- Author: Ben Faber, University of California Cooperative Extension
- Author: Michael Spiers, HortResearch, Ruakura, New Zealand
Biological control of Phytophthora cinnamomi in avocado through the use of mulches was identified by an Australian grower and later described as the "Ashburner Method" by Broadbent and Baker. The technique uses large amounts of organic matter as a mulch along with a source of calcium. Control of avocado root rot in the Ashburner method was attributed to the presence of Pseudomonas bacteria and Actinomycetes. Multiple antagonists are more likely the cause of biological control, since no single organism has been found to be consistently associated with soils suppressive to P. cinnamomi.
The use of organic mulches has multiple effects, such as altered soil nutrient and water status and improved physical structure. Any improvements in plant status resulting from improvements in the growing environment can improve plant health. The effect of organic amendments on soil physical and chemical properties can vary considerably depending on soil texture and the environment. One of the most consistent effects of organic amendments is an increase in biological activity. Increases in organic substrate lead to increased fungal and bacterial populations. In numerous cases, this increase in biomass has been associated with disease suppression. This biological control can be ascribed to several mechanisms: competition, antibiosis, parasitism, predation and induced resistance in the plant.
The microbial biomass is responsible for release of enzyme products and polysaccharides in soils. The microbially-produced enzymes cellulase and glucanase have been demonstrated to have a significant effect on Phytophthora populations. This mechanism of antibiosis is possible because the microbes are releasing these enzymes to solubilize organic matter. Unlike other fungi, Phytophthora have cell walls that are comprised of cellulose and in the process of decomposing organic matter with enzymes, an environment is created that is also hostile to the pathogen.
In order to see if there might be potential differences in organic materials being better at combating avocado root rot, a little field trial was established with 23 different types of materials. The mulch materials were obtained from nearby hedges and chipped or obtained from commercial sources of mulch. Some of these materials would be difficult to get in large amounts, such as manuka (Leptospermum scoparium), but others are commercially available chipped greenwaste. The materials were then spread on the ground to a depth of five inches, in separate plots that were 36 X 36 inch squares. Decomposition was measured over a two year period and then cellulase was measured in the mulch, at the soil / mulch interface and at a two inch depth in the soil at the end of 2 years.
Since cellulase production is part of the decomposition process, the rate of decomposition should be a partial indicator of the amount of cellulase present. After a mulch application there is generally settling due to rainfall-caused compaction, but much of the decline by the second year is due exclusively to decomposition. The more recalcitrant materials, such as bark, wood chips and sawdust have barely lost half their depth after two years, while others such as shredded eucalyptus, manuka, avocado and willow are less than 20% of their initial depth. Much of the shredded/chipped material, such as eucalyptus had a significant fraction of leaves in the mulch. The wool disappeared a little after one year. The greenwaste + chicken manure compost is nearly the same depth as the wood chips, since it is a material that had gone through a decomposition process prior to its application and much of the easily digestible materials had already been decomposed.
The rate of decomposition has some bearing on the rate of cellulase production. Eucalyptus and manuka had the two greatest rates of decomposition and show the highest levels of cellulase production. The cellulase levels were consistent with all the different mulch materials. Using decomposition rate alone is not a complete indicator of cellulase production since, poplar, willow and avocado had high rates of decomposition, but their cellulase rates were half those of manuka and eucalyptus.
It is clear that the cellulase effect is limited to the layer of mulch and not to depth within the soil. There is some effect at the soil surface, but at 5 cm (2 inches) cellulase activity drops to background levels. There is earthworm activity at the test sites and one idea was that earthworm incorporation of organic matter would move the cellulase production into the soil. Maybe with further time this would occur. As it is, when mulches are applied to avocado, the roots tend to proliferate in the mulch, out of the soil where the cellulase activity is the least.
Something to keep in mind is that we do not know what levels of cellulase are necessary to control the root rot fungus. It may be that levels seen with pine bark are more than adequate. Also we have measured cellulase production at only one time in a two-year period and it is quite likely that this is not the best snapshot of what is happening before and after. A further reminder is that cellulase is only one of the many byproducts associated with decomposition and many of the antagonistic properties that are associated with the microbial biomass are not being measured in this trial. Having developed this screening procedure what needs to be done next is to take high, medium and low cellulase producing mulches and challenge the fungus to verify that this is a good way to evaluate mulches.
