Estimating water use by sugar maple trees: considerations when using heat-pulse methods in trees with deep functional...

Area of Science:

• Forestry
• Plant Physiology
• Hydrology

Background:

• Accurate estimation of tree water use is crucial for understanding forest ecosystems and water resource management.
• The heat pulse velocity (HPV) technique is widely used to measure sap flow, but its accuracy depends on precise sapwood property estimations.
• Sapwood volumetric water content (V(h)) is a key parameter that can significantly influence HPV-based sap flow calculations.

Purpose of the Study:

• To investigate the seasonal and radial variation of sapwood volumetric water content (V(h)) in Acer saccharum (sugar maple) trees.
• To assess the impact of V(h) variability on sap velocity and sap flow estimations using the HPV technique.
• To compare different methods for calculating sap velocity and flow, particularly in large trees where sapwood constitutes a significant portion of the stem.

Main Methods:

• Wood cores were collected from Acer saccharum trees over three years to determine V(h) across different seasons, tree sizes, and radial depths.
• Heat pulse velocity (HPV) data were collected from sap flow gauges inserted at four depths.
• Two modified versions of Hatton's weighted average technique (zero-step and zero-average) were compared with the original method for calculating sap velocity and flow.

Main Results:

• Sapwood V(h) exhibited significant variations with the time of year, tree size, and radial depth, with notable interactions between these factors.
• Using a mean whole-tree V(h) instead of depth-specific values led to sap velocity and flow estimation errors ranging from -6% to +47%.
• The zero-average method showed potential for reducing errors in whole-tree sap flow calculations for large diameter trees compared to other methods.

Conclusions:

• Sapwood V(h) in Acer saccharum is dynamic, changing seasonally (50% in winter/early spring to 20% in growing season) and varying with tree size and radial depth.
• Inaccurate V(h) estimations can lead to substantial errors in sap flow calculations, particularly when scaling up to whole-tree or landscape levels.
• For large trees, the zero-average modification of Hatton's method is recommended to improve the accuracy of sap flow measurements and subsequent water balance estimations using the HPV technique.