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Intercropping with banana to improve fractional interception and radiation-use efficiency of immature rubber plantations
Field Crops Research 69 (2001) 237±249
Intercropping with banana to improve fractional interception and radiation-use ef®ciency of immature rubber plantations V.H.L. Rodrigoa, C.M. Stirlingb,*, Z. Teklehaimanotb, A. Nugawelaa a
b
Rubber Research Institute of Sri Lanka, Darton®eld, Agalawatta, Sri Lanka School of Agricultural and Forest Sciences, University of Wales, Bangor, Wales LL57 2UW, UK
Received 15 May 2000; received in revised form 14 November 2000; accepted 21 November 2000
Abstract Intercropping provides an important means of raising not only productivity and land-use ef®ciency of smallholder rubber lands, but also income generation during the unproductive immature phase of the rubber tree. To evaluate current recommendations for intercropping rubber in Sri Lanka, we assessed the effects of a range of planting densities of banana, the most common companion crop of rubber on productivity and resource capture. In this paper, we test the hypothesis that rubber/banana intercropping, even at high densities of banana, results in an increase in biomass per unit land area and per crop plant due to an increase in both radiation capture and radiation-use ef®ciency. Five treatments were imposed: sole crop rubber (R); sole crop banana (B) and three intercrop treatments comprising an additive series of one (BR), two (BBR) and three (BBBR) rows of banana to one row of rubber. Dry matter production in the rubber-based treatments was directly related to planting density, being least in the sole rubber and greatest in BBBR intercrop. A more than four-fold increase in dry matter across treatments derived from an increase not only in light capture (270%) but also radiation-use ef®ciency (RUE, 230%). Neither R nor BR treatment, which is currently recommended for intercropping in Sri Lanka, achieved full ground cover with fractional interception remaining below 40 and 50%, respectively. Fractional interception was greatest in BBBR treatment and by the end of the measurement period, total intercepted radiation was 23 and 73% greater than that in the BBR and BR intercrops, respectively. Shade did not limit either photosynthesis or growth of component crops in the intercrops, even when planting density of banana was increased three-fold. In fact, intercropping increased growth of both rubber and banana components suggesting that shade associated with the denser intercrop canopies, moderated the microclimate and alleviated plant stress. These results highlight the potential gains that can be made by intercropping and optimising planting density for improved resource capture in immature rubber plantations. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Agro-forestry; Plantation; Light use; Planting; Density
1. Introduction Recommendations for planting density of plantation crops such as rubber, are based on the resource *
Corresponding author. Tel.: 44-1248-382438; fax: 44-1248-354997. E-mail address: [email protected] (C.M. Stirling).
requirement of mature trees, with the result that during the establishment phase both land-use ef®ciency and resource capture are small. In terms of light capture, rubber plantations require 4±5 years to achieve a maximum light interception of ca. 80% and even after 30 months of growth only ca. 40% of incoming radiation is intercepted by the canopy (Ibrahim, 1991). Moreover, the time lag between planting and
0378-4290/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 2 9 0 ( 0 0 ) 0 0 1 4 7 - 7
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onset of tapping for latex poses major problems to smallholder farmers as no income is generated from the rubber during this period, which lasts anywhere between 5 and 7 years. One solution is to intercrop with food or cash crops to improve not only productivity and resource capture, but also income during the establishment phase of the plantation (Rodrgio, 1997; Rodrigo et al., 1997, 2001). The potential advantages of intercropping are well documented (Willey, 1979a,b; Vandermeer, 1989). In situations where intercropping has led to higher yields than sole cropping, advantages have often been attributed to the component crops complimenting each other in their use of resources (Willey, 1979a,b; Vandermeer, 1989; Tournebize and Sinoquet, 1995). Spatial complimentarity is associated with heterogeneity in the canopy and root system of intercrops resulting in improved resource capture, whilst temporal complimentarity arises when crops of differing duration are grown together, making demands for resources at different times of the growing season (Sivakumar, 1993). Willey and Reddy (1981) found that physical separation of the root systems of a millet/ groundnut intercrop resulted in a yield advantage similar in magnitude to the unsegregated control treatment, under both well-watered and stressed conditions. These authors concluded that interactions below ground were less important than those above ground in terms of their contribution to the improved performance of species when intercropped. However, analysis of the radiation-use ef®ciency (RUE) indicated that the greater yields in root-segregated and control intercrops, relative to the sole crops, was due to an increase in RUE only under well-watered conditions. This implies that RUE becomes less important in terms of intercrop advantages under conditions of limited soil moisture supplies (Willey and Reddy, 1981). Intercropping does not, in general, dramatically increase total light interception relative to sole crops unless the planting density of the latter is below the optimum (Monteith et al., 1991). However, the timecourse of growth of component crops in mixtures may differ, resulting in temporal advantages in light capture (Ong et al., 1991). One such example is that of plantation crops, where the wide plant spacing is designed to meet the growth requirements at the mature stage. Consequently, during the early stages of growth when canopy cover is small, intercropping
with a shorter duration crop would be expected to improve total light interception of the stand. Interplanting banana with young rubber is the most common form of intercrop found on smallholder rubber lands in Sri Lanka (Jayasena and Herath, 1986; Rodrigo et al., 2001). Whilst intercropping is designed to increase income generation and land-use ef®ciency during the unproductive immature phase of rubber (Rodrgio, 1997), recommendations for planting density of component crops are often well below that which the crop can tolerate because they are designed to impose minimal risks to latex yield. For example, current recommendations for planting density in rubber/banana intercrops are for a single row of banana planted between rubber trees, with the result that banana occupies only ca. 30% of its density in sole crops (Rodrigo et al., 1997). Prior to this work, there had been no systematic evaluation of the effects of increasing planting density of banana on component crop growth and resource use. In an earlier paper, Rodrigo et al. (1997) reported the effects of increasing planting density of banana from one to three rows on development, growth and yield of component banana and rubber crops. They showed that intercropping, even at high densities, resulted in an improved growth of both the component banana and rubber crops. In the present study, we aimed to test the hypothesis that improved biomass productivity of banana/rubber intercrops, particularly at high densities, can be explained in terms of effects on light use with intercropping and planting density increasing radiation capture and radiation use ef®ciency. 2. Materials and methods 2.1. Experimental site, design and planting material The experiment was established on a 5 ha situated in the low country wet zone of Sri Lanka (68300 ± 78000 N and 808000 ±808300 E). The experiment was rainfed and a full description of climatic conditions is provided by Rodrigo et al. (1997). The soil was acidic (pH 4.84) and belonged to the order Ultisol. The experiment comprised ®ve treatments: sole crop rubber (R); sole crop banana (B) and three intercropping treatments consisting of an additive series of one (BR),
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two (BBR) and three (BBBR) rows of banana to one row of rubber. Treatments were laid out in four randomised blocks and in plots of ca. 0.2 ha, with the exception of the sole banana plots which were restricted to ca. 0.09 ha due to the high planting density and limited number of available propagated plants. Rows were orientated in a north-south direction in order to maximise the degree of mutual shading experienced by component intercrops, as explained by Rodrigo et al. (1997). Crops were planted in the month of August. In all treatments, rubber was planted at a spacing of 2.4 m within, and 8.1 m between rows. A triangular planting pattern was used for banana in both the sole and intercrop treatments, with a plant spacing of 2.4 m 2:4 m in the sole crop. In the intercrop treatments, intra-row spacing was kept constant at 2.4 m whilst varying the inter-row spacing according to number of banana rows, ranging from 4.05 m in the BR, 2.7 m in the BBR to 2 m in the BBBR treatments. Planting density of banana was 500, 1000, 1500 and 1700 plants per ha in the BR, BBR, BBBR and B treatments. One-year-old nursery seedlings of rubber were bud grafted with the Sri Lankan clone RRIC 100. After 4 weeks, successfully grafted plants were transplanted in poly-bags to minimise mortality during ®eld establishment. The Kolikuttu cultivar of banana which belongs to the triploid genome group ``AAB'' and subgroup ``Silk'' (Stover and Simmonds, 1987) was used because of its popularity in Sri Lanka and homogeneity was achieved through propagation using tissue culture techniques. 2.2. Crop husbandry A basal dressing of organic manure (i.e. ca. 5 kg of poultry litter) was applied to each planting hole of banana before planting. Thereafter, starting from 2 months after planting, each plant was supplied with ca. 750 g fertiliser (urea 2:super phosphate 1:muriate of potash 3) at 4 month intervals. Rubber was fertilised with mixture of urea 26:rock phosphate 50:muriate of potash 24. At the start of the season, 50 g of the mixture together with 100 g of rock phosphate and 10 g of kieserite were applied to each planting hole of rubber. Fertiliser was applied in the ®rst and second year of rubber growth to supply 12 and 15 g of MgO per plant.
