Fodder grass productivity and soil fertility changes under fourgrass+tree associations in Kerala, India

B. M. Kumar*, S. J. George & T. K. Suresh

College of Forestry, Kerala Agricultural University, KAU P.O., Thrissur 680656, India (*Author for correspondence: E-mail: kumar1@md3.vsnl.net.in)

Received 17 June 1999; accepted in revised form 18 August 2000

Key words: crown architecture, photosynthetic photon flux density, shade tolerance, silvopastoralism,understorey herbage yield

Abstract

Adapted tree+grass combinations make a valuable contribution to forage production in the Indianpeninsula, but knowledge of the interactive effects between trees and grasses on their production is limited.We, therefore, conducted a field experiment involving combinations of four trees and grasses, besidesmonospecific grass controls, for seven years, to investigate grass productivity in association withleguminous and non-leguminous multipurpose trees (MPT) having disparate canopy architecture, and toassess the end-of-rotation soil fertility changes. Post rotation changes in herbage productivity wereevaluated by growing teosinte (Zea mexicana) for three years. The four MPTs were Acacia auriculiformis,Ailanthus triphysa, Casuarina equisetifolia and Leucaena leucocephala. Grasses included Pennisetumpurpureum (hybrid napier), Brachiaria ruziziensis (congo signal), Panicum maximum (guinea grass) andteosinte. Lower tree branches were pruned from fifth year. Understorey herbage production increased untilthree years in all tree+grass combinations, but declined subsequently, as tree crowns expanded. Overall,casuarina among MPTs, and hybrid napier and guinea grass among forage crops, were more productivethan others. Pruning MPTs generally favoured greater herbage production. Understorey light levels foracacia, ailanthus, casuarina and leucaena were 17, 60, 55 and 55% of that in the open at five years. Duringthe post-rotation phase, MPT plots were characterised by higher soil nutrient capital and consequentlyteosinte yields were higher than in the treeless control treatment. All previous tree-grass combinationsshowed an increasing trend till two years after MPT felling. Yield levels declined subsequently, despiteat variable rates. Careful selection of the tree and grass components is, therefore, crucial for optimisingherbage productivity in silvopastoral systems.

Agroforestry Systems 52: 91–106, 2001. 2001 Kluwer Academic Publishers. Printed in the Netherlands.

Introduction

Grazing or harvesting of forage crops grown inassociation with planted trees represents adominant form of silvopastoralism in many partsof the world (Payne, 1985). Although land usesystems of this kind are thought to be highly pro-ductive owing to the vertical stratification of theabove and below ground components, they are

extremely dynamic with available resources andenvironmental conditions changing over time.Conditions under canopies get progressivelydarker as stand age and biomass increases. Severalauthors describe an inverse proportionalitybetween light transmittance and leaf area indexor leaf biomass (Gleeson and Tilman, 1990).Below ground resources also may be limiting(George et al., 1996). Changes in environmental

conditions alter growth and abundance of under-storey components (Naumburg and DeWald,1999), besides diminishing below canopy herbageproduction (Knowles et al., 1999).

Site resource availability in silvopastoralsystems, however, can be altered through man-agerial interventions. Canopy architecture andorientation of the plant canopy in space play avital role in intercepting the incoming solar radi-ation. Compact, candle-flame shaped tree crownsand sparse crowns with low leaf area densityfacilitate light infiltration (Mathew et al., 1992)and favour understorey herbage production. Treesin managed species mixtures also have a greatpotential to bring about micro-site enrichmentthrough processes such as litter dynamics(Jamaludheen and Kumar, 1999). Nitrogen fixingtrees have the additional potential of bringing insubstantial quantities of atmospheric nitrogen intoa combined form (LaRue and Patterson, 1981).Moreover, N-rich legume litter decomposesrapidly in tropical environments (Kumar andDeepu, 1992). Species attributes, therefore, needto be considered in the design of silvopastoralsystems, as they influence site resource avail-ability.

Understorey herbage production in silvopas-toral systems is also dependent on the shadetolerance of the species involved (Mathew et al.,1992). Tolerant grasses are likely to maintainhigher understorey productivity levels underincreasing levels of canopy closure. Therefore,information on the relative shade tolerance ofimportant tropical grasses is particularly valuablein the design and management of tropical sil-vopastoral systems. However, few workers in thetropics seem to have compared the long-termtrends in productivity of forage crops, differingin their ability to tolerate shade, and grown inassociation with multipurpose trees (MPTs) pos-sessing disparate architectural patterns.

The main purpose of this study was to comparethe long-term trends in productivity of fourtropical grass species under mixed species systemsinvolving MPTs. The particular questions ofinterest were: (i) how forage grasses differing inrelative shade tolerance perform in associationwith leguminous and non-leguminous fast growingMPTs having different canopy architecture, (ii)how light interception differs among such MPTs,

(iii) what the impact is of expanding tree biomassand crown size on understorey herbage mass, (iv)how residual soil fertility (after the tree rotation)is modified by different MPTs and (v) how longsuch fertility improvements last, when a residualfodder crop is subsequently grown without fer-tilisers.

Materials and methods

A field experiment involving sixteen tree-grasscombinations (four grass and four tree species)was initiated in June 1988, at the LivestockResearch Station, Thiruvazhamkunnu, Palakkaddistrict, Kerala, India (between 11°21

′30″ and11°21′50″ N latitude, 76°21′50″ E longitude andat 60–70 m above mean sea level). Monospecificgrass plots (treeless controls) were establishedblock-wise in the contiguous area for comparativepurposes in 1992. The treatments were arrangedin a randomised block design and replicated threetimes.

