How many sensors do I need to use to accurately measure sap flow on a tree?
This is the question that no one person can give a definitive answer to, for any given species at any given location, prior to measurement. Instead it must be empirically answered prior to commencing a measurement campaign. The best way to do this is to run a short pre-experiment by installing a Sap Flow Meter within each quadrant: North, South, East & West, around the tree. Measurements should be collected at 10 minute temporal resolution for a period of at least 7 to 14 days. This allows sufficient diurnal water use curves to be examined, and observe the effects of resource competition, that may result in heterogeneity of water conducting xylem or sap wood circumferentially around the tree, as well as any slight climatic variation and how this may impact water use. This small but rich dataset then enables the statistical variance between each quadrant, using time as the replicate, to be determined. Based on this variance and the quadrants weighting or contribution to whole tree water use, a percentage error can be calculated for each number of Sap Flow Meters used per tree and the impact on this error based on position within the tree. Armed with this invaluable data the scientist can now make a valued judgement based on empirical data as to the importance of the research and the cost benefit of using only one or more than one Sap Flow Meter per tree.
When working with uniform genetic material, in an evenly spaced and theoretically uniform resource competition environment such as a plantation, a single Sap Flow Meter per tree if placed in the correct quadrant may yield highly accurate whole tree water use data. However, when working in natural forests with non-uniform tree distribution and competition for resources more than one Sap Flow Meter is typically required. Dr. Rhiannon Smith from University of New England and Alec Downey from ICT International are using this pre-experiment technique to better understand the circumferential heterogeneity of old growth River Red Gums (Eucalyptus camaldulensis) in the Namoi & Gwydir catchments. To learn more about the impacts of understanding circumferential heterogeneity of old growth trees, read on about the use of sap flow to unlock the secrets of dieback in River Red Gum forests.
Can Sap Flow unlock the secrets of Eucalyptus Die-Back?
By Alec Downey B.Sc. (For)
Head of Plant Science Applications & Research, ICT International Pty Ltd
Adjunct Senior Lecturer, School of Plant Biology,
University of Western Australia
Researchers at the University of New England lead by Dr. Rhiannon Smith School of Environmental & Rural Science are looking to use Sap Flow measurements to unravel the complex environmental interdependency that causes die-back in Eucalyptus trees in Australia. The initial research site will be in the North West of NSW amongst an agriculture dominated flood plain along the Namoi River where Sap Flow Meters will be deployed in ancient River Red Gum trees that predate European settlement in Australia. Dr. Rhiannon Smith speculates that these trees could potentially be 400 years old or even older.
Photo 1: A river Red Gum (Eucalyptus camaldulensis) Tree potentially 400 years old.
Photo 2: A river Red Gum (Eucalyptus camaldulensis) ecosystem in crisis through the effects of die-back.
By using SFM1 Sap Flow Meters the actual volume of water used by these ancient trees along the ephemeral flood plains of the Namoi River can be directly measured. This will enable researchers to understand how much water the trees need to grow, and if in fact, seasonal and environmental changes affect the amount of water they use. Because the Sap Flow Meters can log data continuously at 10 minute temporal resolution, other crucial data such as the time at which water is used by the tree during the day and or at night, can also be recorded. This can be very important in understanding the mechanisms that these eucalypts employee to cope with stresses that may be causing a reduction in water use and therefore possibly be accelerating or accentuating the die-back effect.
In addition to sap flow or water use the water potential or the amount of stress the tree is under can also be measured using the PSY1 Stem Psychrometer. Variations in water potential can then be used to understand the plant water stress response to changing ambient parameters. These changes and critical water potential thresholds may be key conditions to induce flowering, flushes of new growth, or the signal to utilize critical carbon reserves to produce epicormic growth as a survival mechanism.
Overall tree growth will be monitored using DBL60 logging band dendrometers that can record the diurnal expansion and contraction of the stem circumference. This provides an additional confirmation of the changes in water use and water potential as stem circumference is directly affected by plant water relations. Continuous logging of stem increment and stem reduction, provides a record of the amount of biomass laid down or the carbon sequestered per unit of water use under differing levels of plant stress. Thresholds of carbon gain and carbon loss will be able to be established on a seasonal basis as well as in response to environmental events in particular such as drought.
