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Overyield of Taxodium ascendens-intercrop systems
Forest Ecology and Management 116 (1999) 33±38
Overyield of Taxodium ascendens-intercrop systems Wending Huang*, Qifen Xu Department of Forest Ecology P.O. Box 28 (Viikin koetila 20) Fin-00014 University of Helsinki, Helsinki, Finland Received 30 September 1997; accepted 14 July 1998
Abstract A study of yield advantages in agroforestry systems was conducted using 9 years' data from the ®eld experiments of Taxodium ascendens-intercrop systems in Lixiahe, Jiangsu Province, China. In the productive coexistence system of T. ascendensintercrops, the growth of T. ascendens was not signi®cantly affected by wheat, rape, soybean and mung bean. When the trees were young (the ®rst 3 years), they did not depress crop yields, a land equivalent ratio greater than unity was thus obtained together with a high yield of both components. The relative yields of the intercrops in a tree stand density of 1667 stems haÿ1 in four different spacing con®gurations fell below unity in the four-year-old stands. The relative yield of wheat, soybean and rape was well below unity in the ®ve-year-old stands. The relative yields of intercrops rose when ryegrass was intercropped in the six- and seven-year-old stands. This suggested that the relative yield and land equivalent ratio could be increased by substituting the high light-demanding crops with the shade-tolerant crops as tree shade increased. All land equivalent ratios of various intercropping types were above 1.0. The managed agroforestry systems were advantageous as compared with the monocultures of trees or arable crops, even though the relative yields of intercrops were below unity. The realized productive coexistence suggests that rape seems to have a higher ecological combining ability with T. ascendens than wheat and soybean. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Agroforestry; Land equivalent ratio; Productive coexistence; Taxodium ascendens; Overyield
1. Introduction In agroforestry systems, species interactions can be advantageous, leading to an increase in total yield or overyielding, a reduction in yield variance, and the maintenance of resources (Anderson and Sinclair, 1993). Overyielding, or yield advantage, has been examined in relation to intercropping, and it occurs where a greater amount of land would be needed, by monocultures, to produce the same quantity of yield *Corresponding author. Tel.: +1-358-9708-5647; fax: +1-3589708-5646; e-mail: [email protected]
that could be produced on 1 ha of polyculture (Vandermeer, 1989). There are many hypotheses concerning these mechanisms(Sanchez, 1995; Young, 1989a,b). In general, resource sharing or greater `resource capture' and facilitation are thought to be of primary importance for overyielding in agroforestry management (Huang and Wang, 1992, Huang et al., 1993). Managing the resource sharing in agroforestry systems is concerned with minimising the mutual interference and maximising the potential of resource capture. In a monoculture, one species cannot utilize all the available ecological resources, and thus, other species can ®t in without excessively disturbing the
0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0378-1127(98)00440-X
34
W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
®rst one. In this case, two species growing together will overyield if there exist resources enough for both of them to grow. The growth of species requires continuous and balanced access to resources, e.g. light, water and nutrients. The quality and quantity of light are affected by the composition of coexistent systems, e.g. plant stature, canopy closure and canopy structure. Water and nutrients, unlike light, are available to plants to a limited extent, even though they can be compensated for from external input. Facilitation is the process in which two individual plants or two populations of plants interact in such a way that at least one exerts a positive effect on the other (Vandermeer, 1989). Overyielding can be obtained by maximizing niche differentiation or `resource capture' and facilitation. This study, using 9 years' data from the ®eld experiments of T. ascendens-intercrop systems, is to determine whether the yield advantage (overyielding) of a species combination is evident in T. ascendens intercropping systems, and to investigate the possible mechanisms of overyielding. 