The study area experiences a warm humidclimate having a mean annual rainfall of 2570 mm,most of which is received during the Southwestmonsoon season (June to August). The secondpeak in rainfall distribution corresponds to theNortheast monsoon season (September–October).These two monsoon seasons together constitutethe wet season (June–October) with more than200 mm of rainfall every month. Mean maximumtemperature ranges from 28.4 °C (October) to38.0 °C (April) and mean minimum temperaturevaries from 19.5 °C (January) to 25.9 °C(November). Dry season usually corresponds tothe period from February to May (scanty rainfalland a mean maximum temperature > 32 °C). Thesoil at the experimental site is an Oxisol (verydeep, clayey, mixed Ustic Palehumults; KSLUB,1995) with an average pH of 5.1.

Multipurpose trees

The four fast growing multipurpose tree speciesused in the study were Acacia auriculiformis A.Cunn. ex Benth. (acacia), Ailanthus triphysa(Dennst.) Alston. (ailanthus), Casuarina equiseti-folia J.R. & G. Forst. (casuarina) and Leucaenaleucocephala (Lam.) de Wit. (K8) (leucaena). All

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except ailanthus are exotic N fixers and wereselected because of their differences in growth andcanopy characteristics. Among the trees, acacia isan evergreen tree with dense foliage and openspreading crowns (TROLL’s model, Halle et al.,1978). Its potential height is up to 30 m, usuallyless and branched. Casuarina grows to a heightof 30 m and possesses a light compact crownwith needle-like cladophyls that facilitate lightpenetration into the understorey (ATTIM’s model).Ailanthus, a deciduous tree with a tall cylindricaltrunk attaining a height of about 30 m and ischaracterised by a small compact crown(KORIBA’s model) that intercepts only a smallfraction of the incident radiation (Mathew et al.,1992). Leucaena occurs both as a branched shruband a tall single trunked tree growing to a heightof 5 m to 20 m in four to five years (presumablyTROLL’s model). The leaves are feathery andpermit a high amount of sunlight to reach theground.

Trees were planted in plots of 6 m × 6 m intwo rows, 4 m apart (within row spacing: 1 m) inJune 1988. Each row consisted of six trees spacedat 1 m distance, giving a population of 12 treesper plot (see layout in George et al., 1996). Fiftycm wide risers on all sides and unplanted 2 m widebuffer strips separated plots within a block while3 m wide buffer strips isolated blocks. To avoidany possible plot to plot interaction throughshading of neighbouring plots on understoreyherbage yield, all samplings were restricted to themiddle of the 4 m wide inter row space. Weassumed that there would not be significant belowground interactions amongst plots in our samplingzones.

Trees in the experimental field were routinelypruned at the beginning of every fodder-plantingseason (June) from 1992 (crown closure for mostMPTs, see Table 1) onwards. Pruning involvedremoval of lower branches to a height up to 5 mwith the objective of allowing uniform quantitiesof filtered light to reach the understorey (approx-imately 60% of open). It left the main stem intact.Foliage fraction of the pruned materials, thoughvariable among the MPTs, was incorporated in therespective plots (Table 1) Consequent differentialnutrient inputs may have only a modest influenceon understorey herbage production, as the grassesare adequately fertilised (see below).

Forage grasses

The four grass species were Pennisetum pur-pureum Schumach. (hybrid napier), Brachiariaruziziensis Germain & Everad. (congo signal),Panicum maximum Jacq. (guinea grass) and Zeamexicana (Schrad.) Reeves & Mangelsd.(teosinte). They were selected to represent shadeintolerant (teosinte), fairly shade tolerant (hybridnapier) and shade tolerant (guinea grass and congosignal) grasses (Skerman and Riveros, 1990).

Fodder planting

The fodder species were planted in the alleys oftree seedlings and managed as per local recom-mendations (KAU, 1996). Except teosinte, anannual crop, all others were planted twice in theexperiment – first in June 1988 and subsequentlyin June 1992. Plots were hoed twice and levelledbefore planting. By 1992, grass growth in plotsinvolving the three perennial fodder species hadbecome uneven and hence they were replanted.Prior to replanting, however, previous cropstubbles were removed.

A spacing of 60 cm × 30 cm was followed inthe case of hybrid napier and congo signal, whilefor guinea grass and teosinte the spacings adoptedwere 40 cm × 20 cm and 30 cm × 15 cm respec-tively (KAU, 1996). Two cuttings of hybrid napier,congo signal and guinea grass were planted perhole. In the case of teosinte, two seeds per holewere dibbled. The plots were weeded at regularintervals and gap-filled by planting cuttings/seedlings. Regarding fertiliser application, ouridea was to fertilise the forage crops adequatelyso that inter specific competition for nutrientscould be excluded as a possible cause for anyobserved yield reduction. For this we followed thelocal recommendations in respect of dosages andmethods of application (KAU, 1996). Fertiliserswere applied uniformly to the forage crops at therate of 200, 50, 50 kg N, P2O5 and K2O ha–1 yr–1

till 1992 (stand age: 5 years). N was given in twoequal split doses (June and August) and P and Kwere basal. No fertilisers were, however, subse-quently applied in the experiment owing to theperceived advantages of nutrient cycling associ-ated with MPTs besides the relatively lower forageyield levels.

