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Physiological behavior and yield evaluation of agronomic crops under agroforestry system

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  • Save International Journal of Plant Research 2013, 3(1): 1-8 DOI: 10.5923/j.plant.20130301.01 Physiological Behaviour and Yield Evaluation of Agronomic Crops Under Agri-horti-silviculture System S. K. Chauhan1,*, W. S. Dhillon2, N. Singh2, R. Sharma1 1Department of Forestry and Natural Resources 2Department of Horticulture, Punjab Agricultural University, Ludhiana, 141 004, India Abstract An agri-horti-silvicultural model involving poplar (Populus deltoides Bartr. Ex Marsh.) as timber tree component, fruit trees and agronomic crops viz, turmeric (Curcuma longa L.) and moong (Vigna radiata L.) were evaluated for yie ld. Net photosynthesis, stomatal conductance and transpiration in both crops were higher in open areas than in shaded ones. Agronomic crops showed initially better performance under partial shade in yield and yield contributing parameters, and decreased as poplar canopy advanced in age. Changes in these parameters showed inverse relationship with canopy age and vice versa with mo re yield reduction under fifth year old canopy follo wed by preceding years and control. The results of studies on the micro-climatic interaction and resultant effect on physiology, yield and economics of agrono mic crops under poplar tree canopy are presented in this paper. The t ranspiration (E) rate of crops was lo west under shade conditions irrespective of the crop used in the experiment leading to more water use efficiency in the shade conditions than in open. There was gradual reduction in crop yield with advancement of age but the economic benefits of intercropping were two to three times higher than traditional crop rotation. It is suggested that to min imize resource competit ion and imp rove physiological processes of crops, canopy management is essential to ensure better yield under poplar-based agri-horti-silv icultural system. Keywords Inter-Cropping, Agronomic Crops, Poplar, Agroforestry, Shading Effect, Physiology, Yield 1. Introduction Agroforestry can be defined as an approach to land use that incorporates trees into farming systems, and allows for the production of trees and crops or livestock fro m the same piece of land in order to obtain economic, environmental, ecological, and cultural benefits[1]. Diversificat ion of e xisting fa rming systems by developing suitable agroforestry models seems to be the need of the day to cope up with ever increasing demand for diversified p roducts. Diversification in trad itional crop rotation (rice-wheat) has been accorded a high priority in irrigated agro-ecosystem of North-western states of India due to several socio-economic and ecological problems i.e., insufficient storage space for rice and wheat produce, declining soil health, depleting underground water resources, indiscriminate use of agrochemicals, etc. Farmers have tried alternatives to rice-wheat rotation like other crops (pulses, oilseed, fruits, vegetable, etc.), poultry, pisciculture, piggery, dairy, etc. but much success has not been achieved because of inadequate market ing, technical and financial s u pp o rt[2]. Agroforestry offers an economical and ecologically viable * Corresponding author: chauahnpau@redi (S. K. Chauhan) Published online at Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved option for large scale diversification in agriculture on one hand and environmental amelioration on the other. The increasing population and rapid industrialization has increased pressure on the traditional fo rests for timber and other related wood products. Therefore, to save forests and meet the growing demands of wood, there is need for large scale plantations of fast growing tree species outside forests to make country self reliant in its timber requirements. Fast growing tree species with rotation of less than ten years like poplar, eucalypts, leucaena, casuarina, willo w, etc. have gained preference due to their higher productivity and acceptability in the market. On-farm t imber tree plantations can also benefit fro m the global environ mental facilities like carbon trading[3,4]. Agricultural crop inputs in India are subsidised (varies fro m state to state) and governments are under pressure to phase out the subsidies to reduce the fiscal burden. Therefore, farmers are looking for co mpetit ive alternative options, and short rotation timber t rees based integrated systems have been found remunerative than traditional crop rotations. North-western Indian states have the potential to meet the