2. Methods Agroforestry systems were set up in the Lixiahe wetlands (328420 ± 338960 N and 1198150 ±1208510 E), Jiangsu Province, China, at an altitude of 1±5 m above sea level, average annual rainfall of around 1000 mm, and mean annual temperature of 14±158C. Experiments with a complete randomized block design at Zhaoguan forestry farm were established in 1984. Four planting spacings of T. ascendens Brongn., with the same overall planting density (1667 trees haÿ1), were 23 m, 1.54 m, 1.25 m, 1.526 m (double-row con®guration, 1.5 mthe distance of inter-plants, 2 mthe distance of narrow inter-rows, and 6 mthe distance of wide inter-rows). There were three replicates for each plant spacing. A total of 12 plots were designed with equal size (0.33 ha/plot). A control plot of trees without intercropping was randomly chosen in each replicate. The arable crop monoculture with three replicates was conducted side by side of each replicate of the four tree spacings. Intercropping was conducted in the four con®gured tree stands. Wheat and rape were intercropped in spring, after wheat (in June) and rape (in
May) were harvested, soybean and mung bean were intercropped immediately. These high light-requiring arable crops were intercropped during the ®rst ®ve years, and ryegrass was intercropped in the following two years. When the tree stands were 3-years-old, ®ve densities of soybean and mung bean (Vigna radiata L. Wilczek) were intercropped in the four stands. Both density experiments of soybean and mung bean were conducted with a complete randomized block design (®ve densities and three replicates). Using bunch seeding, there were three seeds sowed in each bunch. Five sowing densities of soybean seeds were 67.5, 75, 82.5, 90, and 97.5 kg/ha. Five sowing densities of mung bean seeds were 30, 33.75, 37.5, 41.25k, and 45 kg/ha. The trees was randomly sampled. The height, diameter (DBH: stem diameter over bark at 1.3 m) and the crown width and length of the sample trees were measured annually from 1985. Intercrops and monocultural crops, i.e. wheat (Triticum aestivum L.), rape (Brassica napus L.), soybean (Glycine max L.), mung bean (Vigna radiata L.) and ryegrass (Lolium perenne L.) were randomly sampled at harvesting time (ripe for agricultural harvesting), with ®ve replicates (total 15 plots in each spacing, each plot: 11 m). Their grain yields and biomass were measured both in the ®eld and laboratory (oven-drying at 1058C for 24 h). All of the inputs (costs of seeds and labours etc.) and outputs (yields) of the above intercrops were recorded and calculated to determine the threshold of crop production in terms of the productive coexistence criterion. The monetary value of intercrop output included grain yields and straw, but for the trees only the timber volume was calculated. Budgets of inputs and outputs in the whole forest farm were annually recorded and calculated, including investment cost, labour, seeds of intercrops, electricity, irrigation and intercrop yields. Facilitation and resource sharing lead to a bene®cial coexistence in multi-species systems (Huang, 1998). Generally it can be evaluated by the land equivalent ratio (L) (Mead and Willey, 1980): X (1) L Pi Miÿ1 where Pi is the production of species i in the multispecies system, Mi is the production of species i in the monoculture. The land equivalent ratio can be de®ned
W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
35
(, ÿ) shows that the total yield is higher than that in the monoculture, but while, the former implies that the forest is facilitated and its yield is higher than that in monoculture, the intercrop is not, and the latter denotes that the crop is improved and its yield is higher than that in monoculture but the forest is not. The Ef is the ratio (Ef1) of the output of production to the input of the forest management in agroforestry systems, and the Ec is that due to intercropping. The line EfÿEc, using rectangular coordinate systems, denotes the critical risk criterion where output and input are equal in economic value or pro®t index (PI) equals 1.0. If the PI is below the line EfÿEc, it shows that both the components are in negative pro®t, which is unacceptable in practice. It is only acceptable if both components are above 1.0, de®ning the region of productive coexistence (Fig. 1). Fig. 1. Graphical evaluation of productive coexistence in agroforestry systems. Quadrant I, II, III and IV are the facilitation or competition types. L is the land equivalent ratio, and PI is the profit index. Tm is the yield of a tree species in monoculture, and Cm is the yield of an arable crop in monoculture. The Ef is the ratio of the output of production to the input of forest management in agroforestry systems, and the Ec is that due to intercropping. The line EfÿEc denotes the critical risk criterion where output and input are equal in economic value.
