Contour hedgerow intercropping in the mountains of China:a review
Hui Sun ?Ya Tang ?Jiasui Xie
Received:26July 2006/Accepted:31January 2008/Published online:21February 2008óSpringer Science+Business Media B.V.2008
Abstract Hedgerow intercropping systems were introduced in China in early 1990s.Achievements in research and extension of contour hedgerow inter-cropping in China during the past 15years are reviewed here.Results reported in over 70published papers have shown that hedgerow intercropping contributes to soil and water conservation,soil fertility amelioration,land productivity improvement,bio-terrace formation,and gives more options for income generation based on local resources in mountain areas.Research and demonstration works on contour hedge-row systems have achieved success by integrating local resources and needs into the system,especially in the dry valleys of the upper reaches of the Yangtze River,and the Three Gorges region.Contour hedge-row intercropping has attracted the attention of researchers,policy-makers,and farmers,and has been taken as an alternative to implementation of the Grain for Green policy,and ecological reconstruction and restoration today.To date,hedgerow intercropping has been demonstrated and applied practically on sloping land in more than six provinces of China,particularly Sichuan,Guizhou,Shanxi,Shaanxi,as well as in the Three Gorges region of Chongqing and Hubei Prov-ince.The intercropping system has also been practiced
as an optimized technology for conserving farming on sloping lands,improving cash income,and reducing agricultural risks in depressed mountainous regions in southwest and northern China over recent years.Some misunderstandings and problems in studies and exten-sion of the system in China are summarized and clari?ed,and some recommendations for further research and expansion of the system are also presented in this paper.
Keywords Contour hedgerow intercropping áAgroforestry áSoil and water conservation áSustainable agriculture áSubtropical mountains áChina
Introduction
The contour hedgerow intercropping system is a simultaneous agroforestry system which involves planting double hedgerows of nitrogen-?xing plants along the contour lines of a slope at a distance of 4–6m (Tang 2000a ).The space between the contour hedgerows,the alley,is used to grow agricultural and cash crops (Kang et al.1981,1987;Nair 1993;Tang 2000a ).Hedgerows are usually pruned to reduce shading of crops and to supply biomass for mulching and other purposes.Effects of the system on soil conservation,soil fertility improvement,nutrient cycling,belowground interactions,and enhancement
H.Sun (&)áY.Tang áJ.Xie
Department of Environmental Science and Engineering,Sichuan University,24,South Section 1,Yihuan Road,Chengdu,Sichuan 610065,P.R.China e-mail:sunhui?les@https://www.doczj.com/doc/3e3187966.html,
Agroforest Syst (2008)73:65–76DOI 10.1007/s10457-008-9113-x
of production have been widely studied in the humid and semi-arid tropics(Kang et al.1981;Kang et al. 1990;Kiepe1995;Lal1989;Sanchez1995;Palmer 1997;Ong et al.1996;Alegre and Rao1995;Lupwayi and Haque1998).Contour hedgerows are reported to be very ef?cient in soil erosion control and soil fertility improvement on sloping agricultural land, although there is some dispute about the extent of belowground interactions(i.e.,competition for soil resources between crop and hedgerow species)of the system(Sanchez1995;Imo and Timmer2000).
Since in China there is a vast expanse of slopelands from the subtropics to the temperate zone suffering seriously from soil erosion and deterioration of fertility,(especially in the Loess Plateau and south-western China),research and extension work on contour hedgerow systems has been carried out since the early1990s in those areas(Tang1994;Shi et al. 1996).Some experiment stations have been estab-lished to test the impacts of contour hedgerows on soil erosion control and productivity enhancement during the past?fteen years.Moreover,many demonstration sites have also been set up for dissemination and extension objectives at the same time.Many innova-tions on hedgerow intercropping have been successfully made in extension projects in most target regions.However,few reports on the system in China have been summarized and presented internationally. Studies on contour hedgerow intercropping in China
Owing to pressures of population increase and eco-nomic development,more and more slopelands have been exploited in western China since the1980s.In some mountainous counties,more than80%of arable lands are classi?ed as sloping,and half of these are steeper than25°.The contour hedgerow system was introduced into China in the early1990s;subsequently it was tested and modi?ed initially for reducing the serious soil erosion of the arable slopelands in the Three Gorges region and dry valleys of the upper reaches of the Yangtze River(Tang1994;Shen1998).
