and monitoring of the mainnutrient fluxes throughout the ecosystem (Laclau et al., 2010).
The objectives of our study were: (1) to quantify the effects ofharvest residue management on tree growth and soil nutrient poolsover a full Eucalyptus rotation, (2) to gain insight into the way nutri-ents deposited at the soil surface at harvesting affect tree growthover the next rotation, and (3) to assess the potential of soil analysesto predict yields of fast-growing plantations established on tropicalsandy soils.
2. Materials and methods
2.1. Study site
The Eucalyptus plantations studied were located on the coastalplains around Pointe-Noire, Congo (4◦S, 12◦E), where the climateis sub-equatorial with a rainy season from October to May and adry season from June to September. Mean annual rainfall is around1200 mm, and the mean annual temperature is 25 ◦C with sea-sonal variations of about 5 ◦C. The plantations were established onFerralic Arenosols (FAO classification) in which Eucalyptus rootswere found at depths exceeding 9 m, 6 years after afforestationin native herbaceous savanna (Laclau et al., 2001). The soils werecharacterised by a homogeneous sandy texture, acidic pH, verylow amounts of exchangeable base cations, and very low levels oforganic matter (Table 1). The soil mineralogy of the study site was Ta
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dominated by quartz and kaolinite and nutrient bearing mineralswere very scarce (Nzila et al., 2004). Many experiments dealingwith fertilizer applications in Congolese eucalypt plantations haveshown a strong tree response to nitrogen inputs, a low response toK application and a consistent lack of response to added P fertilizer(Bouillet et al., 2004).
2.2. Experimental design
The original savannah was burned before afforestation, andregrowth was treated with glyphosate 2 months later. Trees wereplanted in April 1990 at a spacing of 4.0 m by 4.7 m. The plantingstock was a clone of a Eucalyptus hybrid. Fertilizers were applied atplanting at a rate of 14 kg ha−1 N, 6 kg ha−1 P and 18 kg ha−1 K. Afurther 26 kg ha−1 N, 11 kg ha−1 P, and 35 kg ha−1 K was applied 3years after planting. At harvesting in January 1998, the stand had amean height of 26.1 m, a basal area of 12.9 m2 ha−1 and a standingvolume of 129 m3 ha−1 (Nzila et al., 2002).
The second rotation crop was planted in April 1998, usingthe same clone. The spacing was 2.65 m × 4.70 m, superimposedon the previous rows. NPK fertilizer (16 kg ha−1 N, 16 kg ha−1 P,and 25 kg ha−1 K) was applied at planting. No additional fertilizerwas applied. Weeds were chemically controlled by glyphosateapplications. The experimental design was a randomised completeblock with four replications. Each plot had a gross area of 0.26 ha(204 trees) and an inner plot of 0.15 ha (120 trees) with twoborder rows. The treatments were similar to those in the otherexperiments belonging to the CIFOR project. They were establishedin April 1998 as follows:
R: All aboveground organic residues removed from the plot (litter layerfrom the previous rotation exported from the experiment).
WTH: Whole-tree harvest. All aboveground components of the commercialtrees were removed.
TH: Trunk harvest. Only the commercial-sized boles (top-end over-barkdiameter > 2 cm) and associated bark were removed.
SWH: Stemwood harvested. Only the commercial-sized boles, debarked,were removed (harvesting method used in Congolese commercialplantations).
DS: Double slash. All the trees were logged as in the TH treatment. Theresidues of the treatment were distributed on the ground and theresidues of a trunk harvest collected in the WTH plots were added.
B: Burning. TH + residue burned.
2.3. Plant and soil sampling
2.3.1. Biomass and nutrient contentTree height and diameter at breast height were measured in the
previous stand, then 6 (only tree height), 12, 17, 24, 30, 36, 48, 62,72, and 84 months after treatment establishment. Twelve trees ofthe previous stand, distributed in 6 basal area classes defined fromthe inventory, were sampled before harvesting to develop predic-tive models for biomass and nutrient content of the stands. Thetrees were separated into compartments: leaves, living branches,dead branches, stemwood and stembark. Diameters, lengths andweights were measured in the field. Subsamples were taken fromall the compartments for moisture content measurements and indi-vidual chemical analyses were performed for each component ofeach tree sampled. The same methodology (sampling 12 trees pertreatment at each age in the inner buffer rows) was used 1, 3, and7 years after planting to estimate the aboveground biomass andnutrient contents of the R, SWH, DS and B treatments (except for theB treatment at age 7 years). A low-intensity accidental fire sweptthrough several plots in blocks 1 and 2, in April 2004 (6 years aftertreatment establishment). There was no evidence showing that theaccidental fire greatly affected tree growth in the last year of therotation. ANOVAs performed on tree height and basal area at age 6years (before the fire) and at the end of the study period showed
the same significant differences between blocks and treatments.Biomass and nutrient contents in this paper are therefore given forthe 4 blocks inventoried at each age.
