Growth and nutrientaccumulation of Brazilnuttrees(Bertholletia excelsa)in agroforestry at different fertilizer levels

Go¨tz Schroth?Maria do Socorro Souza da Mota?Maria Elizabeth de Assis Elias

The Brazil nut tree(Bertholletia excelsa)is a frequentcomponentof agroforestry systems in the Amazon because of its adaptation to nutrient-poor upland soils and multiple uses.We investigated the aboveground biomass production(kg dry weight),nutrient uptake and requirements(N,P,Ca,Mg,K)of Brazil nut trees of different sizes grown under agroforestry conditions and fertilized at different levels.Eight of 70 experimental trees with different size were harvested and stem,branches and leaves were separated.Nutrient contents were determined for three trees of varying size.Average tree growth was fast, but variability was high,suggesting considerable potential for the improvement of this economically important species.The trees responded to increased levels of fertilizer and lime with significantly increased foliar nutrient contents and growth,probably because of the improved availability of Mg and Ca for which the species seems to have a relatively high demand.In contrast to Brazil nut trees grown in forest or dense plantations,the agroforestrytrees invested a substantial part of their biomass and nutrients in large branches and developed spreading crowns.To improve stem form,reduce competition with associated crops for light and recycle nutrients,regular pruning of lower branches or planting arrangements that favor self-pruning are recommended.These measures would also increase the recycling of Ca and Mg,large quantities of which are contained in the branches.

Allometric equation·Amazon·Nutrient competition·Nutrient recycling·Plant mineral nutrition· Soil fertility·Silviculture

Introduction

Brazil nut trees(Bertholletia excelsa Humb.&Bonpl.)are widespread in the Amazon basin and are well adapted to the clayey and nutrient-poor Oxisols and Ultisols which cover large areas in the region(FAO 1986).In the rainforest,the tree height of Brazil nut can reach 60 m.The species is of economic importance because of its edible nuts that are mostly collected from wild trees(Scoles and Gribel2011).However,lack of regeneration among wild or semi-wild trees threatens future yield and has been attributed to over-exploitation of the nuts and abandonment of traditionalagroforestry practices(Pereira 2000;Peres et al. 2003).The tree also produces high-quality timber and its bark is used in folk medicine.Brazilian law prohibits logging of naturally grown but not of planted Brazil nut trees.

Brazil nut trees are frequently found in agroforestry plantings in the region because of their multiple uses.For example,Smith(1996)reported their presence in homegardens in upland areas of the eastern Amazon,but not onthe floodplain since the tree is sensitive to waterlogging. The silviculture of Brazil nut trees planted for wood production has been the object of long-term experiments (Fernandes et al.1993).The species has also frequently been used for replanting of poor pastures and abandoned or degraded areas(Fernandes etal.1999;Nepstad etal.1991).

However,little is known aboutthe nutrientrequirements and nutrient accumulation of this relatively fast-growing tree when grown under agroforestry conditions,and this may affect production in agroforestry plantings.Dinkelmeyer et al.(2000)reported that Brazil nut trees took up a relatively large share of their nutrients from the soil under associated tree crops,suggesting a risk of competition when these nutrients are not replaced from external sources.Schroth et al.(2001a)found that Brazil nut trees were relatively efficient in concentrating nutrients from rainfall, dry deposition and crown leaching near their own stem, which may be seen as a strategy for conserving nutrients, but also for reducing their accessibility to associated species.

The purpose of this study was to determine the accumulation of biomass and nutrients in different parts of Brazil nut trees grown under agroforestry conditions on a central Amazonian upland soil.We also tested the response of the trees to different fertilizer levels and used soil and foliar analysis to obtain information on their nutrient requirements.We hypothesized that on acidic,nutrient poor soils the trees would respond positively to higher soil fertility levels with faster growth and nutrient accumulation,but that this might also lead to greater competition with associated crops for light and nutrients.This might be expected if higher nutrient availability and reduced intraspecific competition in agroforestry plantings leads to greater investment of the trees in leaf area and a spreading growth habit.Based on our results,we provide recommendations for the management of the tree species in agroforestry plantings to improve nutrient cycling and reduce competition with associated crops.