as the amount of land needed to produce as much in monocultures as can be produced on the same area of polyculture (Vandermeer, 1989; Anderson and Sinclair, 1993). To determine whether each species is facilitated, the relative production, Ri, (or relative yield) was calculated as R Pi Miÿ1
(2)
In productive coexistence systems, the existence or removal of one species or population is limited by the balance of its output and input (Fig. 1). The line Tm± Cm is the criterion where L1.0. If L is higher than 1.0, it indicates overyielding (i.e. the polyculture is more productive than monoculture). The quadrant I (, ) shows that the yield of each species in agroforestry is higher than that in the monoculture. III (ÿ, ÿ) denotes that the yield of each of them in the agroforestry is lower than that in the monoculture, but it is still advantageous if L is above 1.0. II (ÿ, ) and IV
3. Results 3.1. Effect of tree stand spacings on intercrops The results did not show signi®cant differences in tree height (p 0.412) and diameter growth (p0.079) between four tree stand spacings when tree stands were under the age of eight. The height growth was statistically different between tree spacings at the age of nine (p0.022). There was no distinct effect of different tree stand spacings on the growth of intercrops, e.g. wheat, rape, soybean and mung bean (p>0.05). However, the yields of wheat, rape, soybean and mung bean seemed to be lower in the spacing of 2 m3 m than in the spacings of 1.5 m4 m, 1.2 m5 m and 1.5 m2 m6 m. These may be caused by the narrow tree inter-rows (3 m between two tree rows), which was the narrowest row among the four spacings. The arable crops were intercropped between tree rows, about 50 cm away from the tree line. On analysis of the sowing density experiments for soybean and mung bean in the four tree spacings, there were no signi®cant differences (p>0.05) in the yields of soybean and mung bean between the ®ve sowing densities. However, the highest yield of the soybean was at the sowing density of 82.5 kg haÿ1, and that of mung bean was at the sowing density of 37.5 kg haÿ1. The results did not show signi®cant differences in the
36
W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
Fig. 2. Relative production of different crops in intercropping systems in Jiangsu Province, China. R denotes the relative production of arable crops.
Fig. 3. Relative production of two-season-crops in intercropping systems in Jiangsu Province, China. R denotes the relative production of arable crops.
yields of soybean and mung bean between four tree spacings (p>0.05).
0.4 (Fig. 3) in the 5-year-old stands, a strong negative interaction.
3.2. Relative production
3.3. Yield advantages of productive coexistence systems
When the tree stands were from 2 to 9-years-old, the relative production of T. ascendens was around 1.0 in all four different spacings. This suggests that tree species was not affected by intercrops. On the contrary, the relative productions (R) of wheat, soybean, rape, mung bean and ryegrass were negatively affected when the ages of tree stands were four years and more (R<1.0) (Fig. 2). The relative productions of wheat, soybean and rape were markedly reduced in the ®veyear-old stands. The relative productions of intercrop rose when the ryegrass was intercropped in the 6- and 7-year-old stands. This suggests that the relative production and land equivalent ratio could probably be increased by substituting the high light-demanding crops with the shade-tolerant crops. This is of signi®cance for extending the period of intercropping when the tree canopy is nearly closed. In the two seasonal intercroppings, the relative productions of wheatsoybean, wheatmung bean, rapesoybean and rapemung bean were below 1.0 when tree stands were 3-year-old and more, and those of wheatsoybean and rapesoybean reduced below
All land equivalent ratios of various intercropping types were above 1.0 (Fig. 4). This suggests that the managed agroforestry systems were advantageous as compared with the monocultures, even though the growth of most intercrops was depressed by tree species. The facilitations or competitions between T. ascendens and arable crops appeared in quadrant II (ÿ, ), III (ÿ, ÿ) and IV (,ÿ) (Fig. 4). There was very little evidence to identify whether or not trees and arable crops were facilitated because these values were only just on the boundary of quadrant I (, ), II (ÿ, ) and IV (, ÿ). The realized productive coexistence (RPC) in three intercropping types, i.e. the T. ascendenswheat, the T. ascendenssoybean and the T. ascendensrape. The squarely shaded area of RPC in quadrant I (, ) and IV (, ÿ) were larger in the T. ascendensrape than in the T. ascendenswheat and the T. ascendenssoybean, suggesting that rape may have a higher ecological combining ability with T. ascendens
W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
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Fig. 4. Relationship between the relative production of trees and the relative production of arable crops in Jiangsu Province, China. The symbol & is the relative biomass productions in the 2-year-old tree stands, * is the relative biomass productions in the 3-year-old stands, is the relative biomass productions in the 4-year-old stands and * is the relative biomass productions in the five-year-old stands. L denotes the land equivalent ratio. Square areas represent the realized productive coexistence. LER denotes the land equivalent ratio.