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2.3. Growth analysis Whole plant samples of both rubber and banana were harvested at ca. 4-month intervals between 8 and 28 months after planting. Full details of sampling protocol are given by Rodrigo et al. (1997). Brie¯y, two rubber and two banana plants were harvested at a time from adjacent rows in each plot of R and B treatments. In the intercropping treatments, two rubber and a single banana plant were harvested from their respective row positions. The tap root and lateral roots of rubber were removed at each harvest by loosening the earth around the plant and pulling the roots out. Banana roots and rhizome and roots of rubber were sampled by excavating soil from a hole with an area of 0.81 m2 and a depth of 0.9 m. In later harvests this depth was increased to 1.2 m. Above and below ground components were oven-dried at 808C to a constant weight and then removed for dry weight analysis. 2.4. Radiation capture To measure fractional light interception (f) across the wide row spacing used in the present study solarimeters, 2 m in length, were constructed locally in collaboration with the National Engineering Research Development Centre (NERD), Sri Lanka. Full details are given by Rodrgio, (1997). Seasonal f of treatments was calculated from measurements made using tube solarimeters mounted above the canopies and at ground level in all experimental blocks. Measurements commenced 16 weeks after planting (WAP). The solarimeters were connected to a data logger (21X Micrologger, Campbell Scienti®c Ltd., UK) sited at the weather station (Campbell Scienti®c Ltd., UK) using a multiplexer (Campbell Scienti®c Ltd., UK). Measurements were made every 5 min and hourly means recorded. A commercially made solarimeter (Delta-T Devices Ltd., Cambridge, UK) 1 m in length, was mounted above the crop at a height of 4.5 m in each experimental block to monitor incoming radiation. Each replicate contained two 2 m solarimeters placed at ground level to measure transmitted radiation. Interrow spacing of rubber was constant at 8.1 m in both the sole and intercrop treatments and to obtain representative measurements of transmitted radiation two
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2 m length solarimeters were placed lengthwise across a transect from the centre of the rubber row. In the case of the sole banana crop where inter-row spacing was 2.4 m, the two 2 m solarimeters spanned over one and a half rows. During the early stages of growth (i.e. up to 8 months after planting, MAP), a 1 m solarimeter was placed above the rubber trees in the intercrops (one per each replicate) in order to separate radiation intercepted by the taller banana canopy. However, the banana and rubber crops grew at similar rates and the banana canopy was not wide enough to extend over the rubber component and so overhead shading of young rubber plants by banana was found to be minimal. Fractional interception, f, was calculated from the daily average of incoming (I0) and transmitted radiation (I). Total radiation was monitored by the weather station (Campbell Scienti®c, UK) installed at the experimental site and daily total intercepted radiation was calculated as the product of total radiation and fractional interception measurements. In order to separate fractional interception and radiation use by component crops, transmitted radiation across a horizontal pro®le of each treatment was measured using a ceptometer (Delta-T Devices Ltd., UK) at ca. 2-month intervals starting from 35 WAP. Measurements were con®ned to the period 11.00± 13.00 h to avoid the problems associated with solar angle and rapid changes in light levels. Measurements were made at 0.5 m intervals across the inter-row spacing of rubber in the sole crop and intercrops and at 0.3 m intervals across two rows of the sole crop banana. At the start of each set of measurements, incoming radiation was measured twice and the mean recorded. Fractional interception of each point in the horizontal pro®le was calculated from the incoming (I0) and transmitted (I) radiation. Data of fractional interception across the horizontal pro®le mirrored the row positions of banana and rubber and so it was possible to demarcate areas predominantly covered by the canopy of each component crop. Based on this, the percentage interception of each component was estimated and component fractional interception (fcomp) was calculated from knowledge of whole intercrop fractional interception (fcrop) as measured by the solarimeters. Although ceptometer measurements were taken at mid-day, when direct radiation domi-
nated diffuse, this would have been the same for both crops and so would have had little effect on the relative percentage interception of crop components. Ceptometer measurements began 35 WAP and estimation of fractional and cumulative interception of component crops were calculated from this week onwards. Unlike solarimeters, ceptometer measurements were not continuous and, therefore, in order to calculate seasonal radiation interception for component crops, it was assumed that the ratio of fcomp to fcrop changed linearly between two time points. As was the case for the whole crop, component cumulative intercepted radiation was estimated as the product of total radiation measured by the weather station and component fractional interception measured by the ceptometer and solarimeters. Vertical pro®les of transmitted radiation were made between 50 and 61 WAP using the ceptometer. Measurements were taken at 0.2 m intervals along the stem of the plant starting at the top of the canopy. At each point, ®ve measurements were taken around the circumference of the plant and the average recorded. In the case of banana, measurements included three plants for each row position of all treatments. Measurements of rubber included six plants per treatment measured over 2 days. All vertical pro®le measurements were con®ned to a single block due to time limitations and to the period of 11.30±12.30 h in order to avoid problems associated with changes in incoming radiation due to solar angle. 2.5. Conversion of radiation to dry matter The ef®ciency of conversion of intercepted radiation to dry matter or RUE was calculated at the whole treatment and component crop level using regression analysis of total dry matter production against cumulative intercepted radiation. RUE of both banana and rubber was estimated for two periods corresponding to the mother and ratoon crop periods of banana. For the mother crop period, RUE was estimated from destructive harvests at 8, 12 and 16 MAP and for the ratoon crop on harvests at 20, 24 and 28 MAP. Fusarial wilt disease resulted in severe foliage loss in the sole crop banana at 28 MAP (Rodrigo et al., 1997) and so RUE in this treatment was based on the ®rst two harvests of the ratoon crop (i.e. at 20 and 24 MAP).
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2.6. Leaf gas exchange Preliminary studies were required to develop an appropriate methodology for measuring photosynthesis of the large banana and rubber canopies. Gas exchange measurements were performed using a portable closed system infra-red gas analyser, IRGA (LI6200, Li-Cor, Lincoln, USA). Leaves of rubber are produced in clusters, referred to as whorls, along the stem. Leaves in the whorl were divided into three size categories and variation in photosynthetic capacity between categories was assessed. Gas exchange measurements were restricted to one leaf per category for the most recently expanded whorl and for eight replicate plants, with measurements taken three times per day (10.00, 12.00 and 14.00 h). No signi®cant difference was observed for either photosynthetic rate or stomatal conductance between the different size categories within a whorl and so, two leaves were randomly selected from the east and west directions within each whorl for canopy gas exchange studies. By 103 WAP, the whorl system on the main trunk had been replaced by side branches forming a well-developed canopy. Consequently, measurements thereafter (i.e. 103 and 105 WAP) were restricted to three levels corresponding to the top, middle and bottom, of the rubber canopy. Canopy leaf photosynthesis of rubber was recorded at intervals starting from 35 WAP. Two replicate plants per treatment in the sole crop and extreme density BR and BBBR intercrops, were measured at three times during the day (09.00±10.00, 11.30±12.30, 14.30±15.30 h). Given the small size of the cuvette (1000 cm3) relative to the banana leaf, it was necessary to characterise variation in photosynthetic rates along the leaf in order to develop a protocol for measurement of whole leaf photosynthesis. Measurements were made on the most recently expanded leaf of 11 replicate plants. Each leaf was divided along the midrib and for each half, gas exchange was measured in eight positions, four along the length of the leaf and two across the lamina. Results indicated that CO2 assimilation rates were similar across the width of the leaf lamina, but signi®cantly decreased from the petiole to the tip (P 0:01; Rodrgio, 1997). Since it was not feasible to measure photosynthesis along the whole length of the leaf each time gas exchange measurements were
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undertaken, measurements were con®ned to the central position of the leaf where rates approximated to the average for the whole leaf. Changes in water status of the banana leaf caused changes in the angle between two leaf laminae halves and so gas exchange measurements were made on both sides of the leaf to avoid any discrepancy in incident light. Preliminary measurements showed that photosynthetic rates varied signi®cantly (P 0:001) with position in the canopy (Rodrgio, 1997) and so canopy studies were based on measurements of each green leaf of the banana plant. As with rubber, diurnal canopy photosynthesis measurements of banana were made three times per day (09.00±10.00, 11.30±12.30, 14.30±15.30 h), and were restricted to the sole crop, BR and BBBR treatments. Also, because of time limitations, measurements were restricted to one plant per replicate. Considering the row positions of banana, two plants were selected in the BBBR treatment in order to provide a representative sample of the crop, one from the edge row on the eastern side and the central row, for each set of measurements. 2.7. Data analysis The statistical package `SAS' (ASA Institute Inc., Cary, NC, USA) was used for the data analysis. The number of measurements made on each treatment varied due to different number of leaves/whorls and so the unbalance model ``Proc. GLM'' procedure was used for statistical analysis. To evaluate all possible pairs, mean separation of treatments was performed with Duncan's multiple range test (DMRT). 3. Results 3.1. Dry matter productivity Productivity of rubber cropping systems increased signi®cantly with planting density and by 28 MAP total dry matter (TDM) was over 200% greater in the intercrops than sole crop (Fig. 1). The poor performance of B resulted from damage by the Fusarial wilt virus. Analysis at the individual plant level indicated a signi®cant improvement in growth of banana in the double and triple row intercrops,
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Fig. 1. Relationship between total dry matter production after 28 months after planting and planting density for the sole crop rubber (&; R), sole crop banana (*; B), single (~; BR), double (5; BBR) and triple (^; BBBR) row banana/rubber intercrops. Data represent the mean of four replicate experimental blocks.
