A sap flow monitoring project commenced in Sydney in March 2017. The project is intended to measure and understand the water use patterns of Moreton Bay Fig trees.
As little has been published on plant water relations of Moreton Bay Fig trees, it was proposed that an initial pilot study be conducted to understand the basic anatomy and physiology that must be addressed to develop a successful long term monitoring program.
The complexity and form of the Moreton Bay Fig tree, with multiple stems developed over years of aerial roots reaching the ground, means a total water use figure for the tree requires an incremental approach to measurement. The initial measurement campaign consisted of the deployment of 4 SFM1 Sap Flow Meters on two large stems. Sap Flow Meters were installed on the east and west of each stem to evaluate the circumferential variability from east to west.
A PSY1 Stem psychrometer was installed on a young, small diameter (25 mm diameter), aerial root that had reached the soil and formed a functional stem of the tree. The stem psychrometer measures water potential, the energy used by the tree to transpire water for plant growth.
The instruments are powered by a 24V DC power supply connected to mains power, inside the base of a lamp post. A single run of 2-core low-voltage cable links all the instruments together. Power from the lamp post is supplied only at night and this is used to charge the instruments, they operate on internal battery during the day.
All instruments have built-in proprietary 2.4GHz wireless, which is used to transmit data to a 3G Telemetry Hub for upload to the cloud. The Telemetry Hub has been installed in the adjoining glasshouse.
Initial results show that the circumferential variation from stem to stem looks to be relatively low; although it must be understood that both stems sampled were on the southern side of the tree. This may change when stems on other aspects of the tree are sampled.
An estimate of the whole tree’s diameter was made using a stem diameter tape measure, but no attempt was made to account for all stems that ultimately make up the total diameter of the fig tree. This should be done at a later stage.
The total stem diameter of the tree was 313 cm. Estimated water conducting xylem depth was 4.0 cm, estimated based on the empirical sap flow data collected using a 35 mm long needle.
As the inner measurement point was still recording a high sap velocity and the radial sap velocity gradient was relatively low, it is possible that this estimate of sap wood depth is conservative. If the water conducting tissue is deeper, the water use reported will be underestimated.
Note: the diameter of the two stems upon which sap flow was measured were not included in the overall stem diameter measurement for ease of calculation at this stage of the project.
Two SFMs were installed on the east and west of Stem 1, on the eastern side of the Moreton Bay Fig tree.
Two SFMs were installed on the east and west of Stem 2, on the western side of the Moreton Bay Fig tree.
Averaged sap flow data for the two Sap Flow Meters on Stem 1 is shown below:
Averaged sap flow data for the two Sap Flow Meters on Stem 2:
Based on the average of the measured velocity for Stem 2 applied to the overall stem diameter of the Moreton Bay Fig, the tree is using a maximum of 350 litres of water per day. This occurred on March 31st and April 1st, 2017, following a rainfall event. The (expected) increase in soil moisture content from the rainfall was immediately taken up by the tree.
It is noticeable that even within the last 30 days (April 9th to May 8th) water use has declined by approximately 20%. This reduction is most likely due to the reduction in solar radiation duration and ambient temperature as the season transitions from early- to mid-Autumn.
However, these measurements (even over a six week period) still constitute a discrete moment in time. To fully understand the plant water relations of the Moreton Bay Fig tree, sap flow should be monitored continuously for at least 12 months, preferably 3 years. Such a monitoring program will establish diurnal patterns seasonally throughout the year, clearly showing how the tree responds to abiotic stimuli of each season.
Continuing the monitoring program over 3 successive years will filter out atypical occurrences for any given season or year, building a very strong database upon which to confidently develop a horticultural or arboricultural management plan.
Once a plant-based monitoring system, with an upper and lower threshold of optimum water use, has been established for each season, arborists can make management decisions with more confidence. If the tree begins to experience levels of water stress it becomes more susceptible to attack from pest and disease. This bio-feedback can be used to ensure a healthy water status is maintained, minimising the risk of such attacks.