Plant and Soil208: 135–147, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

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Growth and mineral nutrition of the native trees Pollalesta discolorandthe N-fixing Inga densiflorain relation to the soil properties of a degradedvolcanic soil of the Ecuadorian Amazon

Robert Davidson1,2∗, Daniel Gagnon1,2 and Yves Mauffette21Research and Development Division, Biodôme de Montr´eal, 4777 Pierre-De Coubertin, Montr´eal, (Quebec),Canada H1V 1B3;2Groupe de recherche en ´ecologie foresti`ere interuniversitaire, Universit´e du Quebec aMontreal, C.P. 8888, succ. Centre-ville, Montr´eal, (Quebec), Canada H3C 3P8

Received 26 May 1998. Accepted 8 February 1999

Key words:Foliar nutrients, Hydrandept, mineralization, silviculture, soil organic matter, tropical trees

Abstract

Land reclamation in the humid tropics, using native tree plantations, requires a better knowledge of plant–soilinteractions, and of patterns of growth of several poorly known species. We examined the establishment and mineralnutrition of two early-successional native tree species,Inga densiflora(N-fixing) andPollalesta discolor, in relationto properties of a degraded Hydrandept volcanic soil in Ecuadorian Amazon. Initial content of organic matter wasthe most significant soil variable in explaining the growth of pure stands of both species and was strongly relatedto effective cation exchange capacity (ECEC) and to net total N mineralization. Leaves ofPollalesta discolorhad greater concentration of nutrients thanInga densiflora, which led to a litter-layer rich in nutrients. Deficientconcentrations of foliar P, detected on plots with low soil organic matter, were linked to poor growth ofIngadensiflora, and indicate that this species may be P-limited. The inclusion ofInga densifloradid not stimulate thegrowth of Pollalesta discolorin mixed stands. This study indicates that soil organic matter management is animportant issue on these degraded volcanic soils. We suggest that a reduction of the pool of labile organic matterappears to hamper tree productivity through a nutrient shortage. The high variability of the degraded soil studiedproved to be an obstacle to tree growth and establishment.

Introduction

Among reported consequences of deforestation in thehumid tropics are losses of biodiversity, alteration ofgeochemical and hydrological cycles, and degradationof soils (Anderson, 1990; Brown et al., 1994). InEcuador, 238 000 ha year−1 were deforested between1981 and 1990, or 1.8% year−1 of the natural forestcover (FAO, 1993). During the same period, only2100 ha year−1 were planted with forest stands. Inthe southern region of the Ecuadorian Amazon, smalllandholders settled predominantly on Hydrandepts,which are infertile and waterlogged volcanic soils(MAG-PRONAREG and ORSTOM, 1983), prone to

∗ Fax: +1 514 868-3065.E-mail: rds_biodome@ville.montreal.qc.ca

degradation once forest cover is removed (CañadasCruz, 1983).

Degraded soils, as experienced in abandoned pas-tures, are inhospitable environments for young treeseedlings (Nepstad et al., 1990). Although some re-search has been conducted with native tree speciesplanted on such sites, most studies have concentratedon survival and on characterization of growth (But-terfield, 1995; Gonzalez and Fisher, 1994), withoutrelating these performances with initial soil properties.Given that the biodiversity of Ecuadorian Amazontrees is possibly the highest in the world (Valenciaet al., 1994), and that a number of the tree speciesmay prove interesting for reforestation (Davidson etal., 1998), there remains a basic lack of understand-ing of their silvicultural requirements (Sawyer, 1993).Moreover, few plantations have been established in the

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Ecuadorian Amazon and instead have been establishedon the coast (90%) or in the Andes (10%) (BancoInteramericano de Desarollo, 1982).

As a complementary tool to the standard meas-urements of growth of studied species, foliar nutrientanalysis is useful in assessing the nutritional stressof plantation tree species (Van Den Driessche, 1974).Comparing foliar nutrients of vigourously and poorlygrowing specimens of the same species, in combina-tion with soil analysis, can help to determine mineraldeficiencies related to growth (Mead, 1984), which isespecially useful when no available literature exists onthese species.

The objective of this experiment was to study theestablishment, growth and foliar nutrients of two nat-ive early-successional tree species, one N-fixing andthe other non N-fixing, in relation to the initial soilcharacteristics of a degraded volcanic soil.

Methods

Study site and planting stock

The experimental site is located approximately 16 kmnorth of the city of Macas, the provincial capital ofMorona Santiago (2◦12′S, 78◦05′W), at 1300 m el-evation. Precipitation is well distributed throughoutthe year, and averages 3000 mm per annum (datafrom INECEL (Instituto Ecuatoriano de Electrifica-ción), 1980–1992). The average annual temperatureis 22 ◦C (data from Macas airport, 1990–1992). Soilsare Hydrandepts (Humic Andosols according to FAO-Unesco, 1974), order Inceptisol (now probably Hy-drudands, order Andisols, according to the Soil SurveyStaff, 1994), and can be described as per-humid andof low fertility. Parent material is a conglomerate ofrocks of volcanic origin overlaid by a thick layer ofvolcanic ashes (MAG-PRONAREG and ORSTOM,1983). The bioclimatic zone is very humid, sub-tropical (Cañadas Cruz, 1983), which corresponds tothe premontane, wet forest life zone (Holdridge et al.,1971). The surrounding forest is a multilayered trop-ical rainforest with a high biodiversity (CLIRSEN andDINAF, 1991).