2.3.2. Forest floorBefore harvesting the stand in January 1998, 20 litter samples
were taken within a 50 cm × 50 cm frame in each treatment plotin blocks 1 and 3. Roots adhering to decomposing organs werecarefully removed by hand. Each litter sample was oven-dried(65 ◦C) and weighed individually before combining for chemicalanalysis (120 pooled samples in total). The amounts of harvestresidues deposited at harvesting in each plot were estimated fromallometric equations of biomass and nutrient content establishedbefore harvesting and applied to the inventory (Bouillet et al.,1999).
Litter decomposition was assessed in the WTH, TH, SWH, and DStreatment plots of blocks 1 and 3 by placing nets (50 cm × 50 cm)over the original organic residues to prevent any input of new mate-rial. Twelve samples were collected every 3 months in each plotfrom September 1998 to September 1999 to quantify the remain-ing biomass. Three composite samples were prepared in each plotto estimate mineral contents (6 individual analyses per treatmenton each sampling date).
Forest floor dry mass and nutrient contents were quantified7 years after treatment implementation. Eighteen samples wererandomly taken within a 50 cm × 50 cm frame in each treatmentplot in 4 blocks (a total of 432 samples). Roots adhering to decom-posing organs were removed by hand and four components wereidentified within each sample: dead branches, leaves, bark and amiscellaneous fraction. Each litter fraction was oven-dried (65 ◦C)and weighed individually. However, the results showed that thefire that occurred in April 2004 in blocks 1 and 2 led to a for-est floor dry mass that was lower than in blocks 3 and 4 at age7 years. Consequently, results of forest floor dry mass and nutri-ent contents at age 7 years are only given for blocks 3 and 4.Each fraction of forest floor sampled in blocks 3 and 4 was com-bined within each plot for chemical analysis (48 pooled samples intotal).
2.3.3. Soil samplingSoils were sampled to study the relationships between stand
growth and soil chemical properties for the different site manage-ment practices. Soil samples were taken before harvesting the firstrotation at depths of 0–10, 10–20, 20–50, 50–70, and 70–100 cm inthe plots where the treatments R, B and DS in blocks 1 and 3 wereplanned for the next rotation. Four samples were collected at dif-ferent distances from the trees for each treatment, block and depth.Soil samples collected in each plot before harvesting were pooledper soil layer prior to analysis. Soil was re-sampled 1, 3 and 8 yearsafter treatment establishment, in blocks 1 and 3, at about 50 cmfrom the initial locations. Sampling at age 8 years was performedjust before clear-cutting the stands (second rotation). Individualchemical analyses were carried out for all the samples collectedafter treatment establishment in the 0–10 and 10–20 cm soil layers(8 replications for each layer per treatment). One composite sam-ple per plot was analysed at each age for the 20–50, 50–70, and70–100 cm layers (2 replications per treatment).
Nutrient stocks in the 0–100 cm soil layer for each treatmentwere computed from mean contents in each soil layer and bulkdensities measured in a radius of 500 m from our experiment. Dif-ferences in soil bulk density depending on the amount of residuesretained at the soil surface were observed under E. globulus plan-tations in Australia (Mendham et al., 2003). However, extensivesampling of soil bulk density close to our experiment showed lowspatial variability in this sandy soil and similar values under nativesavanna and Eucalyptus plantations (data not shown). Mean bulk
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density values per layer measured in this area were used to calcu-late nutrient stocks.
2.4. Chemical analysis
In plant samples, N was determined by thermal conductivityafter combustion (FP-428) and P, K, Ca, Mg using an ICP sequentialspectrophotometer (JY 24) after digestion by hydrofluoric acid anddouble calcination. The ash content of all the litter samples wasdetermined by combustion for 4 h at 450 ◦C and the results are givenon an ash-free basis.
In soil samples, exchangeable Ca, Mg, Mn, Na, Fe, H and Alcontents were determined by ICP spectrometry (JY 38 Plus) aftercobaltihexamine extraction. Cation exchange capacity (CEC) wasmeasured at ambient soil pH after cobaltihexamine extraction.Total organic carbon and total nitrogen were determined by drycombustion (Thermo Quest NC2100, Soil Analyzer). Phosphoruswas determined by colorimetry after Olsen extraction (IntégralPlus, Alliance Instruments), for just one composite sample per layerin each plot (pooling the samples collected in block 1) before plant-ing and at age 7 years.