- Author: Ben Faber
Irrigation efficiency requires not only uniform irrigation, but also the proper timing and amount of applied water. It is important that the irrigator know the system water application rate, either in inches per day, inches per hour, or gallons per hour.
Irrigation scheduling which determines the time and amount of water to be applied can be accomplished through a variety of methods, including measuring soil moisture, determining plant moisture status and determining evapotranspirational loss (ET crop or ETc). Evapotranspiration values are a measure of the actual amount of water well watered plants would use. This information is available in many areas of California from newspapers, irrigation districts, and over the Department of Water Resources CIMIS network California Irrigation Management Information System, or CIMIS Help Line (800) 922-4647).
Evapotranspiration varies seasonally and from year to year for a given location. DWR has developed a map of the average daily ET for various zones in California. These zones are distinctive because total sunlight, wind, relative humidity and temperature are the parameters that drive water loss and differ in each zone. Where the Central Valley becomes hot and cloudless in the summer, along the coast the intensity of the marine layer and its effect on sunshine differs from year to year.
Scheduling, as opposed to a fixed amount applied at a fixed time, is especially important in Southern California coastal valleys. Although the average annual irrigation requirement is about 2 feet of applied water per year (2 acre-feet per acre or 651,702 gallons per acre), this value varies tremendously from year to year, from as little as 18 inches to as much as 3 feet.
One of the most important variables in the quantity of applied water is the length of the rainfall season and the effectiveness of the rainfall. The rainfall season determines the length of the irrigation season and effective rainfall determines how much the plant can use. Effective rainfall is defined as the amount of rainfall, which is retained in the root zone of the tree. For example, consider a rooting depth of 2 feet and each foot holds 1 inch of available water. If you have just irrigated or if it rained 2 inches yesterday and it rains 2 inches today, none of today's rain is effective since the soil was already moist. It did leach salts out, however. Rain events of less than 0.25 inches are also not considered effective.
Determining an irrigation schedule based on tree water requirement falls into three broad categories of technology - plantbased, soil-based and weather-based. Many of these technologies are proven and have been in use for years. Others are more experimental and have not been fully tested. In several cases improved electronics and digitalization have put a new spin on older technologies. A method of determining when to irrigate should be learned by all growers and often a combination of techniques can be employed.
Plant-based Scheduling Methods
The plant is the ideal subject to measure, since it is integrating all the various factors driving water loss as well as soil moisture and any stresses such as soil salinity and plant health. To be a useful tool in irrigation scheduling, plant-based measuring devices must provide indicators of stress before that stress reaches levels that result in yield decreases. The methods include:
- Pressure chamber (pressure bomb or Schollander pressure chamber) measures plant water tension by applying a comparable air pressure to a leaf or stem. The amount of pressure required to equilibrate with the plant sap indicates how much stress the plant is under.
- Trunk diameter fluctuations (shrink/swell), measured continuously with linear variable displacement transducers (LVDTs), can be used to calculate parameters that are directly related to tree stress.
- Stem flow gauge estimates transpiration by placing a heat source on the trunk of the tree and then measuring the temperature differential along a trunk.
- Porometer measures the ability of a leaf to transpire, so when the leaf is under water stress then less water is transpired.
- Infrared thermometry measures the canopy temperature as affected by the rate of transpiration, so as the plant goes under water stress, the leaves gets warmer.
- Visual symptoms (wilting, leaf curling) are the cheapest method, but the most expensive in the long run.
- While these techniques can be valuable for scientific use, there has been little adoption in commercial agriculture. With the exception of the pressure chamber and LVDTs, this is due to the aforementioned problem of being able to identify mild water stress. Another reason for their lack of use by commercial agriculture, specifically subtropicals, is that there are logistical problems with mature trees, such as with the stem flow gauge and infrared thermometry. At this time, the pressure chamber is the state of the art in measuring tree water stress in subtropicals while recent research indicates that the LVDTs show promise for automating irrigation scheduling.
Soil-based Scheduling Methods
A rule of thumb is that irrigation timing should occur when about 50% of the water available to the plant has been depleted from the soil. The 50% figure is arbitrary; it allows a buffer of water in the soil in case the weather suddenly turns hot and windy.