We chose two early-successional native tree spe-cies that grow commonly in the region of the plant-ation site: (1) Inga densifloraBentham (Fabaceae– Faboideae), a N-fixing tree producing large ed-ible pods; (2)Pollalesta discolor(Kunth) Aristeguieta(Asteraceae), an invasive timber species found on dis-turbed soils and in abandoned fields.Inga densiflora

pods were collected in June 1993 from a single par-ent tree established in a pasture. Seeds were plantedin small black plastic bags to reduce transplantationshock, and grown for approximately 5 months underthe light shade of small trees in a temporary nursery,near our site, to avoid excessive light at the seedlingstage. Seedlings ofPollalesta discolorwere collectedin November 1993 from recently disturbed soil along-side a nearby road because our attempts to germinateseeds failed.

Plantation and experimental design

The experimental site was deforested approximately30 years ago. It was subsequently channeled to preventexcessive water accumulation and used as a pasturefor about 20 years. In 1983, after a superficial plow-ing with a small tractor, the site was converted intoa sugar cane plantation for alcohol production; us-age which continued until 1989 without fertilizer ormechanical harvesting. From 1989 until 1993, invad-ing trees were removed as they appeared in order tomaintain theAxonopus scoparius((Flügge) Kuhlm)pasture that was sown after the sugar cane field wasabandoned. An area of 150× 150 m was weeded bymachete in July 1993 and fenced to exclude cattle. InAugust, glyphosate herbicide was applied at a dosageof 6 L ha−1. A more complete description of the studysite can be found in Davidson et al. (1998).

The experimental plantation was laid out in an areaof 148 × 64 m, using a randomized block designconsisting of three blocks, each subdivided into 12plots (16× 16 m). In November 1993, the two treespecies were planted, 64 trees per plot, spaced 2 mapart (within row and between row). In each block,three plots were planted withInga densifloraalone,three plots withPollalesta discoloralone and threeplots with an even mix of both species (checkerboardpattern). The remaining three plots were left withouttrees. A total of 864 trees of each species were planted.One month after the initial planting, 175Pollalestadiscolor died and were replaced. We also replacedanother 182 individuals ofPollalesta discolorand 29Inga densiflorabetween August 1994 and November1994, in order to achieve a more uniform tree cover;but these plants were not included in our growth ana-lysis. When needed, glyphosate herbicide was appliedon average two to three times per year, always 2 to 3weeks after a weeding by machete. Height of all treeswere measured at plantation time and after 2.5 years.Trees were measured with a steel ruler when under 1 m

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in height, and with a graduated telescopic measuringrod when above 1 m in height.

Soil analyses

Soil samples (0–10 cm) used to characterize initial soilproperties were collected in November 1993. Five sub-samples were collected in each plot, and mixed into acomposite sample, for a total of 36 samples each time.The samples were air-dried, dried in the oven at 70◦C and then sieved to 2 mm (all analyses except P).A subsample was sieved to 150µm (after grindingwith mortar and pestle) for P extraction. Mineral N(NH4 and NO3) was extracted with KCl 2M (Maynardand Kalra, 1993) and analyzed by flow injection ana-lysis (Tecator FIAstar 5020 Analyser). Exchangeablecations (K, Ca, Mg, Mn, Fe and Al) were extrac-ted with BaCl2 0.1 M (Hendershot et al., 1993) andanalyzed by atomic absorption. Exchangeable P wasextracted by the Bray II method (McKeague, 1978),using NH4F 0.03 N + HCl 0.1 N, and analyzed us-ing the Murphy and Riley (1962) procedure. Organicmatter content was estimated by loss on ignition inthe oven at 550◦C for 2 h (Grimshaw, 1989). The pH(H2O) was determined using a glass electrode in a 1:2soil:water solution. Soil density was measured usingsamples taken in metal cans of known volume.

N mineralization and nitrification were determ-ined with a laboratory incubation of dry soil samples(Paré and Bergeron, 1996). Soil used for this ana-lysis was collected in May 1996, according to thesame sampling design used in November 1993, butwas not sieved (it was relatively homogeneous withoutapparent organic fragments) and was air-dried only.Soil samples were split into two subsamples, non-incubated and incubated, and non-incubated sub-samples were extracted for initial mineral N (NH4 andNO3) with KCl 2 M. Incubated subsamples were re-wetted to saturation, and kept at 22◦C for 6 weeks inthe dark. Each week, soil samples were adjusted to theoriginal weight to compensate for water loss. After theincubation period, samples were extracted with KCl2 M and analyzed for NH4+ and NO3

− by flow in-jection analysis (Tecator FIAstar 5020 Analyser). NetN ammonification was calculated by the differencebetween initial and final NH4+, net nitrification bythe difference between initial and final NO3

−, and netN mineralization by the sum of net N ammonificationand net N nitrification.

