针改阔对杉木林温室气体通量和固碳功能的影响

As the plantation tree species with the largest area in China, Chinese fir plays a significant role in carbon sequestration and mitigating climate change because of its rapid growth and strong carbon fixed capacity. Some problems have emerged in pure Chinese fir plantation during continuous planting, such as stand productivity declining and soil degradation. Its ability of carbon uptake is weakened. The mixed conifer and broadleaved-oriented transformation of low quality pure coniferous forest has been proved as an effective way to improve stand structure, production and ecological service function. At present, it has been popularized in large area as the major method of plantation ecological restoration with low effect. The stand transformation practices may increase soil greenhouse gas emission. On the other hand, it may improve carbon sequestration capacity through changing the downtrend of stand productivity. However, there are few researches focusing on such effects of stand transformation practices. In this project, we intend to examine the change in soil greenhouse gas (CO2, CH4 and N2O) emission, soil organic carbon storage and stand biomass carbon uptake in Chinese fir plantation ecosystem under different stand transformation models. It is expected to reveal the influence mechanism of stand transformation on soil greenhouse gas fluxes and net carbon uptake function of ecosystem, based on the changes of stand growth, soil physical and chemical properties, the composition and structure of soil microbes. And it will demonstrate the potential effects of stand transformation to increase carbon fixed capacity of low effect Chinese fir plantation. This study will provide the comprehensive scientific understanding and evaluation of the effects from stand transformation.

作为我国面积最大的人工林树种，杉木生长速度快，固碳能力强，对固碳减排、减缓气候变化具有重要作用。然而多代连作的杉木人工纯林出现了林分生产力下降和地力衰退等问题，也大大削弱了杉木林的固碳功能。低效针叶纯林阔叶化改造是改善林分结构、提高林地生产力和改善生态服务功能的有效途径，目前作为低效林生态修复的主要手段得到了大面积的推广应用。改造活动一方面可能会增加温室气体的排放，另一方面可能会扭转林分生产力下降的趋势而可望提高生物量固碳能力。但是，这种影响并未受到应有的关注。本项目拟研究不同杉木林阔叶化改造模式对土壤CO2、CH4和N2O三种温室气体通量、土壤有机碳储量与林分生物量碳储量的影响，从林分生长、土壤理化性质及土壤微生物特征的变化揭示阔叶化改造对温室气体通量和生态系统净碳汇功能的影响机制，探讨阔叶化改造提升低效杉木林固碳功能的潜力，为全面认识评估阔叶化改造的作用提供科学依据。

低效针叶纯林阔叶化改造是改善林分结构、提高林地生产力和改善生态服务功能的有效途径，但是这种改造活动对土壤温室气体排放和林分碳储量的影响并未受到应有的关注。本研究探讨了皆伐阔叶化改造和间伐阔叶化改造带来的相关影响。研究发现皆伐改造会对CO2、N2O和CH4三种温室气体的排放产生显著影响，且这种影响与补植密度有关。整体而言，随着皆伐改造时间的增长，皆伐后补植均会显著增加CO2排放、抑制CH4吸收，但皆伐后低密度补植（CL）能显著降低N2O的排放，且对CO2和CH4的影响显著小于皆伐后高密度补植（CH）。皆伐后补植会显著增加土壤温度，补植密度升高会加剧这一趋势。皆伐后改造降低了林分生物量碳，对土壤碳储量影响不大。皆伐后进行合理密度补植阔叶树有利于减少排放。间伐改造也会影响温室气体的排放。间伐强度和年份会显著影响CO2、N2O排放和CH4的吸收，但同等采伐强度下，补植常绿或落叶树种对温室气体通量无显著影响。本研究发现，间伐显著增加了灌木层和草本层的碳储量，显著降低了凋落物层碳储量。本研究发现土壤温度是影响三种温室气体排放最主要的因子，中度间伐和重度间伐对土壤温度影响显著，但与间伐强度和补植树种无关。此外，间伐对土壤理化性质影响显著，但补植不同树种对土壤理化性质无显著影响。本研究对皆伐后的杉木伐桩进行不同处理，探讨萌孽更新对土壤呼吸的影响。研究发现，萌孽能力总体上增加了20%左右的土壤呼吸，萌蘖树种在皆伐挖壕沟后伐桩处会萌蘖新苗且逐渐生长。采用挖壕沟法在皆伐 2 年后依然不能完全区分具有萌蘖能力树种的土壤异养呼吸。由于观察时间相对于轮伐期来说还是太短，皆伐改造和间伐改造补植的阔叶树的碳汇能力还有待于进一步观测。研究结果为全面评估针叶林阔叶化改造的作用、减缓全球气候变化的人工林经营提供了科学依据。