The?rst testing and demonstration project spon-sored and funded by the International Center of Integrated Mountain Development(ICIMOD)was carried out in Ningnan County,in a dry valley in southwest China in1991.Initial objectives were to control soil loss in slopelands and screening of promising nitrogen-?xing tree species for hedgerow usage in the subtropics(Tang1994;Shi et al.1996). Another study was conducted in1994,in Luodian in the mountainous area of Guizhou,funded by the International Board for Soil Research and Manage-ment(IBSRAM)to test the effects of hedgerows on soil and water conservation(Yin et al.1996).At the same time,hedgerows were tested for reducing soil erosion in the Three Gorges Reservoir region(Shen et al.1995;Shen1998).A so-called shrub hedge system was also tested and demonstrated in Northern Shaanxi’s loess plateau,one of the more seriously eroded areas(Wang1994).Since then,research on the system has gradually been carried out in the subtropics of China.
Screening of hedgerow species
A key factor in design of intercropping systems is to select appropriate species for the hedgerows.Three characteristics are commonly considered in China in screening of species:ease of production for hedgerow establishment;high biomass production for enhance-ment of soil nutrient cycling and improvement of soil fertility,and multi-purpose uses of the hedgerow prunings as fodder,?rewood,green manure and so on.Hence,preferred species are generally woody nitrogen-?xing species,which are easy to propagate and widely adaptable to a range of environmental conditions,with deep root and taproot systems, tolerance to cutting,resistant to drought and low fertility soils(Tang2000a;Zhang2001).
To date,Leucaena leucocephala,Desmodium rensonii,Indigofera dosua,Flemingia microphylla, Albizia yunnanensis,Tephrosia candida,Acacia mearnsii and A.dealbata have been selected for hedgerows in dry valleys of the Jinsha basin in southwest China(He et al.1997;Zhang2001),and L.leucocephala and A.yunnanensis have been found to be the most appropriate leguminous species in this area due to their high biomass and resistance to serious seasonal drought.In the Three Gorges region, L.leucocephala,Coriaria sinica,Vitex negundo, T.candida,Alnus spp.,F.microphylla,and Hibiscus syriacus seemed to be suited to the natural conditions (Li et al.1998).As for North China,some native multipurpose species,such as Caragana spp,Amor-pha fruticosa,Hippophae rhamnoides,Salix
psammophila,and S.cheilophila,have been selected for the semi-arid and highly erosion-prone area of the Loess Plateau(Wang1994).
Effects of contour hedgerows on soil and water conservation
Effects of hedgerows on soil and water conservation have been continuously reported in recent years. Surface?ow from arable slopelands with contour hedgerows was reduced by26–60%and soil loss by more than97%in the Jinsha Basin(Sun et al.1999a, 2001a;He et al.2000,2001),and runoff was reduced by18%and soil loss by90%on karst slopelands in Guizhou compared with slopelands without hedge-rows(Yin et al.1996,2001).Xu et al.(2000)reported that runoff and soil loss from contour hedgerow intercropping were only24.8%and16.9%of that from agriculture slopelands under traditional man-agement in the Three Gorges area.Another result indicated that contour hedges can reduce runoff by more than30%and soil loss by more than50%,and increase vegetation cover by15–20%on loess slope-lands in temperate mountains(Wang2000).On the Loess Plateau of Shanxi,runoff and soil loss were reduced by66.2and77.2%respectively by A. fruticosa hedgerows(Zhou et al.1997).