Of course a sandy soil will hold less water than a clay soil, so irrigation will be more frequent. A common perception is that it takes more water to grow plants in sandy soil than clay soil. The total amount required for the whole year by the tree will not be changed by the soil type. This is because it is the sun, wind, temperature and humidity, which decides how much water the tree, will need. The soil is only the reservoir.
To check the water content in the soil, take a trowel, shovel, or soil tube and dig down 8 to 16 inches. A soil that has about 50% available water remaining will feel as follows:
- coarse - appears almost dry, will form a ball that does not hold shape;
- loamy - forms a ball, somewhat moldable, will form a weak ribbon when squeezed between fingers, dark color;
- clayey - forms a good ball, makes a ribbon an inch or so long, dark color, slightly sticky.
Irrigation timing can be determined and also mechanized with the use of a tensiometer. These water filled tubes with a pressure gauge accurately reflect the amount of energy a plant needs to extract water from the soil. The pressure gauge measures "tension values" in centibar units (cbars). When the gauge reads 30 cbars, it is a good time to irrigate.
Placement of the tensiometers requires that they be within the root zone, between the emitter and the tree trunk. Having two tensiometers next to each can be helpful in deciding both when to turn the system on and when to turn it off. A tensiometer at a one-foot depth tells when the water should be turned on and a tensiometer at three feet tells when to turn the system off. Placing a plastic milk crate over the device will prevent pickers from kicking them over.
There are other devices on the market for measuring soil moisture. Gypsum blocks are very effective. Although the part in the ground is inexpensive, the reading device costs in the $250 range. This cost means a large enough acreage is required to spread out the cost of the system.
There are portable meters on the market for measuring soil moisture. These meters rely on an electrical current carried by water in the soil. Even the cheap $10 ones can give a rough estimate of the soil water content. None are very effective in rocky ground, because their sensitive tips break easily.
The amount of water to apply at an irrigation depends on the amount of water held within the root zone. A loamy soil where a microsprinkler with a 20-foot diameter throw has wetted a twofoot depth will hold about 200 gallons of water at 50% of the soils water holding capacity. Exceeding this amount of water will help leach salts; but if far in excess, additional water is only pushing existing water out of the root zone.
It is best to follow one or two irrigation cycles to find out how long to run the system to achieve a certain depth of infiltration. This can be done with a shovel or more easily with a pointed rod or tensiometers. Water moves in a wetting front, and the wetted soil will allow the rod to be pushed in to the depth of dry soil. The system should be run to find out how long it takes water to infiltrate to a depth of two and three feet. That information will indicate how long to run the system when irrigating.
Applying water to achieve a two to three foot depth may take several hours. If run-off occurs, the system may be turned off for a few hours, then turned on again to get the total run time required to infiltrate to a given depth. If run-off is severe, use emitters with a smaller flow rate.
Soil-based methods monitor some aspect of soil moisture which, depending on the method, requires some correlation to plant water use. Some of the methods are well understood and inexpensive, others are expensive, inaccurate, inappropriate or not well researched. Some of the techniques allow multiple site readings while others require a device to be left in place. Some measure soil water directly, like oven-drying and others measure some other parameter with is associated with water content, such as electrical conductance. Some are affected by salts or soil iron content and others have limited value in the desired soil moisture range. Some, like tensiometers and gypsum blocks, give a reading from a porous material, which comes to equilibrium with soil moisture, while many others use the soil directly as the measured media. This is an important distinction since discontinuities in the soil caused by rocks or gopher holes can affect readings when the soil is used to carry a signal. Also, times have changed and some of the old techniques have been improved. For example, gravimetric oven-drying can now be done by microwave, considerably speeding up the process. Tensiometers and gypsum blocks can now be found with digital readouts and connections to data loggers, which make data easier to manage. There are quite a number of devices on the market and the following chart will shed some light on their differences.
As with any tool, the value of these devices increases with use and familiarity. Even though several of these are listed as stationary devices, by placing them in representative positions in the orchard, they can accurately reflect the rest of the orchard. Several of the devices are listed in the table as being both stationary and portable; this is because there are various models that can act one way or the other. The "Ease of Use" category in the table indicates not just the ease of reading the device, but also the maintenance required for it.
