Written by Victor Blanco, Postdoctoral Research Assistant WSU TFREC- Horticulture; Jenny L. Bolivar-Medina, ITT WSU Extension; Erica Casagrande-Biasuz Postdoctoral Research Assistant WSU-TFREC Horticulture; Noah Willsea, Graduate Student Horticulture WSU-TFREC Horticulture; and Lee Kalcsits, Endowed Chair WSU Horticulture. June, 2022.
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We all have heard that the early bird gets the worm, and that can be applied when you are scheduling the irrigation of your apple orchard. Early detection of undesirable water deficit is important for avoiding any penalization in fruit size, yield and tree growth. Early visual cues indicating water stress in apple trees are not so perceptible once they appear, it is often too late to avoid negative effects of severe water stress causes on fruit quality, yield, and tree growth. Precision sensors such as dendrometers can be crucial and make that task much easier.
What is a dendrometer and what does it measure?
Dendrometers are well-studied, plant-based sensors that continuously measure small fluctuations (shrinkage and swelling) in trunk or fruit diameter resulting from variation in sap flow. Trunk and fruit dendrometers (Figure 1) can be used to detect and quantify water stress to improve irrigation scheduling in fruit trees.
Trunk dendrometers
Trunk dendrometers (Figure 1A, B) are devices that are attached to the trunk or branch of a tree to measure daily variation in trunk diameter. Trunk diameter varies diurnally and seasonally but are also affected by soil water availability, environment, tree transpiration, crop load, and the age and health of the tree, among others. Consequently, by registering how the diameter of the trunk of the tree changes, it integrates many other important factors with water availability to indicate perceived stress. Those daily changes in trunk diameter of an adult apple tree are measured in micrometers (µm), 1 µm = 0.0000394 inches.
How do you interpret trunk diameter fluctuations?
The most interesting indexes derived from trunk diameter fluctuations are:
- Maximum daily shrinkage (MDS)
- Trunk growth rate (TGR)
MDS is calculated as the difference between the maximum and the minimum trunk diameter recorded during the same day. On the other hand, TGR is calculated as the difference between the maximum trunk diameter recorded on two consecutive days (Figure 2). Generally, the maximum trunk diameter is daily recorded at dawn and the minimum daily trunk diameter is measured between midday and early afternoon when the evaporative demand is the highest.
How can trunk dendrometers help you in water stress detection and irrigation scheduling?
Soil water availability and evaporative demand affect daily trunk shrinkage and swelling patterns. If trees are under no water stress conditions, the TGR values will be positive and the MDS values can vary depending on environmental conditions. However, under early water stress, trunk growth stops and TGR values decrease to values near zero. MDS values increase as a result of depletion of trunk water storage. Thus, elevated MDS values and TGR values close to zero indicates a tree under a water deficit (Figure 3). If the water deficit continues, the TGR values will become negative and the MDS values will increase until a threshold values where the trunk of the tree has no water reserves. At that point, MDS values will rapidly decrease under severe water stress. This level of water stress might have a negative effect on long-term tree health and productivity.
Instead of comparing absolute MDS values, to assess tree water status and decrease the effect of factors such as the soil and weather conditions, the crop load, or the tree phenology, Goldhamer and Fereres (2001) proposed the use of the MDS signal intensity calculated as the ratio between the MDS values of those trees that we want to assess and the MDS of non water-stressed trees which are located in the same orchard under the same environmental conditions and management with the same practices (reference tree).
MDS-Signal intensity is a dimensionless variable, in which values of one or close to it indicate that the tree that we are assessing has no water stress, and values above 1 indicate water stress levels (Figure 4).
Fruit Dendrometers
Fruit dendrometers monitor fruit growth by measuring real-time fluctuations of fruit diameter. They are helpful to know how the irrigation strategy may be affecting fruit growth and to detect early water stress and avoid penalizing fruit size (Figure 1C, D).
How do you interpret fruit diameter fluctuations and how can fruit dendrometers help you?
Contrary to what happens with the trunk diameter fluctuations, fruit maximum daily shrinkage (MDS) is not used in apple since it is not sensitive enough to detect water stress. Instead, we focus on the fruit growth rate (FGR). FGR is calculated as the difference between the maximum fruit diameter recorded on two consecutive days (Figure 5). If FGR decreases to numbers close to zero, it indicates that fruit growth is limited, and it might be related to a water deficit. Moreover, FGR values are different among cultivars and for different fruit development stages.
Strengths and weakness of trunk and fruit dendrometers
While both trunk and fruit dendrometers can be extremely helpful tools for water stress monitoring in your orchard, there are strengths and weaknesses for each tool that should be considered before shifting your management strategies. The following is a table of these considerations.
| Dendrometers | Strengths | Weaknesses |
|
Trunk |
– Real-time, continuous and direct measurements of the tree water status.
– Early water stress detection. – Rapid response to changes in the tree water status. |
– Need to calculate the MDS and TGR.
– It is difficult to interpret absolute values, need to compare the trees with a reference tree in the orchard. – Highly dependent on other factors, not only water stress. |
|
Fruit |
– Direct measurement of fruit growth and how it is affected by irrigation.
– Precise and accurate technology |
– High variability among fruit even in the same tree.
– High maintenance, need to check that the dendrometer is attached to the fruit. |
References
D.A. Goldhamer and E. Fereres. 2001. Irrigation scheduling protocols using continuously recorded trunk diameter measurements. Irrigation Science, 20, pp. 115-12.
Acknowledgements 
Washington Tree Fruit Research Commission – Project: Validation of plant-based sensors for making irrigation decisions
Contacts
Victor Blanco, Ph.D.
Postdoctoral Research Associate
Tree Fruit Physiology- Kalcsits Lab
WSU Tree Fruit Research & Extension Center
Wenatchee, WA
Fundación Seneca
(Región de Murcia, Spain, 21261/PD/19)
email: victor.blanco@wsu.edu
Jenny L. Bolivar-Medina, Ph.D
ITT- WSU Extension- – Tree Fruit Horticulture
WSU-Irrigated Agriculture Research and Extension Center
24106 North Bunn Road
Prosser, WA 99350
phone: 509-786-9201
email: j.bolivarmedina@wsu.edu
Lee Kalcsits
Associate Professor
Endowed Chair
Tree Fruit Environmental Physiology and Management
WSU Tree Fruit Research & Extension Center
Wenatchee, WA
phone: 509-293-8764
email: lee.kalcsits@wsu.edu
Erica Casagrande-Biasuz
Postdoctoral Research Assistant
WSU-TFREC Horticulture
e.casagrandebiasuz@wsu.edu
Noah Willsea
Graduate Student Horticulture
WSU-TFREC Horticulture
noah.willsea@wsu.edu
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