93

94

Tab

le 1

. D

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f st

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num

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rees

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past

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ala,

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Spe

cies

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ight

M

ean

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rea

Tre

es s

ampl

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y w

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t of

pru

ned

(m)

(cm

)(m

)in

dex

(n)

mat

eria

ls i

n 19

92 (

kg h

a–1)

5 yr

7 yr

5 yr

7 yr

5 yr

7 yr

5 yr

7 yr

5 yr

7 yr

Fol

iage

Tw

igsc

Bra

nche

sd

Aca

cia

auri

culi

form

is10

.912

.3(9

.311

.3(4

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.714

.517

.721

113

1970

1576

2888

(6.1

)(2

.7)

(3.9

)(3

.5)

(4.1

)(2

.06)

(201

)(3

91)

(715

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Ail

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riph

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4.2

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(1.5

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2012

011

2553

657

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.9)

(1.8

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.2)

(2.2

)(0

.31)

(0.8

1)(1

73)

(222

)(3

85)

Cas

uari

na e

quis

etif

olia

8.2

9.4

(5.5

5.5

(2.4

(2.2

00.0

5a00

.9a

1508

623

786

10

(2.8

)(3

.5)

(2.1

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.0)

(0.6

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.82)

(48)

(164

)

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110

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5(2

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914

5113

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(3.0

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(1.5

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17)

(184

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28)

DB

H =

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.

Fodder harvesting

Two fodder harvests were made in the first yearand three or four in subsequent years except forteosinte. Number of harvests in any given yeardepended on vegetative growth of the perennialcrops. Being rain-fed crops this in turn wasdependent on the rainfall distribution and/or inten-sity. Teosinte was harvested about three monthsafter planting every year. Herbage in three 1 m ×1 m quadrats in the central zone of each plot wasclipped and their fresh weights recorded in thefield. Hand separated sub-samples were dried to70 °C for 48 hours, plot-wise and actual compo-nent mass was determined. Using the net harvestedarea, dry matter percentage and fresh weight,fodder yield was scaled up to a per hectare basis.

Tree felling and assessment of post rotation soilfertility

Trees in eight 6 m × 6 m plots (two plots perspecies in replication III) were felled in May 1993to assess tree biomass accumulation in five-yearold trees (see Kumar et al., 1998). Number of treesharvested per species ranged from 15 to 22(Table 1). These eight plots were fallowed duringthe next planting season (June 1993) and were thusexcluded from the 1993 forage yield assessment.Furthermore, as the tree crowns were very dense,fodder growth in the remaining plots was gener-ally poor in 1993. Fodder crops almost entirelyfailed or were extremely patchy in several of theacacia plots and irregular in others during 1994,presumably because of intense shading (despitethe June pruning). Hence no reliable fodder yielddata could be gathered in that year. In May 1995trees in the remaining 40 plots were also felled toevaluate end-of-rotation tree biomass production(tree age: 7 years; Kumar et al., 1998). Totalheight, crown diameter, girth at breast height andbiomass yield of the felled trees were recorded.In addition, representative leaf samples (ca 500g) were collected from the felled trees for esti-mating the total leaf area (three samples perspecies) using a LI 3100 Leaf Area Meter (Li Cor,Lincoln, Nebraska, USA). Total leaf area wascalculated by multiplying the sample leaf area(per g fresh weight) with the total fresh foliageweight of individual trees (Table 1).

To evaluate the residual soil fertility built up bythe four tree species, teosinte was raised in allplots (including tree-less control plots) for threeyears from 1995 to 1997 (June to August).Cultural practices followed were similar to thepre-felling period except that no chemical fer-tilisers were applied.

Understorey photosynthetic photon flux densitymeasurements

We made continuous understorey measurementsof photosynthetic photon flux density (PPFD) inrepresentative plots of each tree species during theperiod from 25 January 1991 to 1 February 1991(tree age: 32 months) and from 13 April 1993 to2 May 1993 (tree age: 59 months) using a linequantum sensor (LI 191SA, LI COR Inc., Lincoln,Nebraska). Within each plot, we measured PARfrom 6 a.m. to 6 p.m. For this, the line quantumsensor was installed in the alleys (herbagesampling zone) on wooden platforms at 1 m abovethe ground in 1991 (two consecutive days withina plot) and at 0.5, 1.5, 2.5 and 3.5 m heights in1993 (four consecutive days within a plot corre-sponding to the four heights). The line of thesensor was oriented toward magnetic south as treerows were oriented east west. A battery powereddata logger (LI 1000, LI COR Inc) integrated themean flux of PPFD at hourly intervals. Radiationincident on the top of the canopy of each plot wassimultaneously recorded by the data logger witha point quantum sensor (LI 190SA, LI COR Inc)mounted on a 12 m pole rising above the level ofthe canopy. Hourly integrated values (mean) abovecanopy ranged from 40.69 µ moles m–2 s–1

(standard deviation = 16.85) between 6 to 7 a.m.to 1532.1 µ moles m–2 s–1 (standard deviation =128.71) between 12 noon to 1 p.m. Correspondingunderstorey PAR levels were 19.79 µ molesm–2 s–1 (standard deviation = 11.75) and 1029.92µ moles m–2 s–1 (standard deviation = 391.35).Hourly integrated values in respect of the 1991measurements are presented in Mathew et al.(1992). The relative proportion of PPFD reachingthe understorey was calculated as mean dailyunderstorey radiation level divided by that in theopen.

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Soil chemical analyses

Soil samples from all plots (three randomly chosenpoints in each plot; 0–15 cm layer) were collectedafter removing the undecomposed and partiallydecomposed litter on the ground in February 1993,May 1995 and August 1995. Two sampling inter-vals in 1995 correspond to before sowing and afterharvest of the teosinte crop. The samples were airdried and ground to pass through a 2 mm sieve.Two sub-samples per plot were drawn from thecomposite samples for chemical analyses asdescribed below: pH in soil and water in the ratio1:2, organic C by Walkley and Black method, totalN by micro-Kjeldahl method, available P by thechloromolybdic acid blue colour method andavailable K by flame photometry using 1N neutralammonium acetate solution as extractant.