Materials and methods

Study site and experimental layout

The study was carried out on the research station of Embrapa Amazo?nia Ocidental near Manaus in the Brazilian Amazon.The climate is lowland humid tropical with mean annual precipitation of 2,600 mm,mean air temperature of 26°C and mean atmospheric humidity around 85%.The driest months are July to September,and the wettest months are February to April.The soil is a Xanthic Ferralsol with clayey texture and kaolinitic mineralogy (Schroth et al.2000a).

A field experiment was conducted in an area of about 13 ha in 1993.This area was cleared for a rubber plantation in 1980,but it was abandoned a few years later and developed into secondary forest.The experiment consisted of different agroforestry systems with important tree crops of the region.In one of the agroforestry systems,Brazilnut was associated with peach palm(Bactris gasipaes),cupuac?u(Theobroma grandiflorum)and annatto(Bixa orellana).The Brazil nut trees were planted in mixed rows in which one Brazil nut tree alternated with a cupuac?u tree at 6.7 m spacing.These rows were planted at 8 m distance, and rows of either peach palm or annatto were planted between these rows.The average distance between Brazil nut trees was about 10.5 m(Fig.1).During the first year, cassava(Manihot esculenta)was grown as temporary intercrop between the trees,followed by an N-fixing leguminous cover crop(Pueraria phaseoloides)which remained in the plots as a partial ground cover throughout the experiment and was shown in other research to have increased N cycling and availability in the soil(Schroth et al.2000b,2001b).

Fig.1 Layout of a polyculture plot with Brazil nut(Bertholletia excelsa,B),cupuac?u(Theobroma grandiflorum,C),annatto(Bixa orellana,A)and peach palm(Bactris gasipaes,P)on a ferralitic upland soil in central Amazonia.The soil between the trees is partially covered by Pueraria phaseoloides(after Schroth etal.1999)

Table 1 Fertilizer applied per plot(kg·ha-1)aduring 7 years in multistrata agroforestry plots with Brazil nut trees(Bertholletia excelsa)on a xanthic Ferralsol in central Amazonia

The experiment was initiated at two fertilization levels (‘low’and‘high’)thatwere later expanded to four levels (Table 1).Since fertilizer responses and optimum fertilizer levels were not known for any of the species used in this experiment,an approximate fertilizer rate was defined for each species,including Brazil nut trees,by local soil experts and this was termed‘high’.The‘low’fertilizer rate was 30%of this level,intended as a comparison to measure the fertilizer response to the‘high’level and to reduce the risk of plantdeath thatwould have been possible in the absence of any fertilization.After the first years,a‘low minus N’treatment was added to see if the leguminous cover crop alone was able to supply the N needed by the plants,and a‘high plus P’level was added to test the effect of an additional 50%of the P rate of the‘high’’level(Table 1).Since the Brazil nut trees did not respond to these latter treatments,the two‘low’and the two‘high’treatments are presented together in the following. Table 1 shows fertilizer rates for the entire plots(not only to Brazil nut trees)since later research showed that Brazil nut trees also had access to nutrients applied to associated species within these agroforestry systems(see below). Nitrogen was applied as urea(45%N)until 1996,then as ammonium sulfate(21%N);in‘low minus N’no N was applied after May 1996.Phosphorus was applied as triple super phosphate(22%P),except in 12/1996 when we applied North Carolina Phosphate‘Atifos’(13%P). Potassium was applied as potassium chloride(50%K).In the‘low minus N’treatmentno dolomite was applied after May 1996.

The plots were arranged in a randomized complete block design.Three replicate blocks with four plots each(12 plots in total)were included in the study.Plot size was 48×32 m,and each plot contained six Brazil nut trees in the central measurement area,giving a total of 72 trees in the experiment.

Estimation of aboveground biomass and nutrient contents

Aboveground biomass of all trees was estimated through allometric equations from their diameter at breast height (DBH,130 cm).Eight trees of different sizes were harvested for development of an allometric equation as described by Schroth et al.(2002).These activities were carried out between December 1999 and February 2000, when the trees had been 7 years in the field.The selected trees were cut at ground level and separated into stem, branches of diameters＞2 cm,1–2 cm and＜1 cm,and leaves(subdivided into old,mature and immature leaves). Each fraction of the trees was weighed and representative samples collected for dry matter determination by drying at 65°C(leaves)or 105°C(woody structures)to constant weight.To estimate the nutrient accumulation in the aboveground biomass of the trees,one small,one medium and one large tree were selected,and dried subsamples of all plant parts were ground and digested according to Novozamsky et al.(1983).N and P were measured colorimetrically,and K,Ca and Mg by atomic absorption spectrometry.

