Mechanical Pruning Direction and Frequency Impacts on Light Interception in Southwestern Pecan Orchards

Pecan (Carya illinoinensis) tree performance is highly sensitive to sunlight availability.  As orchards mature, tree canopies become tall and grow together, so that large portions of canopies receive inadequate sunlight for pistillate flower production, kernel development, and shoot survival.  Under these conditions, alternate bearing intensifies and both average yields and nut quality drop dramatically.  Over the past two decades in the southwestern US mechanical pruning has been widely accepted as a method for sunlight management in mature pecan orchards. Mechanical pruning, however, also reduces canopy interception of photosynthetically active radiation (PAR), which might counteract the benefits of improved canopy light distribution by reducing total canopy carbon fixation.  The objectives of our studies were to characterize effects of mechanical pruning direction and frequency on PAR interception in mature pecan orchard canopies.  An all-terrain-vehicle was instrumented with a 7.6-meter-wide array of 18 light bars (each composed of 80 PAR sensors) and a global positioning satellite (GPS) unit.  As the vehicle moved through the orchard research plots PAR and GPS locations in the orchard were logged at a rate of 2 Hz, which translated to simultaneous data recording from all 18 light bars every 0.75m along the vehicle transect.  Experiment 1 was conducted in a mature ‘Wichita’ pecan orchard in Sahuarita, AZ where tree rows were mechanically pruned in either only the east-west (E-W) direction or only in the north-south (N-S) direction.  Experiment 2 was conducted in a mature ‘Western’ pecan orchard in Las Cruces, NM where trees have been mechanically pruned at three frequencies:  every year (Trt 1), every other year (Trt 2), or every third year (Trt 3).  Experiment 2 also included a control which was mechanically pruned last in winter 2005/2006. Historical yield data collected in a 6-year period in Experiment 1 show 9.5% higher productivity in the ‘Wichita’ blocks with N-S tree-row orientation, while PAR measurements in the summer of 2014 showed that N-S orientated rows had a 4.31% increase in overall light interception compared with E-W oriented rows.  In Experiment 2 ‘Western’ pecan 9 year cumulative yield decreased with increasing pruning frequency.  Canopy PAR interception in 2014 for the Control was 63.4% of full sun, while the canopies in Trt 1, Trt 2, and Trt 3 had 48.5, 41.3, and 44.9% PAR interception, respectively. These results help clarify how pecan growers in the Southwest may use mechanical pruning to optimize both canopy PAR interception and distribution.