industrial requirements of the country and state Govern ments are supporting farmers through liberalized harvesting and market ing policies for on-farm timber. Farmers are looking for species, which are low energy requiring with h igh water and nutrient use efficiency. Populus deltoides is one such deciduous species, which is econo mically the most important 2 S. K. Chauhan et al.: Physiological Behaviour and Yield Evaluation of Agronomic Crops Under Agri-horti-silviculture System being grown under agroforestry plantations in Punjab, Haryana, Uttar Pradesh, Uttrakhand, Himachal Pradesh and Bihar states of India[5-7]. Poplar based agroforestry offers a better livelihood to farmers due to relatively lo w input costs, flexib le labour requirements, compatibility with understorey crops, higher productivity/profitability, etc. Pop lar based agroforestry has been found to give better economic returns than sole annual crops[5,8]. Traditionally, agro-forestry had its origins in developing nations where high population densities coupled with scarce land resources have required that concurrent food and wood production may be produced on the same land base with litt le compro mise on principal of sustainability. Furthermore, tree- based inter-cropping systems can result in mo re diversified economies for both short- and long-term products and provide a market for both agronomic and forest crops. Inter-cropping systems can also play a vital role in sequestering carbon below- and above-ground plant components, thereby addressing present and crit ical societal concerns about global climate change[1,9-11]. With these potential benefits, successful tree based inter-cropping systems will minimize co mpetitive interactions between non-woody (annual agricultural crop) and woody (tree) components while exp loit ing beneficial interactions between these components. Increasing our understanding of these interactions will provide a scientific basis for both improvement and adoption of tree- based inter-cropping s ys tems . Apparently, integration of trees and crops leads to co mp le x in t eract io n s amo n g t h e co mp o n en t s at v ario u s b io -p h y s ical d o ma in s such as light, space, water, nutrients, etc. The co mplementary effect among these factors is the key for success of an agroforestry system. The modifications in micro -environ ment due to growing of trees, directly or indirectly influence various vital physiological processes of the plants grown under tree canopy. Generally, Photosynthetically Active Radiat ion (PA R) and temperature (air/soil) are reduced, wh ile the humidity is increased. Among these, PAR is important as the radiant energy captured by plants is utilized in the photosynthesis, which is the prima ry process governing biomass production and yield. Therefore, investigations on the physiological processes especially those related to photosynthesis are crit ical for understanding the plant growth under the canopy of trees. This article deals with the physiological response of under-storey crops and biological/economic performance of turmeric and moong crops under poplar-fru it based agroforestry system. 2. Materials and Methods The study was conducted in the experimental area of Depart ment of Hort iculture, Punjab Agricultural University (PAU), Ludhiana located at 30o 45 N lat itude and 75o 18 E longitudes at an elevation of 247m above mean sea level. The climate is sub-tropical with dry season fro m late September to early June. The area received an annual rainfa ll of 704 mm. The experimental p lantation was taken up in January, 2006 involving four fruit trees species (Kinnow, guava, peach and plum at 6 x 6m spacing) and poplar trees (6 x 6m, N-S d irect ion) grown alternatively. Understorey crops such as turmeric (Curcuma longa L.) and moong (Vigna radiata L.) were sown in the month of April in the inter-ro w spaces of two ages of poplar trees (G48) planted in north-south direction in comp letely randomized design with three replications (each row as replicate of 30m). Arab le crops, turmeric and moong grown in tree less plots served as control. The recommended agronomic practices of PAU were followed throughout the growing period of the crop. The variables measured include photosynthetic active radiation (PAR), stomatal conductance and intercellula r CO2, transpiration rate with portable photosynthesis system (CID 340, CID Inc., USA). The diurnal variat ion in these parameters was measured to arrive at average values. The canopy characteristics such as the light interception and leaf area index of poplar trees was measured using digital lu x meter and canopy analyser (CID 310, CID Inc, USA), respectively. The per cent light intercepted by trees was calculated as the reduction in the average light intensity under tree cover over control. The crop y ield and yield contributing parameters were recorded on 1mx1m quadrat basis to extrapolate the yield on hectare basis. Since the fruit plants were sma ll and under canopy of poplar therefore they were not included for observations. Average girth at breast height and height of the plants at the age of five years was 83cm and 22.4m, respectively. Accordingly the econo mics was worked out for co mparison. Systat-11, statistical software[12] was used for computation of descriptive. 