Diffusion and seasonal dynamics of O2 in woody debris from the Pacific Northwest, USA

Abstract

O2 is an important regulator of physiological processes involved in the decomposition of woody debris, yet O2 levels and diffusion rates within decomposing logs are largely unknown. We examined how O2 diffusion rates in decayed and sound wood varied with moisture and density, and we compared predicted with observed seasonal changes in oxygen concentration in logs in a Pacific Northwest old-growth Pseudotsuga menziesii forest. In the laboratory, the oxygen diffusion coefficient (DO2) was determined in the longitudinal and radial (or tangential) directions on wood cores of varying moisture content and density. In the field, O2 was measured in tubes inserted to three radial depths (2, 6 and 15 cm) within logs of two species (Pseudotsuga menziesii and Tsuga heterophylla) and five decay classes (where class 5 = most decayed). In both the radial and longitudinal directions, DO2 increased exponentially as the air filled pore space (AFPS) increased and as density decreased. In the field, mean O2 concentrations in logs were not significantly different between species. Mean O2 concentrations were significantly lower in the least decayed logs as compared to the most decayed logs. Mean O2 concentrations decreased with radial depth only in decay class two logs. Seasonal O2 levels did not consistently vary with log moisture, respiration, or air temperature. The comparison of the results from a model that assumes oxygen diffuses only in the radial direction to field data indicates that laboratory measurements of oxygen diffusion may underestimate field oxygen concentrations. Cracks, insect galleries and other passages in decayed logs, and longitudinal oxygen diffusion may account for this discrepancy. In the field, log oxygen concentrations were rarely as low as 2%, indicating anaerobic conditions may not be as common in logs as we previously thought. Oxygen limitations on decomposition may occur in relatively sound and/or water soaked wood, but probably not in decayed logs in a terrestrial setting.