More systematic studies,especially on the spatial distribution of soil physical parameters(Sun et al. 2004;Shi et al.2005),temporal changes in soil moisture(Sun et al.2002d;He et al.2004),root distribution and nutrient patterns in contour hedgerow systems(Sun et al.2005a,2005b;Shi et al.2005;Bu et al.2006),have investigated the mechanisms of runoff and erosion control in hedgerow intercropping systems.Ma et al.(2006)showed that different hedgerow species(L.leucocephala,V.negundo,C. sinica)reduced soil loss signi?cantly,by modeling soil erosion using a cellular automata approach and measuring soil loss on steep lands(25°)for three years. Effects of contour hedgerows on soil fertility improvement
In terms of maintenance and improvement of soil fertility and moisture regimes of slopelands in the subtropics,Sun et al.(1999b)and Xu et al.(1999a, 1999b,2000)reported that loss of soil organic matter, available nitrogen and exchangeable potassium could be signi?cantly reduced by hedgerows on arable slopelands in China.In addition,topsoil(0–30cm layer)fertility was also improved signi?cantly after 4years of application of leucaena prunings on degraded slopelands(Sun et al.2001b;2002a;Shen 1998;Yin et al.1996).Wang(2000)obtained similar results in temperate slopelands in northern China. Further quantitative study indicated that79–258kg N, 6–21kg P,39–207kg K,7–34kg Ca and7–30kg Mg had been added to alley soil on a per hectare basis annually by hedgerow prunings of six nitrogen-?xing species(Sun et al.2005b).In these studies,it has been demonstrated that increases in topsoil organic matter, nitrogen,and available potassium are clear after3–5years of application of prunings from nitrogen-?xing hedgerows,whereas it is less clear whether hedgerow systems enhance available phosphorus in the topsoil or not(Sun et al.2001c;Wang et al.2000).
Mineralization of organic matter,nitrogen,phos-phorus,potassium,calcium and magnesium of hedgerow prunings of D.resonii,T.candida, A. yunnanensis,A.dealbata,A.mearnsii,and L.leuco-cephala,has been studied under different application methods(Sun et al.2002b,2002c,2003).Nutrient release patterns of different species under different conditions has also been monitored in?eld experi-ments with different methods for practical application of hedgerow prunings in the intercropping system (Sun et al.2003).
Belowground interactions in contour hedgerow systems
Nutrient competition between hedgerows and crops, along with nutrient redistribution in the soil pro?le has been studied in detail in hedgerow intercropping systems(Sun et al.2005a,2005b).In a hedgerow system,levels of soil nutrients,organic matter, nitrogen and potassium beneath the hedgerow were higher at0–40cm depth than those beneath the alley, indicating that competition for nutrients does not occur between hedgerows and crops(Sun et al. 2002a).Available P and exchangeable K at40cm below the hedgerow were lower than beneath the alley,which suggested that the root system of the hedgerow may be functioning as a‘‘nutrient pump’’to draw K and P from the deep soil beyond the crop root system and which may be transferred to the topsoil through hedgerow prunings(Sun et al.
2002a).More complicated relationships between hedgerow species and companion crops were reported by Ding et al.(2004),who discovered that different hedgerow species could either compete for nutrients or provide N and P by varying the annual crops in the system.However,P absorption of crops was always improved in the hedgerow system.
Field measurements of soil moisture dynamics of a contour hedgerow system were reported by Sun et al. (2002d).Hedgerows depleted soil moisture below 50cm depth during the late phase of the dry season, and promoted in?ltration of rainfall into deep soil during the rainy season.This led not only to signi?cant reduction of surface?ow in the rainy season,but also to a great improvement of the soil water regime of deep layers in the monsoon and early phase of the dry season,which meant that more water was stored in the deep soil of hedgerow systems in the rainy season which may increase availability of water to annual crops in the early stages of the dry season(Sun et al.2002d,2005b).
Effects of contour hedgerows on bio-terrace formation
Hedgerows on slopelands act as a physical barrier and risers to hold up eroded and soil dislodged from the upper alley due to traditional plowing.Terraces,also called bio-terraces,can be formed gradually as contour hedgerow systems mature.Sun et al. (1999a)and Tang(2000b)reported that bio-terraces could be formed on15–38°slopes after4–7years by planting hedgerows.Similar results have also been obtained in other mountain areas(Li et al.1998;Shen 1998;Yin et al.2001;He et al.2004).
Compared with engineered terraces,bio-terraces formed by hedgerow are more stable and cost less. The cost(including cash investment and labor)is only3–17%of that of the engineering method(Tang 2000b;Tang et al.2001;Ma and Wang2006).Bio-terraces can also be established on steep slopelands with highly-weathered or sandy soil,and on terrain in which it is dif?cult to construct engineering terraces in the subhumid region(Tang et al.2001).