3. Results and Discussion 3.1. Physiology of Under-Storey Crops Grown Under Popl ar Canopy The average photosynthesis of crops (turmeric and moong) grown under poplar plantation varied with the light interception. Among the physiological parameters, the Pn was maximu m under control conditions in all the crops with the maximu m value of 18.15 µmolm-2s-1 in moong, follo wed by turmeric 5.53 µmo lm-2s-1 and. The Pn/E ratio, which indicates water use efficiency (W UE) was higher in 4th year poplar plantation in turmeric (0.0046) and 0.0035 for moong in 5th year old plantation (Table 1). The min imu m values 0.0029 and 0.0031 of WUE were recorded in turmeric and moong, respectively grown under open conditions. The carboxy lation efficiency (Pn/Ci), which indicates the productivity potential, was higher in open grown crop and minimu m in turmeric (0.0103) and moong (0.0243) g rown under 5th year plantation. The photosynthesis capacity is influenced by stomata, which is sensitive to the environmental variations, especially air temperature and water stress. Under these circu mstances, the stomatal functioning appears to be major limitat ion to CO2 influ x, International Journal of Plant Research 2013, 3(1): 1-8 3 causing reduction in photosynthesis during noon. Photosynthesis is a physiological process that is affected by the environmental factors. All the under-storey crops in general show changes in photosynthetic rate with a maximu m photosynthetic activity during afternoon depending upon prevailing weather conditions during their growth period. Proportional changes in photosynthesis rate in rhizo matous crops with available PAR has been reported earlier, which was not observed in open condition[13,14]. Yield in arable crops are attributed to the reduction in light transmission under canopy[15-17]. It is evident from the Fig. 1 that all parameters viz., net photosynthesis rate, transpiration and stomatal conductance were higher in the open except internal Co2, which was higher in the shade (5 years old plantation). The transpiration (E) rate was minimu m under shade conditions irrespective of the crop used in the experiment. Maximu m WUE (Pn/E) was recorded under open conditions because there was decline in photosynthesis under shade conditions. Moong crop recorded better net photosynthesis, stomatal conductance as compared to turmeric (Fig. 1). Carboxylation efficiency (CE) of under-storey crops started declining after four years of poplar plantation as there was more decline in photosynthesis in shade and also there was increase in inter-cellular carbon. Under heavy shade, the photosynthetic rate declined because carboxylation efficiency decline after shading. High Ci suppressed the response of conductance to sunlight. However, there was a large response of conductance to light at low Ci, indicating an interaction between the effects of light and CO2 on stomata. Table 1. Eco-physiological parameters recorded in turmeric and moong grown under poplar trees *P aramet ers/ Canopy age P AR (µmolm-2s-1) Photosynthesis rate - Pn (µmolm-2s-1) T air (° C) RH (%) WUE (Water use efficiency) Carbox y lat ion efficiency C int/C atm IV year* - Moong (2008) - - - - - - III year 433.56 5.62 34.27 69.74 0.0024 0.0188 0.78 Control 700.32 15.23 36.81 77.30 0.0029 0.054 0.85 T urmeric (2008) IV year 497.88 2.31 32.19 81.99 0.0013 0.0063 0.93 III year 654.07 3.50 33.15 72.25 0.0036 0.0100 0.84 Control 875.59 5.50 36.56 69.88 0.0022 0.0173 0.80 Moong (2009) V year 318.71 9.39 34.82 72.54 0.0035 0.0243 0.82 IV year 468.49 13.14 37.29 65.38 0.0033 0.0370 0.86 Control 715.77 18.15 41.04 58.76 0.0031 0.0603 0.60 T urmeric (2009) V year 237.03 3.55 31.00 62.50 0.0032 0.0103 0.62 IV year 354.94 4.74 34.29 54.45 0.0046 0.0162 0.58 Control 807.29 5.53 37.11 60.37 0.0029 0.0183 0.54 *Moong crop failed due to heavy rains aft er crop sowing 20 15 10 5 0 Open Shade Open Shade Transpiration 500 400 300 200 100 0 Open Shade Open Shade Inter Cellular CO2 Cint (ppm) Stomatal conductance Moong Turmeric Moong