云南松林枯落物计划烧除对坡面产流产沙的影响机理研究

Litters plays an important role in hydrological processes and soil erosion in forest ecosystems, but litters in forest ground also prove to be the combustible for forest fire. The long-term accumulation of litter in the forest results in an increasing risk of forest fire and prescribed burning of litter is one of the effective methods to prevent forest fire. However, prescribed burning of litters can lead to considerable progress changes of hydrology and soil erosion, both directly by decreasing of litters cover and changing of soil structure and properties, and indirectly through the effects of changes to the soil and litters layer on hydrological and soil erosion processes. The objective of this study was to reveal the influence of prescribed burning on mechanism of runoff and sediment yield in Pinus yunnanensis forest. The typical prescribed burning Pinus yunnanensis forest with a forest area of 30% in Yunnan Province was taken as the research object and normal litter inputs Pinus yunnanensis forest as control, based on the basic investigation including community characteristics of Pinus yunnanensis forest, litter fall , litter accumulation soil properties and rainfall dynamics, the situ flume scouring experiment and long-term positioning observation runoff plot were used to observation the annual and interannual variation of runoff and sediment yield, to study the effect of prescribed burning on process of runoff and sediment yield in Pinus yunnanensis. And then based on the theory of water balance and erosion dynamics on slope, three key effects of prescribed burning were focused, including (a) the hydrological effects of litter layer and soil, (b) the hydrodynamic characteristics of slope and (c) the change of soil erodibility in prescribed burning and normal litters treatments, in order to analyse the effects of mechanism of prescribed burning on mechanism of runoff and sediment yield in Pinus yunnanensis forest. It is believed that the findings and the approach proposed in this study will provide more interesting and useful hints to support effective forest litter management.

枯落物是森林生态系统发挥水文生态功能的重要组成部分，也是森林火灾的可燃物，枯落物在林内的长期积累造成森林火灾风险不断增加。枯落物人为计划烧除是预防森林火灾的有效方法之一，但同时影响森林生态系统的水文生态和土壤侵蚀过程。本研究以云南松林典型火烧强度的枯落物计划烧除样地为研究对象，以枯落物正常输入样地为对照，在样地云南松林群落特征、枯落物及燃烧剩余物、土壤和区域降雨特征调查、测定基础上，采用原位水槽模拟实验及野外径流小区长期定位观测相结合的方法，分析云南松林枯落物计划烧除后坡面产流量和产沙量的年内和年际变化。基于坡面水量平衡和侵蚀动力学原理，通过云南松林枯落物计划烧除对枯落物及燃烧剩余物水文生态效应、土壤水文生态效应、坡面水动力学特征及坡面产流产沙的影响的研究，揭示云南松林枯落物计划烧除对坡面产流产沙的影响机理，为我国第二大林区用枯落物计划烧除方法预防森林火灾技术的不断完善和科学决策奠定基础。

计划烧除会减轻可能发生的森林火灾，但林内植被、土壤等受到影响。以滇中高原长期连续典型低强度计划烧除的云南松林为研究对象，探讨了计划烧除后森林群落、水文、土壤及抗蚀、抗冲、抗剪性能的变化，采用原位水槽模拟实验和径流小区长期定位观测相结合的方法，揭示了坡面产流产沙的变化及机理。研究结果表明：（1）计划烧除后，乔、灌和草本层物种数量均呈下降趋势，乔木层生物量无明显变化，灌、草层生物量均显著下降，小乔木烧除后很难恢复，烧除2年后，灌木层物种数量基本恢复，草本层生物量仍然显著下降，但植株数量极显著增加。因此，云南松林低强度计划烧除，应每间隔2年后再进行下一次计划烧除。（2）林内枯落物自然蓄积量为18.64 t/hm2，烧除后仅剩3.84 t/hm2，自然含水量由39.76%下降为6.22%，最大持水量由60.62 t/hm2下降为14.00 t/hm2。（3）计划烧除后，0～10cm表层土壤理化性质、酶活性及微生物数量、入渗性能均受到显著影响，随土层加深影响逐渐减小；0～40cm土层＞2mm机械稳定性团聚体增加、水稳性团聚体减少，＜0.25mm水稳性团聚体增加，团聚体直径减小、结构破坏率增大，抗蚀性能减弱；计划烧除后，直径＜0.50 mm的根系明显减小，抗冲系数显著降低，0～20cm土层的中根和细根生物量密度和根长密度明显减小，土壤黏聚力下降，抗剪强度显著降低。（4）计划烧除缩短了林地坡面产流历时，提升了水流速度，增加了径流损失量和土壤侵蚀量。雨季5～10月份未烧除林地土壤侵蚀量和径流损失量分别为3.20t/km2和27.46mm，烧除后分别增加到42.24t/km2和59.93mm，但土壤侵蚀量仍明显小于该地区土壤侵蚀容许值500t/km2·a。因此，计划烧除不会产生明显水土流失危害。研究结果将为云南松林计划烧除的生态环境效应正确评价和制定计划烧除制度提供科学依据。