2.5. Statistical analysis
The model used to assess the biomass and the nutrient contentof each tree compartment was:
Yi,j = a + bj(D2i Hi)
cj + εi,j
where Yi,j is the biomass (or nutrient content) of compartment jof tree i, Di is the diameter at breast height of tree i, Hi is theheight of tree i, aj, bj and cj are the parameters to be estimatedand εi,j is the residual error not explained by the model. As age wasfound to significantly affect this relationship, the equations werefitted by treatment for each age and each tree compartment. Fittingwas performed using PROC NLP of the SAS software and maximumlikelihood estimations (Saint-André et al., 2005). The allometricequations were applied to the inventories to assess biomass andnutrient contents per hectare at each age. Stem taper equationswere used to assess over-bark trunk volumes. These equations wereestablished by sampling >700 trees at ages ranging from 1 to 7 yearsin commercial stands of the same clone with contrasting produc-tivity (Gomat et al., accepted for publication).
A general linear model procedure was used at each age ina two-way analysis of variance to test for differences in treeheight, over-bark trunk volume, aboveground biomass and nutri-ent contents due to the treatment and block (SAS Institute, 1999).Homogeneity of variances was checked at each age by Levene’stest and the normal distribution of residues was tested with theKolmogorov–Smirnov test. The Newman Keuls range test was usedfor multiple comparisons.
We used the repeated measures option of the Mixed statementin SAS to test the effects of treatments, standing age, and interac-tions between the main factors, on the concentrations of elementsin each soil layer. Blocks were considered as a random factor. Pear-son correlation coefficients were computed in SAS between meanstand biomass at age 7 years in treatments R, WTH, TH, SWH andDS, and organic carbon, total nitrogen, exchangeable cation con-centrations, and soil CEC in the 0–10 cm soil layer, 1 and 3 yearsafter planting. Pearson correlation coefficients were also calculatedbetween stand aboveground biomass at age 7 years in the R, WTH,TH, SWH and DS treatments, and dry mass, N, P, K, Ca and Mg con-tents in the litter and harvest residues at planting. A 5% significancelevel was used.
Fig. 1. Time-course of dry matter (DM) decomposition and nutrient contents in theorganic matter above the mineral soil over 20 months after harvesting in treatmentSWH.
3. Results
3.1. Organic matter and nutrients in harvest residues and litter atplanting
Site management practices led to an amount of C within organicresidue (harvest residues + litter) and at the soil surface in the WTHtreatment (6.8 Mg ha−1) that was about half the amount in TH(12.6 Mg ha−1) and SWH (15.7 Mg ha−1), and a third of the amountin DS (23.3 Mg ha−1). The amounts of N, P, K, Ca and Mg containedin organic residues before planting ranged from approximately0 kg ha−1 in the R treatment to 369, 43, 101, 95, and 59 kg ha−1,respectively, in treatment DS (Table 2).
Sequential samplings showed that a 75% loss in harvest residueand litter mass occurred within the first year after clear-cutting,irrespective of the treatment. Nitrogen, P and K were releasedrapidly during the decomposition process, but Ca and Mg releaseswere slower and approximately followed the changes in dry mat-ter contents (Fig. 1). Most of the leaves and bark had decomposedwithin 8 months after clear-cutting, and the remaining slash mainlyconsisted of dead branches. The amounts of nutrients remaining inlitter and harvest residues 1.7 years after harvesting the first rota-tion stand were <5% of the initial pool before planting for P and K,about 10% for N and Mg and about 20% for Ca in each treatment(data not shown).
3.2. Influence of harvest residue management on tree growth
Harvest residue management had a large impact on heightgrowth (Fig. 2A). The ranking among treatments remainedunchanged throughout the rotation, except for the B treatment.Burning harvest residues enhanced mean tree height by 13% 6months after treatment establishment in comparison with the SWHtreatment, but height growth was similar in these two treatmentsfrom age 2 years onwards. Mean tree height was consistently higherin the DS treatment than in the R treatment over the stand rotation.
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Table 2Organic carbon and nutrient stocks in the forest floor + harvest residues and the 0–100 cm soil layer before harvesting the previous stand (age 0) and 7 years after treatmentimplementation (age 7). Nutrient contents within aboveground tree components 7 years after treatment establishment are indicated. Differences in total stocks between age7 and age 0 are indicated in brackets for each treatment. Organic C expressed in Mg ha−1 and nutrient stocks in kg ha−1. Different letters in the same row indicate significantdifferences between treatments (p < 0.05), when the treatments were compared by ANOVA.
