气候变暖背景下林火干扰对中国北方森林地上碳储量的影响

Boreal forests in China cover about 30% of the total forest area and account for one third of forest carbon. They play irreplaceable roles in mediating China's carbon budget under climatic change. Predicted warming climate will lead to the 5-10% increase in forest productivity in the boreal forests. At the mean time, with high biomass broadleaf species (such as Mongolian oak and maple) migrating north, species composition of boreal forests in China will change and the total forest carbon will further increase. However, boreal forests are within high fire frequency region. About 46% of carbon released from forest fires is from boreal forests. The most recent researches indicate that fire occurrence frequency and fire size will substantially increase with prolonged growing seasons under warming climate, and fire occurrence frequency will increase 100-200% with increasing hazardous fire weather periods. Currently, most forest carbon estimations do not directly take forest fire effect into consideration. China is a complied country under Kyoto Treaty, in which no differentiation is made regarding anthropogenic and natural factors that influence the country's carbon budget. In other words, with China's obligated carbon emission reduction target, not only should China compensate carbon emissions from the human causes, it should also count for the carbon loss due to natural and uncontrollable forces such as forest fires. Thus, a thorough understanding about the effects of forest fire on forest carbon is a key issue that need to be revolved for China's carbon trade negotiations under warming climate. This proposal intends to investigate three key research aspects integrating historical data, field data collection, and model simulations, 1) forcast fire regimes (frequency and size) under warming climate, 2) prediction of the spatiotemporal dynamics of boreal forest carbon storage under warming climate, and 3) effects of warming climate and fire on carbon. The project will help understanding the interacting mechanisms of forest fire and boreal forest, explore the research frontiers in global change ecology and disturbance ecology, quantify boreal forest carbon and its dynamics, provide scientific basis for China's carbon trade negotiation and forest fire management policies.

我国北方森林碳储量占我国森林碳储量的1/3以上，气候变暖及具有较高生物量的阔叶树种（如蒙古栎、槭树）的北移将导致北方森林碳储量进一步增加。然而，北方森林是林火的高发区，最新研究表明，受气候变暖的影响，高纬度地区林火发生的频率与火烧面积将随着生长季的延长而大幅度增加（100-200%）。目前大多数森林碳储量的估算没有直接考虑林火的作用，而且京都议定书中规定的减排目标并没有区分人为影响与自然干扰对国家碳收支的影响，所以，我国承诺的减排任务中，由林火造成的碳损失需要通过增碳与减排方式进行平衡。因此，本项目拟结合历史数据、野外调查、模型模拟，开展三方面研究：1）预测气候变暖背景下林火状况；2）预测北方森林碳储量时空动态；3）气候变暖与林火对森林碳储量的影响。本研究探讨气候变暖后林火对森林碳储量的影响，定量我国北方森林碳储量及其动态，为应对气候变暖，国家碳排放贸易谈判和林火管理策略制定提供依据。

中国北方森林主要分布在大兴安岭地区，面积占全国森林天然林总面积的29.9%，碳储量占我国森林碳储量的1/3以上。现有研究表明未来气候变化可能改变森林树种组成、提高大兴安岭森林生产力，进而影响北方森林碳储量。因此模拟未来大兴安岭森林碳储量动态变化，揭示气候变化、林火和采伐对森林碳储量的影响，提高我国北方森林碳储量估算精度，对准确评估我国北方森林固碳在减缓全球气候变化的作用和有效实施森林生态系统管理有着重要的参考意义。.本研究通过LANDIS PRO和LINKAGES模型来模拟林火、采伐及其交互作用下大兴安岭森林碳储量动态变化，量化气候变化、林火和森林采伐对我国北方森林碳储量的影响，并分析气候变化、林火和采伐共同作用下森林碳储量估算的不确定性。得出以下结论：. （1）虽然采伐和林火将会抵消掉一部分由气候变化增加的碳储量，但温度和降雨增加仍然能够增加大兴安岭森林碳储量，使得未来100年该地区森林持续固碳。与模拟初始年相比，现行气候、B1和A2气候变化情景下2100年森林碳储量将会增加5%-13%。在短期，气候变化对森林地上和土壤有机碳的相对影响大于同期林火的影响。在中期和长期气候变化对森林碳储量的直接影响小于林火和采伐的影响。受气候变化的影响，林火发生频率与火烧面积将会改变，在未来100年会进一步降低该地区森林地上和土壤有机碳储量。这意味着气候变化可以改变干扰发生频率和强度来间接降低大兴安岭森林碳储量。. （2）由于未来温度和降雨量变化幅度存在不确定，受此影响未来大兴安岭林区林火干扰状况也存在较大的不确定性，导致未来100年该地区森林碳储量变化存在较大的不确定性。敏感性分析结果表明未来100年呼中林区地上碳储量不确定性为12.4-16.2%，土壤有机碳储量不确定性在6.6-10.4%之间。在考虑气候变化、林火、采伐及其交互作用对森林碳储量影响的情况下，未来100年植被和土壤固碳潜力分别在4.3-6.4和1.7-6.5 Tg之间。本研究表明为准确估算森林生态系统碳储量，需要考虑种子传播、林火、采伐等景观过程的影响。