than wheat and soybean in agroforestry systems (Fig. 4). 4. Discussion The results demonstrated that the diameter (DBH) and height growth of T. ascendens were not markedly in¯uenced by intercropped wheat and soybean, indicating that intercropping under trees produced extra yields but did not depress the tree growth, and suggesting that the resource sharing was operative, i.e: intercropping arable crops under trees is able to tap the
available resources to increase the productivity without signi®cantly interfering with the growth of principal species. Obviously, it was of interest in productive systems to increase the land-use ef®ciency. When the trees were young (the ®rst 3 years), they did not depress crop yields. A higher L was thus obtained together with high yield of trees and arable crops. After the tree stands were 3-years-old, the trees exerted a negative in¯uence on intercrops, so that although the L remained higher than 1.0, the yield of the crop component was lower in the agroforestry than in the monoculture.
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W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
Rape seems to have a higher ecological combining ability with T. ascendens than wheat and soybean (Fig. 4). Possible reasons for this are that the competition of rape with trees for light is less than that of wheat or soybean. In this context, the leaves of T. ascendens appear from 10th to 20th of April, and rape is ripe for harvesting from 15th to 25th of May, while wheat is ripe for harvesting from 10th to 20th of June. Soybean is sown after rape or wheat is harvested, and the whole growth period of soybean is in competition with trees for light. In the present study, the facilitation of tree species on intercrops were not identi®ed, but this does not mean that trees do not have any positive effects on intercrops, because the competitive effect of trees on intercrops is often stronger than the effect of the trees on environmental improvement. For instance, the air humidity in the 4-year-old stands of T. ascendens increased by 0.7±4% points as compared to the open ®elds (Huang and Huang, 1991a, b), and the wind in the tree belts of ®ve rows (tree height: 8.5 m; canopy closure: 0.8±0.9) was 1.14 m sÿ1 while the wind in open ®elds was 2.3 m sÿ1(Gao, 1987). It is possible that tree stands can protect the intercrops from wind and drought stress. Acknowledgements We are grateful to Professor Olavi Luukkanen for valuable comments on the manuscript. We thank Zhang Lijun, Hua Baiyin and Wang Nian for ®eld
assistance. The authors are indebted to the Academy of Finland for ®nancial support. References Anderson, L.S., Sinclair, F.L., 1993. Ecological interaction in agroforestry systems. Agroforestry Abstracts 6(2), 57±91. Gao, L., 1987. Microclimate comparison in agroforestry systems in the Lixiahe wetland. In: W. Shiong (Ed.), Proc. National Symp. Agroforestry Ecol. Syst. North-east Forestry University Press of China, Harbin, pp. 34±36. Huang, B., Huang, W., 1991a. Ecology systems of agroforestry. Chinese J. Ecol. 10(3), 27±32. Huang, B., Huang, W., 1991b. Agroforestry in Wetlands. Science and Technology Press of Jiangsu, Nanjing, China. Huang, W., 1998. Modelling the coexistence gain and interactions of populations in Taxodium ascendens-intercrop systems, ecological modelling 107, 189±212. Huang, W., Zhang, X., Tang, Y., 1993. Compound Agriculture in China. Science and Technology Press of Jiangsu, Nanjing, China, p. 375. Huang, W., Wang, H., 1992. Agroforestry Management. China Forestry Press, Beijing, China, p. 193. Mead, R., Willey, R.W., 1980. The concept of a Land Equivalent Ratio and advantages in yields from intercropping. Exp. Agric. 16, 217±228. Sanchez, P.A., 1995. Science in agroforestry. Agroforestry Syst. 30, 5±55. Vandermeer, J., 1989. The Ecology of Intercropping. Cambridge University Press, Cambridge, p. 237. Young, A., 1989a. Ten hypotheses for soil-agroforestry research. Agroforestry Today 1, 13±16. Young, A., 1989b. Environmental basis of agroforestry. In: Reifsnyder, W.S., Darnhofer, T.O. (Eds.), Meteorology and Agroforestry, Proceedings of An International Workshop on the Application of Meteorology to Agroforestry Systems Planning and Management, ICRAF, pp. 29±49.