In May 1994, soil drainage at the base of eachtree was estimated according to the following scale: 1,

rapidly-drained and compacted; 2, well to moderately-well drained; 3, locally poorly drained (puddle at baseof tree); and 4, very poorly-drained (standing waterafter heavy rainfall). These observations were madeduring a rainy period.

Foliar and litter nutrient analysis

In May 1996, a total of 10 leaves per species per plotwas collected (maximum of one leaf per tree). Forevery selected tree, the first mature leaf of a selectedbranch was collected, and leaves from within plotswere pooled for each species. The litter was sampledonce, in May 1996. Three quadrats of 625 cm2 eachwere placed randomly in every plot, and collected lit-ter was pooled to provide one sample per plot. Leafand litter samples were air-dried in the field, then driedagain at 60◦C in the laboratory. Litter samples wereweighed to estimate the litter biomass (kg ha−1). Allsamples were ground and sent to the Ministry of Agri-culture Laboratory (Rock Forest, Qué.) for analysis ofP, K, Ca, Mg, Mn, Fe, Al, B, Cu and Zn by inductivelycoupled plasma (ICP) emission spectroscopy (Dono-hue and Aho, 1992). C and N were determined with aNitrogen/Carbon Analyser NA1500 Series 2.

Data collection and statistical analyses

Survival at 2.5 years, in relation to species and pureversus mixed stands, was analyzed with a frequencytable analysis using a log-linear model. Growth(height, height annual increment [HAI]) and foliar nu-trients (N, P, K, Ca, Mg, N:P and N:K ratios, B, Zn,Fe, Mn, Cu and Al), both at 2.5 years, were analyzedby analysis of variance, followed by a Tukey meanscomparison test. The growth ANOVA model includedthe effects of block, species, block× species, mixedstand, block× mixed stand, species× mixed stand,block× species× mixed stand and tree row. A treerow term was included in the model to confirm thelarge intra-plot variability present on the plantationsite. Pearson correlation coefficients were calculatedfor a range of growth, soil [initial (NO3-N, NH4-N, P,K, Ca, Mg, Fe, Mn, Al, ECEC, pH, Al (%), OM (%)and bulk density), and final (soil N mineralization andnitrification)] and plant variables. A factorial analysiswas done with the final height and the initial soil dataset. Soil litter data (biomass, C:N ratio, C, N, P, K,Ca, Mg, B, Zn, Fe, Mn, Cu and Al) were analyzed on asub-sample consisting only of plots where a tree coverhad formed. Four plots per treatment were considered,mostly from block 1, and a few from blocks 2 and 3,

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for a total of 16 plots. A Kruskal–Wallis test was used,followed by a Tukey grouping. SAS was used for allanalyses (SAS Institute, 1997).

Results

Soil characteristics

Initially, soils were characterized by a low ECEC,in spite of a high percentage of soil organic matter(Table 1). Calcium was the most abundant cation.There was no aluminum toxicity. Available P waslow and the dominant N form was NH4. There was ahigh soil variability, especially for the nutrient status.Among the three blocks, block 1 had better chem-ical soil properties, with an ECEC twice that of block2, and 1.5 times that of block 3. Block 2 was thepoorest of the three. At plantation time, organic mat-ter was highly correlated with a range of other soilvariables (results not shown), especially ECEC (r =0.81,P< 0.0001), concentrations of available Ca, Mgand Al, and inversely correlated with pH (r = –0.68,P < 0.0001). The average value of the net total Nmineralization (NH4 + NO3) measured during labor-atory incubation of samples collected after 2.5 yearswas 106.1 mg Kg−1 of soil for a 30-day period,and showed a close relationship to initial soil organicmatter (r = 0.75,P< 0.0001).

Tree survival and growth, in pure and mixed stands

Inga densifloraand Pollalesta discolordid not dif-fer in survival in pure stands, which was approxim-ately 75%. In mixed stands, the survival ofPollalestadiscolor fell significantly to about 69% (results notshown). Pollalesta discolorgrew significantly fasterthan Inga densiflorain either pure or mixed stands(Table 2). The strong block× mixed-stand effect andlack of a mixed-stand effect suggests considerablevariation among blocks. The strong tree row effectreflects the large intra-plot variability.

