The generous support of the Federal Crop Insurance Corp., U.S. Department of Agriculture, Kansas City, MO. for the preparation and publication of this handbook is gratefully acknowledged

For multiple reasons, the Texas Citrus Industry regularly needs estimates of fruit yields and tree growth. Long term historical averages or trends are usually the best source for making these estimates. When such data are nonexistent or unreliable, however, other methods must be employed. This handbook arose from such a case. Its primary objective was to provide the data needed for insurance adjusters in Texas to assess freeze damage and projected yield losses of young citrus trees with inadequate bearing histories. However, the information and procedures herein can be used wherever assessment of these tree growth and yield relationships have relevance.

When a freeze occurs, Texas citrus crop insurance covers not only the fruit hanging on the tree but also the spring bloom that would set the next season's crop. Estimating what this crop might be on 3 to 6 year old trees which had borne only one or two commercial crops led to the methods described in this handbook. Using the data and methods which follow plus minimum additional training, even the inexperienced adjuster or orchard appraiser can quickly learn to estimate the age, size and yield of young Texas grapefruit and orange trees.

Background

The method is based on research that has shown trees of a given canopy size or volume will generally have fruit yields that usually fall within a specific range. In their first and second years of bearing, young trees' yields range widely. When the trees reach 6 years and beyond, the range tends to narrow. Still by using the average yields given here and adjusting them if circumstances dictate, realistic yield estimates for young citrus trees can be made.

Procedure

1. Canopy volume is determined by first measuring the diameter and height of the tree's canopy. Using a stiff pole graduated in foot and half foot markings, take two diameter readings at right angles to each other and 2-3 feet above ground level. See diagram inside of back cover. Since the canopy sides are often quite irregular, the measurements should represent the diameters of an imaginary half sphere enclosing the canopy and containing the bulk of the tree's leaves and branches. These "approximated" diameters should account for a few extra long branches or a large cavity in some part of the canopy. The two diameters should then be averaged and rounded to the nearest half foot.

With the same pole measure the tree height to the nearest half foot. Make sure the bottom of the pole and trunk are on the same level. The seasoned appraiser may eventually be able to visually estimate canopy diameters and heights with acceptable accuracy, but the use of a graduated pole or other suitable device is recommended for beginners.

From the diameter and height, find the cubic feet of canopy volume in Table 1. Canopy Volume (cu.ft) of Texas Citrus Trees. For half foot values, interpolate volumes for trees 8 foot in diameter and height or under. These dimensions may be rounded to the nearest foot for larger-trees.

2. From the canopy volume obtained in Table 1, the average yield in pounds of fruit per tree can be found in the following tables: Table 3. Estimated Yields for Texas Grapefruit Trees by Canopy Volume; Table 4. Estimated Yields of Texas Marrs Orange Trees by Canopy Volume; and Table 5. Estimated Yields of Texas Navel Orange Trees by Canopy Volume.

Yields for canopy volumes in between those given in the tables may be interpolated or rounded to the nearest volume given in the table. Since these yields are averages statistically, each has an associated standard deviation. The standard deviation is used to calculate the range within which the estimated yields are expected to fall 67% of the time.

If reliable information or observation indicates an orchard is above or below average in production, yields may be adjusted up or down but should fall within the range given in the tables.

Sample Selection

Even in the most uniform orchards yields will vary from tree to tree. Unless the yield from every tree is recorded, the average yield per tree is determined by harvesting a sufficient number of randomly selected trees as a sample. How many trees to sample depends on natural variability, the accuracy wanted, and time or money constraints. Within the orchard features like soil type, drainage, and microclimate or topography may cause variation in tree growth, and yields in addition to natural variability. Sampling procedures need to account for natural variability as well as that contributed by any other special factors.

1. For the Uniform Orchard. First survey the orchard to confirm that the trees are truly uniform in size and condition throughout. If so, sampling 3 to 10 percent of the trees should be sufficient. Trees must be chosen at random according to some predetermined sampling pattern, e.g., the trees sampled form a "Z" design or sample 3 trees in the third of 20 rows, go down 6 trees, skip a row, sample 3 more trees, etc. It's important that the sampling pattern insures that the trees selected are randomly chosen and are representative of all those in the orchard. For orchards 10 acres and less, sample 10% of the trees, then for each additional 5 acres reduce the number by 1 % until the minimum of 3 % is reached for orchards of 45 acres or more. Rarely, however, would orchards of this size have trees that are uniform throughout, in which case, the next paragraph applies.

