Tree-Crop Competition

Competition between trees and crops
Plants require light, nutrients and water for their growth and survival; trees, crops and weeds are not unlike in this regard. Thus, hedgerow trees, just like weeds, can compete with companion crops for available light, nutrients and water in an alley cropping system. Tree-crop competition is often believed to be responsible for declining crop yields at the hedgerow-crop interface commonly observed in many alley-cropping trials (Singh et al. 1989, Fernandes 1990, MacLean et al. 1992). Studies on tree-crop competitions in alley cropping have mostly focused on indirect competition through exploitation of shared resources. Direct competition such as allelopathy, if present, can exert significant detrimental pressure on the crop; however, its role in alley cropping has not been studied in detail (MacLean et al. 1992).

Above-ground competition
Competition for solar radiation is the most prominent above-ground competition between hedgerow trees and companion crops. While rapid growth is often considered a desirable characteristic in selecting hedgerow species in order to obtain higher pruning biomass, shading of crop by hedgerow perennials at the tree-crop interface is also greater if pruning -- a labor-costing activity -- is not intensified. In an interhedgerow spacing trial with four woody perennials in the humid tropics, Lawson and Kang (1990) observed that maize yield decreased with increased total dry matter yield of prunings from the associated hedgerow species. Maize grain yield was found to be positively correlated with % light transmission incident on maize at cob height. Yield with 2-m interhedgerow spacing was lower than that with the 4-m spacing, due to greater shading of maize at the narrow spacing. A higher degree of shading was observed with vigorously growing species such as Leucaena leucocephala, as compared to the other species tested. Additionally, on the shorter-stature crops the shading effect became more pronounced. While the negative correlation between shading and crop yield was well established in the study by Lawson and Kang (1990), the relative importance of below-ground root competition in reducing crop yield remained unknown, as it is often difficult to assess in above-ground light-competition studies. In a maize alley cropping study in southern Nigeria, Kang et al. (1981) attributed decreased yields from maize rows adjacent to L. leucocephala hedgerows to shading because there were adequate amounts of moisture in the soil and few tree roots were found within 1 m from the maize row.

Hedgerow shoot prunings can alleviate shading of crops while providing biomass for mulch or green manure. A study in Sumatra by Duguma et al. (1988) showed that less frequent pruning and higher pruning height increased hedgerow biomass yields but at the same time reduced the companion crop yield. Shading can be minimized by intensifying the hedgerow pruning regime, i.e., more frequent pruning and lower pruning height, but this also limits the capacity of hedgerows for biomass production and nutrient recycling (Kang 1993). Studying root biomass dynamics in an alley cropping system with G. sepium on an Alfisol in Côte d'Ivoire, Schroth and Zech (1995) observed that during the cropping season when the hedgerows were regularly pruned, the live root mass decreased in all soil depths. Pruning places a high demand on hedgerow trees to use their energy and nutrient reserves for the regeneration of the lost photosynthetic biomass. The reduction of stem starch reserves after repeated pruning has been reported for G. sepium (Erdmann et al. 1993 in Schroth and Zech 1995). Schroth and Zech (1995) suggested that these energy losses led to a progressive decline in the ability of hedgerows to maintain their fine root mass, which subsequently resulted in decreasing tree productivity. This also supports the assumption that shoot prunings can reduce root competition as well due to the death of some hedgerow roots immediately following shoot pruning (Fernandes 1990).

Below-ground competition
Hedgerow tree roots can compete with crop roots for available water and nutrients in the topsoil. Alley cropping experiments in semi-arid India demonstrated significant water competition between Leucaena leucocephala hedgerows and castor, cowpea, and sorghum (Singh et al. 1989). Crop yields declined from 30 to 150% of the sole crop when distance from the hedge was reduced from 5 to 0.3 m. Competition for water is often considered more important than shading effects under arid and semi-arid conditions, but generally in the humid tropics water is not as limiting as nutrients in the soil. As mentioned earlier, LAC soils in the humid and semihumid tropics are often deficient in a range of nutrients essential for plant growth. One of the often cited benefit of trees in agroforestry is their capacity to intercept and recycle nutrients in the soil that would otherwise be lost through leaching, thereby making a tighter nutrient cycle. This evidently entails active nutrient uptake by trees, regardless of the soil nutrient status, i.e., the availability of nutrients for crop growth. Furthermore, the assumption that woody perennials can efficiently utilize nutrients in deeper soil layers unavailable to shallow-rooting crops is not always true, depending on the rooting pattern of the species used (shallow or deep, spreading or geotropic) and the local soil profile. In the humid zone on acid soils, competition between the hedgerow trees and alley crops for nutrients can be very severe; both woody species and crops have the tendency to concentrate their roots in the topsoil because of the subsoil's high acidity and associated aluminum toxicity (Fernandes 1990, Nair 1993).

In alley cropping systems, continuous application of prunings to the alleys might create a fertility gradient extending from nutrient-depleted soils at the base of the hedgerow into the alley, consequently promoting the growth of hedgerow roots into the alley (Fernandes 1990). Schroth (1995) reviewed tree root characteristics favorable for use in agroforestry and identified tree species selection and root pruning as two of the possible strategies to minimize tree-crop root competitions. Ruhigwa et al. (1992) studied the root distribution patterns of four woody perennials grown on an acid Ultisol, and found Dactyladenia barteri (syn. Acioa barteri) to be a promising hedgerow species for alley cropping in acid soils of the humid tropics. It has the desirable rooting system with fewer fine roots in the surface soil. Its roots that are concentrated close to tree trunk, decrease markedly away from the tree base, and penetrate deeper soil horizons. Rooting patterns of other extensively used or potential woody perennial species have not been studied in detail.

Root pruning -- trenching periodically between hedgerow and crops -- can reduce root competition by limiting laterally spreading fine tree roots in the topsoils where crops are restrained to obtain nutrients. Fernandes (1990) studied the effect of root pruning of Inga edulis hedgerows (4 m apart) on yields of alley-cropped rice on an Ultisol in the Peruvian Amazon. Root pruning was done immediately after sowing the crop and again just prior to tillering -- periods of greatest crop demand for nutrients -- by trenching 25 cm from the base of the hedgerow to a depth of 20 cm, which resulted in an average increase in grain yield of 30% in the row closest to the hedgerow and 15% in the center of the alley (Figure).