Relationships between growth and soil characteristics

There were no statistically significant relationshipsamong the survival after 2.5 years and the initial soilvariables. A factorial analysis showed that for bothspecies planted in pure stands, the most importantsoil variable at planting time was soil organic matter(Figure 1), followed by the ECEC. Soil pH was negat-ively related to growth. For pure and mixed stands ofPollalesta discolor, the initial content of NO3-N was

related to the final height. This was not so clearly thecase for pure stands ofInga densiflora. Only whenInga densiflorawas planted withPollalesta discolor,did the initial NO3-N become an important soil vari-able in explaining the height ofInga densiflora. Theinitial content of phosphorus has been excluded fromthe factorial analysis because concentrations weremostly non-detectable.

Table 3 shows that net nitrification was stronglycorrelated with the final height of both species, es-pecially that ofInga densiflorain mixed stands. NetN mineralization was strongly correlated with the fi-nal height ofPollalesta discolor, in mixed and purestands, and ofInga densiflorain mixed stands.

The close link between soil organic matter andgrowth is illustrated in Figure 2. An average decreaseof 1% in initial soil organic matter reduced mean fi-nal height by 11% for pure stands ofInga densifloraand 7% for pure stands ofPollalesta discolor, and by6% for both species in mixed stands. Between the bestand the worst plots, height varied by about 60% (treesapproximately 2.5 times smaller), for both species.

Foliar nutrients

Differences between species were more importantthan between mixed and pure stands (Table 4). Con-centrations of nutrients were always greater in leavesof Pollalesta discolorthanInga densiflora, and the dif-ference was significant for P, K, Ca, Mg, B, Zn, Cu andAl (p < 0.05). Leaves ofPollalesta discolorcontainedup to 60 times more calcium thanInga densiflora, buttheir N content was equivalent. Because these two spe-cies had similar concentrations of N, the N:P ratio wassignificantly greater forInga densiflora.

Foliar N and P showed a strong relationship withthe final growth in height forInga densiflora, and werealso closely related to the initial soil organic mattercontent (Figure 3), as well as with N mineralization(Table 5). There was an inverse relationship betweenthe foliar N:P ratio and the final height. This ratiowas also negatively related to the initial soil organicmatter content and to N mineralization. Foliar Zn wascorrelated with final height and N mineralization.

ForPollalesta discolor, growth was strongly linkedwith higher concentrations of foliar N and P and lowerconcentrations of K (Table 5). Foliar N, P (Figure 3)and the N:K ratio were positively linked to the initialsoil organic matter content, as well as with N miner-alization, and foliar K was negatively linked to thesesame soil variables.

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Table 1. Initial soil properties of the experimental site (November 1993), Macas, Ecuadorian Amazon

Block N-NO3 N-NH4 P K Ca Mg Fe Mn Al ECECa pH Alb OMc Dens.d

(mg kg−1) (cmol kg−1) (H2O) (%) (g cm−3)

1 (n=12) 13.2 30.0 0.03 0.26 2.76 0.55 0.02 0.05 0.52 4.16 5.28 12.7 34.1 0.39

2 (n=12) 8.3 21.6 n.d. 0.18 1.47 0.06 0.01 0.03 0.35 2.09 5.40 16.5 31.5 0.39

3 (n=12) 7.3 83.1 0.13 0.16 1.22 0.30 0.03 0.15 0.97 2.84 5.40 33.2 33.6 0.38

Mean 9.6 44.9 0.05 0.20 1.82 0.31 0.02 0.08 0.61 3.03 5.36 20.8 33.1 0.39

S.D. 10.5 39.2 0.20 0.10 1.73 0.39 0.02 0.13 0.49 2.18 0.22 13.3 3.5 0.04

C.V. 110 87 380 47 95 129 108 171 80 72 4 64 11 9

a ECEC, Effective cation exchange capacity (sum of all cations: K, Ca, Mg, Fe, Mn, Al).b Al (%), % of Al saturation (percentage of Al relative to total cations).c OM, organic matter.d Dens., density; data arefrom May 1994.

Table 2. ANOVA for height of Inga densifloraandPollalesta discolorat 2.5 years,in pure and mixed plantations, Macas, Ecuadorian Amazon

Source df F value P> F

Block 2 34.5 0.0001

Species 1 3970 0.0003

Block× species 2 0.03 0.9681

Mixed stand effect 1 0.16 0.7252

Block× mixed stand effect 2 27.6 0.0001

Species×mixed stand effect 1 0.40 0.5905

Block× species× mixed stand effect 2 3.5 0.0323

Tree row (block× species×mixed stand effect) 273 5.2 0.0001

Error 981

Litter characteristics

The litter tended to be richer in N under a tree coveras compared to the control (Table 6). Litter carbon didnot differ between treatments. As a consequence, theC:N ratios tended to be lower under a tree cover, withvalues ranging from 17 to 20, as opposed to 26 in opencontrol plots. In general, the litter was richer underPollalesta discolorand mixed stands of both species,and was reflected in a higher content of Ca, Zn, Mn,and Cu.

