REFINING THE RELATIONSHIP BETWEEN CANOPY LIGHT INTERCEPTION AND YIELD IN WALNUT

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1 REFINING THE RELATIONSHIP BETWEEN CANOPY LIGHT INTERCEPTION AND YIELD IN WALNUT Bruce Lampinen, Shrini Upadhyaya, Vasu Udompetaikul, Greg Browne, David Slaughter, Samuel Metcalf, Bob Beede, Carolyn DeBuse, Janet Caprille, Bill Coates, John Edstrom, Rachel Elkins, Joe Grant, Janine Hasey, Kathy Kelley and Bill Krueger ABSTRACT Data collected by the authors over the past several years has provided a rough upper limit to productivity in walnut and almond based on the percentage of the available midday canopy photosynthetically active radiation (PAR) that is intercepted and the age of trees. However, most of the data that was collected previously had limitations. The methods of measuring percent PAR interception using a handheld lightbar (Decagon Devices, Pullman, WA 99163) were relatively slow and labor intensive. For this reason, much of the lightbar data that was used to develop the relationship was based on sampling of relatively small samples of trees. Often the area for the yield and PAR interception data did not match (i.e. PAR data from 5 trees and yield data from either one tree or from an entire row). We have recently retrofitted a Kawasaki Mule with a light bar that is able to measure light across an entire row (up to 26 feet wide). The data can be stored on a datalogger at intervals of less than 1 foot down the row at a travel speed of about 4.5 mph giving us a much better spatial resolution in much less time than was possible in the past. These data are useful for any studies that aim to quantify the impact of treatments on yield. By measuring canopy light interception on a large scale, the impacts of differences in canopy development can be separated out from other treatment impacts allowing much more robust data interpretation. OBJECTIVES There were two goals for this project. The first was to provide support to Shrini Upadhyaya and David Slaughter to modify the lightbar design to make it more robust as well as user friendly and also to work toward automating the data processing. The lightbar setup was developed with funding from the almond industry as the well as the USDA almond methyl bromide areawide project. The plan is to use the funding from the California Walnut Board to improve the design of the lightbar and to study its utilization in walnuts. The second goal will be to find orchard sites, in cooperation with farm advisors, throughout the walnut growing areas of California for testing the lightbar. These sites will be chosen in orchards that have a variety of ages and light interception levels. The goal will be to find orchards that are well managed and as productive as possible to attempt to place upper limits on the light interception/yield relationship. California Walnut Board 13 Walnut Research Reports 29

2 PROCEDURES Lightbar modifications- The Mule mounted lightbar was modified during 29 to make it more able to withstand the rough environment driving through heavily canopied walnut orchards (Fig. 1). A piece of aluminum irrigation pipe was added in front of the lightbar and it worked well to Fig. 1. Current design of Kawasaki Mule mounted lightbar showing adjustable end section, aluminum pipe branch deflector, and new support system under lightbar to replace former cable system. deflect branches up and over the bar without scratching or otherwise damaging the surface. A CR 3 data logger with a flash memory card was also added to allow storage of larger data sets. Software was also developed to aid in processing the data to make it more readily usable. Overall, the lightbar and software setup performed very well. Further refinements for next season will include making the bar wider to better accommodate wider walnut spacings in one pass. In addition, LIDAR will be added to allow assessment of canopy shape and height. During mid-summer, two infrared thermometers were added to the Mule lightbar to measure soil temperature in the middle of the drive row and under the tree row. These data will be useful in assessing food safety risk of different canopy management regimes. Walnut orchard mapping- The plan was to obtain lightbar data on individual rows in a variety of walnut orchards and to then harvest these same rows to obtain yield data to compare to the lightbar data. During the 29 season, a total of eighteen walnut orchards were mapped with the mule lightbar (Table 1). Several of the orchards were mapped multiple times during the season to investigate the impact of time of year and environmental conditions on canopy light interception (Table 1). Data was collected on 11 orchards, 4 orchards, 2 Howard orchards, one Serr orchard and 1 Vina orchard. Light bar measurements were done in 5-1 row middles (depending on orchard size and variability) in representative areas of the orchard. These rows were then flagged and at harvest time, rough field weights were taken from each row middle. California Walnut Board 14 Walnut Research Reports 29

3 Subsamples from each row middle were taken, hulled and dried to estimate in shell crop weight for each row. Table 1. Description of sites used for walnut light interception and yield mapping in the 29 season. County Site Variety Date mapped County Site Variety Date mapped Colusa Nickels pruning trial Arbuckle 7/29/9 and 8/8/9 Stanislaus Modesto 9/3/9 Colusa Nickels Howard pruning trial Arbuckle Howard 7/29/9 and 8/8/9 Rio Oso #1 5/26/9 and 9/16/9 Kings Kings County retain Serr 7/25/9 5/26/9 and trial Hanford Rio Oso #2 9/16/9 Kings Kings County PDS 7/25/9 5/26/9 and site Hanford Rio Oso #3 9/16/9 Lake 6/25/9 Own-rooted trial Rio /Vina 9/15/9 #1 Upper Lake Oso Lake 6/25/9 Yolo 8/1/9 #2 Upper Lake Madison Yuba Surround trial 9/24/9 Lake 6/25/9 Rio Oso #3 Upper Lake Yuba Howard Surround trial Howard 7/14/9 Rio Oso San Joaquin Farmington 8/11/9 Yuba replant trial Rio Oso 9/15/9 Solano Surround trial Winters 6/2/9, 6/16/9, 6/17/9, 7/13/9 RESULTS Light interception and yield- The data previously collected with the handheld lightbar is shown in Figure 2 and the data collected in 29 with the Mule lightbar is shown in Figure 3. The units Walnut Almond Midday light interception (%) Fig. 2. Midday canopy light interception versus yield relationship from various almond and walnut trials from throughout state measured with the handheld lightbar California Walnut Board 15 Walnut Research Reports 29

