Irrigation support: The role of trunk dendrometersResearch & Extension
Now available commercially, trunk dendrometers could assist growers with their irrigation decisions. AgVic researchers have been putting this plant-based sensor to the test at Tatura SmartFarm.
This article was written by Ian Goodwin, Lexie McClymont and Alessio Scalisi from Agriculture Victoria. It was first published in the Spring 2022 edition of AFG.
An intended outcome of the PIPS3 Program’s Developing smarter and sustainable pear orchards to maximise fruit quality, yield and labour efficiency project (AP19005) is for the pear industry to adopt technology to increase profitability.
A trunk dendrometer is a tool to better inform irrigation decisions. It enables assessment of tree water stress by monitoring micro-variations in trunk diameter – trunks shrink during the day and expand during the night in response to changes in the tree’s water potential. Both the soil moisture and vapour pressure deficit influence tree water potential and hence how much a trunk shrinks and expands. While weather data and an irrigation budgeting tool (www.hin.com.au, pear irrigation budgeting tool) are used to estimate water use and inform irrigation decisions, trunk dendrometers help to monitor the outcome of these decisions.
Decision-support to schedule irrigation can be achieved by the use of three main types of sensor:
- weather sensors
- soil moisture sensors
- plant-based sensors.
A combination of weather sensors that measure temperature, humidity, wind speed and radiation are used to calculate reference crop evapotranspiration (ETo) – the base for estimating orchard water use and irrigation requirements. For most horticultural regions, the local long-term historical ETo values reported by the Bureau of Meteorology (BoM) are sufficient to prepare seasonal water budgets and monthly irrigation plans (see For more information for a link to an educational video on water budgeting). Real-time daily ETo reported by the BoM can provide additional guidance for irrigation adjustments during periods of atypically high evaporative demand, such as during heatwaves.
Various types of soil moisture sensors have been used in horticulture over the last few decades. They have aided improvement of irrigation scheduling by providing growers with a visual report, primarily in a graphed format, to demonstrate the drying out of the soil associated with root water uptake and the effects of irrigation run-times on movement of water into and beyond the root zone. At the beginning of a season and after rainfall, decisions about withholding and restarting irrigation are improved by knowledge of the current soil moisture status through the soil profile. Similarly, use of deficit irrigation strategies during the season can be guided by soil moisture thresholds.
Trunk dendrometers are a type of plant-based sensor. Plant-based sensors provide a measure of the water potential or water use of plants, integrating the effects of both the availability of soil moisture and weather conditions. Until recently, these have largely been used in research. Indeed, sap flow sensors (measuring rate of water movement through sections of tree trunk), canopy temperature sensors (indicating opening or closing of stomata in response to water availability) and pressure chambers (measuring the pressure required to extract water from a leaf) continue to be most commonly used by researchers. However, a number of companies are currently marketing trunk dendrometers for use in commercial orchards as either standalone units or as a component of an irrigation control system.
Dendrometers provide an indication of tree water potential by measuring fluctuations in trunk circumference or diameter. Trunks shrink during the day when the vapour pressure deficit of the air (the difference between how much moisture the air could hold and how much it is holding) drives movement of water through the tree by a gradient in water potential. At night, when vapour pressure deficit falls, the tree is able to ‘refill’ and the trunk swells. Both the shrinkage of the trunk during the day (maximum daily shrinkage, MDS) and the daily growth rate of the trunk (change in maximum trunk diameter per day, DG) can be examined to assess water potential (Figure 1).
The amount of trunk shrinkage over the course of the day indicates the degree of water deficit experienced by the tree – greater shrinkage corresponds to greater water deficit within the tree. Similarly, trunk growth will slow, or even regress, with increasing water deficits. While these parameters are considered to be reliable indicators of the water potential of the tree, an understanding of a number of factors is required before making decisions about whether or not to irrigate. Generally, when vapour pressure deficit is similar between days, the irrigation requirement is indicated by increasing MDS and a reduction in DG. However, on days of high vapour pressure deficit (i.e. hot days with low humidity), MDS will be relatively large even if the soil is wet and the tree is able to ‘refill’ during the night – irrigating further will simply result in water losses by drainage below the root zone or surface runoff and evaporation.
Trunk growth is expected throughout most of the season when trees are young, whereas less trunk growth occurs in mature trees; the need for irrigation may be indicated by a slowing of trunk growth in young trees, but by regression of trunk diameter in older trees. Consequently, thresholds for triggering irrigation events need to be calibrated for vapour pressure deficit and differ between crop types (potentially even between cultivars and rootstocks) and growth stages. For this reason, most trunk dendrometer systems operate as decision-support feedback and are not used to control irrigation systems directly.
Purchasing a trunk dendrometer
Trunk dendrometers currently retail for $2,000 to $3,000 as standalone units with capability for real-time monitoring. This price usually includes installation and subscription to cloud-based software to display the data. Software may provide guidance regarding interpretation of shrinkage or growth parameters; however, be mindful that this information is decision-support and thresholds for your block may differ from those used by the software. Understanding that soil moisture availability and vapour pressure deficit will influence trunk shrinkage and expansion is key to interpreting your data.
Point dendrometers measure fluctuations in trunk or branch diameter. They are installed with the sensor arm perpendicular to the trunk or branch; the head of the sensor contacts the trunk and moves linearly with the expansion and shrinkage (Figure 3). Some point dendrometers can be attached to young trees or branches, while the design of others requires a trunk large enough to drill and insert a stabilising arm. Band dendrometers encircle the trunk or stem, and measure fluctuations in trunk circumference, with different models suited to particular trunk sizes.
- Only install on a healthy, representative tree and not near the edge of a block.
- Position the sensor to avoid knots or damaged areas of the trunk.
- Remove loose bark that would be under the sensor head or band (Figure 4). Data is likely to become erratic after rainfall if loose bark is present.
- Remove weeds that could interfere with the sensor.
- As the tree grows, adjustment of the initial sensor position will be required.
- If data ceases showing daily fluctuations, check if a point dendrometer is too close to the trunk, or if the trunk has reached the upper size limit of a band dendrometer.
View a short video on how Dr Ian Goodwin is using this technology as part of the PIPS3 Program research in Agriculture Victoria Tatura SmartFarm’s Sundial Orchard.
For more information see:
- Irrigation scheduling in the Sundial Orchard – PIPS3 update
- Plant-based sensors for irrigation management – Agriculture Victoria
- Plant based sensors for irrigation scheduling – Irrigating Agriculture
- The Role of Dendrometers for Irrigation Management in Apple – Washington State University Tree Fruit
The PIPS3 Developing smarter and sustainable pear orchards to maximise fruit quality, yield and labour efficiency project (AP19005) has been funded by Hort Innovation, using the apple and pear research and development levy, contributions from the Australian Government and co-investment from Agriculture Victoria. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.