Discussion

Growth and foliar nutrients in pure and mixedplantations

Species response

Pollalesta discolorgrew quickly, attaining an aver-

age final height 1.5 times that ofInga densiflora. Theaverage HAI of 122 cm year−1 is very close to theaverage value of 116 cm year−1 measured by Diaz(1994) on naturally regenerating trees of that species,and virtually the same as values measured in an adja-cent experiment (Davidson et al., 1998). This growthis comparable to the lowest rates measured on nativespecies in experimental plantations in Costa Rica (But-terfield, 1995, 1996; Espinoza and Butterfield, 1989;Gonzalez and Fisher, 1994). The average HAI ofInga densiflorawas 86 cm year−1, which was slightlygreater than in the adjacent experiment, which pro-duced a value of 72 cm year−1 (Davidson et al., 1998),but much less than other leguminous trees tested inCosta Rica (Butterfield, 1995, 1996; Espinoza andButterfield, 1989; Gonzalez and Fisher, 1994). Themean HAI of Inga densiflorawas almost three timesless than that of another species,Inga edulis, testedin the Atlantic lowlands of Costa Rica (Gonzalez andFisher, 1994). The generally slower growth measuredin our plantation may reflect the state of soil degrad-

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Figure 1. Factorial analysis of the influence of initial soil variables on height ofInga densifloraandPollalesta discolorin pure and mixedplantations after 2.5 years, Macas, Ecuadorian Amazon. Ht, height; OM, % soil organic matter; ECEC, effective cation exchange capacity; pH,soil pH (H2O); NO3, soil NO3-N; NH4, soil NH4-N; Dr, soil drainage index.

Table 3. Pearson correlation coefficients between mean height ofInga densifloraorPollalesta discolor(in pure and mixed plantations) and soil N mineralization, at 2.5years, Macas, Ecuadorian Amazon

Species Treatment Net NO3-N Net N mineralization

(month−1) (NH4-N + NO3-N) (month−1)

Inga densiflora Pure 0.79∗ 0.59

Inga densiflora Mixed 0.96∗∗∗ 0.90∗∗∗Pollalesta discolor Pure 0.77∗ 0.85∗∗Pollalesta discolor Mixed 0.80∗∗ 0.98∗∗∗

∗ P≤ 0.05,∗∗ P≤ 0.01,∗∗∗ P≤ 0.001.

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Figure 2. Relationships between height ofInga densifloraandPollalesta discolorin pure and mixed plantations after 2.5 years, and the initialsoil organic matter, Macas, Ecuadorian Amazon.

ation, as well as the fact that our site is situated at anelevation of 1300 m, with a cooler climate than the oneprevailing in the Costa Rican plantations.

Pure stands of both species had worse survival thanin an adjacent multiple species trial (Davidson et al.,1998), and are in the lower range for measured sur-vival rates of native species growing in experimentaltree plantations in Costa Rica (Butterfield, 1995, 1996;Espinoza and Butterfield, 1989; Gonzalez and Fisher,1994). The lower survival ofPollalesta discolorinmixed plots was unexpected. No mortality can be at-tributed to overshading fromInga densiflora, because

Pollalesta discoloroutgrew the leguminous species onevery mixed plot.

The two tree species maintained markedly differ-ent nutrient concentrations in their leaves, but con-centrations within each species were similar to thosemeasured on trees from an adjacent plantation (Dav-idson et al., 1998).Pollalesta discolorhad low con-centrations of Al, intermediate concentrations of mostfoliar nutrients (P, K, Mg, Fe, Zn, Cu and B), and highconcentrations of Mn. Concentrations of foliar N wererelatively high for a non N-fixing species, althoughsuch concentrations are sometimes reported (Drech-

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Figure 3. Relationships between foliar N and P ofInga densifloraandPollalesta discolorin pure and mixed plantations combined after 2.5years, and the initial soil organic matter, Macas, Ecuadorian Amazon.

sel and Zech, 1991; Montagnini et al., 1995; Reichet al., 1995).Inga densiflorahad very low concen-trations of Ca, low concentrations of P, K, Al, Zn,Cu and B, and intermediate concentrations for N, Mg,Mn, and Fe, when compared to other N-fixing species(Drechsel and Zech, 1991). Mean concentrations offoliar nutrients P, K, Ca and Mg were similar to thosemeasured on tree colonizers of Amazonian Venezuelafor Pollalesta discolor, whereasInga densiflorahadconcentrations more typical of the later successionaltree species (Reich et al., 1995). Compared toIngadensiflora, Pollalesta discolorappears to be a morenutrient demanding species.