R SWHa DS B
Organic carbonAge 0
Slash and litterb 0 a 15.7 b 23.3 c n.d.0–100 cm soil layer 49.6 a 47.5 45.2 a 47.9 aTotal 49.6 63.2 68.5 ≈47.9
Age 7Forest floorb 6.7 a 8.9 b 9.3 b 8.2 ab0–100 cm soil layer 46.4 a 46.0 a 44.8 a 43.6 aTotal 53.1 (+3.5) 54.9 (−8.3) 54.1 (−14.4) 51.8 (≈+3.9)
Total nitrogenAge 0
Slash and litter 0 a 250 b 369 b n.d.0–100 cm soil layer 2024 a 2090 2141 a 2105 aTotal 2024 2340 2510 n.d.
Age 7Forest floor 99 a 130 a 152 a 114 a0–100 cm soil layer 2030 a 2110 a 1998 a 2083 aAccumulation within trees 107 a 154 b 173 b n.d.Total 2236 (+212) 2394 (+54) 2323 (−187) n.d.
PhosphorusAge 0
Slash and litter 0 a 28 b 43 c n.d.0–100 cm soil layerc 134 n.d. 73 n.d.Total 134 n.d. 116 n.d.
Age 7Forest floor 5 a 6 a 7 a 5 a0–100 cm soil layerc 106 53 80 92Accumulation within trees 26 a 30 b 33 b n.d.Total 137 (+3) 89 120 (+4) n.d.
PotassiumAge 0
Slash and litter 0 a 63 b 101 c n.d.0–100 cm soil layerd 137 a 171 157 a 105 aTotal 137 234 258 n.d.
Age 7Forest floor 6 a 7 a 7 a 6 a0–100 cm soil layerd 108 a 122 a 135 a 201 aAccumulation within trees 43 a 61 b 68 b n.d.Total 157 (+20) 190 (−44) 210 (−48) n.d.
CalciumAge 0
Slash and litter 0 a 79 b 95 b n.d.0–100 cm soil layerd 154 a 133 137 a 109 aTotal 154 212 232 n.d.
Age 7Forest floor 17 a 29 b 31 b 28 b0–100 cm soil layerd 58 a 106 a 68 a 91 aAccumulation within trees 29 a 65 b 92 c n.d.Total 104 (−50) 200 (−12) 191 (−41) n.d.
MagnesiumAge 0
Slash and litter 0 a 45 b 59 c n.d.0–100 cm soil layerd 44 a 38 32 a 38 aTotal 44 83 91 n.d.
Age 7Forest floor 7 a 11 ab 12 b 10 ab0–100 cm soil layerd 27 a 26 a 26 a 29 aAccumulation within trees 19 a 30 b 38 c n.d.Total 53 (+9) 67 (−16) 76 (−15) n.d.
n.d.: not determined.a Soil analyses were not performed at age 0 in the SWH plots. The values indicated here are the mean of the values in the other plots before treatment implementation.b Organic carbon in slash and litter was estimated as 50% of dry matter.c Olsen P determinations only for the samples collected in block 1. ANOVAs were thus not performed for P contents in the 0–100 cm soil layer.d Exchangeable K, Ca and Mg determined after cobaltihexamine extraction.
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Fig. 2. Mean height in each treatment as a percentage of the mean tree height in theSWH treatment at each age (A) and mean trunk volume over bark increments overstand rotation (B). Bars indicate LSD values when differences between treatmentsare significant (p < 0.05).
Trunk volume increments were largely affected by harvestresidue management throughout the rotation (Fig. 2B). Over-bark trunk volume ranged from 96 m3 ha−1 in the R treatment to164 m3 ha−1 in the DS treatment 7 years after planting. The vol-ume growth curves merged the second year after planting for theB treatment and the treatments with large amounts of organicresidues deposited at the soil surface before planting (DS, SWH andTH).
3.3. Carbon, nitrogen and base cations in upper soil layers
Chemical analysis showed that organic carbon, total nitrogen,and exchangeable K+, Na+, Ca2+, Mg2+ contents in the 0–10 cmlayer were little affected by harvest residue management overthe full rotation (Fig. 3). The only significant differences betweentreatments showed by ANOVAs performed at each age werefound 8 years after planting, with higher Ca2+ and Mg2+ con-centrations in the B treatment than in the R, DS and SWHtreatments. Repeated measures ANOVAs confirmed that organiccarbon, total nitrogen, and exchangeable K+, Na+ and Ca2+ con-centrations where not significantly modified by treatments overstand rotation. A significant time effect was observed for all theelements (except for exchangeable K+), but most of the variations
were low and the time × treatment interaction was not significant(except for exchangeable Mg2+). The most pronounced time effectwas observed for exchangeable Ca2+, with a decrease from about0.07 cmolc kg−1 before planting to about 0.03 cmolc kg−1 from age1 year onwards.