Turmeric Figure 1. Physiological parameters of turmeric and moong as affected by shade of poplar tree canopy and open 4 S. K. Chauhan et al.: Physiological Behaviour and Yield Evaluation of Agronomic Crops Under Agri-horti-silviculture System 3 .2 . Diurnal Vari ation in Physiological Parameters of Crops and Microcli mate Modification under Popl ar Trees Reduction in solar rad iation influences the physiological processes more importantly the photosynthesis. Plants responses to light include adaptation at physiological and biochemical levels[18]. Plants have ability to adapt to different light regimes (quality/quantity) through changes in physiological behaviour. Rate of photosynthesis/ transpiration/stomatal conductance was proportional to PAR received. The d iurnal variat ions in eco-physiological parameters recorded in under-storey crops viz., moong and turmeric inter-planted with five year poplar are presented in Table 2. The Photosynthetic Active Radiation (PA R) was highest during afternoon irrespective of type of under-storey crops with an average o f 494.42 µmo lm-2s-1 and 412.54 µmolm-2s-1 for moong and turmeric, respectively. Physiological behaviour of crops under control conditions were a lso investigated and it was found that the PAR re ma in almost in the same range with the maximu m of 1103.35 µmolm-2s-1 and minimu m of 1100.2 µmo lm-2s-1. The least PAR was recorded during evening (4 p m) with 123.11 µmolm-2s-1 and 96.25 µmo lm-2s-1 for moong and turmeric, respectively, whereas, in open condition, maximu m and minimu m PA R during evening hours was 575.39 µmolm-2s-1 and 435.67 µmo lm-2s-1, respectively. The photosynthesis rate 21.54 µmolm-2s-1 was highest during morn ing hours in moong under control condition, but its value remained maximu m during noon for turmeric (5.85 µmo lm-2s-1) and moong (12.12 µmo lm-2s-1) under tree canopy and under control condition its value was highest (9.78 µmo lm-2s-1) for turmeric. The lowest rate of photosynthesis was recorded during evening for both open as well as shad conditions with minimu m value of 2.02 µmolm-2s-1 and 7.24 µmolm-2s-1 in turmeric and moong, respectively. The variations in temperature and relative hu midity under four and five year old poplar plantations for the period of April to September, 2009 are presented in Fig. 2. The difference between open and shade condition for both parameters increased with increase in canopy age and modification in micro-climate became more pronounced. These changes in the micro-climatic conditions influenced the physiological process in the under-storey crops, thus affecting the crop yield. Under canopy, PAR availab ility varies with the tree species and this in-turn affects the under-storey crop growth and productivity[16], which was recorded in the present study as well. Table 2. Diurnal variation in eco-physiological parameters of crops grown under poplar trees canopy and open condition during 2009 Time P AR (µmol photonsm-2s-1) Transpiration rate (mmolm-2s-1) (E) St o mat al con duct iv ity (mmolm-2s-1) T air °C (C) Moong under poplar canopy T leaf °C Photosynthesis rat e (µmolm-2s-1) (P n) Internal CO2 (ppm) (Ci) 9:00 AM 338.61 2.65 299.84 34.1 34.6 8.82 345.32 12 Noon 494.42 3.31 243.54 35.56 36.23 12.12 349.23 4:00 PM 123.11 2.07 167.44 35.45 36.15 7.24 465.33 Moong in open 9:00 AM 611.43 5.94 435.28 36.8 38.43 21.54 229.02 12 Noon 1100.2 6.45 351.43 41.04 42.48 17.23 311.21 4:00 PM 435.67 5.37 155.36 39.68 39.65 15.67 361.26 T urmeric under poplar canopy 9:00 AM 202.3 0.63 95.85 32.68 34.32 2.78 229.08 12 Noon 412.54 1.77 164.88 32.96 34.4 5.85 348.14 4:00 PM 96.25 0.85 93.94 23.34 24.18 2.02 455.14 T urmeric in open 9:00 AM 743.12 1.03 137.29 35.65 39.13 4.24 255.24 12 Noon 1103.35 3.45 211.75 38.12 39.82 9.78 286.34 4:00 PM 575.39 1.31 35.46 36.84 37.39 2.56 363.13 International Journal of Plant Research 2013, 3(1): 1-8 5 T-5yr T-4 yr T-Open RH-5 yr RH-4 yr RH-Open 90 37.5 Relative Humidity (%) Temprature (oC) 80 35 70 32.5 60 50 30 40 27.5 30 25 20 10 22.5 0 April May June 20 July August September October Months Figure 2. Micro-climatic modification under poplar tree canopy 3.3. Yiel d Parameters of Turmeric and Moong Crops It is evident fro m Fig. 3 that rhizo me weight of turmeric was found more in open conditions than under shade. The rhizo me weight was more under three year old canopy than four year. The rh izo me thickness and length also showed the same trend. The per cent yield reduction in turmeric was more under five year than in the four year old poplar canopy. An earlier study revealed that fresh rhizo me yield of turmeric (Curcuma longa L.) decreased significantly as the age of poplar