Oxygen in trees

Oxygen can decrease to very low levels also in above-ground parts of plants, particularly in tree stems. In many trees, a major proportion of the oxygen supplied to the live sapwood appears to be transported with the transpiration stream rather than diffuse through the nearly impenetrable cambium. Short-time dynamics of oxygen concentrations in the wood are however not known at all. A main reason why relatively little attention has been paid to stem oxygen is that it is very difficult and cumbersome to measure with traditional methods. An oxygen electrode based on fluorescence quenching makes instantaneous, prolonged and small-scale measurements of O2 concentrations possible and promises to be a most important tool for a better understanding of oxygen relations in plants. The first part of our work should lead to an overview of diurnal and seasonal variations in oxygen content of tree stems of diverse species. We hypothesise that the oxygen courses will have a connection to tree anatomy and habitat requirements, especially to the flooding tolerance of the species. Furthermore we hypothesise here that oxygen is a major factor in tree stress and particularly in tree - pathogen interactions. Wood-infecting fungi may lower oxygen concentrations through increased respiration or by additionally clogging xylem vessels and tracheids and thus preventing transport via sapflow. If oxygen is supplied via the sapflow, several types of stress may result in decreasing wood oxygen concentrations or even anoxia: waterlogging, which reduces soil oxygen content, severe drought, which reduces sapflow, and infections, which increase respiration and may decrease sapflow. Anoxia frequently results in the production of ethanol, which is an important attractant for bark beetles. The hypothesis to be tested is that various forms of stress decrease oxygen content, which increases ethanol and attracts bark beetles. However, transpiration, stem respiration and oxygen solubility in water are strongly affected by climatic factors even without stress, which makes predictions in this poorly understood system difficult.

Although the stems of trees are surrounded by air at a concentration of 20.9% oxygen, the re-supply of O 2 consumed by living cells in wood is by no means guaranteed. There are obviously two possible pathways for this gas (diffusion via bark and cambium, or transport in the xylem water lifted from the soil by transpiration), but little is known about their relative contribution in the field and under variable environmental conditions. The present project, which employed oxygen-sensitive optodes for measurements in the stemwood of trees, concentrated on two hypotheses: a) oxygen may be depleted in stressed spruce trees to a concentration where living cells will switch to fermentation and produce ethanol, which is strongly attractive for bark beetles; b) red heartwood in beech is formed when oxygen enters through injuries and prevented in uncoloured wood by the absence of oxygen. Hypothesis a) could not be supported on young potted trees subjected to drought and flooding. Stress had very little effect on stem oxygen concentrations and none on ethanol emissions. Oxygen is necessary for beech wood discoloration and concentrations were somewhat higher in trees with than in trees without red heartwood. However, concentrations in all trees were high enough for wood discoloration and oxygen alone is not sufficient to explain red heartwood formation in beech. Measurements for the project brought a wealth of new methodological information and some rather unexpected results.