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Trunk and Fruit dendrometers: Detecting early signs of water stress in fruit trees before visual cues.

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.


For Spanish version, click here.


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.


Four images of two types of dendrometers. A, and B show a point dendrometer for a tree, before installation in A and after installed in an apple tree in B. Pictures C and D show fruit dendrometers. Before installation in image C and after in image D. installed in the trunk of an apple tree . Figure B shows an apple fruit attached to a fruit dendrometer
Figure 1. Point dendrometer before (A) and installed in the trunk of an apple tree (B) and fruit dendrometers before (C) and attached to an apple (D) in a commercial orchard located in Washington State, season 2021.


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:

  1. Maximum daily shrinkage (MDS)
  2. 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.


Graph showing data analyzed from dendrometers showing data points used for calculating MDS and TGR
Figure 2. Evolution of the trunk diameter from July 16th and July 18th in 2021 and calculation of the maximum daily shrinkage (MDS) and trunk growth rate (TGR) of an apple tree in Washington State.


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.


Two graphs showing data of TGR and MDS . Figure A shows data from well watered trees. and figure B shows data from a stress tree. Graphs are divided vertically and each division has a different color. Color yellow, shows values of TGR =0, Green is TGR higher than 0; orange, TGR less than 0 and MDS high; and red, TGR lower than 0 and MDS low
Figure 3. Maximum daily shrinkage (MDS), and trunk growth rate (TGR) of two young pear trees under two different water scenarios: (A) Well-watered and (B) Water stress.


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).


Two graphs. First graph shows trunk diameter fluctuation. Blue line represents data from a tree with no water stress. Red line data from water stressed tree. Figure B MDS data.
Figure 4. Trunk diameter fluctuation of two apple trees under no water stress conditions (blue) and submitted to water stress (red) from July 15th to August 15th, 2021 in Washington state (A), and the signal intensity of the maximum daily shrinkage (SIMDS) of the same trees (B). The figure highlights how the same MDS value (140 µm) can be recorded in well-watered trees under some environmental conditions (July 25th, 2021) and by water stressed trees under different environmental conditions (August 10th, 2021). Absolute MDS values cannot be used as a reference for irrigation management and signal intensity corrects for orchard-orchard variation in tree performance.


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.


figure formed by three graphs. graph A, fruit diameter changes through time from july 5 to july 26, 2021. it also has a close up from the data close to july 12. Graph B, MDS data , and graph C, fruit growth rate.
Figure 5. Fruit diameter fluctuations of a ‘Honeycrisp’ apple from July 5th to July 24th, 2021 in Washington state (A) and the maximum daily shrinkage, MDS (B) and the fruit growth rate, FGR (C) calculated for the same period.


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



– 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.



– 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.




D.A. Goldhamer and E. Fereres. 2001. Irrigation scheduling protocols using continuously recorded trunk diameter measurements. Irrigation Science, 20, pp. 115-12.



Washington Tree Fruit Research Commission – Project: Validation of plant-based sensors for making irrigation decisions




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)


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


Lee Kalcsits
Associate Professor
Endowed Chair
Tree Fruit Environmental Physiology and Management
WSU Tree Fruit Research & Extension Center
Wenatchee, WA
phone: 509-293-8764


Erica Casagrande-Biasuz
Postdoctoral Research Assistant
WSU-TFREC Horticulture

Noah Willsea
Graduate Student Horticulture
WSU-TFREC Horticulture articles may only be republished with prior author permission © Washington State University. Reprint articles with permission must include: “Originally published by Washington State Tree Fruit Extension Fruit Matters at” , and a link to the original article.

Washington State University