increases[19]. The study indicated that turmeric can be grown in poplar p lantation up to three years though the yield decreases in the third year, which is in lines with earlier observations[20], however quality is least affected under shade[21]. The reduction of 35.25 and 48.36 per cent was observed under three and four year old tree canopy (Fig.3). Whereas, in case of moong crop, it had less vegetative growth under the four year old than the three year old poplar canopy and in open. Nu mber of pods per plant, pod weight and seed weight per plant reduced almost to half in four year old p lantation as compared to control (Fig. 3). Yield reduction was 57.91 per cent and 36.87 per cent in five and four years old plantations, respectively. Similar results have also been reported for temperate agro-forestry systems. For examp le, maize and soybean yields were reduced to 73 per cent and 79 per cent of the sole crop yield, respectively, when grown in alley-cropping configurations involving poplar and silver map le in Canada[22]. In the present study, the tree reduced crop yield as the canopy advanced in age. Radiat ion is an important factor affecting crop photosynthesis, development and yield. Shade imposes a limitat ion on growth and development of crop plants but varies with shade tolerance of crops. To avoid these possible yield losses, canopies of trees should be pruned to reduce shading and also root-pruned to reduce possible co mpetition for water, nutrients and light and increase crop yield in agroforestry systems[23,24]. PA R under canopy is crucial in vegetative as well as reproductive growth. Higher reduction in y ield in moong in co mparison to turmeric indicates suitability of rhizo matous crops under shade than seed/grain crops. The root crops respond well to the changed micro -climate under tree canopy i.e., soil/air temperature, relative humid ity, light (quality/quantity), etc. Co mparatively higher yield reduction in soybean than turmeric has also been recorded earlier[25], exh ibit ing better suitability of turmeric under tree canopy. Therefore, it is essential to promote light conditions under canopy through managing geo metry of p lantations or exert ing judicious pruning and identificat ion of suitable crops and their specific varieties under prevailing light conditions because when photon flux density decreases to approximately 40 per cent, the carbon assimilation beco mes light limited[26]. The specific responses are also dependent on the arable cro p s [27]. 6 S. K. Chauhan et al.: Physiological Behaviour and Yield Evaluation of Agronomic Crops Under Agri-horti-silviculture System 140 Moong 120 100 80 60 40 5th year 20 4th year 0 Control Figure 3. Yield and yield contributing parameters of turmeric and moong under agri-horti-silvicultural system 3.4. Economic Analysis Productivity cannot be criteria for the making co mparison in different farming systems. Farmers itself will adopt the system on its economic sustainability. Therefore, an economic analysis was made to assess balance sheet so that the economically viable farming system is adopted. The income fro m moong crop followed by garlic/onion (spice crops) + fru it crops (still not in co mmercial bearing) and poplar timber had maximu m income (US$ 5725) followed by turmeric + fruits and poplar (US$ 4814), wh ich was much higher than to traditional rice-wheat rotation (Tab le 2). However, it is interesting that still farmers prefer to raise rice-wheat crops, because of assured government purchase, whereas, the rates of other crops keep fluctuating and they have to compete in the market. Govern ments are stressing hard to make farmers aware of the negative ecological and economic impacts of rice-wheat rotation and the results obtained in the present study will certainly help in convincing the farmers to adopt new rotations. Turmeric though is less affected under shade but the additional inco me fro m onion/garlic in moong based system makes the rotation more remunerative. The success storey of agroforestry in general has already been accepted by the Ministry of Environment and Forests, Government of India to achieve the targets of 33 per cent tree cover and farmers in specific for economic sustainability. Poplar alone makes a b ig difference in profitability in the system[8,28] but our conscious efforts in selecting appropriate crop/variety and management of co mponents will actually decide the sustainability and adoption of the system. The reduced yield of the crops under the tree canopy, lowers down the annual profitability marg in than sole crop cultivation but the overall profitability of the intercropping system after tree harvesting