Arrangements of crops in contour hedgerows
Contour hedgerow systems provide more options for farmers to generate cash income and reduce agricultural risks due to control of runoff and soil erosion,increase in soil fertility,and enhancement of multidimensional niches(Tang2000a;Cai and Bu 2004).Therefore,appropriate arrangements of crops in the system play an important role in hedgerow systems in different agricultural zones with different socio-economic demands,i.e.prunings may be used as fodder,mulch,green manure,and?rewood depending on household needs(Tang et al.1999, 2001).
Many models of crop arrangements have been tested and demonstrated on slopelands in dry valleys in the upper reaches of the Yangtze River since1991 (Tang1994;He et al.1997).Economic assessments have indicated numerous arrangements that are very bene?cial in cash generation and reduction of external inputs(Yuan et al.2000b).As a result, farmers have adopted and copied some models voluntarily,such as hedgerow systems integrated with mulberry trees,food crops,cash crops,or fruit trees(Yuan et al.2000a,2001a).Alternative crop arrangements of the system have also been carried out subsequently in other agricultural zones(Wang 2000;Zhou et al.1997;Zhangpeng2001b),and more local fruit trees,nuts and timbers have been planted into hedgerows for economic reasons.These experiments and demonstrations have also achieved some success.
Economic assessment of contour hedgerows Generally,other than researchers and policy-makers who are interested mostly in ecological and social bene?ts from the system,farmers always focus their attention on how much economic return they may gain after adopting the system.Therefore,economic assessment of hedgerow systems is essential to its adoption and extension,especially in depressed or marginal regions.
According to most studies,the?rst economic bene?t of the system is that productivity is improved very effectively through enhancement of soil fertility and moisture regime.In a four-year study,compared with slopelands without hedgerows,yields of maize and peanuts in leucaena hedgerows on average increased by10%and22%without chemical fertil-izer,and increased24%and69%with fertilizer, respectively;in tephrosia hedgerows maize and peanut yields increased11%and24%with fertilizer
(Sun et al.2001b).Wang(2000)reported that yields of annual food crops can be improved14.9–21.2%on average by hedgerows.Bene?ts of hedgerow inter-cropping in the Three Gorges region and northern China had also been analyzed by Cai and Bu(2004) and Shi et al.(2005),respectively.
The second advantage is that cash crops and fruit trees can be integrated into the system to provide products with higher yield or quality(Yuan et al. 2001a;Tang et al.2003b).Farmers may be able to harvest more fruit production in hedgerow intercrop-ping than in traditional orchards(Yin et al.1996, 2001;Shi et al.1996).Cash and labor input to hedgerow orchards may also be reduced because of fertility improvement and suppression of weeds by mulching with hedgerow prunings.Input/output ratio of a hedgerow orchard was1:4.27while that of a traditional orchard was1:3.27(Yuan et al.2001a). The yield and quality of mulberry leaves for sericul-ture were also increased and chemical fertilizer input was decreased when mulberry trees were planted into hedgerow lines(Zhang et al.2001a;Yuan et al. 2000b;Tang et al.2003b).
The third bene?t of a hedgerow intercropping system is fodder https://www.doczj.com/doc/3e3187966.html,pared with common herbaceous fodders,prunings of nitrogen-?xing hedgerows(e.g.leucaena,tephrosia,acacia,albizia) have higher nutrition content,such as crude protein (Tang et al.1999,2001,2002a;Tang2000b). Farmers who adopted hedgerow systems had already used hedgerow prunings for husbandry development and gained additional cash income (Zhang et al.2001b).Production of14–30.5Mg fresh prunings can be obtained on a1ha slopeland with4m interval hedgerows in the upper reaches of the Yangtze River(unpublished data).These prunings are very suitable for raising pigs,cows, goats,and biogas can be produced from the livestock slurry,which in turn can be developed as a clean energy to reduce fuel shortages.Biogas manure can also be returned to the hedgerow systems,so forming an organic agriculture system (Zhang et al.2001b).
