Pest outbreaks are driving tree dieback and major changes in forest carbon cycles. Predicting if and when these changes feedback to the climate system requires understanding which factors limit wood decay and greenhouse gas production. Recent analyses include stem diameter among the most important predictors, but report effects that vary in magnitude and direction. This complexity may reflect interactions between different mechanisms by which diameter influences decay including (1) the proximity of wood to the soil surface (2) the geometry of exchange between wood and its surroundings (2) and the ratio of stem tissues with different properties. Using a three way partially crossed experiment, we examined the independent and combined effects of soil proximity, stem geometry and bark presence on wood decay and greenhouse gas production in Fraxinus americana, an iconic North American species threatened with extinction by an invasive beetle.
Results/Conclusions
Soil contact accelerated decay rates by an order of magnitude with an effect that varied with stem shape, not bark presence. After experimentally controlling surface area to volume ratio and position relative to the soil surface, wide stems decayed more slowly than narrow stems when both were half-buried in the soil, but more quickly than the average decay rates of fully buried and fully suspended narrow stems. These results closely matched variation in moisture content within and among samples, suggesting that limited vertical movement of moisture from the soil through dead wood is a key factor that mediates the effect of stem diameter on wood decay. Soil contact and conductivity also influenced the internal concentrations of CO2, CH4 and N2O. Carbon dioxide and methane concentrations in suspended dead wood exceeded atmospheric levels and were even higher in buried samples suggesting that carbon mineralization from dead wood involves methanogenesis. Internal nitrous oxide concentrations showed the opposite patterns indicating that dead F. americana may be a sink for this gas. Our results suggest that managing F. americana dieback for greenhouse gas mitigation requires understanding traits that mediate wood permeability and diffusivity to soil moisture and greenhouse gases metabolized by saprotrophs.