Overyield of Taxodium ascendens-intercrop systems Wending Huang*, Qifen Xu Department of Forest Ecology P.O. Box 28 (Viikin koetila 20) Fin-00014 University of Helsinki, Helsinki, Finland Received 30 September 1997; accepted 14 July 1998
Abstract A study of yield advantages in agroforestry systems was conducted using 9 years' data from the ®eld experiments of Taxodium ascendens-intercrop systems in Lixiahe, Jiangsu Province, China. In the productive coexistence system of T. ascendensintercrops, the growth of T. ascendens was not signi®cantly affected by wheat, rape, soybean and mung bean. When the trees were young (the ®rst 3 years), they did not depress crop yields, a land equivalent ratio greater than unity was thus obtained together with a high yield of both components. The relative yields of the intercrops in a tree stand density of 1667 stems haÿ1 in four different spacing con®gurations fell below unity in the four-year-old stands. The relative yield of wheat, soybean and rape was well below unity in the ®ve-year-old stands. The relative yields of intercrops rose when ryegrass was intercropped in the six- and seven-year-old stands. This suggested that the relative yield and land equivalent ratio could be increased by substituting the high light-demanding crops with the shade-tolerant crops as tree shade increased. All land equivalent ratios of various intercropping types were above 1.0. The managed agroforestry systems were advantageous as compared with the monocultures of trees or arable crops, even though the relative yields of intercrops were below unity. The realized productive coexistence suggests that rape seems to have a higher ecological combining ability with T. ascendens than wheat and soybean. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Agroforestry; Land equivalent ratio; Productive coexistence; Taxodium ascendens; Overyield
1. Introduction In agroforestry systems, species interactions can be advantageous, leading to an increase in total yield or overyielding, a reduction in yield variance, and the maintenance of resources (Anderson and Sinclair, 1993). Overyielding, or yield advantage, has been examined in relation to intercropping, and it occurs where a greater amount of land would be needed, by monocultures, to produce the same quantity of yield *Corresponding author. Tel.: +1-358-9708-5647; fax: +1-3589708-5646; e-mail: [email protected]
that could be produced on 1 ha of polyculture (Vandermeer, 1989). There are many hypotheses concerning these mechanisms(Sanchez, 1995; Young, 1989a,b). In general, resource sharing or greater `resource capture' and facilitation are thought to be of primary importance for overyielding in agroforestry management (Huang and Wang, 1992, Huang et al., 1993). Managing the resource sharing in agroforestry systems is concerned with minimising the mutual interference and maximising the potential of resource capture. In a monoculture, one species cannot utilize all the available ecological resources, and thus, other species can ®t in without excessively disturbing the
0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0378-1127(98)00440-X
34
W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
®rst one. In this case, two species growing together will overyield if there exist resources enough for both of them to grow. The growth of species requires continuous and balanced access to resources, e.g. light, water and nutrients. The quality and quantity of light are affected by the composition of coexistent systems, e.g. plant stature, canopy closure and canopy structure. Water and nutrients, unlike light, are available to plants to a limited extent, even though they can be compensated for from external input. Facilitation is the process in which two individual plants or two populations of plants interact in such a way that at least one exerts a positive effect on the other (Vandermeer, 1989). Overyielding can be obtained by maximizing niche differentiation or `resource capture' and facilitation. This study, using 9 years' data from the ®eld experiments of T. ascendens-intercrop systems, is to determine whether the yield advantage (overyielding) of a species combination is evident in T. ascendens intercropping systems, and to investigate the possible mechanisms of overyielding. 