4 on the x-axis in Figure 1 are in kernel pounds per acre but since the shelling percentage for walnuts is generally near 5 percent, the x-axis in Figure 2, which is for in-shell pounds per acre, are comparable. 6 5 Own-rooted trial Rio Oso OR /Pdx Own-rooted trial Rio Oso OR- /OR Own-rooted trial Rio Oso OR- Vina/Pdx Own-rooted trial Rio Oso OR- Vina/OR Lake s Lake County- Upper Lake Rio Oso #2 Rio Oso #1 San Joaquin Farmington site Stanislaus site Modesto Yolo County site Madison Yield (dry tons/acre) Midday canopy light interception (%) Fig. 3. Midday canopy light interception versus yield relationship from various walnut trials from throughout state measured with the Mule mounted lightbar in 29. Midday canopy light interception varied from about 1 to 9 percent in the orchards studied in 29 (Fig. 3). Several of the orchards were above the line in Figure 3. Past data has suggested that alternate bearing may allow orchards to alternate about the line in one year but will then tend to be below the line in the following year- we should be able to clarify this next year since we plan to use the same orchards wherever possible. Several of the orchards are not shown in Figure 3, either because data analysis has not yet been completed or because harvest data was not taken due to time constraints or the grower harvesting the plot without warning. Orchard floor temperature- The impact of midday canopy light interception on soil surface temperature is shown in Figure 4. The position of missing trees is shown at the top of the figure and the effect on decreased midday canopy light interception and resulting increased soil surface temperature can be clearly seen. These data will be useful in assessing impacts of different canopy management regimes on soil surface temperature which can relate to food safety risk since ideal temperature for Salmonella survival in the soil is in the range of 3-35 deg C which is what the shaded areas of the orchard reach in the Central Valley of California during summer. Heavily shaded orchards may lead to increased food safety risk. Determining plant water stress using an IR thermal sensor- Preliminary studies were conducted to investigate the possibility of using a thermal infrared gun to measure plant water stress in California Walnut Board 16 Walnut Research Reports 29

5 walnut orchards during the 29 growing season. The sensing system consisted of a thermal IR gun, photosynthetically active radiation (PAR) measuring device, wind speed sensor, humidity Missing trees 12 Midday light interception (%) Fig. 4. Midday canopy light interception (top) and soil surface temperature (degrees C) under the tree row for the Yolo County orchard near Madison on 8/1/9. Air temperature was approximately 32 deg C (9 deg F). sensor, and an ambient temperature sensor. Figure 5 shows the relationship between the measured temperature difference between the canopy and the ambient air and the predicted difference based on plant water potential and photosynthetically active radiation measurements at the leaf level. The results suggest that the sensor suite was able to predict plant water stress as measured using a pressure chamber. During the 21 growing season, we would like to improve this sensing system by adding a leaf detection system for real-time applications and conduct extensive tests in five to six walnut orchards with trees under different water status conditions. Pred (Tc-Ta) Soil surface temperature under tree row (degc) Distance (meters) Walnut: Nickels 7/29/9 & 8/8/9 y =.7656x R 2 = Tc-Ta Fig. 5. The difference between walnut leaf and ambient temperatures as a function of leaf water potential as measured by a pressure chamber and amount of PAR incident on the leaf. Regression equation is Pred (Tc-Ta) = PAR SWP SWP PAR. California Walnut Board 17 Walnut Research Reports 29

6 Plans for 21- The data for light interception and yield will be used to further refine the relationship shown on the graph below for walnut. A secondary goal will be to assess the impact that alternate bearing has on the relationship. With a better estimate of the maximum productivity per unit light interception, these data can be used to assess potential orchard yield and will allow separating out canopy light interception as a variable in other research projects. For example, if a pruning study is being conducted, this tool will allow the separation of the effect of the pruning treatment on overall canopy light interception as opposed to the effect of the pruning treatment on productivity per unit canopy. It will also allow block to block variability to be assessed before or after a research trial in initiated. The plan in 21 is to continue monitoring the same orchards that were monitored in 29 for midday canopy light interception, orchard floor temperature and yield. Additional measurements will be done on canopy temperature versus water stress effects in the orchards with the greatest variability in water potential as determined in spring 21. California Walnut Board 18 Walnut Research Reports 29