with leaf area and TDM per plant at least 293 and 188% greater, respectively, than in the sole banana crop (Table 1a). Similarly, there was no evidence that high density intercropping had a detrimental effect on growth of rubber, in fact the opposite, with TDM and leaf area per plant signi®cantly greater in Table 1 Summary of treatment effects on dry matter production (TDM) and leaf area per plant of (a) component banana and (b) rubber grown either as sole crops or intercropsa TDM (g per plant) (a) Component banana Sole crop Single row intercrop Double row intercrop Triple row intercrop (b) Component rubber Sole crop Single row intercrop Double row intercrop Triple row intercrop
Leaf area (m2 per plant)
2964a 3128a 5571b 6483b
3.10a 4.27a 9.08b 9.68b
7199a 12319b 13320b 14279b
7.9a 12.1ab 14.3b 13.5ab
a Values refer to the mean of four replicate experimental blocks measured at ®nal harvest at 28 months after planting. Means with a different letter within a column are signi®cantly different at the P 0:05 level.
the intercrop treatments relative to the sole crop (Table 1b). 3.2. Fractional and cumulative interception The sole rubber crop was least ef®cient in capturing light, with fcrop averaging only 20% over the measurement period (Fig. 2a). Fractional interception, fcrop, increased by more than 25% in all intercrops over the measurement period, compared with 23% in the sole rubber crop. Fractional interception of the sole banana crop was comparable with the BBBR treatment until ca. 60 WAP but thereafter declined; from 115 WAP onwards, fcrop, decreased further reaching values similar to those of sole rubber crop and only ca. 60% of the maximum achieved by the mother crop. Fractional interception was greatest in BBBR and by the end of the measurement period, total intercepted radiation was 6374 MJ mÿ2, 23 and 73% greater than that in the BBR and BR intercrops, respectively (Fig. 2b). Analysis of light attenuation within individual trees indicated that the rubber canopy was more effective than banana in capturing light (Fig. 3). Light was attenuated more rapidly in the rubber than banana canopy, with maximum attenuation achieved
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Fig. 2. Summary of (a) seasonal changes in weekly fractional interception and (b) cumulative intercepted radiation of sole and intercrops. Treatment codes refer to the sole rubber (R), sole banana (B), single (BR), double (BBR) and triple (BBBR) row banana/rubber intercrops. Data represent the mean of four replicate experimental blocks.
within the ®rst 1 m depth in both canopies (Fig. 3). Whilst there was no treatment effect on the vertical distribution of light within the rubber canopy, in the case of banana, light was attenuated more rapidly by banana in the intercrop relative to sole crop (Fig. 3).
Fractional interception (fcomp) and cumulative interception of component crops in the intercrop and sole crop are summarised in Fig. 4. As expected, fcomp, and, hence, cumulative interception of banana increased with planting density of banana, been greatest in the sole crop and least in the single row banana
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well below those of the sole crop (Fig. 4c and d). The overall increase in fcomp of rubber during the measurement period was 15, 11 and 14% in the BR, BBR and BBBR intercrops, respectively, compared with 27% in the sole rubber crop. At the end of the experiment, cumulative interception of component rubber in the BBR and BBBR crops were comparable and 90 and 75% of the values for the BR and sole crop, respectively. 3.3. Radiation-use ef®ciency Radiation-use ef®ciency (RUE) tended to be greater for the intercrops relative to sole crop rubber, although the effect was only signi®cant during the mother crop period of banana when the value for the BBBR intercrop was ca. 100% greater than that of the sole crop rubber (Fig. 5a). The component banana crop showed a greater RUE for the mother than ratoon crop (Fig. 5b). Double and triple row banana intercropping raised the RUE of component banana relative to the single row system, particularly for the ratoon crop when values were 375 and 450% greater, respectively, than in the BR intercrop. In contrast to banana, RUE of the component rubber crop was higher during the ratoon than mother crop period and tended to increase with planting density of banana in the intercrop (Fig. 5c). During the ratoon crop period, component RUE of rubber in all three intercrops showed a dramatic increase of more than 200% over the sole crop (Fig. 5c). 3.4. Photosynthetic performance of intercropped banana and rubber Fig. 3. Changes in fractional interception with canopy depth in (a) component banana and (b) component rubber crops measured between 50 and 61 weeks after planting (WAP). Treatment codes refer to the sole rubber (R), sole banana (B), single (BR), double (BBR) and triple (BBBR) row banana/rubber intercrops. Error bars represent the S.E. of means where n 3 in the B and BR, n 6 in the R and BBR and n 9 in the BBBR.
intercrop. In the case of rubber, fcomp and cumulative interception was similar in all treatments during the mother crop period of banana (i.e. 8 to 16 months after planting, MAP). However, during the ratoon crop period of banana (i.e. 20 to 28 MAP), fcomp and cumulative interception of intercropped rubber were
There was no signi®cant effect of row position on CO2 assimilation rates (A) of banana in BBBR treatment and so measurements for the central and outer rows were combined to give a mean value for ease of comparison with the single row measurements of the sole and BR intercrop (Fig. 6). Despite increased mutual shading in the high density intercrops, no consistent treatment effects on A of either banana or rubber was observed. In general, A was similar for both crops, but varied signi®cantly over time (P 0:001). Both the rubber and banana crops showed a dramatic decrease in A between 566 and 584 days after planting, due to a prolonged period without rainfall (Fig. 6).
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Fig. 4. Analysis of seasonal changes in fractional interception (a, c) and cumulative radiation interception (b, d) of component banana (a, b) and component rubber (c, d) crops in the sole and intercrop treatments.
4. Discussion Intercropping offers much scope for improving resource capture and productivity of immature plantation tree crops such as rubber, by utilising the wide row spacing needed to meet the growth requirements of the mature tree. With the exception of B, dry matter productivity was directly related to planting density, been least in the sole rubber and greatest in the BBBR crop planted at densities of 500 and 2000 plants per ha,
respectively (Fig. 1). Productivity of B was much lower than expected, due to damage caused during the later stages of the experiment by the Fusarial wilt virus (Rodrigo et al., 1997). The more than four-fold increase in dry matter production in the BBBR compared with R crop derived from an increase not only in light capture (270%) but also RUE (230%). The extent to which resources may be under utilised during the establishment phase of rubber was highlighted by the poor ef®ciency of light capture (Fig. 2).
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Fig. 5. Summary of treatment effects on radiation-use ef®ciency of (a) whole cropping systems, (b) component banana and (c) component rubber crops. Treatments codes refer to the sole rubber (R), sole banana (B), single (BR), double (BBR) and triple (BBBR) row banana/rubber intercrops. Radiation-use ef®ciency was calculated for two periods corresponding to the mother (8 to 16 months after planting) and ratoon (20 to 28 months after planting) crop periods of banana. Error bars represent the S.E. of means of four replicate experimental blocks in whole cropping systems and that of the slope of the linear regression analysis of mean cumulative dry matter and intercepted radiation in the case of component crops.
Sole crop rubber did not achieve full canopy cover and by the end of the experimental period, fcrop was still below 40%, comparable with that found for a 2.5-yearold rubber crop (Ibrahim, 1991). Similarly the BR intercrop, which is currently recommended for inter-
cropping in Sri Lanka, did not achieve full ground cover with fcrop remaining below ca. 50% throughout the experimental period. As expected, fcrop increased with planting density of banana such that values for the BBBR mother crop were similar to that of the B
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Fig. 6. Seasonal variation in photosynthetic rate of the (a) mother and (b) ratoon crops of (c) banana and rubber. Treatment codes refer to the sole crop banana (B), sole crop rubber (R), and the single (BR), double (BBR) and triple (BBBR) row banana/rubber intercrops. Error bars represent the S.E. of means of all measurements of all leaves in the canopy taken on one particular day.