Statistical analyses

Cumulative annual yield data were analysed fordifferences between tree species and time intervalsseparately for the periods before and after treefelling using ANOVA with repeated measures(MANOVA; Moser et al., 1990) employing thestatistical package SPSS/PC+ (Advanced StatisticsV2.0) except for 1992 and 1993. Hierarchicalcluster analysis was performed, as the multivariatetests for tree species-by-year, forage-by-year andyear effects were significant. Clustering was doneusing average linkage between groups (Everitt,1974). The distance measure used was squaredEuclidean distance. Fodder yield data for 1992 and1993 were separately analysed using two-wayANOVA in MSTAT and simple factorial ANOVAin SPSS respectively, owing to addition of tree-less control plots (1992) and/or exclusion of thefelled plots (1993). Data on soil parameters(means corrected for dry weight of the duplicatesamples) were analysed using factorial ANOVA(in MSTAT) and mean daily understorey PPFDlevels using two-way ANOVA (in MicrosoftExcel). To evaluate the performance of teosinteat different stages of tree growth and after treefelling, its yield data for the nine-year period wasregressed on time interval using the SPSS curvefitprotocol. Relative fodder yields were calculated aspercentage of the treeless control fodder yield.

Results

Temporal trends in understorey herbage production

Data presented in Figure 1 clearly indicate ageneral increase in fodder productivity with time,regardless of the associated MPTs in the initialyears after tree planting. Multivariate ANOVAindicated that year (within subject) effects werehighly significant (P < 0.001) for the first fouryears (1988 to 1991). Multivariate (within-subjectrepeated factor) tests of significance – Pillais’trace, Hotellings’ trace and Wilk’s lambda werealso highly significant (P < 0.001) in respect ofgrass-by-year, tree-by-year and year effects.Herbage mass rose until the third year to befollowed by a significant (P < 0.001) drop in thesubsequent year. It, however, increased again inthe fifth year and declined in the sixth year. Allgrasses were similar in respect of the time-courseof productivity changes, despite differences inyield levels.

Herbage yield in diffierent MPT-grass combinations

Multivariate ANOVA indicated that both tree andgrass effects and their interaction with year werehighly significant (P < 0.001). From the third yearonwards, acacia exerted a marked depressingeffect (P < 0.001) on herbage mass of all grasses.Leucaena plots also yielded less than that ofeither ailanthus or casuarina. Hierarchical clusteranalysis using average linkage between tree-grasscombinations showed that there are three distinctclusters in respect of tree-grass combinations(Figure 2). Cluster segregation was mostly alongspecies lines. Teosinte plots (lowest yields),regardless of MPTs formed a single cluster whilecongo signal in association with acacia andleucaena formed the second cluster. Forage yieldswere greatest for hybrid napier. All MPT combi-nations involving hybrid napier and guinea grassbesides congo signal with casuarina and ailanthusconstituted a single cluster.

A comparison of the data for fifth and sixthyear (1992 and 1993) indicates that dry matterproductivity of fodder crops grown in associationwith MPTs was generally lower than that of the

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Figure 1. Cumulative annual biomass yield of four forage grasses grown in association with fast growing multipurpose trees (1988to 1993, Block arrows on the horizontal axis indicate both MPT pruning and replanting of perennial grasses in 1992 while simplearrow represents pruning alone – teosinte was sown annually). Error bars indicate standard deviations. Fifth and 6th year meanswere compared using LSD (same superscripts do not differ significantly). Means for one to four years were compared usinghierarchical cluster analysis (see dendrogram in Figure 2).

treeless control (Figure 1). Results of two-wayANOVA indicate that tree, grass and their inter-action effects were significant (P < 0.01) in 1992while only tree-grass interaction was significantduring the subsequent year (P < 0.05). AmongMPTs, ailanthus and casuarina followed thetreeless controls in respect of herbage mass,

regardless of the associated grasses. Fodder yieldsunder acacia and leucaena were generally lower(23–26% of the control yield in the fifth year,Table 2). A similar trend was also discernible inthe subsequent year although at lower productivitylevels.

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Figure 2. Dendrograms using average linkage between groups for comparing mean dry matter yield of four forage grasses grownin association with fast growing multipurpose trees (year one to four, i.e. 1988 to 1991). ACA = acacia, AIL = ailanthus, CAS =casuarina, LEU = leucaena, HBN = hybrid napier, GUI = gunica grass, CON = congo signal and TEO = teosinte.

Table 2. Relative fodder yields (% of control) for different multipurpose trees and grasses in Kerala, India.

Species Stand age Time after tree felling*

5 yr 6 yr Mean 1 yr 2 yr 3 yr Mean

Forage grassesCongo signal 33 058 46 230 244 188 221Guinea grass 41 084 63 279 318 126 241Hybrid napier 44 040 42 308 260 117 228Teosinte 69 103 86 169 199 133 167

Multipurpose treesAcacia auriculiformis 23 048 36 251 257 134 214Ailanthus triphysa 59 067 63 231 241 135 202Casuarina equisetifolia 66 083 74 186 244 110 180Leucaena leucocephala 26 064 45 284 256 164 235

Relative fodder yield (%) for forage grass = Species-wise mean dry fodder yield under different MPTs/mean yield under treelesscontrol × 100 and for MPT = mean dry fodder yield for an MPT/mean yield in the treeless control × 100.* Trees were felled after seven years and teosinte sown in succession for three years.