Initial sap flow measurements of River Red Gums experiencing die-back have revealed variations of water use within different positions within the tree. These variations conform to published literature on the conventional distribution of water use patterns radially across the sap wood and even circumferentially around a tree. However, significant differences radially at any given point must be more thoroughly examined.
Figure 1: Sap Flow measurements at two quadrants, North and West, and two radial positions (Inner and Outer) across the sap wood depth at each respective location on a River Red Gum Tree experiencing die-back.
Photo 3: A SFM1 Sap Flow Meter installed on a River Red Gum (Eucalyptus camaldulensis) tree experiencing die-back.
Understanding Circumferential Homogeneity
Because the trees are of such great age and suffering die back they have seemingly numerous discontiguous active zones of xylem spread circumferentially around the tree. These zones are the result of insect damage, fire and physical damage to the trees over extended periods of time. This poses an immediate problem for accurate sampling of total water use of the tree. Therefore a robust sampling protocol must be developed and tested that looks at the circumferential homogeneity of water use by the tree, but also accounts for radial homogeneity.
Typically trees conform to a generalised radial profile of water use where the majority of the water is used on the outer edge of the tree relating to the youngest and most active layers of xylem with a negative gradient of water use radially across the xylem reducing to zero upon reaching the sapwood-heartwood interface where the heartwood is fully lignified and no longer actively conducting water. However, with these ancient trees this profile is further complicated due to the effects of dieback where the sapwood is not uniform or homogenous due to issues such as insect damage and other historical damages that are not superficially visible such as pockets of Kino vein scarring from past episodes of severe bush fires.
A sample sap flow data set collected from the Northern side of the River Red Gum tree clearly shows the discrepancy between the outer layer of actively conducting xylem and the inner layer (Figure 2 below) with an order of magnitude difference between them (20 cm hr-1 to 2 cm hr-1 respectively.)
Figure 2: Sap Velocity measurements for two radial points (Outer and Inner) over a period of 2 ½ days during late autumn measured on the Northern side of the tree.
Photo 4: A SFM1 Sap Flow Meter installed on the Northern side of the remaining “living” or active water conducting portion of the tree.
However, on the western side of the tree the discrepancy is not significant and conforms to the more characteristic radial profile of water use within trees.
Figure 3: Sap Velocity measurements for two radial points (Outer and Inner) over a period of 2 ½ days during late autumn measured on the Western side of the tree.
Photo 5: A SFM1 Sap Flow Meter installed on the western side of the remaining “living” or active water conducting portion of the tree.
This potentially error inducing heterogeneity can be overcome with a detailed initial circumferential reconnaissance of each tree using the SFM1 Sap Flow Meter for a period of 1 week measuring diurnal sap flow rates in at least each quadrant of the tree. In trees of this complexity of water conducting tissue perhaps more points are necessary (where living tissue exists). A mean value for the radial sap Velocity can then be used as a comparative tool for assessing circumferential homogeneity and a test of accuracy for the sampling protocol employed.
Figure 4: Mean Radial Sap Velocity measurements of two radial points (Outer and Inner) used to compare the Mean sap velocity between quadrants on a tree.
Monitoring the whole tree water relations using the SFM1 Sap Flow Meter, the PSY1 Stem Psychrometer and the DBL60 band dendrometer will develop a detailed empirical data set of the integrated whole tree, diurnal and seasonal water use, water stress and overall growth patterns of River Red Gum trees experiencing die-back. This information can be analysed to identify the predominant agents and or interrelationship of dominant causal factors of die-back at any given time. Understanding the acute and or chronic dieback stresses will aid in the long-term ecophysiological management of the environment.
An additional benefit of this research is the ability to use this fundamental data to advance current carbon sequestration models, typically based on allometric equations and conventional forest mensuration techniques, to yield more accurate, real time, rates of carbon sequestration. Such improvements in these models would lead to more accurate carbon budgeting. This would aid government planning, international carbon markets, and provide farmers with empirical data that they can use to access, and trade on farm sequestered carbon (as a commodity) on international carbon markets. This would allow farmers to diversify their farming enterprise, and ultimately provide incentive and cash flow to better manage these “revenue producing” ecosystems that are currently under environmental stress.