Table1_Whole-tree harvesting improves the ecosystem N, P and K cycling functions in secondary forests in the Qinling Mountains, China.docx

Forest ecosystem nutrient cycling functions are the basis for the survival and development of organisms, and play an important role in maintaining the forest structural and functional stability. However, the response of forest nutrient cycling functions at the ecosystem level to whole-tree harvesting remains unclear. Herein, we calculated the ecosystem nitrogen (N), phosphorus (P), and potassium (K) absorption, utilization, retention, cycle, surplus, accumulation, productivity, turnover and return parameters and constructed N, P, and K cycling function indexes to identify the changes in ecosystem N, P, and K cycling functions in a secondary forest in the Qinling Mountains after 5 years of five different thinning intensities (0% (CK), 15%, 30%, 45%, and 60%). We showed that the ecosystem’s N, P, and K cycling parameters varied significantly and responded differently to thinning treatments. As the thinning intensity increased, the N, P, and K cycling function indexes increased by 5%~232%, 32%~195%, and 104%~233% compared with CK. Whole-tree harvesting promoted ecosystem N and P cycling functions through two pathways: (a) directly regulated litter biomass, indirectly affected soil nutrient characteristics, and then regulated ecosystem N and P cycling functions; (b) directly regulated plant productivity, indirectly affected plant and soil nutrient characteristics, and then regulated ecosystem N and P cycling functions. In contrast, whole-tree harvesting mainly indirectly affected the plant and soil nutrient characteristics by directly adjusting the plant productivity, and promoting the ecosystem K cycling function. Furthermore, N and P cycling functions were mainly regulated by understory plant productivity while tree and herb nutrient characteristics were key driving factors for K cycling functions. These findings indicated that whole-tree harvesting significantly improved the ecosystem N, P and K cycling functions, and reveals varied regulatory mechanisms, which may aid in formulating effective measures for sustainable forest ecosystem nutrient management.

森林生态系统养分循环功能是生物生存与发展的基础，对维持森林结构与功能稳定性具有重要作用。然而，生态系统尺度下的森林养分循环功能对全树收获的响应机制仍不明晰。本研究计算了生态系统氮（N）、磷（P）、钾（K）的吸收、利用、存留、循环、盈余、积累、生产力、周转与归还参数，构建了氮、磷、钾循环功能指数，以明确秦岭山区次生林在5年5种不同抚育强度（0%（CK）、15%、30%、45%、60%）处理后的生态系统氮、磷、钾循环功能变化。研究结果显示，生态系统氮、磷、钾循环参数存在显著差异，且对抚育处理的响应各不相同。随着抚育强度提升，氮、磷、钾循环功能指数相较于CK分别提升了5%~232%、32%~195%与104%~233%。全树收获通过两条途径促进生态系统氮、磷循环功能：（a）直接调控凋落物生物量，间接影响土壤养分特征，进而调控生态系统氮、磷循环功能；（b）直接调控植物生产力，间接影响植物与土壤养分特征，进而调控生态系统氮、磷循环功能。与之相对，全树收获主要通过直接调整植物生产力间接影响植物与土壤养分特征，从而促进生态系统钾循环功能。此外，氮、磷循环功能主要受林下植物生产力调控，而木本与草本养分特征则是钾循环功能的关键驱动因子。本研究结果表明，全树收获可显著提升生态系统氮、磷、钾循环功能，并揭示了差异化的调控机制，可为制定可持续森林生态系统养分管理的有效措施提供参考。