2. Methods Agroforestry systems were set up in the Lixiahe wetlands (328420 ± 338960 N and 1198150 ±1208510 E), Jiangsu Province, China, at an altitude of 1±5 m above sea level, average annual rainfall of around 1000 mm, and mean annual temperature of 14±158C. Experiments with a complete randomized block design at Zhaoguan forestry farm were established in 1984. Four planting spacings of T. ascendens Brongn., with the same overall planting density (1667 trees haÿ1), were 23 m, 1.54 m, 1.25 m, 1.526 m (double-row con®guration, 1.5 mthe distance of inter-plants, 2 mthe distance of narrow inter-rows, and 6 mthe distance of wide inter-rows). There were three replicates for each plant spacing. A total of 12 plots were designed with equal size (0.33 ha/plot). A control plot of trees without intercropping was randomly chosen in each replicate. The arable crop monoculture with three replicates was conducted side by side of each replicate of the four tree spacings. Intercropping was conducted in the four con®gured tree stands. Wheat and rape were intercropped in spring, after wheat (in June) and rape (in
May) were harvested, soybean and mung bean were intercropped immediately. These high light-requiring arable crops were intercropped during the ®rst ®ve years, and ryegrass was intercropped in the following two years. When the tree stands were 3-years-old, ®ve densities of soybean and mung bean (Vigna radiata L. Wilczek) were intercropped in the four stands. Both density experiments of soybean and mung bean were conducted with a complete randomized block design (®ve densities and three replicates). Using bunch seeding, there were three seeds sowed in each bunch. Five sowing densities of soybean seeds were 67.5, 75, 82.5, 90, and 97.5 kg/ha. Five sowing densities of mung bean seeds were 30, 33.75, 37.5, 41.25k, and 45 kg/ha. The trees was randomly sampled. The height, diameter (DBH: stem diameter over bark at 1.3 m) and the crown width and length of the sample trees were measured annually from 1985. Intercrops and monocultural crops, i.e. wheat (Triticum aestivum L.), rape (Brassica napus L.), soybean (Glycine max L.), mung bean (Vigna radiata L.) and ryegrass (Lolium perenne L.) were randomly sampled at harvesting time (ripe for agricultural harvesting), with ®ve replicates (total 15 plots in each spacing, each plot: 11 m). Their grain yields and biomass were measured both in the ®eld and laboratory (oven-drying at 1058C for 24 h). All of the inputs (costs of seeds and labours etc.) and outputs (yields) of the above intercrops were recorded and calculated to determine the threshold of crop production in terms of the productive coexistence criterion. The monetary value of intercrop output included grain yields and straw, but for the trees only the timber volume was calculated. Budgets of inputs and outputs in the whole forest farm were annually recorded and calculated, including investment cost, labour, seeds of intercrops, electricity, irrigation and intercrop yields. Facilitation and resource sharing lead to a bene®cial coexistence in multi-species systems (Huang, 1998). Generally it can be evaluated by the land equivalent ratio (L) (Mead and Willey, 1980): X (1) L Pi Miÿ1 where Pi is the production of species i in the multispecies system, Mi is the production of species i in the monoculture. The land equivalent ratio can be de®ned
W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
35
(, ÿ) shows that the total yield is higher than that in the monoculture, but while, the former implies that the forest is facilitated and its yield is higher than that in monoculture, the intercrop is not, and the latter denotes that the crop is improved and its yield is higher than that in monoculture but the forest is not. The Ef is the ratio (Ef1) of the output of production to the input of the forest management in agroforestry systems, and the Ec is that due to intercropping. The line EfÿEc, using rectangular coordinate systems, denotes the critical risk criterion where output and input are equal in economic value or pro®t index (PI) equals 1.0. If the PI is below the line EfÿEc, it shows that both the components are in negative pro®t, which is unacceptable in practice. It is only acceptable if both components are above 1.0, de®ning the region of productive coexistence (Fig. 1). Fig. 1. Graphical evaluation of productive coexistence in agroforestry systems. Quadrant I, II, III and IV are the facilitation or competition types. L is the land equivalent ratio, and PI is the profit index. Tm is the yield of a tree species in monoculture, and Cm is the yield of an arable crop in monoculture. The Ef is the ratio of the output of production to the input of forest management in agroforestry systems, and the Ec is that due to intercropping. The line EfÿEc denotes the critical risk criterion where output and input are equal in economic value.