All the results have shown that contour hedgerow systems are a practical and suitable technology for mountains or depressed areas in mountainous regions of west and north China because of their simplicity, adaptability,pro?tability,sustainability and environ-mental friendliness.Adoption and extension of hedgerow intercropping in China
In the mountains and uplands of west China and the loess plateau in north China,more and more marginal lands have been exploited,and soil erosion has become increasingly serious due to deforestation, intense cultivation of sloping land and other inap-propriate management of natural resources.For example,more than0.53billion tons of soil is eroded from land in the upper reaches of the Yangtze River and its tributaries every year,with60–80%coming from arable slopelands.The Three Gorges dam in China has been endangered by serious erosion from the upper watershed(Chen et al.2000).Such unsus-tainable practices were encouraged by the implementation of the Western China Development policy in1997.Fortunately,since1999the‘Grain for Green’policy of the Chinese Central Government has been enforced.The aim of this policy is to promote the conversion of steep arable land([25°slope)to forestry and pasture for soil and water conservation and restoration of degraded environments.It is hoped to rehabilitate ecological functioning of about36mil-lion ha of slopeland in western China under this initiative.
However,it is not possible to implement the Grain for Green project everywhere and its effects on food security and economic development of local farmers must also be considered.Hence,it is necessary to investigate new options to generate cash income, enhance crop yield for local farmers and restore ecosystems in the mountains.Contour hedgerow intercropping can be used very easily on gently sloping lands to improve productivity through con-serving cultivation.If farmers obtain suf?cient grain yield and cash income through application of hedge-row intercropping in degraded less steep slopelands, then exploitation and re-exploitation of steep lands for agriculture can be avoided.Therefore,the contour hedgerow intercropping system has attracted much attention from politicians,technicians,researchers, and farmers in many mountain areas.
Southwest China
Dissemination and extension of hedgerow intercrop-ping systems has continuously been carried out after a successful seven-year research project sponsored by
the Project Group of Appropriate Technologies for Soil Conserving Farming Systems(ATSCFS).In 1997hedgerow intercropping was certi?ed and approved as an optimized option for sustainable utilization of slopeland in the Dry Valley of the Jinsha River by a senior specialist committee from the Chinese Academy of Sciences.There have been some innovations of hedgerow intercropping in this project.For instance,mulberry trees and many high value fruits trees and annual crops have been incorporated in the system,and many multistrata agroforestry systems based on hedgerow intercrop-ping were introduced in the project(Tang2001;Tang et al.2001,2002b;Zhang2001;Yuan et al.2000a, 2001b),which was important in meeting the demands of local people and adapting to local socioeconomic situations.
Since then,the contour hedgerow intercropping system,supported by specialists,local government bodies,as well as policy,has been extended as a key technology for soil conservation,soil fertility improvement,and mixed farming in the mountains of subtropical China(He et al.1997,2000,2001; Yuan et al.2000a,2001b;Xie et al.2003).To date, about ten demonstration sites have been established in Sichuan,Yunnan,Giuzhou,and Hunan Province. The system has been applied on arable slopelands of more than5,000ha,and more than20,000farmers have been involved with the extension work.Dem-onstration sites of hedgerow orchards,hedgerow fodder systems,hedgerow mixed farming,hedgerow systems for soil fertility improvement of degraded slopelands have been established for training farmers (Yuan et al.2001b;Xie et al.2003).
It is important to attract farmers’attention to hedgerow intercropping by satisfying their demands in slopelands,especially in depressed mountain regions.Yin et al.(2006)developed participatory methods and successfully expanded hedgerow sys-tems over40ha at six sites in4counties of Guizhou province.
Three Gorges region
A demonstration site of contour hedgerow intercrop-ping was set up in Zigui,Hubei Province,located in the Three Gorges Reservoir area,where related studies have also been conducted in recent years. Some crop arrangement models were tested,and economic bene?ts have also been assessed(Cai and Bu2004;Li1995;Li et al.1998;Zhen and Shen 1998;Shen1998;Xu et al.1999b,2000;Zhao et al. 2004).Following this,local farmers gradually adopted the system for slopeland agriculture.
Some extension works have also been put into practice in the Three Gorges region in recent years, principally for soil erosion control.Hedgerow inter-cropping has been adopted in more than200ha of slopelands along the banks of the Yangtze River,in which hedgerows were established at5m intervals (local government report).Soil loss was controlled while farmers gained cash income by integrating fruit trees into the system,such as sweet orange,mulberry and chestnut(Cai and Bu2004;Shi et al.2005). Hedgerow intercropping had also been adopted by a Sino-German afforestation project in Hubei,and more than200ha at24demonstration and extension sites have been established(Ma and Wang2006). Loess Plateau region
This region is located in the semi-arid zone of North China.Contour hedgerows have been applied on seriously degraded slopelands of more than1,000ha. Hemerocallis fulva and other Chinese traditional herbs have been interplanted in the hedgerows for improving farmers’cash income(Xin and Liang 1997).In Shanxi province,hedgerow intercropping has been promoted by local government and estab-lished on700,000ha of slopeland in the valleys of the Sanchuan and Fenhe rivers,as well as on other mountains for soil erosion and runoff control,reduc-tion of evaporation of water from soil,and mixed farming(Li2000).Many local appropriate species, such as Lonicera japonica,Amorpha fruticosa and Caragana microphylla,were chosen for utilization in hedgerows.