Changes in soil chemical properties over stand rotation in the10–20 cm soil layer were similar to the pattern observed in the0–10 cm soil layer (Fig. 4). The concentrations of organic car-bon, total nitrogen, and exchangeable Ca2+, Mg2+, K+ and Na+ inthe 10–20 cm soil layer were not significantly modified by har-vest residue management, but the time effect was significant.The time × treatment interaction was not significant whateverthe element, and the large decrease in exchangeable Ca2+ con-centrations the first year after harvesting in the upper layerwas also observed in the 10–20 cm soil layer (from 0.05 to0.03 cmolc kg−1).
3.4. Changes in carbon and nutrient contents in soils and treesover stand rotation
The amounts of C, N, P, K, Ca, and Mg in the forest floor 7years after treatment establishment were largely dependent on theamounts of the same elements contained in the organic residues atplanting (R2 of linear regressions ranging from 0.78 for K to 0.97for P and Mg) (Fig. 5). In contrast, harvest residue managementdid not significantly modify the amounts of organic C, total N, andexchangeable K, Ca, and Mg in the mineral soil down to a depth of1 m at the end of the rotation (Table 2).
Nutrient accumulation in the trees at the end of the rotationwas significantly influenced by management of the harvest residuesfrom the previous rotation (Table 2). The amounts of nutrients accu-mulated in aboveground tree components at age 7 years were only107, 26, 43, 29, and 19 kg ha−1 for N, P, K, Ca and Mg, respectively, inthe R treatment, for a mean annual increment of 14 m3 ha−1 year−1
at 7 years of age. These amounts were 62%, 27%, and 58% larger intreatment DS than in treatment R for N, P, and K, respectively, for amean annual increment reaching 23 m3 ha−1 year−1 at age 7 years.The effects of harvest residue management on the accumulation ofCa and Mg within trees at the end of the rotation were much morepronounced, with Mg contents in aboveground tree componentsabout twice as large and Ca contents 3 times larger in DS than in R.
The amounts of N, K, Ca and Mg within the ecosystem at the endof the rotation (including nutrient contents within abovegroundtree components, forest floor and specified fractions in soil layersdown to a depth of 1 m) were of the same order of magnitude as thecorresponding amounts at planting (including specified fractionsin soil layers down to a depth of 1 m, litter and harvest residues),whatever the treatment (Table 2). The discrepancies between theamounts at planting and at the end of the rotation differed in abso-lute value by 50–200 kg ha−1 for total N, about 4 kg ha−1 for P,20–50 kg ha−1 for K, 10–50 kg ha−1 for Ca, and 10–15 kg ha−1 for Mgdepending on treatments. These amounts of nutrients accountedfor about 10%, 3%, 15–20%, 6–32% for Ca, and 16–25% of the stocksof N, P, K, Ca and Mg in specified soil fractions and organic residuesat planting, respectively. Whilst a general downward pattern wasobserved for K, Ca, Mg in the SWH and DS treatments from plant-ing to the end of the rotation, the trend was less clear for the Rtreatment with a decrease in the stocks for Ca and a rise for K andMg.
3.5. Prediction of tree growth
Conventional soil analyses performed in the upper soil layerbefore harvesting the previous rotation and the first yearsafter treatment implementation were unable to predict biomass
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Fig. 3. Influence of organic residue management on concentrations of organic C (A), total N (B), exchangeable Ca (C), exchangeable Mg (D), exchangeable K (E), and exchange-able Na (F) in the 0–10 cm soil layer. Bars represent the least significant difference between treatments where differences were significant (p < 0.05). Significance of treatmenteffects at each sampling time is represented by: *, **, ***, **** for p < 0.05, 0.01, 0.001, and 0.0001, respectively. Tables of results for repeated measure ANOVAs are indicated.
accumulation, despite large differences in tree growth betweentreatments. Pearson correlation coefficients computed in the plotswhere soil analyses were performed (blocks 1 and 3) betweenaboveground biomass 7 years after treatment establishment andthe concentrations of organic C, total N, exchangeable Ca2+, Mg2+,K+, and soil CEC were not significant (p < 0.05), whatever the standage when soils were sampled (except for the concentration of Mgin the upper soil layer 3 years after planting). The same pattern
was observed considering a 0–20 cm soil layer or the total stocks ofthese elements down to the depth of 1 m (data not shown).