Effect of fertilizer levels on soil and foliar nutrient levels

To evaluate the effect of the different fertilizer inputs on soilchemistry,soilsamples were collected from 0 to 20 cm depth under Brazil nut trees in April–May 1998.After airdrying and sieving the soil to pass 2 mm,the following analyses were carried out:Total N by Kjeldahl digestion; available P,K,Ca and Mg by extraction with the Mehlich 3 solution at a soil:solution ratio of 1:10(Tran and Simard 1993),followed by colorimetric measurement of P and measurement of K,Ca and Mg by atomic absorption spectrometry;exchangeable acidity by extraction with 1 mol·L-1KCl at a soil:solution ratio of 1:15 and titration with NaOH againsta phenolphthaleine indicator;pH with a glass electrode at a soil:solution ratio of 1:2.5 in distilled water.The cation exchange capacity was calculated by summing the basic cations and the exchangeable acidity.

Mineral nutrition of the trees and its variation over the year were studied on leaf samples collected from all study plots in November 1997(end of the dry season),January 1998(early rainy season),May 1998(mid rainy season) and August 1998(beginning of the dry season).Leaves were collected in unshaded positions from the middle of the crown height from six trees per plot.Three leaf ages were sampled and analyzed separately:The youngest green leaves,which were often still soft(‘new leaf’),mature leaves from the middle part of a branch(‘medium leaf’), and old leaves from the inner partof the crown,which were often covered with epiphylls but still entirely green(‘old leaf’).The leaves were washed in tap water,dried,ground, digested and analyzed as described above.

Statistical analysis

Statistical comparisons of biomass data for the fertilizer treatments were calculated by analysis of variance (ANOVA)for a randomized complete block design using the STATISTICA software package.Data were normally distributed and did notviolate the assumptions of ANOVA (e.g.correlation between treatment means and variances). Fertilizer levels were used as the independent variable and biomass,soil and foliar nutrient concentrations as thedependent variables.If an F-test proved significant at p＜0.05,then means were compared by least significant difference tests at the same probability level(Little and Hills 1978).

Results

Effect of fertilizer on soil nutrient concentrations

Calcium and Mg contents as well as base saturation of the soil increased significantly with increasing fertilizer input. Available P,pH and CEC showed the same tendency, whereas K contents were not influenced by fertilizer levels (Table 2).Total N was 2.1 g kg-1at 0–10 cm depth and 1.1 g kg-1at 10–30 cm depth without significant differences between fertilizer treatments.

Tree biomass and growth response to fertilizer

From the 72 trees originally planted in the field,only two (3%)died during the first 7 years.The remaining 70 trees had an average DBH of 20.6 cm,corresponding to an average diameter growth of almost 3 cm per year.DBH of the eight harvested trees ranged from 8.6 to 26.6 cm and heights ranged from 7 to 14.3 m.This corresponded to an aboveground dry matter of 22–460 kg tree-1,i.e.a more than 20-fold difference(Fig.2).There was no significant difference between the two low fertilization treatments or between the two high fertilization treatments,which were therefore notshown separately in Fig.3.However,average tree biomass was about 20%higher between the two high fertilization treatments than between the two low fertilization treatments(283 vs.221 kg tree-1,p=0.041; Fig.2).

Fig.2 Calcium and magnesium concentrations in three leaf age classes of 7-year-old Brazil nut trees(Bertholletia excelsa)grown under agroforestry conditions on a xanthic Ferralsol in the central Amazon as affected by fertilization level(values with similar letters are not significantly different at p=0.05)

With increasing tree size,the contribution of leaves to aboveground tree biomass decreased from 20%for the smallestto 11%for the largesttree,and the contribution of woody parts(stem,branches and twigs)increased correspondingly(Fig.3).On average,about two thirds of the leaf biomass consisted of old leaves with a pronounced epiphyll cover and often considerable area loss to insects. Despite the wide size range of the eightharvested trees,the contribution of the stem to the aboveground tree biomass remained around 50%,while the contribution of large branches(＞2 cm diameter)increased from 0 to 36%from the smallest to the largest tree(Fig.3).No effect was detected of fertilization rate on the percent leaf and branch biomass.