In Shaanxi Province,hedgerow intercropping,also called shrub hedges by some authors,has been applied on more than10,000ha of slopeland,and hedgerow prunings have been used as green manure, mulch,?rewood,and other materials(Wang1994, 2000).Under the system,the yield of food crops has obviously been improved and soil erosion has been controlled.The system has also provided options for farmers to gain more income from perennial cash crops interplanted into hedgerows,such as?atspine pricklyash,pear and chestnut.
Due to the complicated environmental,socioeco-nomic and cultural backgrounds,extension and dissemination has not yet been carried out in many other mountainous agricultural regions with serious soil erosion,fertility deterioration,and seasonal drought.Hedgerow intercropping systems have been considered as an effective and low-cost alternative for sustainable utilization of agricultural slopelands and economic development in erodible or depressed areas by more and more researchers and policy-makers(Li et al.2001;Zhangpeng2001a;Guo et al. 2001;Ma et al.2006;Yin et al.2006).It is expected that further studies will focus more on innovations and improvements of contour hedgerow intercropping,and that more dissemination and extension of the system can be carried out in the course of implementation of the Western China Development policy for sustainable agriculture, environmental protection and use of natural resources.
Some problems of contour hedgerow intercropping in China
Some misunderstandings about contour hedgerow intercropping
Although its advantages have been recognized by local government of?cials and target farmers,it is dif?cult to expand hedgerow intercropping quickly on a large scale.The problems are partly attributed to the economically-orientated development in most rural areas after enforcement of reform and open-ing-up policy since the1980s.Hedgerow intercropping systems are usually considered as just another ecological model because it cannot produce effects on local economic development instantly or provide a very signi?cant increase in yield or cash income after one or two crop seasons.
Another prejudice against the hedgerow intercrop-ping system is that many farmers in the mountains take it for granted that the hedgerows will compete for soil moisture and nutrients with crops.It is dif?cult to persuade them that the relationship between trees and crops in this system is not the same as other tree-crop interplanting systems in which competition often happens and crop yields are decreased.Contour hedgerow intercropping systems and contour barrier strips
Elsewhere in the world,studies and demonstrations of hedgerow intercropping have usually been carried out in tropics of Africa,America,and Southeast Asia (Kang et al.1981,1987,1990;Kiepe1995;Nair 1993;Lal1989;Comia et al.1994;Sanginga et al. 1994;Ong et al.1996;Palmer1997;Shively1998; Lupwayi and Haque1998).However,in China studies and extensions of contour hedgerows have been conducted principally in subtropical and tem-perate zones.Woody species were commonly used for hedgerows,and it is very interesting that nitrogen-?xing trees and shrubs(NFTS)were used at almost all the test sites in China.
Apart from woody species,grasses have also been used for hedgerows in some places for soil and water conservation in the slopelands of China (Cheng1998;Xia et al.2002).These are usually called contour strips or contour barrier strips (Thapa et al.1999).Vetiver grass(Vetiveria zizanioides)was considered to be the most prefer-able species for hedges(Cheng1998),and some tests and research work has been carried out sponsored by The World Bank and ISBRAM(Chen and Hu1994;Xia and Shu2001).There are three objectives for vetiver systems:soil erosion control (slope stabilization and sediment deposition for reservoirs,dams,roads,and highways,etc.)(Cheng 1998;Xia et al.2002),phytoremediation and eco-system restoration(Xia and Shu2001),and income generation(vetiver leaves for mushroom cultiva-tion)(Cheng1998).