In contrast, the dry mass of organic residues (harvestresidues + litter) at the soil surface before planting was an excel-lent predictor of aboveground biomass throughout the rotation intreatments R, WTH, TH, SWH and DS (Fig. 6). The coefficients ofdetermination of linear regressions between mean abovegroundbiomass and organic residue dry mass at planting were >0.97 at
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Fig. 4. Influence of organic residue management on concentrations of organic C (A), total N (B), exchangeable Ca (C), exchangeable Mg (D), exchangeable K (E), and exchange-able Na (F) in the 10–20 cm soil layer. Bars represent the least significant difference between treatments where differences were significant (p < 0.05). Significance of treatmenteffects at each sampling time is represented by: *, **, ***, **** for p < 0.05, 0.01, 0.001, and 0.0001, respectively. Tables of results for repeated measure ANOVAs are indicated.
ages 1, 3, and 7 years. The increase in the slopes of the regres-sions from 0.052 at age 1 year to 0.728 at age 7 years showed anincrease in tree response to harvest residue management through-out the rotation. Aboveground biomass at age 7 years in eachtreatment was also accurately predicted using N, P, K, Ca, or Mgcontent within organic residues at planting as an independent vari-able in the regressions (R2 ranging from 0.94 to 0.99) (data notshown).
4. Discussion
4.1. Consequences of harvest residue management for tree growth
Mean annual increments (MAI) of aboveground tree compo-nents at the end of the rotation ranged from 7.8 Mg ha−1 year−1
in the R treatment to 13.0 Mg ha−1 year−1 in the DS treatment andwere at the lower range of values reported for commercial Euca-
Author's personal copy
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Fig. 5. Relationship between dry matter (DM) and nutrient contents in the forestfloor sampled in blocks 3 and 4 at harvesting (age 7 years) and the amounts of thesame elements within harvest residues and litter at planting in the R, WTH, TH, SWH,and DS treatments. Regressions indicate the relationship between the amounts of aspecific element contained in the forest floor at age 7 years (dependent variable) andthe amounts of the same element within organic residues at planting (independentvariable).
Fig. 6. Relationship between aboveground biomass at ages 1, 3, and 7 years andorganic residue (harvest residues + litter) dry mass at planting. Bars indicate stan-dard deviations between the blocks for each treatment (n = 4).
lyptus plantations in Australia (Mendham et al., 2003) and India(Sankaran et al., 2008), and low in comparison with most Brazil-ian and South African commercial plantations (e.g. Goncalves et al.,2008b; du Toit et al., 2010). Cloudy weather for much of the year inthis region, leading to low incoming radiation in comparison withother tropical regions (Nouvellon et al., 2009), infertile soils and a5-month dry period might account for the relatively low yields inthese sandy soils despite several decades of Eucalyptus breeding.However, tree growth in our experiment was faster than in similarexperiments set up for Eucalyptus plantations in Spain and Portugal(Jones et al., 1999), and MAIs were higher than in most temperateforest plantations (e.g. Powers et al., 2005).
Tree response to harvest residue management in our experi-ment was much stronger than in most of the studies dealing withthe amounts of harvest residues in forest ecosystems. A decade afterthe installation of 26 experiments across the USA, organic mat-ter removal did not significantly affect productivity (Powers et al.,2005). Harvest residue removal tended to reduce early tree growthin Eucalyptus plantations established in Spain and Portugal, but theeffects were not very marked (Jones et al., 1999). The experimentsestablished under sub-tropical and tropical climates as part of the“Site Management and Productivity in Tropical Plantation Forests”project exhibited an overall upward response to on-site harvestresidue retention when the proportions of nutrients contained inharvest residues increased relatively to the stocks in the upper layerof mineral soil (Saint-André et al., 2008). The largest response toharvest residue management in this network was observed in ourexperimental set-up in the soils with the smallest stocks of C, totalN, and exchangeable K+, Ca2+, and Mg2+.
