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Strategies for Nursery Trees Based on Plant Water Status

Irrigation Management Strategies for Nursery Trees Based on Plant Water Status Measured with Automated Stem Psychrometers.

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Irrigation management consists of two fundamental factors; irrigation timing and amount. Pan evaporation is a classical technique used to measure evaporation rate of an open water surface and provides a strong relationship with plant water use. However, pan evaporation methods do not quantify plant water status between irrigation events it simply estimates the potential amount of plant water loss. To adequately assess water stress, measurements of water potential are required. In the past the pressure bomb provided destructive water potential measurements but at low temporal resolutions. With an automated stem psychrometer higher data resolution is attained with non-destructive methods.

1. Measure cumulative plant water status and concurrent environmental demand (i.e. VPD) under conventional irrigation management using automated stem psychrometers
2. Measure plant water status under modified (reduced) irrigation management
3. Analyze and correlate cumulative water potential and vapour pressure deficit (VPD) with the goal to predict plant water status from environmental demand (i.e. VPD)

This study was conducted over a two year period (2013 and 2014) and carried out in two phases:
1. Assessment of nursery tree water status under conventional irrigation management
2. Assessment of nursery tree water status under modified irrigation management
This study was conducted at a tree nursery located in Waterdown, Ontario, Canada. Thuja occidentalis and Acer rubrum representing two common nursery evergreen and deciduous trees, were selected for trials. Trees were grown in a pot-in-pot production system with drip irrigation (Figure 1A). In both phases, concurrent environmental conditions were measured with a conventional weather station.



Conventional irrigation phase (2013)
Eight trees from each tree species were randomly selected to have psychrometers installed to measure water potential (WP). WP responses were measured every 30 minutes during nurseries normal irrigation schedule throughout the season.

Modified irrigation phase (2014)
Twenty one trees from each species were randomly selected to have psychrometers installed to measure WP response every 15 minutes. Three irrigation treatment based on cumulative water potential integral were assigned to each tree species with seven trees in each treatment level. Treatments were identified as: mild stress, stress, and most stress. These are summarized in table 1.

Table 1 –Thuja occidentalis and Acer rubrum were assigned three treatments which consisted of: mild stress, stress, and most stress. Each treatment level per species was assigned a cumulative water potential integral (MPa∙Hrs) threshold developed from historical data.

Acer rubrum
Thuja occidentalis
Mild Stress -20 -50
Stress -40 -100
Most Stress -60 -150


Water potential measurements
PSY1 Stem Psychrometer sensors were used to measure WP in both 2013 and 2014 trials. Figure 2 is an image of a psychrometer.

Meteorological measurements
Meteorological data was collected using a Vantage Pro2 wireless weather station (Figure 1B). Environmental conditions such as: solar radiation (W/m2), air temperature (oC), wind speed (km/hr), precipitation (mm), and relative humidity (%) were measured simultaneously with WP for each year. VPD was then calculated using the methodology outlined by Allen et al.(1998).



Cumulative WP and VPD integrals
WP integrals are the sums of measured WP responses accumulated for each daylight hour between watering events. VPD integrals were calculated and accumulated for the same periods as WP integrals. These variables were then correlated.



The relationship between VPD and WP integrals for Thuja occidentalis exhibited a slope response of -2.4 MPa∙Hrs/kPa∙Hrs. Similarily, Acer rubrum exhibited a slope response of -1.6MPa∙ Hrs/kPa∙Hrs. These relationships will be confirmed with additional field trials and species added to the catalogue of water status responses based on environmental demand for water. The ultimate goal is to develop a rational approach to irrigation management that accounts for prevailing environmental conditions as well as the species specific responses of plant and requires nothing more than a conventional weather station as a sensor.


1. Measurements of water stress in these species under conventional nursery irrigation resulted in a significant level of stress
2. Preliminary results indicated that conventional practices significantly over-irrigated the trees
3. Relationship between water potential and vapour pressure deficit shows promise as a tool for irrigation management in the nursery sector and others (orchards, viticulture, etc.)


This research was supported by ICT International Pty. Ltd. (Australia), Root Rescue Environmental Ltd. (Canada), and Connon Nurseries C.B.Vanderkruk(Canada) and the Ontario Centres of Excellence.