Aerobic processes in plant metabolism require a constant supply with oxygen. Whereas gas exchange of leaves and roots is a central topic in plant eco-physiology and has been studied in much detail, only few studies have dealt with gas exchange and especially the oxygen content of stems. This was probably due to the lack of a proper methodology. Recently developed micro-sensors, based on fluorescence quenching, allowed the direct measurements of oxygen in the stem of standing trees. The research conducted in this study measured the oxygen concentration in standing spruce and beech trees, investigating how the living parts of the sapwood are supplied with enough oxygen for respiration. It was hypothesized that oxygen is either transported to the parenchyma cells by means of radial diffusion through bark, phloem and cambium or in dissolved form with the transpiration stream upwards from the roots to the crown. It was further investigated what factors can influence the oxygen content of a stem, and if and how a reduction of oxygen can affects a tree. First, the velocity of axial and radial oxygen diffusion was measured in wood of various native trees to determine if diffusion allone can supply the living sapwood with oxygen. Results showed that oxygen diffusion is strongly dependent on wood anatomy and water content, and model calculations implied that active sapwood can be supplied with oxygen under certain conditions. However, if water content and respiration are too high to assure a sufficient supply by radial diffusion, respiration in the sapwood is reduced by oxygen deficiency. Continuous long-term measurements in stems of spruce and beech trees showed distinct diurnal fluctuation of oxygen, and differences in oxygen between various depths and positions in the stem (sapwood versus heartwood, stem height). These results suggest that both transport pathways play a role. On the one hand, the decrease of oxygen with stem height found in spruce and the dependency of oxygen in beech on soil moisture indicated the importance of the transpiration stream as medium of oxygen transport. On the other hand the fact that oxygen replenished even in times of zero sapflow, and it did not decrease when sapflow was reduced suggests that at least some oxygen enters by radial diffusion. Experiments with potted spruce saplings were conducted to investigate if drought stress or flooding induces oxygen deficiency in the stem. A hypothesis was tested, suggesting that the emissions of ethanol and volatile terpenes are enhanced under conditions of hypoxia or anoxia, which successively could lead to an increased attractiveness for bark-beetles that locate stressed individuals by their emission spectrum. In this study oxygen was not reduced in stressed individuals and emissions of ethanol and volatile terpenes were not increased. Further the hypothesis was tested that oxygen, penetrating into the heartwood through injuries in beech trees, induces the formation of red heart, while bacteria and fungi are irrelevant in the process. Oxygen was measured in beech trees with and without red heart and wood samples were secreened for the presence of micro-organisms. In exposure experiments beech wood was stored at various oxygen concentrations in the laboratory. Although wood discolorations were stronger in the presence of elevated oxygen concentrations in the laboratory, discolorations were also found in beech wood exposed to 0% oxygen and in already formed red heart. In the standing tree a difference in oxygen concentration between beeche trees with and without red heart was found only on few occasions, and oxygen was present in both in a sufficient concentration to induce red heart. The results of this study imply that oxygen is not the only factor necessary for the formation of red heart in beech, and the involvement of micro-organisms needs to be studied in more detail.