treatment; towards the end of the experiment fcrop of B declined sharply due to the aforementioned damage by Fusarial wilt (Fig. 4). Leaf area index and, hence, fcrop of banana varies widely with variety, season and growth conditions (Turner, 1972; Lahav and Kalmar, 1981) and previous studies have reported values of fcrop as high as 90% at a LAI of 4 (Turner, 1982). These values are much higher than those reported here, where a maximum LAI of only 2.2 was achieved in
the BBBR; differences re¯ect the use in previous studies of much higher chemical inputs and improved varieties from the Cavendish group (Stover, 1984). Maximum light interception was achieved at a depth of 1 m in the canopy of both rubber and banana. Treatments had little effect on the extinction of radiation in rubber canopies (Fig. 3), in contrast with banana where the extinction of radiation was most rapid in the intercrops, particularly the BBR and
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BBBR treatments (Fig. 3). This may have conferred some advantage in terms of RUE of intercropped banana, as less light would have been wasted in the uppermost canopy layers where the capacity for leaf photosynthesis is greatest (Rodrgio, 1997; Eckstein and Robinson, 1995). Values of RUE were low compared with other crops (Colomb et al., 1995; Morrison and Stewart, 1995;Yunusa et al., 1995), although most studies have been on annual crops whose conversion ef®ciency is generally greater than that of perennials (Jayasekara and Jayasekara, 1995). Where studies have focused on tree crops, most estimates of RUE were based on short-term studies of less than 12 months using seedlings and saplings (Cannell et al., 1998). In the case of more mature trees, calculations of RUE have generally considered above ground biomass only (Grace et al., 1987; Squire and Corely, 1987; Dalla-Tea and Jokela, 1991), unlike the present study in which both above and below ground biomass were included. During the mother crop period, RUE of the intercrops, and in particular BBR and BBBR treatments, was signi®cantly (P 0:05) greater than R (Fig. 5), most likely re¯ecting the advantage of increased LAI and canopy heterogeneity on light capture (Tournebize and Sinoquet, 1995; Yunusa et al., 1995a) and use (Deka and Pal, 1995; Yunusa et al., 1995a,b). A similar increase in RUE of intercrop treatments relative to R was observed during the ratoon crop period, but differences were not signi®cant (Fig. 5). In general, RUE of component banana and rubber crops tended to be greater in the high density intercrops; the most notable response been the ca. 200% increase in RUE of intercropped rubber compared with R during the ratoon crop period (Fig. 5). The shape of the light response curve of photosynthesis of these two C3 crops would explain, in part, the improved RUE when intercropped. Leaf photosynthesis of both banana (Rodrgio, 1997) and rubber (Nugawela, 1989) saturates well below full sunlight, especially for ``shade'' leaves within the canopy with the result that shading would have had a minimal effect on leaf net photosynthesis in the intercrops (Fig. 5). Indeed, photosynthetic ef®ciency of these ``shade'' leaves would have been improved by increasing the proportion of leaves operating in the lightlimited region of the photosynthetic light response curve. With fewer leaves light-saturated, less light was
wasted in the intercrop canopies and so RUE was increased (Harris et al., 1987; Stirling et al., 1990; Monteith et al., 1991). Intercropping, particularly at high densities, was also found to increase partitioning of biomass to above ground structures in both the banana and rubber components (Rodrigo et al., 1997). This is a typical shade response which would contribute to improved radiation capture through an increase in leaf area (Table 1) and to improved RUE, through an increase in net assimilation rate. It is widely assumed in agro-forestry research that shading has a detrimental effect on growth of understorey crops, despite the fact that for a signi®cant portion of the growing season in the tropics radiation loads are high and stress inducing. Provided it is not excessive, shading can have major bene®ts for crop growth by moderating the canopy microclimate and alleviating plant stress (Monteith et al., 1991). This appeared to be the case in the present study where intercropping, with its associated increase in canopy density and mutual shading, not only improved light capture and biomass production per unit land area but also improved growth of both banana and rubber at the individual plant level, even when planting density of banana was increased three-fold. Acknowledgements We wish to thank the Rubber Research Institute of Sri Lanka for providing land for the experiment and staff at the Institute for valuable assistance in the ®eld. This paper is an output from a project (Plant Sciences Research Programme R7212) funded by the UK department for International Development (DFID) and administered by the Centre for Arid Zones Studies (CAZS) for the bene®t of developing countries. The views expressed are not necessarily those of DFID. References Cannell, M.G.R., Mobbs, D.C., Lawson, G.J., 1998. Complementarity of light and water use in tropical agroforests II. Modelled theoretical tree production and potential crop yield in arid to humid climates. For. Ecol. Manage. 102, 275±282. Colomb, B., Bouniols, A., Delpech, C., 1995. Effect of various phosphorus availabilities on radiation-use ef®ciency in sun-
V.H.L. Rodrigo et al. / Field Crops Research 69 (2001) 237±249 ¯ower biomass until anthesis. J. Plantation Nutr. 18, 1649± 1658. Dalla-Tea, J., Jokela, E.J., 1991. Needlefall, canopy light interception and productivity of young intensively managed slash and loblolly-pine stands. For. Sci. 37, 1298±1313. Deka, J., Pal, M., 1995. Bioenergetics of different sole and pigeonpea (Cajanus cajun) based intercropping systems. I. J. Agri. Sci. 65, 639±643. Eckstein, K., Robinson, J.C., 1995. Physiological responses of banana (Musa AAA Cavendish sub-group) in the subtropics. I. In¯uence of internal plant factors on gas exchange of banana leaves. J. Hortic. Sci. 70, 147±156. Grace, J.C., Jarvis, P.G., Norman, J.M., 1987. Modelling the interception of solar radiation energy in intensively managed stands. New Zealand J. Forestry Sci. 17, 193±209. Harris, D., Natrajan, M., Willey, R.W., 1987. Physiological basis for yield advantage in a sorghum/groundnut intercrop exposed to drought. 1. Dry matter production, yield and light interception, yield and light interception. Field Crops Res. 17, 259±272. Ibrahim, A.G., 1991. In¯uence of rubber canopy on intercrop productivity. Trans. Malaysian Soc. Plant Physiol. 2, 75±79. Jayasekara, C., Jayasekara, K.S., 1995. Photosynthetic characteristics of tropical tree species with special reference to palms. Energy Conversion Manage. 36, 919±922. Jayasena, W.G., Herath, H.M.G., 1986. Innovation, receptivity and adoption in rubber smallholding of Sri Lanka. SRRP Research Study No.71, ARTI, Sri Lanka, pp. 79±88. Lahav, E., Kalmar, D., 1981. Studies on the phenology and production potential of Williams banana in a subtropical climate. Subtropica 2, 12±16. Monteith, J.L., Ong, C.K., Corlett, J.E., 1991. Microclimatic interactions in agroforestry systems. For. Ecol. Manage. 45, 31± 44. Morrison, M.J., Stewart, D.W., 1995. Radiation-use in summer rape. Agron. J. 87, 1139±1142. Nugawela, A., 1989. Gas exchange characteristics of Hevea genotypes and their use in selection for crop yield. Ph.D. Thesis, University of Essex, England, UK. Ong, C.K., Corlett, J.E., Singh, R.P., Black, C.R., 1991. Above and below ground interactions in agroforestry systems. For. Ecol. Manage. 45, 45±57. Rodrgio, V.H.L., 1997. Effects of planting density of growth and resource capture of banana/rubber intercrops. Ph.D. Thesis, University of Wales, UK. Rodrigo, V.H.L., Stirling, C.M., Teklehaimanot, Z., Nugawela, A., 1997. The effect of planting density on growth and development of component crops in rubber/banana intercropping systems. Field Crops Res. 52, 95±108. Rodrigo, V.H.L., Stirling, C.M., Naranpanawa, R.M.A.K.B.,
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Herath, P.H.M.U., 2001. Intercropping of immature rubber; present status in Sri Lanka and ®nancial analysis of rubber intercrops planted at three different densities of banana. Agrofor. Syst., in press. Sivakumar, M.V.K., 1993. Growth and yield of millet and cowpea in relay and intercrop systems in the Sahelian zone in years when the onset of the rainy season is early. Exp. Agric. 29, 417±427. Squire, G.R., Corely, R.H.V., 1987. Oil palm. In: Sethuraj, M.R., Raghavendra, A.S. (Eds.), Tree Crop Physiol. 141±167. Stirling, C.M., Williams, J.H., Black, C.R., Ong, C.K., 1990. The effect of timing of shade on development, dry matter production and light-use ef®ciency in groundnut (Arachis hypogaea) under ®eld conditions. Aust. J. Agric. Res. 41, 633± 644. Stover, R.H., 1984. Canopy management in Valery and Grand Nain using leaf area index and photosynthetically active radiation measurements. Fruits 39, 89±93. Stover, R.H., Simmonds, N.W., 1987. Bananas. Tropical Agriculture Series, 3rd Edition. Longman, London. Tournebize, R., Sinoquet, H., 1995. Light interception and partitioning in a shrub/grass mixtures. Agric. Forest Meteor. 72, 277±294. Turner, D.W., 1972. Banana plant growth 2. Dry matter production, leaf area and growth analysis. Aust. J. Exp. Agric. Anim. Husbandry 12, 216±224. Turner, D.W., 1982. A review of plant physiology in relation to cultural practices in the Australian banana industry. In: Proceedings of the Australian Banana Industry Development Workshop, Lismore, New South Wales, pp. 34±58. Vandermeer, J., 1989. The Ecology of Intercropping. Cambridge University Press, Cambridge, UK. Willey, R.W., 1979a. Intercropping Ð its importance and research needs. I. Competition and yield advantages. Field Crop Abstr. 32, 1±10. Willey, R.W., 1979b. Intercropping Ð its importance and research needs: agronomy and research approaches. Field Crop Abstr. 32, 78±85. Willey, R.W., Reddy, M.S., 1981. A ®eld technique for separating above-ground and below-ground interactions in intercropping Ð an experiment with pearl millet-groundnut. Exp. Agri. 17, 257±264. Yunusa, I.A.M., Mead, D.J., Pollock, K.M., Lucas, R.J., 1995a. Process studies in a Pinus radiata-pasture agroforestry system in a subhumid temperature environment. I. Water use and light interception in the third year. Agrofor. Syst. 32, 63±183. Yunusa, I.A.M., Mead, D.J., Lucas, R.J., Pollock, K.M., 1995b. Process studies in a Pinus radiata-pasture agroforestry system in a subhumid temperature environment. II. Analysis of dry matter yields in the third year. Agrofor. Syst. 32, 185±204.
