Orchard canopy coverage has been shown to correlate with yield. As canopy size increases, more sunlight is intercepted by the leaves. This leads to the production of more energy that can be directed to the tree, leading to more growth, and therefore more crop.

Correlation of intercepted light at mid-day (PAR) and kernel yield per acre. Research conducted by Lampinen, et al.

What is often forgotten is the most critical to canopy development: water. Without access to water, tree canopy growth will slow or stop due to the reduction of gas exchange and photosynthesis, leading to smaller canopy size. Water is generally limited within an orchard system by either supply (e.g. reduced access to irrigation water) or delivery issues (e.g. irrigation engineering or water infiltration issues). To complicate this even further, water availability is not necessarily simply the amount of irrigation water available per area of orchard. It also takes into account rainfall that has been stored within the soil, general water availability, system engineering and distribution uniformity, and water infiltration rates. Limitations in any of these will lead to a reduction of the tree’s available water.

Work by researchers in California has shown the correlations of canopy coverage with yield. This research was performed by gathering the amount of light intercepted at mid-day and comparing it to measured yields within orchards in California. This work has identified that high-producing orchards tend to alternate around 50 kernel lbs for every 1% of light intercepted, giving a theoretical maximum yield of 5000 lbs/acre.  Since it takes water to develop the canopy, this correlation can be also be extrapolated to water use: every percent of the tree’s water needs met will provide roughly 50 kernel lbs of crop in California’s growing conditions.

An orchard with >80% light interception requires full irrigation to maximize crop. This orchard required 48″ of applied water/acre (total water use = 54″/acre), produced over 4,000 lbs of kernel/acre. Photo by B. Lampinen.

Based on this logic, water supply should be a consideration when determining tree spacing within a new orchard. Since canopy size is directly correlated with water use, water use will increase as the canopy reaches 85-100% coverage. Once the canopy reaches full coverage, research suggests that water demand will stabilize. This means that increasing density of trees per area will increase early orchard water demand, but not at maturity. Therefore, if water is not a limiting resource, where availability is exceeding 75% of estimated tree demand, trees can be planted at nearly any density desired.

In situations of limited water availability, orchard density should be carefully considered. Wider spacings would increase the potential of stored soil moisture due to a greater soil volume per tree. This can be accomplished by wider aisle or tree spacings, or both. Wider aisle spacings maintain operation and harvesting efficiencies, while wider tree spacings allow for planting patterns that maximize soil volume (and light) per tree.

It is important to distinguish between high-density plantings from higher yields. High-density orchards will reach maximum yields earlier in the life of the orchard. Increasing tree density will lead to faster canopy development, but it will not necessarily increase total canopy size of the orchard. This canopy volume determines the yield potential of the orchard. Once the canopy matches the available water, canopy expansion will stop. Increasing tree density in a water limited environment will lead to smaller trees, not an increase in yield per tree at maturity. 

This wider spaced orchard was planted in a water limited environment. It is ~ 35% light interception, requiring about 24″ of water/acre, 18″-20″ applied/acre. Photo courtesy of B. Lampinen.
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