is substantially high than traditional crop cultivation[5,29], thus encourages the framers to invest in this sector and consider it a best performing low risk asset in near future. International Journal of Plant Research 2013, 3(1): 1-8 7 Table 2. Comparat ive income of different crop rot ations on per hect are basis in UD dollars Rot at io n Income ha-1 (USD) Rice-wheat 1897 T urmeric + fruit crops + poplar 4814 Moong + Garlic/onion* + fruit crops + poplar 5725 * Garlic/onion follows moong crop during winter season, however, not discussed in the paper but included for economic analysis. Turmeric is annual crop, whereas moong is season crop. Rice-wheat rotation is traditionally followed but needs diversification. Journal of Crop Improvement 12 (1–2): 339–363. [2] Chauhan SK and M angat PS (2006). Poplar based agroforestry is ideal for Punjab, India. Asia-Pacific Agroforestry News 28: 7-8. [3] Pandey DN (2007). Multifunctional agroforestry in India. Curr. Sci. 92(4): 455-463. [4] Dogra AS (2007). Contribution of trees outside forests toward wood production and environmental amelioration. Ind. J. Ecol. 38:1-5. [5] Chandra JP (2011) Development ofpoplar based agroforestry system. Ind. J. Ecol. 38:11-14. 4. Conclusions The crop yield is certain ly affected by the shade of the trees in tree-crop co mbinations but the resources use efficiency is better under trees than in open conditions. However, on system basis the productivity of the combination is more than pure cropping. Additionally, the mu ltip le outputs can be realized by the small farmers with limited land holdings. The distribution of benefits and costs are important for the farmers to evaluate the intercropping options. A system may be technically feasible for a biologist/ecologist but may be irrelevant for the farme rs. It is largely the economics, which determines whether tree-crop interventions are an opportunity or burden. The mu ltip le cropping (agri-hort i-silvicultural system) has been found economically feasible {two to three times higher inco me (USD 4814-5725) than traditional rice-wheat rotation (USD 1897)} and many progressive farme rs have adopted the same. The selection of intercropping components however must be based on the principle of minimizing the competition and maximizing co mplimentarily among themselves. Adequate care and proper management are essential to harness appropriate productivity in the intercropping systems. The continuity in research, regulated market and defined policy including framing of “Agroforestry Policy” will be favourable in motivating the farmers to adopt agroforestry for h igher productivity and livelihood security. In the changing climate scenario, tree-crop interface may be an adoption strategy and the carbon market may add to the profitability margins, wh ich are yet not realized. ACKNOWLEDGEMENTS Authors are thankful to Indian Council of Agricu ltural Research, New Delhi (India) for financial support to undertake the present study. [6] Lal P (2004). Integrated development of farm forestry plantations and wood based industries. Ind. For. 130:71-78. [7] Luna RK, Thakur NS and Kumar V (2011). Growth performance of twelve new clones of poplar In Punjab, India. Indian Journal of Ecology 38 : 107-109. [8] Bangarwa KS and Wuehlich G (2009). Using exotic poplar in Northern India for higher returns in agroforestry. Asia-Pacific Agroforestry News 35:3-5. [9] Brandle JR, Wardle TD, and Bratton GF (1992). Opportunities to increase tree planting in shelterbelts and the potential impacts on carbon storage and conservation. In: Sampson, R.N., Hair, D. (Eds.), Forest and Global Change. American Forests, Washington, DC, pp. 157–176. [10] Schroeder P (1993). A groforestry systems: integrated land use to store and conserve carbon. Climate Research 3: 59–60. [11] Kort J and Turnock R (1999). Carbon reservoir and biomass in Canadian prairie shelterbelts. Agroforestry Systems 44: 175–186. [12] Wilkinson L and Coward M (2007). SYSTAT: Statistics-II. (Version 12) Systat software Inc, Sanjose, CA-95110. [13] Dhillon WS, Chauhan SK and Singh N (2009). Physiology and yield of turmeric under poplar canopy. Asia-Pacific Agroforestry News 35: 5-6. [14] Chauhan SK, Dhillon WS, and Nighat Jabeen (2011). Analyzing the performance of Colocasia esculenta in poplar based agroforestry system. Asia-Pacific Agroforestry News 39: 9-10. [15] Peng X, Zhang Y, Cai J, Jiang Z and Zhang S (2009). Photosynthesis, growth and yield of soybean and maize in a tree-based agroforestry intercropping system on the Loess Plateau. Agroforestry Systems 76:569–577. [16] Baig M J and Gill AS (2005). 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