Post-rotation teosinte dry metter productivity

The time-course of teosinte dry matter productionsucceeding MPT-grass combinations is presented inFigure 3. Seasonal trends (year effects) in teosinteproductivity were significant (P < 0.001). Averagedtests of significance (using UNIQUE sums ofsquares procedure of SPSS Release 6.0) indicatethat tree-by-grass-by-year and tree-by-year effectswere highly significant (P < 0.001) although grass-by-year effects were not so (P = 0.394). Multivari-ate tests of significance (Pillais’ trace, Hotellings’trace and Wilk’s lambda) were also significant(tree-by-grass-by-year, P < 0.005, tree-by-year, P <0.001 and year, P < 0.001). In general, yield levelsincreased in the second year after felling, followedby a substantial drop during the third year. Alltree-grass plots were similar in this respect.

Post-rotation teosinte yields (unfertilised) in theMPT plots were markedly higher than treelesscontrols (relative yields of 169% in the first year;Table 2). Among the MPTs, leucaena showed thehighest teosinte yield followed by either ailanthusor acacia (first year following felling), regardlessof previous grass species. A similar trend was alsodiscernible in the third year of cropping. Duringthe second year, however, acacia and casuarina inassociation with hybrid napier and/or teosinteresulted in the highest dry matter yield of suc-ceeding teosinte (Figures 3 A and D). Leucaenaand ailanthus in association with either guineagrass or congo signal also favoured higher drymatter productivity of the succeeding teosinte(Figures 3 B and C). Treeless controls invariablyhad lower values. Hierarchical cluster analysisof teosinte biomass succeeding different MPTsshowed three distinct clusters (Figure 4). Teosinteyield succeeding all tree-less controls and casua-rina-congo signal combination formed a singlecluster. Similarly acacia in association withteosinte and hybrid napier, leucaena-guineagrass, ailanthus-congo signal and casuarina-hybrid napier constituted a second cluster, and allothers constituting the remaining cluster.

Teosinte yield in the MPT plots (before andafter felling) was regressed on time (years) afterMPT planting as an independent variable. F ratioswere not significant and only a non-significanttrend to non-linearity was observed, with rela-tively low R2 values (data not presented)

Soil properties

Changes in soil chemistry associated with MPTsat five years after tree planting, end-of-rotation(seven years) and following a short rotation foddercrop after tree felling were evaluated by com-paring the MPT-grass plots with tree-less grassmonoculture plots (Figure 5). MPT influence wasevident in respect of the soil chemical parametersevaluated (F significant at P < 0.01 unless other-wise stated). Forage and interaction effects were,however, mostly not significant. Regarding soilpH, acacia was associated with lower soil pHvalues at five years and after the teosinte crop(P < 0.05) compared to tree-less control and otherMPTs. However, such a clear trend was notdiscernible at the stage prior to teosinte, whentreeless control plots showed relatively lower soilpH.

Interspecific variations (MPT) in soil organicC, N, P and K concentrations were significant.Acacia plots had the highest soil organic C levelsin the two initial samplings (P < 0.01 and P <0.001 respectively). Subsequently, however,leucaena plots showed the highest (P < 0.001)organic C content. Soil organic C was lowest inthe treeless control. Ailanthus plots had lower Nthan the N-fixing MPTs before felling (Figure 5).Tree felling in general improved the soil organicmatter pool. This continued in all MPT plotsexcept acacia even after the harvest of teosinte.MPTs also manifested higher soil N, P and Klevels than treeless controls during the pre- andpost-felling periods (significance of F rangingfrom 0.005 to < 0.001). Differences among theMPTs, however, did not follow any predictablepattern.

Understorey photosynthetic photon flux density

A comparison of the understorey PPFD data(Figure 6) implies strong interspecific differences(P < 0.001). Clearly, acacia intercepted much ofthe incoming solar radiation at both stages ofobservation. Only 12% of the total incomingradiation reached its understorey at three years.Ailanthus, however, transmitted approximately94% of the incoming solar radiation, whilecasuarina and leucaena were intermediate withunderstorey PPFD levels of 54% and 59% of the

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Figure 3. Cumulative annual biomass yield of teosinte succeeding different tree-grass combinations, for three years after MPTfelling. A to D – Different tree-grass combinations preceding teosinte. A. Hybrid napier (HBN, Pennisetum purpureum) inassociation with four MPTs (ACA = acacia, AIL = ailanthus, CAS = casuarina, LEU = leucaena), B. Guinea grass (GUI, Panicummaximum) in association with four MPTs, C. Congo signal (CON, Brachiaria ruziziensis) in association with four MPTs and D.Teosinte (TEO, Zea mexicana) in association with four MPTs. Respective monospecific grass plots are also shown. Error barsindicate standard deviations. For mean comparison using hierarchical cluster analysis, see dendrogram in Figure 4.

open respectively at three years of age. MeanPPFD levels (at 50 cm and 150 cm) were 17, 60,55 and 55% of the open for acacia, ailanthus,casuarina and leucaena respectively at five years.

Discussion and conclusions

Understorey herbage yield at different stages ofstand development

Year to year variations in herbage yield probablymirror a cumulative effect of intrinsic variationsin MPT canopy development, fodder growth ratesand/or microsite enrichment. Yield levels of theperennial grasses were least in 1988 (first year),which co-incidentally represents the establishmentphase of these crops marked by incomplete swardformation. Tree crowns were relatively smallduring this period (Table 1). As such the MPTsexerted only a modest influence on growth of theassociated grass components early.

