Changes in log quality at different decay stages in an alpine forest

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Aims Log is an important pool of carbon (C) and nutrients in alpine forest ecosystems. Changes in log quality with decay could reveal the process of C and nutrient release during log decomposition. However, little informa- tion is available on this. Therefore, this study aims to understand the changes in log quality during log decaying. Methods Changes in C, nitrogen (N), phosphorus (P), lignin and cellulose concentrations were investigated in the heartwood, sapwood and bark of fir (Abies faxoniana) logs at five (I-V) decay stages in an alpine forest in western Sichuan, China. The stoichiometry of C:N:P and the ratios of lignin:N, lignin:P, cellulose:N, and cellu- lose:P were also calculated. Important findings C content in bark increased from the stage I to stage III of decay and then significantly de- creased, but in the heartwood and sapwood it decreased from the stage I through stage V, especially at stages IV and V. N content increased from the stage I through stage V regardless of the log components. P content in sap- wood also showed tended to increase from the stage I through stage V, but P content in heartwood and bark de- creased following an increase tendency. In comparison with sapwood and heartwood, bark had the lowest C:N:P stoichiometry at the same decay stages. Percentage of the labile to total C (Fm) also inferred that bark was the most decomposable component. The higher C:N:P stoichiometry in sapwood was observed in logs of the stages I and II, but higher Fm in heartwood was detected from the stage III to stage V. Critical values of C:N in sapwood and bark and C:P in heartwood, sapwood and bark were negatively correlated with the initial N and P concentra- tions, respectively. Cellulose concentration decreased from the stage I to stage V regardless of log components,