Weather-based Methods of Irrigation Scheduling
Another scheduling technique that has become popular is the use of weather data that has been converted to a crop water use value. This value is the estimated amount of water an orchard would use. The value is often referred to as the evapotranspiration (ET) of the crop. ET is the amount of water that can be lost by a well-watered crop either through the leaves (transpiration) or evaporation from the surface of the soil. By applying the ET amount at an irrigation, the trees are kept at optimum moisture content. The technique is often called the water budget method or checkbook scheduling.
The CIMIS network of over 50 weather stations calculates reference evapotranspiration (ETo). This value is an estimate of the amount of water lost from a well-watered field of grass. Grass is the standard or reference for all other crops. ETo is modified for the specific crop with a crop coefficient (kc). The formula for converting ETo to crop ET is: ETo X kc = ETcrop.
For a full-grown subtropical orchard a kc of 0.65 is used in most of the State, but in the desert growing areas, 0.56 is used. With smaller trees, a smaller kc is used. When trees are young and intercept little energy to drive water loss, a kc of 0.05 works well. As the trees increase in size to where their shade covers about 65% the soil surface, the kc is gradually increased each year. With rapidly growing trees, the kc increase is usually about 10 % each year, until about year 8 when the 65% figure is reached. A correction factor needs to be incorporated for the irrigation system distribution uniformity, as well.
If the orchard is cover cropped for part or all of the year, the period during which the cover is present needs to be recognized in the water use calculation. A soil that is covered by a cover crop and trees uses water just like a mature orchard. Therefore, if the young orchard is covered by a perennial cover crop a kc of 0.65 is used regardless of tree size. If a winter annual cover is used, that uses only rainfall for its growth, correction is not usually necessary in a high rainfall year. But in low rainfall years, the water requirements of the cover need to be recognized in the irrigation program.
Reference evapotranspiration values are available from many irrigation districts, CIMIS, several weekly journals and magazines. In Ventura County, the values are available through County Flood Control, and in San Diego County, they are available from the Resource Conservation Districts.
One of the drawbacks of the centralized weather stations is that in hilly terrain with different sun exposures, the station values can be quite different from the water loss at a grove. When using evapotranspiration figures it is always important to back up the estimates with field checks in the grove. An alternative to using the centralized weather stations is establishing one of your own. These electronic stations cost in the range of $5,000 and require regular maintenance as well.
A simpler weather station can be developed with an evaporation pan or an atmometer (atmosphere meter). Both of these devices actually measure the loss of water due to evaporation and since the physics of evaporation and transpiration are very similar, the values can easily be used in a water budget.
The major drawback to the evaporation pan is the maintenance required to keep birds, coyotes, and bees from causing inaccurate readings. Algae also needs to be kept free of the pool. An atmometer is a closed system with a ceramic head, much like a tensiometer. As water is drawn out of a reservoir, a sight tube shows how much water has been evaporated. The atmometer is more expensive (~$300) than a pan, but it is much easier to maintain.
Regardless of what scheduling technique or combination of techniques is used, a thorough evaluation of the system needs to be performed so that a known amount of water is being applied. Until volume and distribution of water are known, it makes little sense to schedule applications.
- Author: Gary S. Bender
The wildfires in San Diego and Ventura Counties during the fall of 2003 were certainly devastating to many avocado groves adjacent to burning native chaparral. Many of the avocado trees were singed in the canopy without extensive damage to the large scaffold branches; these trees will re-grow new foliage with some relatively minor pruning to clear out smaller dead branches. However, other groves have had extensive damage, complete with charring of the bark in the trunk and boiling of the sap through the bark of the trunk. In these cases, the sap became hot enough to steam the cambium layer (the layer of living cells just beneath the bark), killing the tree above the soil line.
In the latter case, the tree above the soil line is dead, but the roots are still alive. Beginning about the first of March 2004, we have noticed that many of these trees are sending up rootstock suckers near the trunk. If left to grow un-grafted, these suckers will become an avocado tree, but not a known cultivar. The question is: should these burned trees be removed and replanted with a new tree? Or should a sucker be tip-grafted back to a known cultivar?
Sucker grafting in avocado is a well-known practice and has been used extensively in the industry when a grower desired to change cultivars. Generally, the tree is cut down leaving a threefoot stump, which is used as a stake for the new tree. A strong sucker growing from the base of the tree is selected (the sucker should be about 3/4 to 1" in diameter and stiff, not rubbery), and the other smaller suckers should be removed. The sucker is cut with a horizontal cut about 6-8" above the soil line, a 2”vertical slit is made down through the center of the sucker, and 3" to 4" long piece of budwood, cut like an arrowhead at the bottom end, is slipped into the slit, matching the cambium layers together on at least one side, and preferably on both sides. The graft is wrapped tightly with grafting tape, and the entire budstick is wrapped with Parafilm to prevent moisture loss, and grafting tape is used to tie the new grafted sucker to the stump (used as a stake).