2. For the Non-uniform Orchard. Survey the orchard to identify the areas of non-unformity and estimate the acreage involved in each. Differences in tree size, canopy density, leaf color, branch die-back and insect or disease infestation could all be potential causes of non-uniformity. Determine what proportion of the whole orchard the non-uniform areas represent, and sample trees within that area in proportion to the total number of trees to be sampled. E.g., a 20 acre orchard of 5 year-old trees planted 140 trees/acre, has 4 acres of 3 year-old trees and 2 acres of trees suffering from poor drainage. Total trees to sample = 224 (20 acres x 140 trees/acre x .08). Randomly select 45 (4/20 x 224) from the 3 year-old area and 22 (2/20 X 224) from the poorly drained group. The remaining 157 or so should be selected as described above.

Use Table 2. Citrus Tree Planting Densities (Trees/acre) to help determine the number of trees per acre by row and tree spacing.

Trunk Diameter and Canopy Size

There may be occasions when tree canopy volume needs to be estimated from the trunk size. These two values are quite closely related in trees up to 12 years old. After that, however, due to freezes, pruning and other factors the correlation is less reliable. Table 6. Canopy and Trunk Size Relationships for Texas Grapefruit Trees and Table 7. Canopy and Trunk Size Relationship for Texas Marrs Orange Trees, show these correlations along with ranges covered by the standard error.

Insufficient data was available to determine the trunk diameter and canopy size relationships for navel and Valencia orange trees, but their growth tends to be more like grapefruit than Marrs oranges.

Trunk Size and Tree Age

In their early years, citrus tree trunk size and tree age are quite closely correlated. After about 15 years, the relationship weakens for the same reasons the trunk size/tree age

correlation becomes inconsistent. Nevertheless, using Table 8. Tree Age to Trunk Diameter Relationships for Texas Grapefruit or Table 9. Tree Age to Trunk Diameter Relationships for Texas Marrs Oranges, a tree's age can be estimated by measuring its trunk diameter, or conversely, if a tree's age is known, one can determine whether it meets average growth standards by measuring its trunk diameter. Trunk diameter, or alternatively, circumference is normally measured 2 to 6 inches above the bud union. See diagram on inside of the back cover. On old trees where the bud union is not clearly visible trunk measurements should be taken 10 to 12 inches above the ground.

Lack of long term records precluded constructing tables like the above for navel and Valencia oranges. Again, however, both these cultivars tend to produce trees more similar to grapefruit than Marrs oranges.

Number of Fruit per Tree and Average Weight of Each Fruit

When grapefruit or orange trees set a large number of fruit, the average size of each fruit tends to be reduced. Contrarily, the fruit on trees with a light crop tends to be larger than average. This relationship is given for grapefruit and oranges, respectively, in Table 10. Grapefruit. The Average Weight per Fruit vs. Number of Fruit per Tree and Table 11. Marrs Oranges: The Average Weight per Fruit vs. Number of Fruit per Tree.

CAUTION. These relationships do not apply to trees 4 years old or under.

While the number vs. average fruit size phenomenon applies generally to navel and Valencia oranges, lack of data precludes providing tables of these relationships.

...In Conclusion

When using the above tables, keep in mind the data for them came primarily from trees in Citrus Center orchards which are not necessarily representative of all Texas' commercial orchards. Remember as well the values and relationships are averages which, when applied to other trees and situations, must be considered estimates or approximations. They should never replace valid, accurate and appropriate yield records and growth measurements.

Like all living things, however, tree growth and yield will vary both within and between orchards and particularly from season to season. Grapefruit and Marrs orange trees, especially, have a tendency toward biennial bearing, i.e, having heavy and light crops in alternate years. For historical perspective and comparison, some average yields of Texas grapefruit and oranges are given in Table 12. Reported Acreage, Tree Populations and Production of Texas Citrus. The yields in this table were derived from data published in Texas and U.S. Department of Agriculture statistical reports.

N.B.

All values derived from these tables and methods, can be appropriately modified IF sound reasons and accurate, reliable supplemental information justify doing so.

TABLES