Relationships with soil properties

The initial soil content of organic matter proved tobe the best indicator of productivity for both speciesplanted in pure stands. Final height on plots with highsoil organic matter was more than twice that of plotswith low soil organic matter. ECEC ranked secondas a factor related to growth. The inverse relationshipbetween growth and soil pH reflects the link betweenthe soil organic matter and pH, suggesting the import-ance of organic matter as the source of acidity for theseAndisols. The acidity of humus-rich allophanic An-disols is known to be determined by carboxyl groupsof their humic acids (Nanzyo et al., 1993). The strong

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Table 4. Mean concentrations of nutrients in foliage ofInga densifloraandPollalesta discolorin pure and mixed plantations after 2.5 years,Macas, Ecuadorian Amazon

Treatment N1 P1 K1 Ca1 Mg1 N:P N:K B2 Zn2 Fe2 Mn2 Cu2 Al2

Inga densiflora

Pure 27.7 1.10 5.12 0.21 1.77 25.5 5.77 11.9 13.8 49.6 297 4.98 9.19

Mixed 26.3 0.99 4.01 0.10 1.77 27.6 7.28 13.7 13.0 51.6 338 4.08 8.88

Mean 27.0a 1.04b 4.57b 0.16b 1.77b26.5a 6.52a 12.8b 13.4b 50.6a 318a 4.53b 9.03b

S.D. 3.7 0.3 1.5 0.2 0.3 3.6 2.18 2.6 2.9 5.8 65.8 1.6 5.5

Pollalesta discolor

Pure 26.9 2.23 19.2 9.73 4.02 12.1 1.54 29.0 62.3 69.3 758 19.9 15.9

Mixed 27.5 2.20 18.7 9.84 3.84 12.5 1.56 25.7 63.9 73.4 789 18.6 21.2

Mean 27.2a 2.22a 19.0a 9.79a 3.93a 12.3b 1.55b 27.3a 63.1a 71.4a 774a 19.3a 18.5a

S.D. 3.8 0.3 4.1 1.7 0.7 1.2 0.61 3.9 11.1 16.1 244 2.9 7.1

Letters within columns indicate significant differences from a Tukey grouping (P ≤ 0.05). Significantly higher means or ratios are in bolditalics.1(mg g dry matter−1).2(mg kg dry matter−1).

correlation between the ECEC and the soil organicmatter content reflects the importance of soil organicmatter in supplying nutrients in Andisols and its majorrole in their productivity (Nanzyo et al., 1993). Ourresults showed that a decrease of 25% in the soil or-ganic matter content (between richer and poorer plots)was related to a 77% reduction in the ECEC. Mixedstands showed similar trends, except that NO3 avail-ability and N mineralization were more tightly relatedto the growth ofInga densiflora, suggesting a possiblecompetition withPollalesta discolorfor N nutrition.Inga densiflorawas probably partly independent ofNO3 supply in pure stands, being a N-fixer.

Despite soil organic matter contents of between25 and 30%, some of these soils showed a markeddecrease in soil fertility. A large portion of the totalsoil organic matter is probably stabilized and not eas-ily available for microbial decomposition. Amongproposed mechanisms for this stabilization are theformation of Al–humus complexes (Shoji et al., 1993)and the presence of non-crystalline clays protectingthe organic matter from degradation by enzymes andmicroorganisms (Oades et al., 1989). Indeed, the nettotal N mineralization of plots with a 30% soil organicmatter content was 57% inferior to that of plots with40% soil organic matter content. Changes of a fewpercentage units in soil organic matter content mayhave an important impact on the nutrient status, as sug-gested by the high coefficient of variation for cationsin the field, as well as on plant productivity. Similarvariability in P availability is also likely, since an im-

portant pool of this element is found in organic formsin volcanic ash soils (Shoji et al., 1993).

The variation of soil organic matter, through itsinfluence on plant nutrition, is also reflected in thevariation in concentrations of foliar nutrients for bothspecies. The relationship between foliar P and growthwas especially helpful in understanding the import-ance of P nutrition for our species, since our method ofmeasurement of available soil P yielded extracts withundetectable concentrations of PO−3

4 and prohibited aproper analysis of this variable in relation to growth.Foliar N, P and K were the most important nutrientsassociated with the growth ofPollalesta discolor. Onsites with low soil organic matter, foliar K was 20.5–22 mg g−1, which are considered to be intermediate tohigh concentrations (Drechsel and Zech, 1991), com-pared to 16.5–17.5 mg g−1 on sites with higher soilorganic matter. The N:K ratio varied from 1.0–1.3 onplots with low soil organic matter to 1.8–2.0 on plotswith higher soil organic matter. It is unclear why foliarK was greater on sites with low soil organic matter.Because it was negatively correlated with Ca, Mg, Zn,Cu and N, high concentrations of K may have createdproblems of nutrient uptake. Zech and Drechsel (1991)found an inverse relationship between K and Ca in teak(Tectona grandis). Finally, in spite of the variation inP uptake between plots with lower and higher soil or-ganic matter, this species always maintained relativelyhigh foliar concentrations of P. Roots ofPollalestadiscolor were found to have a density of vesicular–arbuscular mycorrhiza that is almost 20 times higher