However, as the tree canopy developed in the

subsequent years, a strong negative influence onunderstorey herbage yield was discernible(Figure 1 ). At or after three years the trees beganto significantly shade the grasses (Figure 6).Shading has two opposing effects on pasturegrowth: it reduces the photosynthetic productionand may improve soil water status and nitrogenmineralisation, especially under dry conditions(Wilson, 1996). The negative effect of reducedPPFD levels, however, could be temporarilyreversed through canopy manipulation. The mostgeneral effect of any canopy reduction treatmentis enhanced understorey light availability and aconsequent increase in herbage production(Schacht et al., 1988). Understorey light avail-ability, however, may be variable depending on thequantity of biomass removed in the pruningprocess. Also the effects often may be temporaryas re-growth consequent to pruning may continueto intercept incoming solar radiation at the sameor higher levels. The general increase in under-storey herbage production in the fifth year aftertree planting can, therefore, be partly attributed

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Figure 4. Dendrograms using average linkage between groups for comparing mean dry matter yield of teosinte in differenttree-grass combinations for three years after MPT felling (ACA = acacia, AlL = ailanthus, CAS = casuarina, LEU = leucaena,HBN = hybrid napier, GUI = Guniea grass, CON = Congo signal and TEO = teosinte, grass alone are treeless control plots).

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Figure 5. Soil chemical properties of different MPT plots at three stages of cropping (1995 = Five years after tree planting,1995(1) = After clearfelling but before sowing the teosinte crop and 1995(2) = After teosinte harvest). Error bars indicate standarddeviations. MPT and forage effects were significant at P < 0.01 for all parameters except soil pH (1995-2) which was significantat P < 0.05. Bars bearing the same superscripts do not differ significantly within a time interval.

to higher PPFD levels after pruning (initially about60% of the open). In addition, agronomic practicessuch as grass replanting with a view to maintainoptimal herbage density also may have contributedto the observed increase in under storey herbageproductivity (Figure 1). It is hard to separate thesetwo effects based on the results of the presentstudy. Nonetheless, our results imply that bothunderstorey and overstorey components should becarefully managed to optimise production inmixed species systems.

Below-ground interference amongst plots waspossible in the present experimental set-up. Lateralroot spread of trees especially in the arid andsemi-arid regions is often much further than thecrown, and reductions in lateral root spread due toshoot pruning vary amongst species (Jones et al.,1998). However, tree root systems being highlyplastic, proximity of two or more trees/speciesgenerally favours diminished lateral spread and ordeeper root penetration of the woody perennialcomponents (Lehmann et al., 1998). Hence thebelow ground interactions amongst the plots is notexpected to have a major impact on present treat-ment differences.

Comparing forage species (pre-felling period)

It can be deduced from Figure 1 that overall forageyields (combined grass means over years) werein the order hybrid napier > guinea grass > congosignal > teosinte. A similar trend was observable

for treeless controls also, despite higher yieldlevels. Variations in the magnitude of yieldreductions (Table 2) in tree-grass combinations, incomparison to grass monocultures, can probablybe attributed to the differential shade tolerance ofthe forage grasses. Although forage plants differwith respect to their ability to tolerate shade(Eriksen and Whitney, 1981), the general effect ofreduced light intensity is reduction in yield levels(Figure 1). Productivity in the open reflects theinherent potential for growth and yield whenlight is not limiting. Teosinte, is an annual shadeintolerant crop, so this producing the lowestoverall yield is not surprising. Congo signal, albeitbeing shade tolerant, yielded less in comparisonto hybrid napier and guinea grass, perhapsimplying its lower productive potential.Interspecific variations in biomass yield onaccount of genotype × environment interaction asobserved presently is well documented in theliterature (Martiniello and Ciola, 1995).

Yield loss (Table 2) for grasses grown in asso-ciation with MPTs was highest for hybrid napier(fairly shade tolerant). This was followed bycongo signal and guinea grass (both consideredas shade tolerant). Teosinte, however, had thehighest relative yields in both years. Although thisimplies that teosinte is a better choice for sil-vopastoral systems than other grasses evaluatedpresently, its overall productivity was lower thanthe perennial grasses (Figure 1). There is also thepossibility that the grasses were tolerant to

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Figure 6. Relative proportion of understorey photosynthetic photon flux density (PPFD, as % of the open) in four MPT standsat two stages of stand growth and at different heights above the ground level (Mean per cent understorey PPFD level to that ofthe open from 6 a.m. to 6 p.m.). Measurements were made for the period from 15 January 1991 to 1 February 1991 and 13 April1993 to 2 May 1993). Error bars indicate standard deviations. Species effects were significant at P < 0.01 and height and standage effects were not significant. Bars bearing the same superscripts do not differ significantly.

reduced levels of nutrients and/or water avail-ability. Hence proper testing of species and culti-vars is of utmost importance in the managementof silvopastoral systems, which integrate trees andlivestock on the same land management unit.

Effect of associated multipurpose tree components

MPT induced variations in understorey herbageyield (strongly negative effects of acacia andleucaena and less negative effects of casuarina andailanthus, Figure 1) can be explained as a cumu-lative outcome of the differences in crownarchitecture, understorey PPFD levels and soilfertility. Although casuarina, an actinorhizal plant,was associated with higher understorey herbageyield, this was not a general result since otherN-fixing trees with denser crowns (acacia andleucaena) did not exert such a favourable effect.Soil fertility assessment when the trees were five-years-old (1993) showed no conclusive evidencein this respect (Figure 5). Despite the highest Nlevels, understorey herbage yields were particu-larly low in the leucaena plots. Nitrogen gain withN-fixing species is probably negligible in land usesystems like the present one where the field cropcomponent is adequately fertilised. However, inN-limited systems this process could providesubstantial N inputs.

