Measuring tree stress by pressure bombing is an increasingly popular way for orchard managers to monitor and manage the impacts of drought and reduced water allocations, says Richard Buchner, Tehama County orchard advisor and University of California Division of Agriculture and Natural Resources (UCANR) extension agent.
However, using this method alone may not capture the variability that impacts orchard health, Buchner says.
Different approaches to pressure bombing
Pressure bombing is a method of measuring water stress in plants by using pressure chambers. It's often compared to taking a person's blood pressure.
Pressure bombing provides a good measure of stress placed on a tree by a range of factors: soil moisture, humidity, wind and direct sunlight or heat load, according to “Using the Pressure Chamber for Irrigation Management in Walnut, Almond and Prune,” a UCANR publication Buchner co-authored.
Buchner lists four methods of pressure bombing:
- Leaf water potential—Measuring an uncovered leaf mid-afternoon, when water tension is greatest.
- Pre-dawn leaf water potential—Measuring an uncovered leaf before the sun rises, to reflect the tree's water status after trees are most recovered from the previous day’s stressors.
- Stem water potential —Measuring a leaf on the lower, shaded canopy that has been bagged for 10 minutes, typically between noon and 4 p.m., when the tree is experiencing maximum water demand.
- Shaded leaf water potential —Similar to stem water potential, but the sample leaf is wrapped with a damp cloth instead of being bagged.
The stem water potential technique in particular reduces variability versus using bare leaves and is the most common approach, “pretty well adopted” by orchard managers in Tehama County, Buchner says.
Advantages and disadvantages of pressure bombing
Stem water potential is useful for assessing trees' water needs directly, versus other techniques—such as water budgeting and soil moisture monitoring—that do so indirectly.
However, it doesn’t provide a complete picture of what’s going over time and across different locations, Buchner says. For example, pressure bombing in one part of the orchard can't account for the effect of different soil types on moisture levels elsewhere.
“The huge problem you have in orchards is there is a large amount of variability [in water retention]. You can have class one to class four soils and all kinds of things that could be impacting the variations,” Buchner said. “And once you get into these more marginal soils that we’re planting on the west side (of California), it gets even more [variable].”
Pressure bombing is time- and labor-intensive, and requires specialized equipment. There's also the issue of timing: according to the UCANR report, trees pressure-bombed in the spring may exhibit low water stress simply because they are consuming winter rainfall or winter irrigation stored in the root zone—and then experience high stress in the summer months when those water stores run out. By the time pressure bombing reveals that stress, typical drip or micro-sprinkler systems may not be able to recharge soil moisture adequately to relieve it.
Combining techniques and interpreting results
For that reason, combining stem water potential readings with soil moisture or tree evapotranspiration (ET) data—or both—can be a better approach to predicting water needs later in the season, Buchner says. The addition of probes or aerial imagery can help mitigate the blind spots left by pressure bombing alone.
“Aerial imagery can really show you where that variation is based on a bird’s eye view,” Buchner says.
Understanding the strengths and limitations of each source of data is important to interpreting it—especially when they seem to conflict. For example, the exact placement of soil moisture sensors determines whether they reflect what's really happening at the root zone. They may suggest stress when stem water potential shows the opposite if trees are getting water from lower soil depths. Conversely, in areas where rainfall plays a large part in determining irrigation needs, stem water potential might show that trees are stressed even when soil moisture sensors indicate high soil moisture content.
“Soil is so variable it is hard to guess what the tree is doing just based on the soil moisture,” Buchner said. “Having several different sources of information is always nice to confirm what is going on. It gives you a better look,” Buchner says. “It boils down to how much do you want to do.”