as the amount of land needed to produce as much in monocultures as can be produced on the same area of polyculture (Vandermeer, 1989; Anderson and Sinclair, 1993). To determine whether each species is facilitated, the relative production, Ri, (or relative yield) was calculated as R Pi Miÿ1
(2)
In productive coexistence systems, the existence or removal of one species or population is limited by the balance of its output and input (Fig. 1). The line Tm± Cm is the criterion where L1.0. If L is higher than 1.0, it indicates overyielding (i.e. the polyculture is more productive than monoculture). The quadrant I (, ) shows that the yield of each species in agroforestry is higher than that in the monoculture. III (ÿ, ÿ) denotes that the yield of each of them in the agroforestry is lower than that in the monoculture, but it is still advantageous if L is above 1.0. II (ÿ, ) and IV
3. Results 3.1. Effect of tree stand spacings on intercrops The results did not show signi®cant differences in tree height (p 0.412) and diameter growth (p0.079) between four tree stand spacings when tree stands were under the age of eight. The height growth was statistically different between tree spacings at the age of nine (p0.022). There was no distinct effect of different tree stand spacings on the growth of intercrops, e.g. wheat, rape, soybean and mung bean (p>0.05). However, the yields of wheat, rape, soybean and mung bean seemed to be lower in the spacing of 2 m3 m than in the spacings of 1.5 m4 m, 1.2 m5 m and 1.5 m2 m6 m. These may be caused by the narrow tree inter-rows (3 m between two tree rows), which was the narrowest row among the four spacings. The arable crops were intercropped between tree rows, about 50 cm away from the tree line. On analysis of the sowing density experiments for soybean and mung bean in the four tree spacings, there were no signi®cant differences (p>0.05) in the yields of soybean and mung bean between the ®ve sowing densities. However, the highest yield of the soybean was at the sowing density of 82.5 kg haÿ1, and that of mung bean was at the sowing density of 37.5 kg haÿ1. The results did not show signi®cant differences in the
36
W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
Fig. 2. Relative production of different crops in intercropping systems in Jiangsu Province, China. R denotes the relative production of arable crops.
Fig. 3. Relative production of two-season-crops in intercropping systems in Jiangsu Province, China. R denotes the relative production of arable crops.
yields of soybean and mung bean between four tree spacings (p>0.05).
0.4 (Fig. 3) in the 5-year-old stands, a strong negative interaction.
3.2. Relative production
3.3. Yield advantages of productive coexistence systems
When the tree stands were from 2 to 9-years-old, the relative production of T. ascendens was around 1.0 in all four different spacings. This suggests that tree species was not affected by intercrops. On the contrary, the relative productions (R) of wheat, soybean, rape, mung bean and ryegrass were negatively affected when the ages of tree stands were four years and more (R<1.0) (Fig. 2). The relative productions of wheat, soybean and rape were markedly reduced in the ®veyear-old stands. The relative productions of intercrop rose when the ryegrass was intercropped in the 6- and 7-year-old stands. This suggests that the relative production and land equivalent ratio could probably be increased by substituting the high light-demanding crops with the shade-tolerant crops. This is of signi®cance for extending the period of intercropping when the tree canopy is nearly closed. In the two seasonal intercroppings, the relative productions of wheatsoybean, wheatmung bean, rapesoybean and rapemung bean were below 1.0 when tree stands were 3-year-old and more, and those of wheatsoybean and rapesoybean reduced below
All land equivalent ratios of various intercropping types were above 1.0 (Fig. 4). This suggests that the managed agroforestry systems were advantageous as compared with the monocultures, even though the growth of most intercrops was depressed by tree species. The facilitations or competitions between T. ascendens and arable crops appeared in quadrant II (ÿ, ), III (ÿ, ÿ) and IV (,ÿ) (Fig. 4). There was very little evidence to identify whether or not trees and arable crops were facilitated because these values were only just on the boundary of quadrant I (, ), II (ÿ, ) and IV (, ÿ). The realized productive coexistence (RPC) in three intercropping types, i.e. the T. ascendenswheat, the T. ascendenssoybean and the T. ascendensrape. The squarely shaded area of RPC in quadrant I (, ) and IV (, ÿ) were larger in the T. ascendensrape than in the T. ascendenswheat and the T. ascendenssoybean, suggesting that rape may have a higher ecological combining ability with T. ascendens
W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
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Fig. 4. Relationship between the relative production of trees and the relative production of arable crops in Jiangsu Province, China. The symbol & is the relative biomass productions in the 2-year-old tree stands, * is the relative biomass productions in the 3-year-old stands, is the relative biomass productions in the 4-year-old stands and * is the relative biomass productions in the five-year-old stands. L denotes the land equivalent ratio. Square areas represent the realized productive coexistence. LER denotes the land equivalent ratio.