Vetiver,and all other grass species used in hedges,is suitable for water and soil control in gently sloping lands.However,it is very dif?cult to use successfully in steeply sloping lands and mountain areas because its shoots are not strong enough to hold up large amounts of soil from the upper slope or to act as bio-terrace risers that are formed by woody hedgerow species.In addition, vetiver does not provide suf?cient options for farmers to diversify cash income,and belowground competition may also occur between vetiver hedges and companion annual crops.Therefore,it is very dif?cult for farmers to adopt and apply vetiver in agricultural zones,especially in intensive agriculture in mountain regions,even if vetiver hedges may be
ef?cient in conserving soil and water.It seems that grass species for hedgerow will require much further modi?cation before they will?nd practical application in China due to the socioeconomic factors.
Height and density control of hedgerows
Contour hedgerow intercropping is characterized by a double-row of plants with at high density along the contour.The distance between the rows of double hedgerows varies from30to60cm(Li and Shi1996; Tang2000a).When double hedgerows grow to over 1m in height,they are pruned down to about50cm to reduce shading of crops in the alleys and at the same time providing prunings for green manure or other utilization(Tang1994;Tang et al.1999,2001; Sun et al.1999a,2001a).If the interval between individual plants within the hedgerow row is more than10cm,the effect on soil conservation is very poor(Li and Shi1996;Sun et al.1999a,2001b). Therefore,management of hedgerow intercropping systems involves replanting and gap?lling to make double hedgerows denser(Tang2000a).
Hedgerow intercropping is very different from another agroforestry system,which intercrops tim-ber or fruit trees with food crops in arable lands in the plains of the north and northeast of China.This system,in which trees are scattered on arable land and act as windbreaks to protect food crops from wind,is also called a silvo-agricultural system. Although some researchers have used fruit trees as hedgerow species in these areas and also called the system hedgerow intercropping or alley cropping, in which fruit trees are planted along contour lines in gentle slopelands(Chen et al.2001,2002;Zhu and Chen2006),this is not the same as a hedgerow system in hilly and mountain regions.Trees in this system cannot be cut or pruned to restrain their height,which leads to intense competition for light and moisture between trees and annual crops,and a signi?cant decrease in crop yield(Chen et al.2001, 2002).Nevertheless,it is possible that cash returns could also be improved and soil and water conservation enhanced after application of the system according to the report from Zhu and Chen (2006).However,the results of soil erosion control under this tree/crop system should be examined carefully.Distance between hedgerows
The space between the contour hedgerows is used for agricultural and cash crops.The distance of neigh-boring hedgerows,or width of the alleys,should be determined primarily by the biomass the hedgerow supplies and the width of the bio-terrace formed by the system.Hedgerow prunings should be suf?cient for the alley to be mulched to conserve soil and water and meet the nutrient demands of crops.Stable bio-terraces are formed3–5years after establishment of contour hedgerows,and alley width of4–8m is suitable for routine cultivation by farmers(Sun et al. 2002b;Xu et al.1999a).
The distance between hedgerows ranges from3m to6m in the subtropics,varying with hedgerow growth and gradient of slope(Tang1994,2000a; Tang et al.2003b),whereas the maximum alley width in the Three Gorges area of the Yangtze River exceeded15m(Li et al.1998).Distance between hedgerows determines the ef?ciency of erosion control occurring in the alley,hence alley width should be varied with the slope,soil erodibility, runoff and other parameters of the slope erosion process(Li et al.1998;Xu et al.2000;Cai and Bu 2004).
Soil erosion always occurs to some extent on slopes,even on gentle ones.However,in contour hedgerow systems,soil eroded from the upper part of the alley is deposited at the lower end,given that hedgerows supply a stable barrier strip dense enough to obstruct runoff and suf?cient biomass to mulch the alley.Therefore,instead of rill erosion control, supply of prunings and appropriate alley width for cultivation,should be taken into account when planning alley spacing or distance between hedge-rows on slopelands.
Further study and extension of contour hedgerows in China
Prospects for further research
More than two-thirds of China’s land is mountainous and hilly.Local people and governments focus more on economic development today,though environ-mental protection also needs more attention in the fragile mountain areas.Contour hedgerow
intercropping,as a feasible alternative to sustainable agriculture,has been proven through extensive research to have great potential in economic and environmental advancement.However,many prob-lems remain and research on basic understanding of the system is still needed for its application in the subtropical mountains of China due to diverse socioeconomic needs,natural backgrounds and cul-tural conditions.