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Table 5. Pearson correlation coefficients between mean final height, initial soil variables, final soil N min-eralization and foliar nutrient concentrations ofInga densifloraand Pollalesta discolorin pure and mixedplantations taken together, after 2.5 years, Macas, Ecuadorian Amazon

Foliar nutrients

Variable N P K N:P N:K Zn

Inga densiflora

Final height 0.81∗∗∗ 0.87∗∗∗ 0.03 –0.70∗∗∗ 0.16 0.62∗∗NO3-N 0.59∗∗ 0.67∗∗ 0.46 –0.53∗ –0.23 0.63∗∗NH4-N 0.00 –0.28 –0.50∗ 0.54∗ 0.45 –0.14

P 0.00 –0.14 –0.16 0.25 0.12 –0.03

ECEC 0.41 0.46 –0.05 –0.39 0.04 0.32

pH (H2O) –0.74∗∗∗ –0.82∗∗∗ –0.28 0.61∗∗ 0.07 –0.77∗∗∗Organic matter 0.61∗∗ 0.74∗∗∗ 0.23 –0.68∗∗ –0.13 0.58∗Net NO3+NH4 month−1 0.71∗∗∗ 0.79∗∗∗ 0.15 –0.73∗∗∗ 0.07 0.71∗∗∗

Pollalesta discolor

Final height 0.81∗∗∗ 0.84∗∗∗ –0.70∗∗∗ 0.13 0.74∗∗∗ 0.62∗∗NO3-N 0.33 0.49∗ –0.31 -0.13 0.32 0.30

NH4-N 0.07 –0.24 –0.13 0.43 0.04 –0.02

P 0.50∗ 0.47∗ –0.48∗ 0.12 0.60∗∗ 0.33

ECEC 0.54∗ 0.60∗∗ –0.54∗ 0.07 0.57∗ 0.36

pH(H2O) –0.44 –0.63∗∗ 0.42 0.14 –0.47∗ –0.36

Organic matter 0.67∗∗ 0.70∗∗ ∗ –0.58∗ 0.10 0.63∗∗ 0.49∗Net NO3+NH4 mth−1 0.71∗∗∗ 0.83∗∗∗ –0.54∗ –0.01 0.62∗∗ 0.53∗

∗ P≤ 0.05,∗∗ P≤ 0.01,∗∗∗ P≤ 0.001.

Table 6. Litter biomass and quality ofInga densifloraandPollalesta discolorin pure and mixed plantations, Macas, Ecuadorian Amazon (on plots with treecover only;n = 4 for each treatment, totaln = 16)

Nutrient Inga Pollalesta Mixed Open control

(n = 4) (n =4) (n = 4) (n = 4)

C:N 20.0ab 19.5ab 16.7b 25.7a

Litter biomass 5094a 4681a 6850a 4368a

(kg ha−1)

C 371a 391a 409a 381a

N 19.3ab 20.8ab 24.7a 15.1b

P 1.1a 1.1a 1.2a 1.1a

K 0.7b 0.9ab 0.9ab 1.3a

Ca 3.0b 14.9a 11.1a 4.8b

Mg 2.8a 3.2a 3.5a 2.6a

(mg g dry matter−1)

B 11.5ab 18.3a 14.0a 4.0b

Zn 39.8b 114a 108a 56.3b

Fe 1855a 2063a 1602a 1553a

Mn 498b 1026a 875a 472b

Cu 21.8b 36.0a 29.5a 15.1c

Al 6386a 7476a 5746a 5263a

(mg kg dry matter−1)

Letters within rows indicate significant differences from a Tukey grouping (P≤ 0.05).

145

than roots ofInga densifloraon our site (C. Vasseur,pers. comm.), which might help explain its efficiencyin thriving on high P-adsorbing soils, where it growsnaturally.

N and P uptake played dominant roles in thegrowth of Inga densifloraat our site. Foliar P rangedfrom 0.75–0.90 mg g−1 on plots with low soil organicmatter, which could be considered to be deficient tolow, to 1.18–1.29 mg g−1 on plots with high soilorganic matter, concentrations considered to be lowto intermediate (Drechsel and Zech, 1991; Zech andKaupenjohann, 1990). Foliar N also varied in relationwith soil organic matter, but concentrations were inter-mediate in all situations. Foliar P increased faster thanfoliar N with increasing soil organic matter content,leading to a decreasing N:P ratio. This ratio variedfrom 28–31 on plots with low soil organic matterto 23–24 on the plots with greater soil organic mat-ter. Zech and Drechsel (1992) observed many mineraldisorders when the N:P ratio exceeded 24 in trop-ical pines and P is probably a limiting nutrient forInga densiflora. Considering the high P-adsorption ofvolcanic soils (Wada, 1985), the availability of thisnutrient is probably dependant upon mineralization ofthe organic matter. Foliar Zn was only moderatelyassociated with the growth ofInga densiflora. Zn con-centrations are usually associated with the P status ofthe tree (Drechsel and Schmall, 1990). Foliar Zn ofInga densifloravaried from 10.5–12 ppm on plots withlower soil organic matter to 14.5–15.5 on plots withgreater soil organic matter. According to Zech andDrechsel (1992), concentrations below 14 ppm havebeen associated with completely chlorotic needles forpines in Liberian plantations. In short,Inga densiflorawas apparently limited by the P availability of the soilon the poorest parts of our site, and up to a point bya poor Zn supply. However, this species seems to beremarkably undemanding as to the soil nutrient status.