Foliar nutrient contents and nutrient accumulation in tree biomass

The foliar analyses did not show any effect of fertilizer levels on concentrations of N,P and K,with concentrations in mature leaves of 18.5,0.84 and 4.4 mg g-1,respectively (means of four fertilization levels and four collection dates).In contrast,the Ca concentrations in old leaves were significantly higher in the two high than in the two low fertilization treatments in August(dry season),and the younger leaves showed the same tendency(p=0.086 for medium leaves;Fig.4).The Mg concentrations showed a significant fertilizer response in both May and August forseveral leaf ages(Fig.4),and in January for the medium leaves(data not shown),also reflecting differences in Mg availability in the soil(Table 2).

Table 2 Nutrient availability at 0–20 cm under Brazil nut trees(Bertholletia excelsa)at four fertilization levels,and statistical probability of differences between treatments(F-test)

Fig.3 Frequency distribution of aboveground biomass within two groups of 7-year-old Brazil nut trees(Bertholletia excelsa)grown under agroforestry conditions on a xanthic Ferralsol in the central Amazon attwo fertilization levels.The trees of the two lowerand the two higher fertilization levels were pooled respectively because they did not differ significantly with regard to aboveground biomass.The biomass of individual trees was estimated from their diameter at breastheight with an allometric equation

The highest nutrient concentrations were found for N in the immature and mature leaves,for P and K in the youngest leaves,for Ca in the old leaves and the smaller branches,and for Mg in the mature and old leaves and the smallest branches(Table 3).The relative contribution of leaves and wood to the average nutrientcontentof the trees at this age was about 40:60 for N and Mg,30:70 for P and Ca,and 20:80 for K(Table 4).Within the woody parts, stem and branches had about equal stocks of N,P and K, but branches had much higher Ca and Mg stocks than stems(Table 4).An average tree with a basal area of 340 cm2(DBH=20.8 cm)and a biomass of 250 kg contained about 1.5 kg of N,1.4 kg of Ca,1 kg of K, 0.26 kg of Mg and 0.12 kg of P.With 93 Brazil nut trees·ha-1in the agroforestry plots,the aboveground nutrient stocks in these trees were about 140 kg·ha-1of N, 130 kg·ha-1of Ca,93 kg·ha-1of K,24 kg·ha-1of Mg and 11 kg·ha-1of P.

Discussion

Tree survival,total tree biomass and fertilizer response

The average DBH(20.6 cm)of the 70 surviving Brazil nut trees after 7 years in this experiment compares favorably with a mean DBH(13.9 cm)of 10-year-old Brazil nuttrees grown under forestry conditions at 3×3 m spacing in the Manaus region(Fernandes et al.1993).When planted in degraded pastures,12-year-old Brazil nut trees planted at 10×10 m in the Manaus region had mean DBH of 25.6 cm and 5-year-old trees in the Eastern Amazon had mean DBH of 5.1 cm(Pereira etal.1998).The fastgrowth of the trees in our experiment can be explained by the absence of intraspecific competition,fertilizer and lime application and less weed competition than in the other experiments due to the initial association with a food crop and subsequent planting of a leguminous cover crop. However,the trees varied enormously in size,indicating a considerable genetic variability of this economically important,but little improved tree species(Buckley et al. 1988)and presumably a pronounced response to smallscale site heterogeneity(Fig.2).

The pronounced increase in the contribution of the large branches(＞2 cm diameter)to aboveground tree biomass (from 0 to 36%,Fig.3)shows an important tendency of Brazil nut trees to develop spreading crowns with very large and heavy branches when grown under agroforestry conditions with little or no lateral shade.In contrast,Brazil nut trees grown in plantations at 3×3 m spacing in the same region exhibited good self-pruning properties(Fernandes et al.1993),and old trees in the forest often have considerable unramified stem length and relatively small crowns(Cavalcante 1991).Besides reducing the timber value of trees grown in agroforestry,the low and spreading branches also competed with associated crops for light.To maintain the option of producing high-value timber and reduce competition with associated crops in agroforestry,Brazil nut trees should be pruned regularly, removing the lower branches of the stem as well as forked stems,which were also relatively common.Specific planting arrangements to encourage self-pruning,such as planting the trees in groups surrounded by smaller trees, could also be tested.On the other hand,the heavy branches could be of interest where fuel wood is of economic value, for example on farms that also produce cocoa(Theobroma cacao)or other crops that require artificial drying.