First,it is crucial that more nitrogen-?xing tree species(NFTs)should be collected and tested for their suitability as hedgerows bearing in mind the demands of farmers from mountain areas with diverse environmental,social and economic backgrounds. Collecting and screening local and exotic species for hedgerows on different soil types and altitudes has been done preliminarily in southwest China by Zhang (2001).More systematic works,such as a NFTs database for different purposes in different regions, should be established,and more hedgerow intercrop-ping systems with appropriate species are expected to be established in different eco-zones for further extension,dissemination and application of hedgerow intercropping at a larger scale.
Secondly,belowground interactions between hedgerows and crops are still poorly known,even in those hedgerow intercropping systems which have been successfully tested and applied.For instance,in the hedgerow intercropping system consisting of hedgerow species,mulberry trees within hedgerows, and sweet orange trees in alleys,the relationships of these three components seem to be very complemen-tary but the basis of this is not understood(Zhang et al.2001a;Yuan et al.2001a;Tang et al.2003a). Another question is how much nitrogen and organic matter is contributed by the underground parts of hedgerows,though spatial heterogeneity of soil nutrients induced by hedgerows has been reported (Sun et al.2002a).Additionally,?ne root turnover of different hedgerow species is not clear yet.
Lastly,for all agricultural systems it is necessary to understand dynamics and cycling patterns of nutrients.Some preliminary studies on nutrient dynamics,such as nutrient mineralization from prunings,and nutrient budgets,have been carried out in recent years(Sun et al.1999b,2002b,2002c, 2003).In mountain regions of China,agriculture was developed primarily based on use of chemical fertilizers and herbicides over the past20years.Some emerging issues,such as non-point pollution, soil degradation,loss of natural enemies of crop pests,and high cash input,have highlighted its disadvantages and unsustainability.Hedgerow inter-cropping provides a promising new option to control soil loss and weed growth,improve cash income and ecological bene?t,and develop organic agriculture or green agriculture in mountain areas in a low-cost, acceptable and simple way.In the opinion of the authors of this review,more studies of the intercrop-ping system,especially the cycling patterns of nutrients,nutrient recovery,nutrient balance,crop arrangement,and nutrient mineralization of prunings, are required in the subtropical mountains of China. Further dissemination and extension
According to past experiences reported in published materials,other than just research,we also?nd demonstrations,training,awareness-spreading and policy-support are necessary for successful extension work of hedgerow intercropping.
Demonstration.Seeing is believing.Demonstra-tion is crucial to extending new technologies because other forms of information are inaccessible in undeveloped mountain areas.Farmers will follow and modify the system voluntarily if it is thought to be practical and pro?table,so if demonstration sites are monitored very well and succeed in raising cash income,the target farmers will be in?uenced strongly and continuously.
Training.Training is another important way of disseminating and extending hedgerow intercropping (He et al.1997,2000).Experienced training groups and simple and detailed training manuals are the basic support for training.It would be excellent to have of?cers,technicians and farmers trained locally or on-site,and to use local materials as tools in the training.More innovations are required for this system in depressed mountain areas.
Awareness-spreading and policy-support.It is very important to spread awareness of contour hedgerow intercropping to high-ranking of?cials and experts. Training materials,videos and information on exten-sion of contour hedgerow intercropping have been released to some related institutions such as the Territorial Bureau,Bureau of Soil and Water Conser-vation in China(He et al.2000).This helps to ensure that the system is taken into account when related
projects and policies are programmed.Meanwhile, application of contour hedgerows is not only an ecological construction,but also an opportunity for local people in the mountains.Because at?rst hedge-row intercropping was generally considered to be an ecological technology by farmers,policy support from government would be helpful to extend the technology on a large scale,especially in the early stages.Contour hedgerow intercropping would be disseminated and extended ef?ciently,if local government or NGOs are able to provide cash,materials(such as seeds for hedgerow establishment)and subsidies,especially for pioneer farmers who bear the risks of failure or output reduction(Yuan et al.2001b).
Acknowledgements The authors gratefully acknowledge the anonymous reviewers for their helpful comments on the manuscript.This work is?nancially supported by the National Basic Research program of China(Grant No. 2005CB422006),and Appropriate Technology for Soil Conserving in Farming System(ATSCFS)Project from the International Center for Integrated Mountain Development (ICIMOD).
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