NOAA/WDS Paleoclimatology - Umbanhowar fire data from Takhilt Nuur, western Mongolia - IMPD MNTHN001

Fires are common in grassland regions of the world, and the frequency and severity of fire is linked to climate-driven changes in fuel loads. Western Mongolia is dominated by grasslands but the fire history of this region is largely unknown. We reconstructed modern fire (48 lakes) and historical fires (9 lakes) using sediment charcoal. Modern fuel loads were estimated using a combination of clipped plots, satellite-based estimates of annual aboveground net primary productivity (NPP) and NPP modeled from annual temperature and precipitation. Loss-on-ignition and environmental magnetics of lake sediments were analyzed as proxies for climate. We found little evidence for modern or historical fire in the landscape, as charcoal was absent from the surface sediments of 34 of 48 lakes. Charcoal influxes were uniformly low, averaging from 0.002 to 0.028 mm2/cm2 per yr, over the past 1200 years at nine lakes, and the past 6000-5000 years at two of the lakes with longer sediment records. In the modern landscape, livestock grazing has eliminated most of the fuels necessary to carry a fire, as measured fuel loads (27.3+-4.9 g/m2) were only ~20% of aboveground annual NPP estimated using MODIS Imagery or modeled from climate data. The historical absence of fire may indicate a longer history of intensive grazing than sometimes assumed, and cultural prohibitions against burning may also play a role. Regional summary indicated a >50% decrease in charcoal influxes since AD 1600 at most sites which may be related to lower temperatures or greater aridity during the 'Little Ice Age'. Alternatively this decrease in charcoal influxes may reflect increases in livestock numbers or increased local concentrations because of restrictions on the movement of animals coincident with the establishment of Manchu rule in the late seventeenth century.

全球草原区域火灾频发，火灾的发生频率与严重程度与气候驱动的燃料负荷变化密切相关。蒙古国西部以草原地貌为主，但该区域的火灾历史却鲜有系统研究。本研究通过湖泊沉积物炭屑，重建了该区域现代（基于48个湖泊）与历史时期（基于9个湖泊）的火灾记录。现代燃料负荷通过刈割样地、卫星反演的年地上净初级生产力（NPP），以及基于年温降水数据模拟的NPP相结合的方式进行估算。研究同时分析了湖泊沉积物的烧失量与环境磁学参数，将其作为气候重建的代用指标。本研究未发现该区域存在近现代或历史火灾的显著证据：48个湖泊的表层沉积物中，有34个未检出炭屑。对9个湖泊的近1200年沉积物记录分析显示，其炭屑通量始终处于较低水平，年均值介于0.002至0.028 mm²/cm²之间；其中2个沉积记录更长的湖泊，其近6000至5000年的炭屑通量同样极低。当前区域内的家畜放牧已移除了大部分维持火势蔓延所需的燃料：实测燃料负荷为(27.3±4.9) g/m²，仅为基于中分辨率成像光谱仪（MODIS）影像反演或气候数据模拟得到的年地上NPP的约20%。历史上火灾的缺失，一方面可能表明该区域的高强度放牧历史比此前假设的更为久远，另一方面当地禁火的文化传统也可能起到了一定作用。区域综合分析显示，绝大多数采样点的炭屑通量自公元1600年以来下降了50%以上，这可能与小冰期（Little Ice Age）期间的气温降低或干旱加剧有关。此外，炭屑通量的下降也可能与17世纪末清朝建立后实施的牲畜活动管制政策有关，该政策导致牲畜数量增加或局部牲畜密度上升。