Advantages from sucker grafting (as opposed to planting a new tree).
- Sucker grafting is cheaper. As recently quoted by a grafter in Fallbrook, sucker grafting usually costs about $2 per tree after the tree has been cut down to a 3 foot stump. If the grafter supplies the budwood and grafting tape, the price will probably be $2.50 per tree. If the grafter has to travel away from Fallbrook, the price will be higher according to the distance traveled. A new replacement tree will cost about $14 on a seedling rootstock, or $19-22 on a clonal rootstock. The labor cost for planting the new tree would be about $2.00 per tree. These costs do not include cutting down the older burned tree, or follow-up care for the young tree.
- The older, burned avocado tree has an extensive root system with a lot of stored energy. When the sucker graft begins to grow it usually grows very rapidly, much faster than a young replant tree. The sucker grafted tree should start to set fruit two years after grafting.
Disadvantages from sucker grafting.
- We are assuming that the sucker-grafted tree is healthy and does not have root rot or some other disease. If the older tree has root rot, it would be better to remove the old tree and replant with a new tree grown on one of the newer root-rot tolerant clonal rootstocks.
- In the system described above, the trunk is used as a stake. When the new tree grows enough to be selfsupporting, the old stump should be cut down close to the ground. The stump should be slightly sloped to drain water away from the new tree. This takes some careful chainsaw work.
- Suckers. Until the new tree gains strength and starts to shade the old stump, there will be other suckers emerging. These must be removed or they will take over and shade the grafted sucker.
- Posted By: Chris M. Webb
- Written by: Gary Bender, David Crowley and Mary Lu Arpaia
This is the story of a remarkable avocado rootstock trial that was set up in 2004, lost to the freeze of January 2007, recovered (mostly) and had its first harvest in spring of 2010. But the real story is how some of the rootstocks bore at a really high rate with water that was so saline that almost killed most of our California rootstocks.
As part of Crowley and Arpaia's salinity rootstock trial, in cooperation with farm advisors and several growers, and funded by the California Avocado Commission, this particular trial was planted in 2004 at the Nick Stehly Ranch in Valley Center. The trial had 10 different rootstocks all grafted with Hass scions. Twenty trees of each rootstock were planted in a randomized and replicated block design: the rootstocks were Duke 7, Spencer, Parida, VC 44, VC 207,VC 801, VC 218, PP14 (Uzi), PP 16 (Rio Frio) and PP24 (Steddom). The VC series are rootstocks selected in Israel for tolerance to salinity, and the PP series are rootstocks selected for root rot tolerance by Dr. John Menge at the Plant Pathology Dept., U.C. Riverside. At the time of planting it was not known how the PP trees would react to salinity.
In Spring 2005 we planted six Hass/Dusa trees into vacant spots in the trial. These trees were left over from a Bender irrigation trial on another part of the ranch.
The trees were grown with highly saline irrigation water with an average EC of 2.5 and chloride levels of approximately 300 ppm. Needless to say, most of these trees suffered greatly with severe tip-burn and some of the trees almost died. But some looked better than others and we were waiting for the first harvest. In Spring 2006 some the trees set fruit and we expected the first harvest to be in 2007. But then disaster struck!
In January 2007 we had a serious freeze in San Diego County. Nick Stehly called us to let us know that he recorded a temperature of 18° F in our plot. All of the trees looked like they had died and we gave up on this plot and went on to other trials.
But the irrigators didn't give up! They kept pruning the dead wood out of the trees that did not die and gradually brought most of the trees back to life. But the trees were still being irrigated with the saline water, except for one important difference.
The Stehly family liked to swim in the reservoir about three times during each summer. So they would fill the small reservoir at the end of the ranch that supplied our trial, with Metropolitan Water District water with an EC of 0.7 – 0.9. After swimming they used this water for an irrigation of the trial. Amazingly, this “leaching” irrigation was apparently enough to keep the trees growing without too much tip-burn, and the irrigator reported to Nick in January, 2010 that we had enough fruit for a harvest.