Fig.4 Aboveground biomass of 7-year-old Brazil nut trees (Bertholletia excelsa)grown under agroforestry conditions on a xanthic Ferralsolin the central Amazon as a function of tree basal area atthree sampling dates

Table 3 Concentrations of main nutrients(in mg·g-1,means and se)in differentparts of 7-year-old Brazilnuttrees(Bertholletia excelsa)grown under agroforestry conditions in the central Amazon

Table 4 Distribution of main nutrients(%of total plant,means and se)between stem,branches and leaves of 7-year-old Brazilnut trees (Bertholletia excelsa)grown under agroforestry conditions in the central Amazon

Effect of fertilizer on soil and foliar nutrient concentrations

The absence of a significantfertilizer effecton foliar N and K concentrations was in accord with the soil data and reflected the high N mineralization rates in this type of soil and the presence of a leguminous cover crop(Schroth etal. 2001b).For P an effectof the fertilizer treatments on foliar levels would have been expected from the soil data (Table 2);its absence may be explained by P uptake from a relatively large area,including fertilized soil under associated tree species(Dinkelmeyer et al.2000).The positive response of the foliar Ca and Mg concentrations to the higher input level was in agreement with the generally low Ca saturation of the soil and increasing soil Ca contents with increasing lime input(Table 2).The foliar analyses suggest that the significant growth response of Brazil nut trees to higher fertilizer input might have been caused by Ca and Mg from the dolomitic lime.

Since about 50%of the total Ca and Mg stocks of the trees were contained in the branches(Table 4),pruning of lower branches,as recommended above,could make a significant contribution to the recycling of these nutrients. If these branches are burned as fuelwood,ashes should be returned to the field to reduce nutrient exports.For the same reason,measures to directthe architecture of the trees towards higher investments in the stem and reduced branch formation could perhaps reduce the Ca and Mg requirements of the trees and increase their overall growth.Such measures are particularly important when designing agroforestry systems with large trees for nutrient poor upland soils in the Amazon region where Ca and Mg are typically deficient in the soil(Schroth et al.2000a).

Conclusions

Brazil nut trees are well adapted to ferralitic Amazonian upland soils and are a valuable component of agroforestry systems in the region because of their multiple uses.However,individualtrees differwidely in growth,suggesting that there is considerable potential for the improvement of this economically important tree species.Despite the ability of the trees to extract nutrients from poor soils,they responded to fertilizer and lime with increased growth,probably because ofthe improved availability of Mg and Ca for which the species seems to have a relatively high demand.

Under agroforestry conditions with little or no shading from taller trees and fertilizer application,Brazil nut treesshow fast growth in height and diameter and rapidly dominate associated,slower-growing species.Under these conditions,the trees invest heavily in large branches and develop a spreading crown.To improve timber quality and reduce competition with associated species for light,regular pruning of lower branches and/or planting designs that favor self-pruning of the trees are recommended.These measures would also increase the recycling of Ca and Mg, of which large quantities are contained in the branches. These recommendations will also hold for many other valuable tree species that could be usefully integrated into agroforestry systems but have been even less studied than Brazil nut trees.

AcknowledgmentsSammya Agra D’Angelo helped in the field work.J.Ferraz made valuable comments on an earlier version of the text.

Buckley DP,O’Malley DM,Apsit V,Prance GT,Bawa KS(1988) Genetics of Brazil nut(Bertholletia excelsa Humb.&Bonpl.: Lecythidaceae).Theor Appl Genet 76:923–928

Cavalcante PB(1991)Frutas Comest?′veis da Amazo?nia(Edible Fruits of the Amazon).Museu Goeldi,Bele′m,p 279

Dinkelmeyer H,Lehmann J,Kaiser K,Teixeira WG,Renck A,Zech W(2000)Fate of applied N fertilizer in mixed cropping systems in the central Amazon.III Congresso Brasileiro de Sistemas Agroflorestais.Embrapa,Manaus,pp 196–197

FAO(1986)Food and fruit-bearing forest species,3rd edn.Food and Agriculture Organization of the United Nations,Rome,p 308

Fernandes ECM,Davey CB,Nelson LA(1993)Alley cropping on an acid soil in the upper Amazon:mulch,fertilizer,and hedgerow root pruning effects.In:Ragland J,Lal R(eds)Technologies for sustainable agriculture in the tropics.American Society of Agronomy,Madison,pp 77–96

