Influence of water availability on orchard spacing and development
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.
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 thereduction of gas exchange and photosynthesis, leading to smaller canopy size. Wateris generally limited within an orchard system by either supply (e.g. reducedaccess to irrigation water) or delivery issues (e.g. irrigation engineering orwater infiltration issues). To complicate this even further, water availabilityis not necessarily simply the amount of irrigation water available per area oforchard. It also takes into account rainfall that has been stored within thesoil, general water availability, system engineering and distributionuniformity, and water infiltration rates. Limitations in any of these will leadto a reduction of the tree’s available water.
Work by researchers in California has shown the correlationsof canopy coverage with yield. This research was performed by gathering theamount of light intercepted at mid-day and comparing it to measured yieldswithin orchards in California. This work has identified that high-producingorchards tend to alternate around 50 kernel lbs for every 1% of lightintercepted, 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 thetree’s water needs met will provide roughly 50 kernel lbs of crop inCalifornia’s growing conditions.
Based on this logic, water supply should be a considerationwhen determining tree spacing within a new orchard. Since canopy size is directlycorrelated with water use, water use will increase as the canopy reaches85-100% coverage. Once the canopy reaches full coverage, research suggests thatwater demand will stabilize. This means that increasing density of trees perarea will increase early orchard water demand, but not at maturity. Therefore,if water is not a limiting resource, where availability is exceeding 75% ofestimated tree demand, trees can be planted at nearly any density desired.
In situations of limited water availability, orchard densityshould be carefully considered. Wider spacings would increase the potential ofstored soil moisture due to a greater soil volume per tree. This can beaccomplished by wider aisle or tree spacings, or both. Wider aisle spacingsmaintain operation and harvesting efficiencies, while wider tree spacings allowfor planting patterns that maximize soil volume (and light) per tree.
It is important to distinguish between high-densityplantings from higher yields. High-density orchards will reach maximum yieldsearlier in the life of the orchard. Increasing tree density will lead to fastercanopy development, but it will not necessarily increase total canopy size ofthe orchard. This canopy volume determines the yield potential of the orchard. Oncethe canopy matches the available water, canopy expansion will stop. Increasingtree density in a water limited environment will lead to smaller trees, not anincrease in yield per tree at maturity.
