大兴安岭森林火灾对冻土环境的影响研究

Forest fires affect the boreal forest ecosystem in many aspects: nutrient cycles and surface water-energy balances, physical and chemical properties of soils, hydrothermal dynamics of soils in the active layer and shallow permafrost, and subsequent soil carbon stock and budgets, carbon and nitrogen cycles and other biogeochemical processes. Under a warming and drying climate and with increasing anthropic activities, the occurrences of the wild and human-induced fires have been increasingly more frequent in boreal forests during the last few decades. Previous studies indicate that wildfires have significant impacts on the hydrothermal processes of soils in the active layer and shallow permafrost. They could result in an irreversible degradation of permafrost, adverse successions of forests, shrub-lands and wetlands, rapid losses of soil carbon stock, and subsequent hazardous periglacial landforms, such as thermal subsidence and slumping. The existing studies on the impacts of forest fires on the permafrost environment are mainly short-time, small-scale, and qualitative descriptions and empirical inferences, which lack of quantitative analysis and systematic evaluation on the basis of longer-term observations over extensive areas. Up to date, long-term and systematic field observations, laboratory testing and numerical model simulations are very limited, and the cutting-edge research for and environmental management practice in the controlled burn and other fires on boreal forests and the permafrost environment await further development. On the basis of studies on hydrothermal processes of the active layer and permafrost, periglacial phenomena and post-fire re-vegetation and ecosystem successions, using the approach of the study of spatial distributive features as a substitute for the study of temporal changes, the proposed program aims at quantitative studies and evaluation of the long- and short-term hydrothermal impacts of forest fires on permafrost and active layer, and their possible irreversible thresholds for ecological successions in the northern Da Xing’anling (Hinggan) Mountains. The project plans to implement field investigations and site monitoring, geophysical sounding and remote sensing analysis to obtain the original scientific data, to build, validate and improve the coupled hydrothermal models on the basis of observed data and proper parameterization, and to quantitatively study the impacting mechanisms of forest fires on the permafrost environment. The results of these studies may provide key scientific support for and visionary insights to logical environmental impact evaluation of the fires on boreal forests and wetlands, and cold regions environmental restoration and management.

林火不仅影响森林生态系统的营养物质循环、能水流动和土壤理化特性，而且对冻土环境、土壤碳库和碳氮循环等过程有重要影响。随气候干暖化趋势和人为活动不断增强，北方林区火灾日益频繁，并对冻土水热状态影响显著，如导致活动层加深、冻土退化、浅层土壤有机碳大量、快速释放、生态系统逆向演替，及融沉、滑塌等不良冰缘现象。目前，林火对冻土环境的影响还缺乏较长时间、大范围定量比较分析和长期及系统的野外观测与数值模型研究。拟在研究冻土水热过程变化、冻土现象和植被再生与演替等基础上，通过“空间换时间”的思路，定量研究和评估林火对大兴安岭北部冻土和活动层温度、水分变化过程的长、短期影响，以及可能产生的不可逆生态演替阈值等关键科学问题。拟通过野外调查和监测、物探和遥感等，构建和完善冻土水热耦合模型，定量研究林火对冻土环境的影响机理，为生态环境管护和修复提供关键和前瞻性科学依据。

在北方和北极地区，森林火灾对生物地球化学过程、活动层和近地表冻土的水热动态以及随后的养分循环具有重要影响。森林火灾可能导致多年冻土不可逆转的退化、北方针叶林的演替、土壤碳储量的迅速流失以及冻融灾害的广泛增加。森林火灾后，土壤温度升高；活动层增厚；土壤碳氮释放增强；植被由针叶林转变为阔叶林、灌丛或草原。本研究以我国大兴安岭北部地区（火烧后1-32年）为研究区，采用“空间换时间”的思路，通过野外调查与观测、室内试验分析、遥感反演以及数值模拟的方法结合，系统研究了林火对大兴安岭北部地区冻土环境的影响。研究表明：1）林火后，火烧迹地植被种类发生了显著更新和演替：火烧后短期内草本和灌木种类和盖度会迅速增加，随着火后时间的增加，草本和灌木种类和覆盖度开始减少，乔木种类和盖度增加，逐渐向针阔混交林演替；火后，植被NDVI明显降低，但火烧后10-12年植被NDVI基本恢复到火烧前的水平。2）林火对冻融土的水热状况有明显影响：随着火烧程度的增加，温度逐渐升高，活动层增厚，土壤含水量降低；林火对土壤水热状况的影响深度在火烧后9年超过20 m，影响时间超过30年，即林火后30年冻土水热状况并没有恢复到火烧前的水平。3）林火导致多年冻土土壤有机碳、全氮含量明显降低，而且随着火烧强度的增加，含量降低越明显，林火也可导致全磷和速效磷及全钾和速效钾不同程度的增加或降低。4）SHAW模型可很好的模拟不同火烧程度下植被高度、植被叶面积指数和有机层厚度对土壤水热状况的影响，用于定性评价林火对冻融土水热状况的影响。林火导致有机层厚度减薄，植被烧毁导致积雪重分布，地表反照率降低，大量热量进入土壤中，导致土壤温度升高、活动层增厚和含水量降低。结果，多年冻土快速退化，在地表形成热融洼地和湖塘等。这些成果为林火对北方森林生态系统影响系统研究和科学评估及北方森林的环境和资源有效管护提供了重要的科学依据。