Fernandes ECM,Perin R,Wandelli E,Souza SG,Matos JC,Arco-Verde M,Ludewigs T,Neves A(1999)Agroforestry systems to rehabilitate abandoned pastureland in the Brazilian Amazon.In: Jime′nez F,Beer J(eds)Internationalsymposium on multi-strata agroforestry systems with perennial crop.CATIE,Turrialba, pp 24–26

Little TM,Hills FJ(1978)Agriculturalexperimentation.Wiley,New York,p 350

Nepstad DC,Uhl C,Serra?o EAS(1991)Recuperation of a degraded Amazonian landscape:forest recovery and agricultural restoration.Ambio 20:248–255

Novozamsky I,Houba VJG,van Eck R,van Vark W(1983)A novel digestion technique for multi-element plant analysis.Commun Soil Sci Plant Anal 14:239–248

Pereira HS(2000)Castanhais nativos:um caso de domesticac?a?o incidental de uma e′specie dominante do dossel de floresta tropical.III Congresso Brasileiro de Sistemas Agroflorestais. Embrapa,Manaus,pp 353–356

Pereira AP,Graca MAS,Molles M(1998)Leaf litter decomposition in relation to litter physico-chemicalproperties,fungalbiomass, arthropod colonization,and geographicalorigin of plantspecies. Pedobiologia 42:316–327

Peres CA,Baider C,Zuidema PA(2003)Demographic threats to the sustainability of Brazilnutexploitation.Science 302:2112–2114

Schroth G,da Silva LF,Seixas R,Teixeira WG,Mace?do JLV,Zech W(1999)Subsoil accumulation of mineral nitrogen under polyculture and monoculture plantations,fallow and primary forest in a ferralitic Amazonian upland soil.Agric Ecosyst Environ 75:109–120.doi:10.1016/S0167-8809(99)00068-7

Schroth G,Seixas R,da Silva LF,Teixeira WG,Zech W(2000a) Nutrient concentrations and acidity in ferralitic soil under perennial cropping,fallow and primary forest in central Amazonia.Eur J Soil Sci 51:219–231

Schroth G,Teixeira WG,Seixas R,da Silva LF,Schaller M,Mace?do JLV,Zech W(2000b)Effectof five tree crops and a cover crop in multi-strata agroforestry at two fertilization levels on soil fertility and soil solution chemistry in central Amazonia.Plant Soil 221:143–156

Schroth G,Elias MEA,Uguen K,Zech W(2001a)Nutrient fluxes in rainfall,throughfalland stemflow in tree-based land use systems and spontaneous tree vegetation of central Amazonia.Agric Ecosyst Environ 87:37–49

Schroth G,Salazar E,da Silva JP Jr(2001b)Soil nitrogen mineralization under tree crops and a legume cover crop in multi-strata agroforestry in central Amazonia:spatial and temporal patterns.Exp Agric 37:253–267

Schroth G,D’Angelo SA,Teixeira WG,Haag D,Lieberei R(2002) Conversion of secondary forest into agroforestry and monoculture plantations in Amazonia:consequences for biomass,litter and soil carbon stocks after 7 years.For Ecol Manage 163:131–150.doi:10.1016/S0378-1127(01)00537-0

Scoles R,Gribel R(2011)Population structure of Brazil nut (Bertholletia excelsa,Lecythidaceae)stands in two areas with different occupation histories in the Brazilian Amazon.Hum Ecol 39:455–464

Smith NJH(1996)Home gardens as a springboard for agroforestry development in Amazonia.Int Tree Crops J 9:11–30

Tran TS,Simard RR(1993)Mehlich III-extractable elements.In: Carter MR(ed)Soil sampling and methods of analysis.Lewis Publishers,Boca Raton,pp 43–49

17 November 2013/Accepted:25 May 2014/Published online:27 January 2015

?Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

Projectfunding:This research was funded by the German Ministry of Education and Research(BMBF)and the Brazilian Conselho National de Desenvolvimento Cient?′fico e Tecnolo′gico(CNPq).

The online version is available at http://www.springerlink.com

G.Schroth(?)

C.P.513,Santare′m,PA 68109-971,Brazil e-mail:goetz.schroth@gmail.com

M.S.S.da Mota

Centro de Estudos Avanc?ados de Promoc?a?o Social e Ambiental, Av.Mendonc?a Furtado 3979,Santare′m,PA 68040-050,Brazil

M.E.de Assis Elias

Universidade Federal do Amazonas,Manaus,AM 69077-000, Brazil