than wheat and soybean in agroforestry systems (Fig. 4). 4. Discussion The results demonstrated that the diameter (DBH) and height growth of T. ascendens were not markedly in¯uenced by intercropped wheat and soybean, indicating that intercropping under trees produced extra yields but did not depress the tree growth, and suggesting that the resource sharing was operative, i.e: intercropping arable crops under trees is able to tap the
available resources to increase the productivity without signi®cantly interfering with the growth of principal species. Obviously, it was of interest in productive systems to increase the land-use ef®ciency. When the trees were young (the ®rst 3 years), they did not depress crop yields. A higher L was thus obtained together with high yield of trees and arable crops. After the tree stands were 3-years-old, the trees exerted a negative in¯uence on intercrops, so that although the L remained higher than 1.0, the yield of the crop component was lower in the agroforestry than in the monoculture.
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W. Huang, Q. Xu / Forest Ecology and Management 116 (1999) 33±38
Rape seems to have a higher ecological combining ability with T. ascendens than wheat and soybean (Fig. 4). Possible reasons for this are that the competition of rape with trees for light is less than that of wheat or soybean. In this context, the leaves of T. ascendens appear from 10th to 20th of April, and rape is ripe for harvesting from 15th to 25th of May, while wheat is ripe for harvesting from 10th to 20th of June. Soybean is sown after rape or wheat is harvested, and the whole growth period of soybean is in competition with trees for light. In the present study, the facilitation of tree species on intercrops were not identi®ed, but this does not mean that trees do not have any positive effects on intercrops, because the competitive effect of trees on intercrops is often stronger than the effect of the trees on environmental improvement. For instance, the air humidity in the 4-year-old stands of T. ascendens increased by 0.7±4% points as compared to the open ®elds (Huang and Huang, 1991a, b), and the wind in the tree belts of ®ve rows (tree height: 8.5 m; canopy closure: 0.8±0.9) was 1.14 m sÿ1 while the wind in open ®elds was 2.3 m sÿ1(Gao, 1987). It is possible that tree stands can protect the intercrops from wind and drought stress. Acknowledgements We are grateful to Professor Olavi Luukkanen for valuable comments on the manuscript. We thank Zhang Lijun, Hua Baiyin and Wang Nian for ®eld
assistance. The authors are indebted to the Academy of Finland for ®nancial support. References Anderson, L.S., Sinclair, F.L., 1993. Ecological interaction in agroforestry systems. Agroforestry Abstracts 6(2), 57±91. Gao, L., 1987. Microclimate comparison in agroforestry systems in the Lixiahe wetland. In: W. Shiong (Ed.), Proc. National Symp. Agroforestry Ecol. Syst. North-east Forestry University Press of China, Harbin, pp. 34±36. Huang, B., Huang, W., 1991a. Ecology systems of agroforestry. Chinese J. Ecol. 10(3), 27±32. Huang, B., Huang, W., 1991b. Agroforestry in Wetlands. Science and Technology Press of Jiangsu, Nanjing, China. Huang, W., 1998. Modelling the coexistence gain and interactions of populations in Taxodium ascendens-intercrop systems, ecological modelling 107, 189±212. Huang, W., Zhang, X., Tang, Y., 1993. Compound Agriculture in China. Science and Technology Press of Jiangsu, Nanjing, China, p. 375. Huang, W., Wang, H., 1992. Agroforestry Management. China Forestry Press, Beijing, China, p. 193. Mead, R., Willey, R.W., 1980. The concept of a Land Equivalent Ratio and advantages in yields from intercropping. Exp. Agric. 16, 217±228. Sanchez, P.A., 1995. Science in agroforestry. Agroforestry Syst. 30, 5±55. Vandermeer, J., 1989. The Ecology of Intercropping. Cambridge University Press, Cambridge, p. 237. Young, A., 1989a. Ten hypotheses for soil-agroforestry research. Agroforestry Today 1, 13±16. Young, A., 1989b. Environmental basis of agroforestry. In: Reifsnyder, W.S., Darnhofer, T.O. (Eds.), Meteorology and Agroforestry, Proceedings of An International Workshop on the Application of Meteorology to Agroforestry Systems Planning and Management, ICRAF, pp. 29±49.