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Thinking about Light

Auhor: Jon Clements

Journal: "The Apple Press" published by the University of Vermont vol 18, no 1, pp. 10-11


That's right, think about it. Sunlight is a natural resource that fuels the orchard production machine. Without sunlight, every other component of apple production (soil, water, trees, nutrients, etc.) is secondary. And sunlight is free. What a deal! To capitalize on the apple building power of free sunlight, however, every orchardist needs to be well attuned to the interception and distribution of sunlight fn their orchards. Why? Recent research has shown that orchard productivity-in terms of quality fruit yield over the life of the orchard-is dependent on both the total amount of available sunlight intercepted by the orchard canopy and the distribution of sunlight within the tree canopy. Of course life is never that simple, so, just what are sunlight interception and distribution, and how do they interact to affect orchard productivity?

Light interception refers to the total amount of available sunlight that is captured by the orchard canopy. Conversely, sunlight that falls on the orchard floor has not been intercepted. Numerically, light interception can run from 0% (as would be the case with newly planted dormant nursery stock) up to 100% (an amount which, for all practical purposes, is not achievable because of the need for tractor rows, etc.). Researchers including Bruce Barritt, Curt Rom, and Terence Robinson (from whom I've gleaned most of my understanding of light interception and distribution in apple orchards) have shown that per acre production is positively correlated with light interception:

Therefore, should the goal be to maximize light interception, hence yield and profitability? Not exactly. Light interception is only half the picture because light distribution in the orchard canopy also has a profound influence on orchard productivity by having a direct impact on fruit quality.

Light distribution can be thought of as sunlight penetration into the orchard canopy. Sunlight penetration must be sufficient for individual spur leaves throughout the tree canopy to receive at least 30% of the available sunlight. Good sunlight distribution within the orchard canopy is essential for producing quality fruit (good color and larger packed box sizes that will command a premium price.) Unfortunately, via shading, it doesn't take long for light penetration to decrease. In fact, research evidence suggests that spur leaves located just 6 feet into the tree canopy are exposed to less than 30% of available sunlight. Don't expect quality fruit from these spurs:

It appears the secret to unleashing orchard productivity (remember, we're talking about both yield and fruit quality here) is by maximizing both sunlight interception and sunlight penetration, by and within the orchard canopy. Yes, but that's not as easy as it sounds (indeed, light penetration and interception in orchards has been the topic of numerous scientific papers and book chapters- particularly, Chapter 3 "Light Interception and Utilization in Orchards", and Chapter 4 "Spur Quality, Light Management, and Renovation Pruning", In Intensive Orcharding. published by Good Fruit Grower Magazine). What often happens for example, when an orchard ages-is that as sunlight interception increases, sunlight distribution within the tree canopy suffers. Also, as the season progresses, ,lower portions of the tree canopy become more heavily shaded as shoots and leaves grow. Therefore, pruning (both dormant and summer) becomes an important management tool, opening up windows within the tree canopy so that the light can shine in!

Finally, consider that high-density plantings of dwarf trees capitalize on this interaction between light interception, light distribution, and orchard productivity by maximizing both the interception and distribution components of light management, thereby making copious yields of high quality fruit a real possibility. That's right, think about it!


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