Genetic improvement of sugar maple for high sap sugar content. I. Clone selection and seed orchard development

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Abstract
A grafted seed orchard of Acersaccharum Marsh, was established in Ohio in 1957 as part of a program of genetic improvement of sugar yield. During the 3rd decade, the orchard trees averaged 4.2% sap sugar. Although environmental sources of variance in sugar concentration were large, clonal differences were evident and generally consistent over the years; the clone × year interaction was negligible. Sap sugar concentration varied from year to year, but after an increase coinciding with early crown development in the 1st decade, there was no upward or downward trend with age. Inclusion of one seedling family in the orchard made it possible to obtain an approximation of within-family variance by comparison with within-clone variance. It was estimated to be about one-third of total family variance. The clone–family comparison also suggested that about 11% of total clonal variance was a rootstock effect. Sap tests over 3 years in a plantation of seed-orchard offspring showed that an initial thinning to an average 6.3 × 6.3 m spacing would leave trees with an average 4% sap sugar concentration. At final spacing the concentration would be higher. Only a few clones in the orchard produced medium or heavy crops of male or female flowers prior to age 27, but those from which seed was collected in early years averaged at least 4% sap sugar. All clones and the seedling family flowered by age 27.

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