Despite being an N-fixer, acacia had a markednegative effect on herbage yield implying thatcrown size and architecture are more importantdeterminants of understorey herbage yield.Casuarina with its needle-like cladophyls (an LAIof 0.05 at five years of age) and ailanthus with itssmall crown (an LAI of 1.1) favoured higherunderstorey herbage production while acacia,probably because of its spreading crown (Table1) depressed it (Figure 1). Crown architecture andsize determine leaf display, leaf distribution, leafarea index and canopy density, and therefore,influence light interception and understoreyherbage production (Wang and Jarvis, 1990).

Understorey light availability and below groundinteractions

Owing to its orthotropic branching pattern typicalof TROLL’ s model, larger crown dimensions

(Table 1), and/or the dense crown with dark greenfoliage consisting of phyllodes (a high LAI of 14.5at five years of stand age), acacia intercepted morelight than other MPTs (Figure 6). Ailanthus due tothe combination of its small crown and trunk(Table 1) intercepted the least light. Casuarinaand leucaena were comparable in terms of lightpenetration into the understorey. In general, fodderproduction followed a declining trend withincreasing interception of the incoming solarradiation by tree crowns (Figure 1) and trees withdense spreading crowns generally retard under-storey herbage production. Biomass production inthe understorey is a function of the photosynthet-ically active radiation reaching the ground(Naumburg and deWald, 1999). UnderstoreyPPFD levels below 50% of that in the open weredetrimental to the associated forage grassespresently evaluated. Canopy reduction treatmentsshould, therefore attempt to maintain more than50% PPFD in the understorey.

However, it is improbable that differentialPPFD levels could explain all variations in under-storey yield levels. Both casuarina and leucaena,regardless of having similar below canopy PPFDlevels and soil N levels (Figures 5 and 6), weresignificantly different in respect of understoreyherbage mass (Figure 1). This probably implicatesbelow ground interactions. George et al. (1996),using the present experimental set up, showed thatboth acacia and leucaena strongly competed withgrasses for 32P. In addition, both MPTs had aspreading root system with most root activityconcentrated in the surface horizons of the soil.For casuarina, however, the magnitude of com-petitive interactions was of a lower order, owingto the less spreading nature of its roots (Georgeet al., 1996).

Post rotation herbage production and soil fertility changes

Relative yield levels in the MPT plots (Table 2)were clearly greater than the control after MPTfelling and vice-versa before (except for teosinteyield at a stand age of six years). It can be deducedfrom the data on relative yield levels that overallproductivity of succeeding teosinte cropsdecreased in the order leucaena > acacia >ailanthus > casuarina. During the two years

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preceding MPT felling, however, understoreyherbage mass of the four grasses studied followedthe order casuarina > ailanthus > leucaena >acacia. Soil fertility improvement associated withMPTs may explain the observed yield increase inleucaena and acacia plots after MPT felling(Figure 3). Data presented in Figure 5 indicate anoverall increase in the site mineral nutrient capitalof the MPT plots in comparison to treelesscontrols. The effect is perhaps cumulative and maycorrespond to the length of time during which theland is under tree cover. Higher soil organic C,N, P and K contents in 1995 (before the firstteosinte crop) in the MPT plots than the previousmeasurement (1993) exemplifies this. Presumablyprocesses such as nutrient cycling/nutrientpumping, litterfall, root decomposition and/or bio-logical N fixation are implicated. George andKumar (1998) using the present experimental setup showed that litterfall in five year-old MPTstands ranged from 1.92 to 6.25 Mg ha–1 yr–1.Present data, therefore, affirm that nutrientcycling/nutrient pumping associated with MPTsespecially leucaena and acacia in silvo-pastoral systems may enhance residual herbageproductivity (Figure 3).

However, influence of MPTs on succeedingteosinte dry matter productivity was variable withtime after felling and the nature of the associatedgrass component. Data presented in Figure 3indicate that teosinte yields (unfertilised) werelowest for the first crop. They increased in thesecond year and declined finally in the third year.Changes in soil nutrient availability (Figure 5) alsosignified an increasing trend with time after felling(before and after the first teosinte crop) implyingpeak nutrient availability for the second teosintecrop. Subsequently however, it is possible that thesoil nutrient availability in the MPT plots waslower than that of 1995. Because we did notmeasure soil nutrients after the second and thirdteosinte crops (after MPT felling), it is notpossible to make any firm conclusions in thisregard. Nonetheless, the crest in teosinte produc-tivity (Figure 3) implies that peak mineralisationof organic nutrients corresponded to the growthphase of the second crop of teosinte. Nutrientrelease through the mineralisation process mayhave progressively diminished after this.Consequently, yield levels started declining.

We therefore, hypothesise that soil fertilityimprovement brought about by fast growing MPTsat this site may sustain reasonable levels ofherbage production for two crop seasons, even ifno fertilisers are added to the residual crop.Beyond this period yields may decline to non-remunerative levels (Figure 3). The situation isanalogous to shifting cultivation where soil fer-tility restoration by woody perennials during thefallow period may sustain field crop productionfor about two to three years. Although, MPTinfluence was dependent on the previous grasscrop grown in association (Figure 3), there was nopredictable pattern evident in this respect.