The first harvest was completed in March,2010. The data for mean pounds of fruit per tree is presented in Figure 1. The number of surviving trees after the freeze of 2007 is presented in Table 1.
The mean wt of Hass avocados for the VC 801 rootstock was 92.2 lbs, and the mean wt for the Dusa rootstock was 139 lb. If this is compared to the San Diego County average yield of 7000 lbs per acre (about 70 lbs per tree), it would indicate that we might be making progress in finding some better rootstocks for use with some of our saline irrigation waters. However, as we all know, you can't base any conclusions on one year of yield data. We need to have at least three years of yield data to even begin to make a conclusion.
Our hats off to the irrigators at the Stehly Ranch, and to the Stehly family for their cooperation (and their reservoir/swimming pool). You never know what might show up in some of these older rootstock trials.
- Posted By: Chris M. Webb
- Written by: Ben Faber
In numerous publications world-wide, planting hole recommendations for avocado and other subtropical crops are made for large holes from 2 feet by 2 by 2 to as much as a cubic yard. These recommendations also include incorporation of manures or composts comprising 25% by volume with the native soil. I have noted the use of large holes and amendments in several countries, including New Zealand, Guatemala, Brazil, Costa Rica, Mexico and the United States.
The various reasons given for making these large holes are to disrupt any compaction or limiting soil layers and to create a more conducive environment for root growth. In the case of replanting deciduous orchards, McKenry found it to be beneficial in actually replacing the native soil in the hole with pathogen free soil. In many cases, research has shown that holes much larger than the planting ball and using organic amendments can cause problems for many tree species. Improper mixing of the organic amendment can cause anaerobic conditions and settling due to amendment decomposition. Soil that has not been properly firmed in the hole can also lead to plant settling and stems can drop below grade leading to crown rot.
Nonetheless, on the basis of recommendations made in many countries there could be some value in these planting practices, especially in the light of the effect organic matter has on avocado root rot. Numerous studies have shown organic matter suppresses the causal agent of root rot. This study evaluated the effect of hole size and amendments on avocado growth in an ideal environment with excellent soil conditions and in a more harsh one with heavy soil texture and the presence of the root rot pathogen.
Materials and methods
On the north island of New Zealand in the Bay of Plenty, 20 trees each were planted to one of four treatments: a) small holes (12 by 18 inches) without amendment; b) small holes with 25% by volume compost; c) big holes (60 deep by 30 wide by 24 wide inches) without amendment and d) big holes with 25% by volume compost. Big holes were dug with a backhoe, while small holes were dug by shovel. Trees were approximately 2 feet tall at planting. Soil was a deep sandy loam at both sites. Trees were irrigated by drip irrigation. Trees were ‘Hass’ on ‘Zutano’ seedling rootstock. Trees were planted the second week of spring 2000. Tree height, trunk caliper and canopy volume were measured on a monthly basis for eight months and then twice a year for the next year. In Carpinteria, California a similar trial was established using ‘Hass’ on ‘Toro Canyon’ rootstock. Trees were approximately 2 feet tall at planting. The grove had a heavy clay loam soil and a history of root rot. The trees were on drip irrigation. The trees were planted summer 2001 and monitored for 18 months after planting.
Results and discussion
Figures 1and 2 show the results of the different planting treatments at sites in New Zealand on ideal soils and on the heavy soil infected with root rot in California. Only tree height is shown; trunk girth and canopy volume followed similar patterns. From planting onwards, there were no differences in tree growth in any of the treatments at any of the sites. This would lead one to the conclusion that there is no value in and a great expense in making big holes and incorporating amendment. This is especially so in hillside situations where moving equipment and amendments on steep slopes would be very difficult.
The trees at the Carpinteria site, although infested with root rot, all looked good. The addition of organic matter in conjunction with the clonal rootstocks did not apparently provide any greater disease resistance. This is in accordance with work done by John Menge which shows that the greatest benefit derived from mulching are seedling rootstocks. The effect of mulch on disease suppression diminishes with the rootstock’s resistance to root rot.
Figure 1. Tree height (meters) at site 1 in New Zealand 20 months after planting. No differences were found at the 5% level of significance.
Figure 2. Tree height (meters) in California 18 months after planting. No differences were found at the 5% level of significance.