European beech (Fagus sylvatica) facultatively develops red heartwood, which decreases the value of its timber and is difficult to predict in standing trees. According to current theory, the absence of oxygen prevents discolouration in the wood of uninjured trees, and red heartwood forms when oxygen enters the stem through injuries. This theory requires that oxygen concentrations in uncoloured wood are generally very low, and that oxygen can diffuse several metres in the centre of a stem, bypassing the respiring sapwood. Oxygen concentrations measured at different depth in stems with and without red heartwood varied strongly and were generally depleted relative to the air, but rarely close to 0. Concentrations in red heartwood were somewhat, though not significantly higher than in the inner wood of trees without red heartwood. The colour of wood exposed to different oxygen concentrations changed strongly at higher concentrations, but concentrations in standing stems are generally high enough for discolouration. Model calculations suggested that only massive injuries that kill most sapwood at an entry point would allow high amounts of oxygen to penetrate to the core, in which case it may diffuse several metres in the axial direction without being consumed by respiring sapwood. However, given the relatively high diffusion in axial direction, oxygen should spread within a few days, not several years as the development of red heartwood appears to take. These measurements and calculations suggested that, while oxygen is required for beech red heartwood discoloration, it is not the only factor involved but could act by affecting the activity of micro-organisms.
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