Root competition between coconut palms and interplanted multipurpose trees under varying nutrients management regimes

Abstract

Coconut based production systems in the tropics often aim at improved resource capture through incorporating several trees and field crops. However, competition between the system components are probable when multipurpose trees are systematically interplanted in the coconut plantations. Soil fertility regimes are presumably important in determining the magnitude of below ground competitive interactions. Hence a study was conducted to evaluate the influence of interplanted multipurpose trees on coconut productivity along a soil fertility gradient, to assess the performance of multipurpose trees and to determine the nature of below ground interactions between coconut palms and multipurpose trees, at Vellanikkara since 1992. Treatments included combinations of cocounut with anyone of the three multipurpose trees namely, Vateria indica, Ailanthus triphysa and Grevillea robusta, following two planting geometries(randomised block design, replicated thrice). A soil fertility gradient, was super-imposed with high, medium and low fertility levels in 2000. 32p soil injection technique was employed to characterize . root interaction and logarithmic spiral trenching technique for evaluating root distribution pattern. Results show that coconut yield was not adversely affected by multipurpose trees interplanting until the trees reached eight years of age. Vateria, ailanthus and grevillea showed marked variations in their growth rates. Initially vateria recorded higher height and radial growth albeit ailanthus registered higher growth rates subsequently. Isotopic studies reveal that 32p absorption by coconut palms was similar in both sole and mixed cropping situations along the fertility gradient; probably implying the non-interfering nature of multipurpose trees. 32p absorption by vateria and ailanthus suggests that the absorption of radioactive phosphorus declined linearly with increasing distance ie. 32p absorption by multipurpose trees also did not affect the 32p uptake by coconut, suggesting that integrated land use systems involving multipurpose trees and coconut are ideally suited for improved resource capture and increased system productivity. Excavation of multipurpose tree root systems showed that proximal locations recorded higher rooting intensities and that the rooting intensities decreased with increasing distance. Size of the trees showed discernible differences in respect of spatial root distribution pattern. Large sized trees showed higher root distribution compared to small and medium. The first 10cm soil layer recorded the highest rooting intensities. Ailanthus roots were distributed upto a maximum distance of 469 cm, vateria upto 163 cm and grevillea upto 227 cm. Implicit in this is the species-dependent'variations in lateral root spread. In general, vateria and ailanthus have a well developed and ramified root systems. Grevillea, however, had a less spreading root systems. Selection of tree species with low root competitiveness and/or trees with complementary root interactions is of strategic importance in agroforestry.