Epsilon Archive for Student Projects

Abstract

The boreal region of Europe has seen a notable increase in spring temperatures which affects the onset of transpiration. Freeze-thaw temperature cycles are characteristic of early spring, but transpiration is difficult to measure under these conditions. Sap flow sensors can estimate transpiration in trees by using the relationship between wood thermal properties (volumetric heat capacity and thermal diffusivity) and stem water content. Rapid temperature changes that occur during freeze-thaw cycles are known to introduce error in these calculations, and the relationship between stem water content and thermal properties needs validation across temperature gradients to produce reliable estimates of transpiration in spring. The purpose of this study is to achieve the following research aims: 1) Establish a relationship between changes in water content and volumetric heat capacity of three boreal tree species: birch, pine and spruce. 2) Establish a relationship between changes in volumetric heat capacity and thermal diffusivity of the three boreal tree species. 3) Identify whether heat pulse sap flow sensors can reliably estimate thermal properties within a temperature range of -15 to 25ºC. 4) Use point dendrometers to determine the role of stem water content in diameter fluctuations during freeze-thaw cycles. The results showed a temperature-dependent limit to reliable measurements of stem water content using current sensor methods. Thermal properties can be related to water content, but require parameterization near the freezing threshold and for rapid temperature change. Stem expansion from ice formation does occur during freezing, and should be taken into account when using diurnal cycles of stem diameter change to mark the onset of spring transpiration. This study contributes to understanding whole-tree water use and measurements of transpiration during shifting spring conditions, with implications for measuring the effect of climate change on forest functioning.