Burning harvest residues leads to large losses of N byvolatilization into the atmosphere (Raison et al., 1985), but themineralization of large amounts of nutrients contained in organicresidues during burning sharply increases the amounts of nutrientsat the soil surface at planting (e.g. Mendham et al., 2003). Burningincreased early tree growth in Eucalyptus plantations in Australia(Mendham et al., 2003), South Africa (du Toit et al., 2008), andBrazil (Goncalves et al., 2008a). In our experiment, losses of largeamounts of N volatilized in the B treatment, whereas that nutrientwas released by harvest residue decomposition in the TH, SWH,and DS treatments, might explain why growth curves for the lastthree treatments overtook that in treatment B the second year afterplanting. Fertilization experiments demonstrated a strong nutri-ent limitation of tree growth in Congolese Eucalyptus plantations.Yields in these highly weathered sandy soils are mainly dependenton N availability, even though a low response to added K fertil-izer is observed (Bouillet et al., 2004). All the experiments in thisregion showed a lack of tree response to added P fertilizer, unlikethe pattern reported for most Eucalyptus plantations established ontropical soils (Goncalves et al., 2004; Xu et al., 2002). Phosphoruscontents in organic residues were low in relation to the stocks ofavailable P in the mineral soil (Tables 1 and 2) and the lack of treeresponse to added P fertilizer in that soil suggests that P contentsin organic residues were unlikely to strongly influence tree growthafter planting. In contrast, the base cation contents were so low inthe soil that they were probably also involved in the limited treegrowth observed in the R treatment throughout the rotation.
4.2. Consequences of harvest residue management for soil fertility
Early tree growth in the R and B treatments suggested that treeresponse to organic residue management was much more influ-enced by nutrient availability than by changes in soil microclimateresulting from a mulching effect of organic residues. Whilst the soilmicroclimate was similar for trees planted in bare soils in the Band R treatments, the highest mean tree height at age 6 monthswas observed in treatment B and the lowest in the R treatment.
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Early height growth was intermediary in all the treatments withorganic residue retention. Moreover, litter fall led to developmentof the forest floor in all the plots from age 1 year onwards (datanot shown), and a mulching effect was unlikely to account for thegreater differences in tree growth between treatments over therotation.
Particle size fractionation of 72 soil samples collected 7 yearsafter treatment establishment in our experiment showed that theconcentrations of total C, total N and �13C were not significantlydifferent between the R, WTH, TH, SWH, DS and B treatments, what-ever the size fraction (0–20, 20–50, 50–200, or 200–2000 �m) in the0–10, 10–20, and 20–50 cm soil layers (D’Annunzio et al., 2008).This feature confirmed the very low influence of harvest residuetreatments on the chemical properties of mineral soils in thisexperiment. In particular, similar �13C values in the soil for all thetreatments after afforestation in a herbaceous savanna suggestedthat SOM was little affected by organic residue management at thesoil surface at planting. The isotopic composition of soil carbon andsoil CO2 efflux measured on a four-age chronosequence of Eucalyp-tus, and on an adjacent savanna at the same site, showed that theflux of savanna-derived soil CO2 efflux decreased by approximately75% in the 7 years after afforestation (Epron et al., 2009). The largedifferences in tree growth observed were thus unlikely to resultfrom a large enhancement of N availability due to a priming effectof harvest residues on soil organic matter (D’Annunzio et al., 2008).
Small discrepancies between nutrient stocks within the ecosys-tem at planting and at the end of the rotation (upper 1 m ofsoil + litter + trees) in the R, WTH and DS treatments suggested thatthe pools of nutrients within organic residues at planting weretaken up by the trees throughout their development (Table 2). Soilsolution chemistry monitoring in adjacent Eucalyptus plantationsover 7 years showed that nutrient losses by deep drainage werevery low, even after clear-cutting (Laclau et al., 2010). The fairlygood agreement between nutrient pools within the ecosystem atplanting and at the end of the rotation in Table 2, despite the uncer-tainties due to the spatial variability in forest soils and the accuracyof chemical analyses close to the detection limits of the methods,suggested that nutrient uptake below a depth of 1 m was low. Thestrong relationships between the amounts of C, N, P, K, Ca, and Mg inthe forest floor at harvesting and the amounts of the same elementscontained in the organic residues at planting showed that har-vest residue management might affect the sustainability of thesefast-growing plantations in the absence of large mineral fertilizerinputs. This result was consistent with unbalanced input–output Nbudgets in commercial Eucalyptus plantations in the Congo, whichshow that fertilizer requirements are greatly affected by the har-vesting method and that they will have to increase over successiverotations (Laclau et al., 2005).
Our results suggest that tree responses were mainly a result ofnutrient availability in organic residues. The possibility to achievethe same productivity through substitution of the nutrients in theresidues with inorganic fertilizer if the residues were removed is atopic of current interest with residues in many plantation systemsbeing appraised for their value as a biofuel feedstock. Comple-mentary studies would be necessary to check that organic residueremoval has no long-term effect on biological processes in highlyweathered sandy soils.