Intercropping with banana to improve fractional interception and radiation-use ef®ciency of immature rubber plantations V.H.L. Rodrigoa, C.M. Stirlingb,*, Z. Teklehaimanotb, A. Nugawelaa a
b
Rubber Research Institute of Sri Lanka, Darton®eld, Agalawatta, Sri Lanka School of Agricultural and Forest Sciences, University of Wales, Bangor, Wales LL57 2UW, UK
Received 15 May 2000; received in revised form 14 November 2000; accepted 21 November 2000
Abstract Intercropping provides an important means of raising not only productivity and land-use ef®ciency of smallholder rubber lands, but also income generation during the unproductive immature phase of the rubber tree. To evaluate current recommendations for intercropping rubber in Sri Lanka, we assessed the effects of a range of planting densities of banana, the most common companion crop of rubber on productivity and resource capture. In this paper, we test the hypothesis that rubber/banana intercropping, even at high densities of banana, results in an increase in biomass per unit land area and per crop plant due to an increase in both radiation capture and radiation-use ef®ciency. Five treatments were imposed: sole crop rubber (R); sole crop banana (B) and three intercrop treatments comprising an additive series of one (BR), two (BBR) and three (BBBR) rows of banana to one row of rubber. Dry matter production in the rubber-based treatments was directly related to planting density, being least in the sole rubber and greatest in BBBR intercrop. A more than four-fold increase in dry matter across treatments derived from an increase not only in light capture (270%) but also radiation-use ef®ciency (RUE, 230%). Neither R nor BR treatment, which is currently recommended for intercropping in Sri Lanka, achieved full ground cover with fractional interception remaining below 40 and 50%, respectively. Fractional interception was greatest in BBBR treatment and by the end of the measurement period, total intercepted radiation was 23 and 73% greater than that in the BBR and BR intercrops, respectively. Shade did not limit either photosynthesis or growth of component crops in the intercrops, even when planting density of banana was increased three-fold. In fact, intercropping increased growth of both rubber and banana components suggesting that shade associated with the denser intercrop canopies, moderated the microclimate and alleviated plant stress. These results highlight the potential gains that can be made by intercropping and optimising planting density for improved resource capture in immature rubber plantations. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Agro-forestry; Plantation; Light use; Planting; Density
1. Introduction Recommendations for planting density of plantation crops such as rubber, are based on the resource *
Corresponding author. Tel.: 44-1248-382438; fax: 44-1248-354997. E-mail address: [email protected] (C.M. Stirling).
requirement of mature trees, with the result that during the establishment phase both land-use ef®ciency and resource capture are small. In terms of light capture, rubber plantations require 4±5 years to achieve a maximum light interception of ca. 80% and even after 30 months of growth only ca. 40% of incoming radiation is intercepted by the canopy (Ibrahim, 1991). Moreover, the time lag between planting and
0378-4290/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 2 9 0 ( 0 0 ) 0 0 1 4 7 - 7
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onset of tapping for latex poses major problems to smallholder farmers as no income is generated from the rubber during this period, which lasts anywhere between 5 and 7 years. One solution is to intercrop with food or cash crops to improve not only productivity and resource capture, but also income during the establishment phase of the plantation (Rodrgio, 1997; Rodrigo et al., 1997, 2001). The potential advantages of intercropping are well documented (Willey, 1979a,b; Vandermeer, 1989). In situations where intercropping has led to higher yields than sole cropping, advantages have often been attributed to the component crops complimenting each other in their use of resources (Willey, 1979a,b; Vandermeer, 1989; Tournebize and Sinoquet, 1995). Spatial complimentarity is associated with heterogeneity in the canopy and root system of intercrops resulting in improved resource capture, whilst temporal complimentarity arises when crops of differing duration are grown together, making demands for resources at different times of the growing season (Sivakumar, 1993). Willey and Reddy (1981) found that physical separation of the root systems of a millet/ groundnut intercrop resulted in a yield advantage similar in magnitude to the unsegregated control treatment, under both well-watered and stressed conditions. These authors concluded that interactions below ground were less important than those above ground in terms of their contribution to the improved performance of species when intercropped. However, analysis of the radiation-use ef®ciency (RUE) indicated that the greater yields in root-segregated and control intercrops, relative to the sole crops, was due to an increase in RUE only under well-watered conditions. This implies that RUE becomes less important in terms of intercrop advantages under conditions of limited soil moisture supplies (Willey and Reddy, 1981). Intercropping does not, in general, dramatically increase total light interception relative to sole crops unless the planting density of the latter is below the optimum (Monteith et al., 1991). However, the timecourse of growth of component crops in mixtures may differ, resulting in temporal advantages in light capture (Ong et al., 1991). One such example is that of plantation crops, where the wide plant spacing is designed to meet the growth requirements at the mature stage. Consequently, during the early stages of growth when canopy cover is small, intercropping
with a shorter duration crop would be expected to improve total light interception of the stand. Interplanting banana with young rubber is the most common form of intercrop found on smallholder rubber lands in Sri Lanka (Jayasena and Herath, 1986; Rodrigo et al., 2001). Whilst intercropping is designed to increase income generation and land-use ef®ciency during the unproductive immature phase of rubber (Rodrgio, 1997), recommendations for planting density of component crops are often well below that which the crop can tolerate because they are designed to impose minimal risks to latex yield. For example, current recommendations for planting density in rubber/banana intercrops are for a single row of banana planted between rubber trees, with the result that banana occupies only ca. 30% of its density in sole crops (Rodrigo et al., 1997). Prior to this work, there had been no systematic evaluation of the effects of increasing planting density of banana on component crop growth and resource use. In an earlier paper, Rodrigo et al. (1997) reported the effects of increasing planting density of banana from one to three rows on development, growth and yield of component banana and rubber crops. They showed that intercropping, even at high densities, resulted in an improved growth of both the component banana and rubber crops. In the present study, we aimed to test the hypothesis that improved biomass productivity of banana/rubber intercrops, particularly at high densities, can be explained in terms of effects on light use with intercropping and planting density increasing radiation capture and radiation use ef®ciency. 2. Materials and methods 2.1. Experimental site, design and planting material The experiment was established on a 5 ha situated in the low country wet zone of Sri Lanka (68300 ± 78000 N and 808000 ±808300 E). The experiment was rainfed and a full description of climatic conditions is provided by Rodrigo et al. (1997). The soil was acidic (pH 4.84) and belonged to the order Ultisol. The experiment comprised ®ve treatments: sole crop rubber (R); sole crop banana (B) and three intercropping treatments consisting of an additive series of one (BR),
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two (BBR) and three (BBBR) rows of banana to one row of rubber. Treatments were laid out in four randomised blocks and in plots of ca. 0.2 ha, with the exception of the sole banana plots which were restricted to ca. 0.09 ha due to the high planting density and limited number of available propagated plants. Rows were orientated in a north-south direction in order to maximise the degree of mutual shading experienced by component intercrops, as explained by Rodrigo et al. (1997). Crops were planted in the month of August. In all treatments, rubber was planted at a spacing of 2.4 m within, and 8.1 m between rows. A triangular planting pattern was used for banana in both the sole and intercrop treatments, with a plant spacing of 2.4 m 2:4 m in the sole crop. In the intercrop treatments, intra-row spacing was kept constant at 2.4 m whilst varying the inter-row spacing according to number of banana rows, ranging from 4.05 m in the BR, 2.7 m in the BBR to 2 m in the BBBR treatments. Planting density of banana was 500, 1000, 1500 and 1700 plants per ha in the BR, BBR, BBBR and B treatments. One-year-old nursery seedlings of rubber were bud grafted with the Sri Lankan clone RRIC 100. After 4 weeks, successfully grafted plants were transplanted in poly-bags to minimise mortality during ®eld establishment. The Kolikuttu cultivar of banana which belongs to the triploid genome group ``AAB'' and subgroup ``Silk'' (Stover and Simmonds, 1987) was used because of its popularity in Sri Lanka and homogeneity was achieved through propagation using tissue culture techniques. 2.2. Crop husbandry A basal dressing of organic manure (i.e. ca. 5 kg of poultry litter) was applied to each planting hole of banana before planting. Thereafter, starting from 2 months after planting, each plant was supplied with ca. 750 g fertiliser (urea 2:super phosphate 1:muriate of potash 3) at 4 month intervals. Rubber was fertilised with mixture of urea 26:rock phosphate 50:muriate of potash 24. At the start of the season, 50 g of the mixture together with 100 g of rock phosphate and 10 g of kieserite were applied to each planting hole of rubber. Fertiliser was applied in the ®rst and second year of rubber growth to supply 12 and 15 g of MgO per plant.
