Pomological researchers have assessed other approaches to determining potential yield. Since planting density works together with per-tree yield to determine productivity per acre, researchers have used canopy dimensions to estimate planting density and then estimated potential yield per acre. This approach is perfect if the trees used to measure yield and canopy size are not contained by any pruning. Once the canopy is manipulated, and particularly if it is manipulated differently for trees of different sizes, potential yield per acre becomes almost a meaningless calculation.
Another approach to assessing tree size is to simply measure the trunk size near the graft union. Tree size assessed this way is less affected by variation in canopy pruning and is well correlated with the size of an unpruned canopy. Relating yield to trunk size, therefore, can be used to assess relative productivity per acre. Yield efficiency does just this by presenting yield per tree relative to the trunk cross-sectional area about 12 inches above the graft union. Although yield efficiency does not estimate yield per acre, it allows for a direct comparison of trees of varying sizes without the need to plant those trees at optimal densities. The potential pitfall of this approach comes when trees are spaced closer than the canopy dimensions might suggest, such as with the super spindle and tall spindle training systems. Further work is necessary to determine whether yield efficiency is still a valid tool for comparison when trees are in a highly competitive environment.
For more information:
W. Autio, D. Greene, and W. Lord presented a comparison of several apple rootstocks with McIntosh as the scion cultivar. They also evaluated and compared different approaches to measuring apple tree productivity. See this article titled "Performance of `McIntosh' Apple Trees on Seven Rootstocks and a Comparison of Methods of Productivity Assessment" at http://hortsci.ashspublications.org/cgi/content/abstract/31/7/1160.
Dr. Wesley Autio, University of Massachusetts Amherst
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