4.3. Indicators of soil fertility in highly weathered sandy soils
Conventional soil chemical analyses were unable to detect largedifferences in fertility (exhibited by tree growth) in our sandysoil. Soil fertility can result from either large stocks of bioavailablenutrients, or large nutrient inputs into soil solutions, from mineralweathering, biological fixation of atmospheric N2, or mineraliza-tion of organic matter (Ranger and Turpault, 1999). Nutrient stocks
were very low in our soil, as was non-symbiotic N2 fixation in thesemonospecific Eucalyptus plantations (Le Mer and Roger, 2001), andnutrient release through mineral weathering (Nzila et al., 2004).The fertility of this soil was therefore strongly dependent on effi-cient recycling of small nutrient pools within the ecosystem (Laclauet al., 2010). The development of a dense root mat above the mineralsoil in these plantations, adhering to decomposing organic residues,efficiently helped to prevent nutrient losses by drainage despite thehigh hydraulic conductivity of the soil and high rainfall (Laclau etal., 2004).
A relatively limited influence of harvest residue management(except burning) on the chemical properties of upper soil layersis a general feature of temperate plantations (Jones et al., 1999;Powers et al., 2005), and sub-tropical and tropical Eucalyptus plan-tations (Mendham et al., 2003; du Toit et al., 2008; Goncalves et al.,2008a; Sankaran et al., 2008). The strong influence of these treat-ments on tree growth in nutrient-poor tropical soils (Saint-André etal., 2008) suggests that the low ability of chemical analyses to assesssite productivity in our study might be a general pattern in highlyweathered soils. Soil analyses make it possible to rank soil fertili-ties when the ranges of physico-chemical properties are large, andthey are commonly used to drive fertilization regimes in Brazilian(Goncalves et al., 2008b), South African (Herbert, 1996), and Aus-tralian (Smethurst et al., 2004) Eucalyptus plantations. However,our results show that soil analyses failed to detect a large share ofthe fertility derived from organic residues in our tropical sandy soil.Indicators based on soil solution chemistry have been proposedto improve the ability of conventional soil analyses to assess thepotential of forest soils to support tree growth (Smethurst et al.,2001). However, our results suggest that a large share of soil fertil-ity is located above the mineral soil in nutrient-poor tropical forestsoils and the power of any indicator considering only the minerallayers is therefore likely to be limited.
Organic residue mass at planting was an excellent indicator ofstand productivity and might be used in addition to soil analyses toestimate the agronomic potential of nutrient-poor forest soils. Thisindicator is a proxy of nutrient contents in the residues, easy tomeasure in the field (even though ash determination in the labora-tory is needed to estimate contamination by adhering soil particles)and cheap in comparison with soil analyses. Using nutrient contentinstead of the mass of organic residues did not improve tree growthpredictions, probably as a result of the proportionality betweenorganic residue mass and nutrient contents in our treatment plots.The key role of biogeochemical cycles of nutrients has been pointedout in forest ecosystems (e.g. Attiwill and Adams, 1993; Rangerand Turpault, 1999; Laclau et al., 2010). However, estimations oforganic residue mass at planting to predict the production of forestplantations in highly weathered soils have yet to be widely usedby forest managers. This parameter should be useful for improv-ing the predictions of empirical forest growth models in tropicalregions.
5. Conclusion
The productivity of clonal Eucalyptus plantations on the Con-golese coastal plain is highly dependent on management practiceswhich conserve organic matter and nutrients. Removing forest floorand harvest residues at planting reduced stand volume at 7 years ofage by one-third, compared to stemwood-only harvesting. Compar-isons of nutrient stocks at planting and at the end of the rotation forcontrasting harvesting methods suggested that the release of nutri-ents during organic residue decomposition was largely involvedin the large differences in tree growth observed throughout therotation. The mass of organic residues at planting (litter + harvestresidues) was an excellent predictor of tree growth throughout the
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rotation and might be used in addition to conventional soil analysesto assess the fertility of highly weathered forest soils.
Acknowledgements
We thank Christian Cossalter (CIFOR), and Sadanandan Nambiar(CSIRO) for the coordination of the network and all the researchersinvolved in this project for very useful suggestions during succes-sive workshops. We acknowledge CRDPI, the Republic of Congo,EFC, the European Integrated Project on Ultra Low CO2 Steelmak-ing (ULCOS - Contract no. 515960), the CARBOAFRICA project (EUProject no. 037132), and INRA-CIRAD for their financial support.We thank S. Dzomambou and J.C. Mazoumbou for field measure-ments. We are grateful to Karine Alary and the staff of the CIRADlaboratory in France for chemical analyses. We thank Peter Bigginsfor the revision of the English.
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