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2.3. Growth analysis Whole plant samples of both rubber and banana were harvested at ca. 4-month intervals between 8 and 28 months after planting. Full details of sampling protocol are given by Rodrigo et al. (1997). Brie¯y, two rubber and two banana plants were harvested at a time from adjacent rows in each plot of R and B treatments. In the intercropping treatments, two rubber and a single banana plant were harvested from their respective row positions. The tap root and lateral roots of rubber were removed at each harvest by loosening the earth around the plant and pulling the roots out. Banana roots and rhizome and roots of rubber were sampled by excavating soil from a hole with an area of 0.81 m2 and a depth of 0.9 m. In later harvests this depth was increased to 1.2 m. Above and below ground components were oven-dried at 808C to a constant weight and then removed for dry weight analysis. 2.4. Radiation capture To measure fractional light interception (f) across the wide row spacing used in the present study solarimeters, 2 m in length, were constructed locally in collaboration with the National Engineering Research Development Centre (NERD), Sri Lanka. Full details are given by Rodrgio, (1997). Seasonal f of treatments was calculated from measurements made using tube solarimeters mounted above the canopies and at ground level in all experimental blocks. Measurements commenced 16 weeks after planting (WAP). The solarimeters were connected to a data logger (21X Micrologger, Campbell Scienti®c Ltd., UK) sited at the weather station (Campbell Scienti®c Ltd., UK) using a multiplexer (Campbell Scienti®c Ltd., UK). Measurements were made every 5 min and hourly means recorded. A commercially made solarimeter (Delta-T Devices Ltd., Cambridge, UK) 1 m in length, was mounted above the crop at a height of 4.5 m in each experimental block to monitor incoming radiation. Each replicate contained two 2 m solarimeters placed at ground level to measure transmitted radiation. Interrow spacing of rubber was constant at 8.1 m in both the sole and intercrop treatments and to obtain representative measurements of transmitted radiation two
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2 m length solarimeters were placed lengthwise across a transect from the centre of the rubber row. In the case of the sole banana crop where inter-row spacing was 2.4 m, the two 2 m solarimeters spanned over one and a half rows. During the early stages of growth (i.e. up to 8 months after planting, MAP), a 1 m solarimeter was placed above the rubber trees in the intercrops (one per each replicate) in order to separate radiation intercepted by the taller banana canopy. However, the banana and rubber crops grew at similar rates and the banana canopy was not wide enough to extend over the rubber component and so overhead shading of young rubber plants by banana was found to be minimal. Fractional interception, f, was calculated from the daily average of incoming (I0) and transmitted radiation (I). Total radiation was monitored by the weather station (Campbell Scienti®c, UK) installed at the experimental site and daily total intercepted radiation was calculated as the product of total radiation and fractional interception measurements. In order to separate fractional interception and radiation use by component crops, transmitted radiation across a horizontal pro®le of each treatment was measured using a ceptometer (Delta-T Devices Ltd., UK) at ca. 2-month intervals starting from 35 WAP. Measurements were con®ned to the period 11.00± 13.00 h to avoid the problems associated with solar angle and rapid changes in light levels. Measurements were made at 0.5 m intervals across the inter-row spacing of rubber in the sole crop and intercrops and at 0.3 m intervals across two rows of the sole crop banana. At the start of each set of measurements, incoming radiation was measured twice and the mean recorded. Fractional interception of each point in the horizontal pro®le was calculated from the incoming (I0) and transmitted (I) radiation. Data of fractional interception across the horizontal pro®le mirrored the row positions of banana and rubber and so it was possible to demarcate areas predominantly covered by the canopy of each component crop. Based on this, the percentage interception of each component was estimated and component fractional interception (fcomp) was calculated from knowledge of whole intercrop fractional interception (fcrop) as measured by the solarimeters. Although ceptometer measurements were taken at mid-day, when direct radiation domi-
nated diffuse, this would have been the same for both crops and so would have had little effect on the relative percentage interception of crop components. Ceptometer measurements began 35 WAP and estimation of fractional and cumulative interception of component crops were calculated from this week onwards. Unlike solarimeters, ceptometer measurements were not continuous and, therefore, in order to calculate seasonal radiation interception for component crops, it was assumed that the ratio of fcomp to fcrop changed linearly between two time points. As was the case for the whole crop, component cumulative intercepted radiation was estimated as the product of total radiation measured by the weather station and component fractional interception measured by the ceptometer and solarimeters. Vertical pro®les of transmitted radiation were made between 50 and 61 WAP using the ceptometer. Measurements were taken at 0.2 m intervals along the stem of the plant starting at the top of the canopy. At each point, ®ve measurements were taken around the circumference of the plant and the average recorded. In the case of banana, measurements included three plants for each row position of all treatments. Measurements of rubber included six plants per treatment measured over 2 days. All vertical pro®le measurements were con®ned to a single block due to time limitations and to the period of 11.30±12.30 h in order to avoid problems associated with changes in incoming radiation due to solar angle. 2.5. Conversion of radiation to dry matter The ef®ciency of conversion of intercepted radiation to dry matter or RUE was calculated at the whole treatment and component crop level using regression analysis of total dry matter production against cumulative intercepted radiation. RUE of both banana and rubber was estimated for two periods corresponding to the mother and ratoon crop periods of banana. For the mother crop period, RUE was estimated from destructive harvests at 8, 12 and 16 MAP and for the ratoon crop on harvests at 20, 24 and 28 MAP. Fusarial wilt disease resulted in severe foliage loss in the sole crop banana at 28 MAP (Rodrigo et al., 1997) and so RUE in this treatment was based on the ®rst two harvests of the ratoon crop (i.e. at 20 and 24 MAP).
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2.6. Leaf gas exchange Preliminary studies were required to develop an appropriate methodology for measuring photosynthesis of the large banana and rubber canopies. Gas exchange measurements were performed using a portable closed system infra-red gas analyser, IRGA (LI6200, Li-Cor, Lincoln, USA). Leaves of rubber are produced in clusters, referred to as whorls, along the stem. Leaves in the whorl were divided into three size categories and variation in photosynthetic capacity between categories was assessed. Gas exchange measurements were restricted to one leaf per category for the most recently expanded whorl and for eight replicate plants, with measurements taken three times per day (10.00, 12.00 and 14.00 h). No signi®cant difference was observed for either photosynthetic rate or stomatal conductance between the different size categories within a whorl and so, two leaves were randomly selected from the east and west directions within each whorl for canopy gas exchange studies. By 103 WAP, the whorl system on the main trunk had been replaced by side branches forming a well-developed canopy. Consequently, measurements thereafter (i.e. 103 and 105 WAP) were restricted to three levels corresponding to the top, middle and bottom, of the rubber canopy. Canopy leaf photosynthesis of rubber was recorded at intervals starting from 35 WAP. Two replicate plants per treatment in the sole crop and extreme density BR and BBBR intercrops, were measured at three times during the day (09.00±10.00, 11.30±12.30, 14.30±15.30 h). Given the small size of the cuvette (1000 cm3) relative to the banana leaf, it was necessary to characterise variation in photosynthetic rates along the leaf in order to develop a protocol for measurement of whole leaf photosynthesis. Measurements were made on the most recently expanded leaf of 11 replicate plants. Each leaf was divided along the midrib and for each half, gas exchange was measured in eight positions, four along the length of the leaf and two across the lamina. Results indicated that CO2 assimilation rates were similar across the width of the leaf lamina, but signi®cantly decreased from the petiole to the tip (P 0:01; Rodrgio, 1997). Since it was not feasible to measure photosynthesis along the whole length of the leaf each time gas exchange measurements were
241
undertaken, measurements were con®ned to the central position of the leaf where rates approximated to the average for the whole leaf. Changes in water status of the banana leaf caused changes in the angle between two leaf laminae halves and so gas exchange measurements were made on both sides of the leaf to avoid any discrepancy in incident light. Preliminary measurements showed that photosynthetic rates varied signi®cantly (P 0:001) with position in the canopy (Rodrgio, 1997) and so canopy studies were based on measurements of each green leaf of the banana plant. As with rubber, diurnal canopy photosynthesis measurements of banana were made three times per day (09.00±10.00, 11.30±12.30, 14.30±15.30 h), and were restricted to the sole crop, BR and BBBR treatments. Also, because of time limitations, measurements were restricted to one plant per replicate. Considering the row positions of banana, two plants were selected in the BBBR treatment in order to provide a representative sample of the crop, one from the edge row on the eastern side and the central row, for each set of measurements. 2.7. Data analysis The statistical package `SAS' (ASA Institute Inc., Cary, NC, USA) was used for the data analysis. The number of measurements made on each treatment varied due to different number of leaves/whorls and so the unbalance model ``Proc. GLM'' procedure was used for statistical analysis. To evaluate all possible pairs, mean separation of treatments was performed with Duncan's multiple range test (DMRT). 3. Results 3.1. Dry matter productivity Productivity of rubber cropping systems increased signi®cantly with planting density and by 28 MAP total dry matter (TDM) was over 200% greater in the intercrops than sole crop (Fig. 1). The poor performance of B resulted from damage by the Fusarial wilt virus. Analysis at the individual plant level indicated a signi®cant improvement in growth of banana in the double and triple row intercrops,
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Fig. 1. Relationship between total dry matter production after 28 months after planting and planting density for the sole crop rubber (&; R), sole crop banana (*; B), single (~; BR), double (5; BBR) and triple (^; BBBR) row banana/rubber intercrops. Data represent the mean of four replicate experimental blocks.
