In the spring of 1965 a trial was planted with Golden Delicious IX and James Grieve 'aimed' VII, in which tree spacing, deblossoming, Alar sprays and pruning were variable factors, Results are presented over the period 1966-1969.
At the end of 1969, the 5th year from planting, 400 Delicious trees and 160 'aimed' trees were grubbed to measure tree weight and foliation. Tree weight was closely correlated to trunk diameter; for a given diameter pruned trees weighed slightly less than unpruned trees. The relations of leaf weight and leaf area per tree with trunk diameter were less well defined and varied more for different treatment combinations; the same applied to the relations of shoot number and total shoot length over 1966-1969 with trunk diameter at the end of 1969.
The grubbed trees were also used to collect data on dry matter yield for Delicious IX. Estimated dry matter yield per unit area in 1969 still increased with density, largely as a result of increasing 'leaf area indices'. Fruiting greatly enhanced dry matter yield per unit foliage and more than 50% of dry matter yield was recovered in the fruit. Pruning also tended to stimulate dry matter yield per unit leaf area; it raised the share of dry matter accumulated in the tree frame, at the expense of the share for leaves and fruit. A large proportion of dry matter increment in the tree frame was removed by pruning.
Inter-tree competition at close spacing manifested itself in reduced growth and yield per tree, in lower yields per unit growth, in a reduction of mean weight per fruit and in the tendency of the trees to become top-heavy as a result of accelerated height growth and suppressed growth in the basal portion of the trees. These effects became more prominent with increasing age, suggesting that at the closest spacings it will be difficult to control growth and to prevent yield depressions as the trees get older. This leaves the superiority of yield during the early years of orchard life as the main advantage of close spacing.
Initially yield per ha was indeed higher at close tree spacings. Cumulative yields of unpruned Delicious IX over 1966-1969 amounted to 43, 63, 79 and 100 ton per ha for mean densities of 925, 1275, 1820 and 2645 trees per ha respectively; in 1969 yield still increased with density. The corresponding figures for 'aimed' VII were 33, 45, 56 and 58 ton per ha and in 1968 and 1969 yield tended to level off at densities beyond 1275 trees per ha. Yields of Delicious IX virtually all exceeded 1 kg per cm 2
trunk cross section and greatly reduced growth in comparison with the deblossomed controls. Yields of 'aimed' VII generally remained well below this level and only slightly reduced growth. In spite of the large growth reduction caused by the heavy Delicious crops, symptoms of intertree competition were still evident at close spacing. For the less precocious 'aimed' trees fierce inter-tree competition led to serious problems in respect of growth control at close spacing by the end of the period under study.
Alar was sprayed only on Delicious IX. In the first year of application it reduced shoot growth and improved fruit retention. Continued application in subsequent years did not affect yield but slightly reduced growth and changed root distribution; pruning weights of treated trees were much smaller, suggesting a more balanced distribution of growth over the tree. Although the effects of Alar on Delicious IX were small, incidental use of Alar on poorly cropping stock-scion combinations may be much more effective in checking inter-tree competition.
Relieving inter-tree competition by winter pruning had very unfavourable side effects. It did reduce tree size, especially at low yield levels, but yield was reduced even more. Moreover, pruning strengthened the dominating position of the tree top; hence it was extremely difficult to correct the top-heavy habit of closely spaced trees by pruning. Nevertheless some pruning was necessary to rejuvenate the fruiting wood and to increase fruit size, especially for Delicious IX. Fruit thinning was not quite as effective as pruning for improving fruit size and quality. Pruning of deblossomed trees had only adverse effects: it kept the trees much smaller and the growth response was unbalanced, necessitating further pruning and bending of branches to maintain an acceptable tree shape. These findings confirm that pruning is a useful instrument for regulating competitive relations within the tree, at least for fruiting trees, but they also indicate that pruning is of little value for reducing competitive stress between trees.
A high density orchard is defined as an orchard with such a large number of trees per ha that in due course an orchard with a lower tree number per ha would give the same or higher yields. It is concluded that for intensive fruit growing only high densities deserve consideration. For stock-scion combinations such as 'Lired' VII, which do not produce substantial crops before intertree competition makes itself felt, the optimum tree number per ha is close to the lower limit of high density. For precocious stock-scion combinations such as Delicious IX, a wider range of high densities may be attractive, depending mainly on the cost of buying and raising additional trees in relation to the extra fruit produced in the early years. In due course equal yield, representing the ceiling crop for orchards of a given planting system and age, will be attained over this range of densities.
Tree thinning is recommended to prevent overcrowding and yield depressions at the higher densities of the range, in case other measures - such as promotion of regular heavy cropping and judicious pruning - are inadequate.