J.W. Palmer, S.J. Wertheim
In the first one or two cropping years yield is often linearly related to tree density. In later years total yield per tree declines at close spacing compared to wide spacings so that yield per hectare becomes a curvilinear function of tree density or even declines at very high densities. Fruit size is usually large on young trees but in later years although high tree densities can continue to give high yields, the yield per hectare of large fruit often shows no increase over the yields at lower densities, particularly in the case of within-row spacing trials. The yield of large well-coloured fruits can show decline at close within-row spacings.

Unfortunately it has been difficult to extract from the literature detailed records of yield and fruit size from spacing and systems trials. Even where good records for fruit size are available it has not always been appreciated that small differences in mean fruit size have large effects on the yiel of fruit in the economically important size grades and the data has not been presented in enough detail.

The effects of within-row spacing on size and colour suggest that competition for light is the causative factor, as shading is known to reduce fruit size and colour. Computer models of light distribution within trees indicate that, within a given tree size, a certain point is reached where any further increase in leaf area does not increase the well-illuminated area but merely moves it to the periphery of the tree. Consequently the quantity of large fruit borne by those leaves in the well-illuminated parts of the tree reaches a plateau.

Shading is not the only factor known to reduce fruit size, crop load, temperature, competition for water and the age of the spurs bearing the fruit can all affect size and these factors can act differentially at different densities. In 1976 drought at East Malling resulted in a fourfold reduction in the yield per hectare of large Golden Delicious apples at a within-row spacing of 0,9 m compared to 2,7 m, although total yield was slightly higher at the closer spacing.

The relationship between fruit quality and tree density is not a simple one as it depends not only on site, rootstock/scion combination and age of tree but also on tree arrangement. Trials at East Malling and Wilhelminadorp have shown the advantages of close between-row spacing as a more effective way of increasing yield of large fruit compared to close within-row spacings.

Palmer, J.W. and Wertheim, S.J. (1981). EFFECTS OF TREE DENSITY ON FRUIT QUALITY. Acta Hortic. 114, 139-142
DOI: 10.17660/ActaHortic.1981.114.15

Acta Horticulturae