with leaf area and TDM per plant at least 293 and 188% greater, respectively, than in the sole banana crop (Table 1a). Similarly, there was no evidence that high density intercropping had a detrimental effect on growth of rubber, in fact the opposite, with TDM and leaf area per plant signi®cantly greater in Table 1 Summary of treatment effects on dry matter production (TDM) and leaf area per plant of (a) component banana and (b) rubber grown either as sole crops or intercropsa TDM (g per plant) (a) Component banana Sole crop Single row intercrop Double row intercrop Triple row intercrop (b) Component rubber Sole crop Single row intercrop Double row intercrop Triple row intercrop
Leaf area (m2 per plant)
2964a 3128a 5571b 6483b
3.10a 4.27a 9.08b 9.68b
7199a 12319b 13320b 14279b
7.9a 12.1ab 14.3b 13.5ab
a Values refer to the mean of four replicate experimental blocks measured at ®nal harvest at 28 months after planting. Means with a different letter within a column are signi®cantly different at the P 0:05 level.
the intercrop treatments relative to the sole crop (Table 1b). 3.2. Fractional and cumulative interception The sole rubber crop was least ef®cient in capturing light, with fcrop averaging only 20% over the measurement period (Fig. 2a). Fractional interception, fcrop, increased by more than 25% in all intercrops over the measurement period, compared with 23% in the sole rubber crop. Fractional interception of the sole banana crop was comparable with the BBBR treatment until ca. 60 WAP but thereafter declined; from 115 WAP onwards, fcrop, decreased further reaching values similar to those of sole rubber crop and only ca. 60% of the maximum achieved by the mother crop. Fractional interception was greatest in BBBR and by the end of the measurement period, total intercepted radiation was 6374 MJ mÿ2, 23 and 73% greater than that in the BBR and BR intercrops, respectively (Fig. 2b). Analysis of light attenuation within individual trees indicated that the rubber canopy was more effective than banana in capturing light (Fig. 3). Light was attenuated more rapidly in the rubber than banana canopy, with maximum attenuation achieved
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Fig. 2. Summary of (a) seasonal changes in weekly fractional interception and (b) cumulative intercepted radiation of sole and intercrops. Treatment codes refer to the sole rubber (R), sole banana (B), single (BR), double (BBR) and triple (BBBR) row banana/rubber intercrops. Data represent the mean of four replicate experimental blocks.
within the ®rst 1 m depth in both canopies (Fig. 3). Whilst there was no treatment effect on the vertical distribution of light within the rubber canopy, in the case of banana, light was attenuated more rapidly by banana in the intercrop relative to sole crop (Fig. 3).
Fractional interception (fcomp) and cumulative interception of component crops in the intercrop and sole crop are summarised in Fig. 4. As expected, fcomp, and, hence, cumulative interception of banana increased with planting density of banana, been greatest in the sole crop and least in the single row banana
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well below those of the sole crop (Fig. 4c and d). The overall increase in fcomp of rubber during the measurement period was 15, 11 and 14% in the BR, BBR and BBBR intercrops, respectively, compared with 27% in the sole rubber crop. At the end of the experiment, cumulative interception of component rubber in the BBR and BBBR crops were comparable and 90 and 75% of the values for the BR and sole crop, respectively. 3.3. Radiation-use ef®ciency Radiation-use ef®ciency (RUE) tended to be greater for the intercrops relative to sole crop rubber, although the effect was only signi®cant during the mother crop period of banana when the value for the BBBR intercrop was ca. 100% greater than that of the sole crop rubber (Fig. 5a). The component banana crop showed a greater RUE for the mother than ratoon crop (Fig. 5b). Double and triple row banana intercropping raised the RUE of component banana relative to the single row system, particularly for the ratoon crop when values were 375 and 450% greater, respectively, than in the BR intercrop. In contrast to banana, RUE of the component rubber crop was higher during the ratoon than mother crop period and tended to increase with planting density of banana in the intercrop (Fig. 5c). During the ratoon crop period, component RUE of rubber in all three intercrops showed a dramatic increase of more than 200% over the sole crop (Fig. 5c). 3.4. Photosynthetic performance of intercropped banana and rubber Fig. 3. Changes in fractional interception with canopy depth in (a) component banana and (b) component rubber crops measured between 50 and 61 weeks after planting (WAP). Treatment codes refer to the sole rubber (R), sole banana (B), single (BR), double (BBR) and triple (BBBR) row banana/rubber intercrops. Error bars represent the S.E. of means where n 3 in the B and BR, n 6 in the R and BBR and n 9 in the BBBR.
intercrop. In the case of rubber, fcomp and cumulative interception was similar in all treatments during the mother crop period of banana (i.e. 8 to 16 months after planting, MAP). However, during the ratoon crop period of banana (i.e. 20 to 28 MAP), fcomp and cumulative interception of intercropped rubber were
There was no signi®cant effect of row position on CO2 assimilation rates (A) of banana in BBBR treatment and so measurements for the central and outer rows were combined to give a mean value for ease of comparison with the single row measurements of the sole and BR intercrop (Fig. 6). Despite increased mutual shading in the high density intercrops, no consistent treatment effects on A of either banana or rubber was observed. In general, A was similar for both crops, but varied signi®cantly over time (P 0:001). Both the rubber and banana crops showed a dramatic decrease in A between 566 and 584 days after planting, due to a prolonged period without rainfall (Fig. 6).
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Fig. 4. Analysis of seasonal changes in fractional interception (a, c) and cumulative radiation interception (b, d) of component banana (a, b) and component rubber (c, d) crops in the sole and intercrop treatments.
4. Discussion Intercropping offers much scope for improving resource capture and productivity of immature plantation tree crops such as rubber, by utilising the wide row spacing needed to meet the growth requirements of the mature tree. With the exception of B, dry matter productivity was directly related to planting density, been least in the sole rubber and greatest in the BBBR crop planted at densities of 500 and 2000 plants per ha,
respectively (Fig. 1). Productivity of B was much lower than expected, due to damage caused during the later stages of the experiment by the Fusarial wilt virus (Rodrigo et al., 1997). The more than four-fold increase in dry matter production in the BBBR compared with R crop derived from an increase not only in light capture (270%) but also RUE (230%). The extent to which resources may be under utilised during the establishment phase of rubber was highlighted by the poor ef®ciency of light capture (Fig. 2).
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Fig. 5. Summary of treatment effects on radiation-use ef®ciency of (a) whole cropping systems, (b) component banana and (c) component rubber crops. Treatments codes refer to the sole rubber (R), sole banana (B), single (BR), double (BBR) and triple (BBBR) row banana/rubber intercrops. Radiation-use ef®ciency was calculated for two periods corresponding to the mother (8 to 16 months after planting) and ratoon (20 to 28 months after planting) crop periods of banana. Error bars represent the S.E. of means of four replicate experimental blocks in whole cropping systems and that of the slope of the linear regression analysis of mean cumulative dry matter and intercepted radiation in the case of component crops.
Sole crop rubber did not achieve full canopy cover and by the end of the experimental period, fcrop was still below 40%, comparable with that found for a 2.5-yearold rubber crop (Ibrahim, 1991). Similarly the BR intercrop, which is currently recommended for inter-
cropping in Sri Lanka, did not achieve full ground cover with fcrop remaining below ca. 50% throughout the experimental period. As expected, fcrop increased with planting density of banana such that values for the BBBR mother crop were similar to that of the B
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Fig. 6. Seasonal variation in photosynthetic rate of the (a) mother and (b) ratoon crops of (c) banana and rubber. Treatment codes refer to the sole crop banana (B), sole crop rubber (R), and the single (BR), double (BBR) and triple (BBBR) row banana/rubber intercrops. Error bars represent the S.E. of means of all measurements of all leaves in the canopy taken on one particular day.
