A TREE CROP INTERFACE ORIENTATION EXPERIMENT WITH A GREVILLEA-ROBUSTA HEDGEROW AND MAIZE

Pruned triple-row hedges of Grevillea robusta with 11 rows of maize planted in parallel on either side were grown in 12.5 x 16.0 m plots on a flat, freely exposed site. There were two replicates of four orientation treatments and crop yield was recorded row-by-row over 9 seasons. After the hedges had become established there were some marked differences in maize yields between sides, especially in the first rainy seasons. The maize on the downside of prevailing winds frequently showed improved growth and yield over the non-sheltered maize, sometimes by as much as 50% over the whole subplot and up to 80% in the tree/crop interface zone. These differences were significant (p less-than-or-equal-to 0.05) when analyzing the whole experimental period for first and for second rainy seasons separately (prevailing winds more southerly or northerly, respectively), although they varied between seasons and zones. Attributing results to 'shelter' can only be adduced circumstantially because meteorological data were not collected at the plot level. Shading effect were not considered important over the whole sub-plot. Soil water profiles obtained from an array of gypsum resistance blocks in two of the plots (at 000-degrees and 090-degrees) characterized the wetting and drying patterns in this system, but expected asymmetries due to orientation effects were not apparent except, perhaps, fleetingly in the topsoil. At the end of the experiment the distribution of fine roots of both species was examined by means of root trenching. The G. robusta root system extended almost symmetrically to the edges of the plot in the uncropped part, but only to some 4 m from either side of the hedge where cropped. Part of the upper profile to 1.3 m was shared with the maize roots, but Grevillea rooted further down to at least 2 m. as well as rooting deeper. The possible implications of such niche differentiation for below-ground resource capture are noted.