Dynamic variations in bark hydraulics - understanding whole tree processes and its linkage to bark hydraulic function and structure

Abstract

A mature tree stem generally consists of a column of wood that is composed of a series of annual incremental layers and enclosed in a covering of bark. The dynamic variations of the bark are complex due to its structure and function: the thick outer-bark acts as a protective barrier against the abiotic and biotic environment; while the phloem is where sugar transport occurs. Much of the bark variation is due to the transport of sugars and its related processes. The driving force for sugar transport in the phloem is generated by the accumulation of sugars at source sites (e.g. leaves), which creates differences in gradients in turgor pressure along the stem. As a result, mass flow occurs – transporting sugars to sink regions that require it (e.g. stem and roots) for active growth, respiration and storage. The xylem pathway, which transports water in the opposite direction, is connected to the phloem in parallel along the entire length of the stem. The immediate connection between these two transport pathways suggests a functional linkage, as the phloem draws water from the xylem in order for mass flow to occur. The dynamic interactions between the xylem and phloem, and the processes occurring within the bark have great implications for whole tree physiology. The purpose of this thesis is to study the dynamic processes that occur within the bark and its interaction with other internal tree processes and the external environment. This is accomplished by first understanding the bark hydraulic architecture and its linkage to the environment, followed by its linkage to various tree processes. These linkages have not been thoroughly quantified, especially on an intra-annual (e.g. daily) scale. The study of bark hydraulic dynamics is of great interest because it is a relatively new topic with great potential. The changes of the bark in response to the environment may play a large part as a regulator to other tree processes. The thesis consists of four papers, of which one is a modelling paper and three are experimental (field and laboratory) studies. The model estimates growth by using dendrometer measurements as inputs for the model. Growth is estimated by separating the water-related influences from measured inner-bark, revealing a growth signal – proxy for cambial stem growth. Using this growth signal, a correlation study to microclimate variables is examined in one paper; and to assumed growth respiration in a second paper. The remaining two papers explore the seasonality of photosynthesis and respiration, and bark stem dynamics during the spring recovery period in the spring. As a conclusion of this thesis, these four papers show how inextricably linked, the individual tree processes are to the changes within the bark, due to the tight coupling with sap flow-related changes of the xylem. The culmination of this thesis opens new opportunities to further understand the dynamics of bark hydraulics and ecophysiological processes by implementing field measurements and state-of-the-art modelling.