treatment; towards the end of the experiment fcrop of B declined sharply due to the aforementioned damage by Fusarial wilt (Fig. 4). Leaf area index and, hence, fcrop of banana varies widely with variety, season and growth conditions (Turner, 1972; Lahav and Kalmar, 1981) and previous studies have reported values of fcrop as high as 90% at a LAI of 4 (Turner, 1982). These values are much higher than those reported here, where a maximum LAI of only 2.2 was achieved in
the BBBR; differences re¯ect the use in previous studies of much higher chemical inputs and improved varieties from the Cavendish group (Stover, 1984). Maximum light interception was achieved at a depth of 1 m in the canopy of both rubber and banana. Treatments had little effect on the extinction of radiation in rubber canopies (Fig. 3), in contrast with banana where the extinction of radiation was most rapid in the intercrops, particularly the BBR and
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BBBR treatments (Fig. 3). This may have conferred some advantage in terms of RUE of intercropped banana, as less light would have been wasted in the uppermost canopy layers where the capacity for leaf photosynthesis is greatest (Rodrgio, 1997; Eckstein and Robinson, 1995). Values of RUE were low compared with other crops (Colomb et al., 1995; Morrison and Stewart, 1995;Yunusa et al., 1995), although most studies have been on annual crops whose conversion ef®ciency is generally greater than that of perennials (Jayasekara and Jayasekara, 1995). Where studies have focused on tree crops, most estimates of RUE were based on short-term studies of less than 12 months using seedlings and saplings (Cannell et al., 1998). In the case of more mature trees, calculations of RUE have generally considered above ground biomass only (Grace et al., 1987; Squire and Corely, 1987; Dalla-Tea and Jokela, 1991), unlike the present study in which both above and below ground biomass were included. During the mother crop period, RUE of the intercrops, and in particular BBR and BBBR treatments, was signi®cantly (P 0:05) greater than R (Fig. 5), most likely re¯ecting the advantage of increased LAI and canopy heterogeneity on light capture (Tournebize and Sinoquet, 1995; Yunusa et al., 1995a) and use (Deka and Pal, 1995; Yunusa et al., 1995a,b). A similar increase in RUE of intercrop treatments relative to R was observed during the ratoon crop period, but differences were not signi®cant (Fig. 5). In general, RUE of component banana and rubber crops tended to be greater in the high density intercrops; the most notable response been the ca. 200% increase in RUE of intercropped rubber compared with R during the ratoon crop period (Fig. 5). The shape of the light response curve of photosynthesis of these two C3 crops would explain, in part, the improved RUE when intercropped. Leaf photosynthesis of both banana (Rodrgio, 1997) and rubber (Nugawela, 1989) saturates well below full sunlight, especially for ``shade'' leaves within the canopy with the result that shading would have had a minimal effect on leaf net photosynthesis in the intercrops (Fig. 5). Indeed, photosynthetic ef®ciency of these ``shade'' leaves would have been improved by increasing the proportion of leaves operating in the lightlimited region of the photosynthetic light response curve. With fewer leaves light-saturated, less light was
wasted in the intercrop canopies and so RUE was increased (Harris et al., 1987; Stirling et al., 1990; Monteith et al., 1991). Intercropping, particularly at high densities, was also found to increase partitioning of biomass to above ground structures in both the banana and rubber components (Rodrigo et al., 1997). This is a typical shade response which would contribute to improved radiation capture through an increase in leaf area (Table 1) and to improved RUE, through an increase in net assimilation rate. It is widely assumed in agro-forestry research that shading has a detrimental effect on growth of understorey crops, despite the fact that for a signi®cant portion of the growing season in the tropics radiation loads are high and stress inducing. Provided it is not excessive, shading can have major bene®ts for crop growth by moderating the canopy microclimate and alleviating plant stress (Monteith et al., 1991). This appeared to be the case in the present study where intercropping, with its associated increase in canopy density and mutual shading, not only improved light capture and biomass production per unit land area but also improved growth of both banana and rubber at the individual plant level, even when planting density of banana was increased three-fold. Acknowledgements We wish to thank the Rubber Research Institute of Sri Lanka for providing land for the experiment and staff at the Institute for valuable assistance in the ®eld. This paper is an output from a project (Plant Sciences Research Programme R7212) funded by the UK department for International Development (DFID) and administered by the Centre for Arid Zones Studies (CAZS) for the bene®t of developing countries. The views expressed are not necessarily those of DFID. References Cannell, M.G.R., Mobbs, D.C., Lawson, G.J., 1998. Complementarity of light and water use in tropical agroforests II. Modelled theoretical tree production and potential crop yield in arid to humid climates. For. Ecol. Manage. 102, 275±282. Colomb, B., Bouniols, A., Delpech, C., 1995. Effect of various phosphorus availabilities on radiation-use ef®ciency in sun-
V.H.L. Rodrigo et al. / Field Crops Research 69 (2001) 237±249 ¯ower biomass until anthesis. J. Plantation Nutr. 18, 1649± 1658. Dalla-Tea, J., Jokela, E.J., 1991. Needlefall, canopy light interception and productivity of young intensively managed slash and loblolly-pine stands. For. Sci. 37, 1298±1313. Deka, J., Pal, M., 1995. Bioenergetics of different sole and pigeonpea (Cajanus cajun) based intercropping systems. I. J. Agri. Sci. 65, 639±643. Eckstein, K., Robinson, J.C., 1995. Physiological responses of banana (Musa AAA Cavendish sub-group) in the subtropics. I. In¯uence of internal plant factors on gas exchange of banana leaves. J. Hortic. Sci. 70, 147±156. Grace, J.C., Jarvis, P.G., Norman, J.M., 1987. Modelling the interception of solar radiation energy in intensively managed stands. New Zealand J. Forestry Sci. 17, 193±209. Harris, D., Natrajan, M., Willey, R.W., 1987. Physiological basis for yield advantage in a sorghum/groundnut intercrop exposed to drought. 1. Dry matter production, yield and light interception, yield and light interception. Field Crops Res. 17, 259±272. Ibrahim, A.G., 1991. In¯uence of rubber canopy on intercrop productivity. Trans. Malaysian Soc. Plant Physiol. 2, 75±79. Jayasekara, C., Jayasekara, K.S., 1995. Photosynthetic characteristics of tropical tree species with special reference to palms. Energy Conversion Manage. 36, 919±922. Jayasena, W.G., Herath, H.M.G., 1986. Innovation, receptivity and adoption in rubber smallholding of Sri Lanka. SRRP Research Study No.71, ARTI, Sri Lanka, pp. 79±88. Lahav, E., Kalmar, D., 1981. Studies on the phenology and production potential of Williams banana in a subtropical climate. Subtropica 2, 12±16. Monteith, J.L., Ong, C.K., Corlett, J.E., 1991. Microclimatic interactions in agroforestry systems. For. Ecol. Manage. 45, 31± 44. Morrison, M.J., Stewart, D.W., 1995. Radiation-use in summer rape. Agron. J. 87, 1139±1142. Nugawela, A., 1989. Gas exchange characteristics of Hevea genotypes and their use in selection for crop yield. Ph.D. Thesis, University of Essex, England, UK. Ong, C.K., Corlett, J.E., Singh, R.P., Black, C.R., 1991. Above and below ground interactions in agroforestry systems. For. Ecol. Manage. 45, 45±57. Rodrgio, V.H.L., 1997. Effects of planting density of growth and resource capture of banana/rubber intercrops. Ph.D. Thesis, University of Wales, UK. Rodrigo, V.H.L., Stirling, C.M., Teklehaimanot, Z., Nugawela, A., 1997. The effect of planting density on growth and development of component crops in rubber/banana intercropping systems. Field Crops Res. 52, 95±108. Rodrigo, V.H.L., Stirling, C.M., Naranpanawa, R.M.A.K.B.,
249
Herath, P.H.M.U., 2001. Intercropping of immature rubber; present status in Sri Lanka and ®nancial analysis of rubber intercrops planted at three different densities of banana. Agrofor. Syst., in press. Sivakumar, M.V.K., 1993. Growth and yield of millet and cowpea in relay and intercrop systems in the Sahelian zone in years when the onset of the rainy season is early. Exp. Agric. 29, 417±427. Squire, G.R., Corely, R.H.V., 1987. Oil palm. In: Sethuraj, M.R., Raghavendra, A.S. (Eds.), Tree Crop Physiol. 141±167. Stirling, C.M., Williams, J.H., Black, C.R., Ong, C.K., 1990. The effect of timing of shade on development, dry matter production and light-use ef®ciency in groundnut (Arachis hypogaea) under ®eld conditions. Aust. J. Agric. Res. 41, 633± 644. Stover, R.H., 1984. Canopy management in Valery and Grand Nain using leaf area index and photosynthetically active radiation measurements. Fruits 39, 89±93. Stover, R.H., Simmonds, N.W., 1987. Bananas. Tropical Agriculture Series, 3rd Edition. Longman, London. Tournebize, R., Sinoquet, H., 1995. Light interception and partitioning in a shrub/grass mixtures. Agric. Forest Meteor. 72, 277±294. Turner, D.W., 1972. Banana plant growth 2. Dry matter production, leaf area and growth analysis. Aust. J. Exp. Agric. Anim. Husbandry 12, 216±224. Turner, D.W., 1982. A review of plant physiology in relation to cultural practices in the Australian banana industry. In: Proceedings of the Australian Banana Industry Development Workshop, Lismore, New South Wales, pp. 34±58. Vandermeer, J., 1989. The Ecology of Intercropping. Cambridge University Press, Cambridge, UK. Willey, R.W., 1979a. Intercropping Ð its importance and research needs. I. Competition and yield advantages. Field Crop Abstr. 32, 1±10. Willey, R.W., 1979b. Intercropping Ð its importance and research needs: agronomy and research approaches. Field Crop Abstr. 32, 78±85. Willey, R.W., Reddy, M.S., 1981. A ®eld technique for separating above-ground and below-ground interactions in intercropping Ð an experiment with pearl millet-groundnut. Exp. Agri. 17, 257±264. Yunusa, I.A.M., Mead, D.J., Pollock, K.M., Lucas, R.J., 1995a. Process studies in a Pinus radiata-pasture agroforestry system in a subhumid temperature environment. I. Water use and light interception in the third year. Agrofor. Syst. 32, 63±183. Yunusa, I.A.M., Mead, D.J., Lucas, R.J., Pollock, K.M., 1995b. Process studies in a Pinus radiata-pasture agroforestry system in a subhumid temperature environment. II. Analysis of dry matter yields in the third year. Agrofor. Syst. 32, 185±204.