Lower teosinte herbage yields in the first yearfollowing felling (Figure 3) can be explained bythe high C/N ratio of the woody fractions, whichin turn leads to a temporary nutrient immobilisa-tion. Higher soil temperatures after felling and/ortillage associated with forage production mayfavour rapid organic matter turnover, with theresult that soil mineral nutrient availabilityincreases over time (Figure 5). Matson et al.(1987) observed that with increases in inorganicN concentrations after clearfelling/burning, therelease and leaching of inorganic N from surfacesoil also increased. Our data (Figure 3) alsosuggest the possibility of higher leaching lossesfor elements such as K, which is not structurallybound. Relatively lower K levels in the soil afterthe harvest of the teosinte crop may suggest this.

Acknowledgements

This work forms part of the All India Co-ordinatedResearch Project on Agroforestry (KAU Centre).Associate Dean, College of Forestry, Vellanikkaraand the Professor, Livestock Research Station,Thiruvazhamkunnu provided necessary facilities.Mr Thomas Mathew, Dr K. V. S. Babu and Mr K.Umamaheswaran were involved in the initiallayout of the field trial. Dr F. L. Sinclair and twoanonymous reviewers provided useful commentson previous versions of the manuscript.

References

Eriksen F and Whitney AS (1981) Effects of light intensity ongrowth of some tropical forage species. I. Interactions of

105

light intensity and nitrogen fertilisation on six foragegrasses. Agron J 73: 427–433

Everitt S (1974) Cluster Analysis. Heineman EducationalBooks Ltd, London.

George SJ and Kumar BM (1998) Litter dynamics and soilfertility improvement in silvopastoral systems of the humidtropics of Southern India. Int Tree Crops J 9(4): 267–282

George SJ, Kumar BM, Wahid PA and Kamalam NV (1996)Root competition between the tree and herbaceous com-ponents of silvopastoral systems of Kerala, India. PlantSoil 179: 189–196

Gleeson SK and Tilman D (1990) Allocation and the tran-sient dynamics of succession on poor soils. Ecology 71:1144–1155

Halle F, Oldeman RAA and Tomlinson PB (1978) TropicalTrees and Forests: an architectural analysis. Springer-Verlag, New York, 441 pp

Jamaludheen V and Kumar BM (1999) Litter of nine multi-purpose trees in Kerala, India-variations in the amount,quality, decay rates and release of nutrients. For EcolManage 115: 1–11

Jones M, Sinclair FL and Grime VL (1998) Effect of treespecies and crown pruning on root length and soil watercontent in semi-arid agroforestry. Plant Soil 201: 197–207

KAU (1996) Package of Practices Recommendations Crops1996. Directorate of Extension, Kerala AgriculturalUniversity, Mannuthy, Thrissur, India, 267 pp

Kerala State Land Use Board (KSLUB) (1995) LandResources of Kerala State. Government of Kerala,Thiruvananthapura, India, 290 pp

Knowles RL, Horvath GC, Carter MA and Hawke MF (1999)Developing a canopy closure model to predict over-storey/understorey relationships in Pinus radiata sil-vopastoral systems. Agrofor Syst 43: 109–119

Kumar BM, George SJ, Jamaludheen V and Suresh TK (1998)Comparison of biomass production, tree allometry andnutrient use efficiency of multipurpose trees grown inwood lot and silvopastoral experiments in Kerala, India.For Ecol Manage 112/1-2: 145–163

Kumar BM and Deepu JK (1992) Litter production anddecomposition dynamics in moist deciduous forests of the

Western Ghats in Peninsular India. For Ecol Manage 50:181–201

LaRue TA and Patterson TG (1981) How much nitrogen dolegumes fix? Adv Agron 34: 15–38

Martiniello P and Ciola A (1995) Dry matter and seed yieldof Mediterranean annual legume species. Agron J 87:985–993

Lehmann J, Peter I, Steglich C, Gebauer G, Huwe B and ZechW (1998) Below ground interactions in agroforestry. ForEcol Manage 111: 157–169

Mathew T, Kumar BM, Babu KVS and Umamaheswaran K(1992) Comparative performance of some multipurposetrees and forage species in silvopastoral systems in thehumid regions of southern India. Agrofor Syst 17: 205–218

Matson PA, Vitousek PM, Ewel JJ and Mazzarino MJ (1987)Nitrogen transformations following tropical forest fellingand burning on a volcanic soil. Ecology 68: 491–502

Moser EB, Saxton AM and Pezeshki SR (1990) Repeatedmeasures analysis of variance: application to tree research.Can J For Res 20: 524–535

Naumburg E and DeWald LE (1999) Relationships betweenPinus ponderosa forest structure, light characteristics andunderstorey graminoid species presence and abundance.For Ecol Manage 124: 205–215

Payne WJA (1985) A review of the possibilities for integratingcattle and tree crop production in the tropics. For EcolManage 12: 1–36

Schacht WH, Long JN and Malechek JC (1988) Above groundproduction in cleared and thinned stands of semi-aridtropical woodland, Brazil. For Ecol Manage 23: 210–214

Skerman PJ and Riveros F (1990) Tropical Grasses: FAOPlant Production and Protection Paper No. 23 FAO, Rome,832 pp

Wang YP and Jarvis PG (1990) Influence of crown structuralproperties on PAR absorption, photosynthesis and tran-spiration in sitka spruce-application of a model. TreePhysiol 7: 297–316

Wilson JR (1996) Shade stimulated growth and nitrogenuptake by pasture grass in a subtropical environment? AustJ Agric Res 47: 1075–1093

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