A beneficial effect was expected from the presenceof Inga densiflora, a N-fixing species, on the growthof Pollalesta discolorin mixed stands. According toBinkley (1992), the growth of a non-N-fixing tree aswell as the growth of the whole stand tend to be su-perior in mixed stands with a N-fixing tree only on Ndeficient sites. We could not demonstrate any signific-ant mixed-stand effect, possibly due to the young ageof the plantation.

We suggest that the labile pool of soil organic mat-ter on plots with low soil organic matter is reduced tothe point of hampering mineral nutrition and plant pro-ductivity, through a nutrient shortage. The labile pool

of soil organic matter is important in terms of nutri-ent release, and given its smaller size in tropical soilsand its faster turnover under warm and humid tropicalconditions (Duxbury et al., 1989), any disturbance asa consequence of forest removal could cause nutrientlosses. Indeed, our experimental site showed a loss ofsoil organic matter close to 40%, and an ECEC thatwas approximately four times lower than that of anadjacent forest (Thibaudeau-Robitaille, 1997).

Litter biomass and litter quality

Pure stands

The mass of accumulated litter was similar to the leastaccumulation among native species in an experimentalplantation in Costa Rica (Montagnini et al., 1993),but was almost twice that in a nearby natural and ma-ture forest (Thibaudeau-Robitaille, 1997). Litter qual-ity has an important influence on its decomposition,through C and nutrient availability to saprotrophs, Nand P usually being the most critical elements (An-derson and Flanagan, 1989). BothInga densifloraandPollalesta discolorhad N-rich leaves, which was re-flected in a N-rich litter. N concentrations in the litterof Inga densiflorawere similar to those measured forother N-fixing tree species from the Atlantic forest re-gion of Brazil, whereas N concentrations in the litterof Pollalesta discolorwere much greater than thoseof other non N-fixing tree species (Montagnini et al.,1995). The N-enriched litter under both species pro-duced a C:N ratio of around 20, which is associatedwith high N-availability (Vitousek et al., 1982), andhigh decomposition and mineralization rates (Gosz,1984).

Mixed stands

According to Binkley (1992), the rate of cycling ofN is three to eight times greater in mixed stands withN-fixing species than in stands containing only nonN-fixing trees. Our results suggested that the litter ofmixed stands had more N than that of the open con-trol, and there was a trend toward more N than foreach species considered separately. Mixed stands hada richer litter than that ofInga densiflorafor Ca, Zn,Mn and Cu, but these concentrations were not statist-ically different than those ofPollalesta discolorlitteralone. The larger mass of litter in mixed stands, al-though not significant, could play a role in influencingsoil properties in the future.

146

Conclusion

Growth and mineral nutrition of two early-successionalnative tree species were strongly linked to the soilproperties of a former pasture established on a vol-canic soil. Soil organic matter content was the mostsignificant edaphic factor in explaining plant pro-ductivity, indicating that its management is as import-ant an issue on these Andisols, as it is on soils withdifferent mineralogical characteristics, such as Oxisolsand Ultisols, which generally have a much lower soilorganic matter content. Research is urgently neededto characterize the silviculture of native tree species.Species differences in nutrient cycling are likely to bereflected in soil properties, through balance betweenlitterfall accumulation and nutrient uptake.

Acknowledgments

We wish to acknowledge the International Develop-ment Research Centre of Canada, for a Young Ca-nadian Researchers Award to R. Davidson. We alsowish to acknowledge the Biodôme de Montréal andthe International Cooperation Office of UQAM forproviding travel funds for D. Gagnon. We wish tothank the staff of the CREA (Centro de ReconversiónEconómica del Austro), at Macas, for their supportand donation of seedlings, and especially the late P.Fernandez, whose knowledge of local native tree spe-cies made this work possible. We are grateful for thefield help provided by local residents and the Escuelade Biologia del Medio Ambiente, Universidad delAzuay, Cuenca, especially that of H. Hernandez, P.Turcotte, F. Serrano, A. Verdugo, E. Zarate, and G.Chacón; as well as UQAM students H. Thibaudeau-Robitaille and D. Proulx. We would also like to thankSr R. Peña, who kindly allowed us to use facilitieson his farm during the course of our research. Finally,thanks to S. Daigle for statistical advice and C. Vasseurfor help with laboratory analyses. The comments andsuggestions of two anonymous reviewers have greatlycontributed to improving this paper.

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