Carbon storage and understory plant diversity in boreal Larix gmelinii–Carex schmidtii forested wetlands: A comparison of harvest, fire and draining disturbance

ABSTRACT Background: Forested wetlands are an important factor in carbon (C) sequestration. In this study, we compared the short-term effects (5 years) of three harvest intensities and two wildfire severities, and the mid-term effects (25 years) of four groundwater table depth (WTD) drainage for forestry on eco-system C storage and understory vegetation structures in Daxing’anling, northeast of China. Results: We found that: Low intensity harvest slightly enhanced ecosystem C storage, whereas moderate- and high-intensity harvest led to significant reductions (33.2-41.6%) in comparison with the control natural forested wetlands stand (274.54 t C·ha -1), and light and heavy burn caused the ecosystem C storage sig-nificant decreases by 27.8% and 45.2%. As for the drainage for forestry, the ecosystem C storage was higher at the low WTD (316.78 t C·ha -1), and diminished significantly by 24.1-28.1% with the increasing WTD on the forested wetland plantation transect. Compared with the control and high intensity harvest stands (8.28 t ha -1 yr -1, 5.08 t C·ha -1 yr -1 and 6.48 t ha -1 yr -1, 3.52 t C·ha -1 yr -1), significantly higher NPP (net primary productivity, defined by the biomass difference of two measurements) (69.1-83.2%) and annual C sequestration (52.0-78.7%) were observed in the low- and moderate-intensity harvest stands. Significant increases by 48.6% and decreases by 31.5% in NPP or by 52.9% in annual C sequestration were in light and heavy burn stands, respectively. In contrast, the NPP and annual C sequestration (3. 67-10. 34 t ha -1 yr -1 and 1. 59-4. 87 t C·ha -1 yr -1) showed a significant in-creasing trend with increasing WTD, respectively. The understory species diversity indices were generally lower in the harvest and burned stands than the control natural forested wetlands and more pioneer tree species regeneration occurred after the disturbances. Conclusion: Therefore, it seems that low intensity harvest and wildfire could sustain the ecosystem C sink and drainage for forestry is an effect way to restore C sequestration for this forested wetland type.

摘要 背景：森林湿地是碳（C）固存的重要载体。本研究以中国东北大兴安岭地区为研究区域，对比分析了三种采伐强度、两种野火烈度的短期效应（5年），以及四种林业地下水位埋深（water table depth, WTD）排水措施的中期效应（25年）对生态系统碳储量及林下植被结构的影响。结果：本研究发现，低强度采伐对生态系统碳储量略有提升，而中、高强度采伐相较于对照天然森林湿地林分（274.54 t C·ha⁻¹）会导致碳储量显著降低，降幅达33.2%~41.6%；轻度和重度火烧则使生态系统碳储量分别显著下降27.8%和45.2%。就林业排水措施而言，地下水位埋深较浅时生态系统碳储量更高（316.78 t C·ha⁻¹），随着森林湿地人工林样带地下水位埋深增加，碳储量显著降低24.1%~28.1%。与对照组及高强度采伐林分（对应数值分别为8.28 t ha⁻¹ yr⁻¹、5.08 t C·ha⁻¹ yr⁻¹，以及6.48 t ha⁻¹ yr⁻¹、3.52 t C·ha⁻¹ yr⁻¹）相比，低、中强度采伐林分的净初级生产力（net primary productivity, NPP，定义为两次测量的生物量差值）及年固碳量分别显著提升69.1%~83.2%和52.0%~78.7%。轻度火烧林分的净初级生产力显著提升48.6%，年固碳量提升52.9%；重度火烧林分的净初级生产力下降31.5%。与之相反，随着地下水位埋深增加，净初级生产力和年固碳量分别呈现显著上升趋势，其数值范围分别为3.67~10.34 t ha⁻¹ yr⁻¹和1.59~4.87 t C·ha⁻¹ yr⁻¹。采伐与火烧林分的林下物种多样性指数普遍低于对照天然森林湿地，且扰动后会出现更多先锋树种的自然更新现象。结论：综上，低强度采伐和野火可维持生态系统碳汇功能，林业排水措施是恢复该类森林湿地碳固存能力的有效途径。