Pre-industrial particulate emissions and carbon sequestration from biomass burning in North America

Spatial trends in pre-industrial biomass burning emissions for eastern North America were reconstructed from sediment charcoal data. Petrographic thin sections were prepared from varved lake sediments along a transect of sites extending from NW Minnesota eastward to NE Maine. Results showed an exponential decline in charcoal abundance with distance east from the prairie/forest border. This result quantifies burning along the broad climate/vegetation gradient from xeric woodland to mesic eastern deciduous forest. Post-settlement charcoal accumulation showed no such geographic pattern, varying from site-to-site, likely reflecting local variability in land use and combustion sources. Results suggest the total emissions of large (> 10 m diameter) charcoal particles decreased by a factor of three during the twentieth century.     

An example of Holocene vegetation stability from Camerons Lagoon,a near treeline site on the Central Plateau,Tasmania

Abstract A discontinuous record of vegetation over the past 7500 years was obtained through pollen analysis of pond sediments from an extensive treeless plain on the relatively dry Central Plateau of Tasmania. The results demonstrate continuity of treeless conditions, which probably persisted from the Pleistocene throughout the Holocene, up to the present day. Some changes to the structure of the grassland were observed, especially over the past 200 years. Analysis of carbonized particles showed that fires had been infrequent over the period examined. It seems that this area, close to the altitudinal tree limit, has remained a natural grassland, and the hypothesis of Jackson (1973), that unreliable summer conditions may be a major factor in maintaining open conditions, is supported. Parts of the Central Plateau may thus preserve plant communities with some of the floristic elements and structure of widespread Bassian grasslands of the Late Pleistocene.     

Carbon emissions from fires in tropical and subtropical ecosystems

Global carbon emissions from fires are difficult to quantify and have the potential to influence interannual variability and long‐term trends in atmospheric CO₂ concentrations. We used 4 years of Tropical Rainfall Measuring Mission (TRMM) Visible and Infrared Scanner (VIRS) satellite data and a biogeochemical model to assess spatial and temporal variability of carbon emissions from tropical fires. The TRMM satellite data extended between 38°N and 38°S and covered the period from 1998 to 2001. A relationship between TRMM fire counts and burned area was derived using estimates of burned area from other satellite fire products in Africa and Australia and reported burned areas from the United States. We modified the Carnegie‐Ames‐Stanford‐Approach (CASA) biogeochemical model to account for both direct combustion losses and the decomposition from fire‐induced mortality, using both TRMM and Sea‐viewing Wide Field of view Sensor (SeaWiFS) satellite data as model drivers. Over the 1998–2001 period, we estimated that the sum of carbon emissions from tropical fires and fuel wood use was 2.6 Pg C yr^?1. An additional flux of 1.2 Pg C yr^?1 was released indirectly, as a result of decomposition of vegetation killed by fire but not combusted. The sum of direct and indirect carbon losses from fires represented 9% of tropical and subtropical net primary production (NPP). We found that including fire processes in the tropics substantially alters the seasonal cycle of net biome production by shifting carbon losses to months with low soil moisture and low rates of soil microbial respiration. Consequently, accounting for fires increases growing season net flux by ～12% between 38°N and 38°S, with the greatest effect occurring in highly productive savanna regions.     

浙江省1991~2006年森林火灾释放黑碳量的估算

黑碳气溶胶不仅造成环境污染危害人的健康,同时通过吸收太阳和大气辐射产生的辐射强迫对全球和区域产生影响,成为影响全球变暖的仅次于CO2的重要成分.在全球变暖及大气中CO2浓度不断升高的趋势下,各国开始采用造林再造林来减缓全球变暖的趋势,随着森林面积和林内生物量的不断增加,以及森林火灾的频发,林火带来的黑碳排放量也不容忽视.然而在世界上大部分地区,这方面的工作却很少被人所关注.根据1991～2006年浙江省森林火灾统计资料和浙江省各种森林类型地上生物量的数据,计算出了浙江省每年的森林火灾导致的生物量损失量.同时采取释放因子法,对1991～2006浙江省每年森林火灾释放的黑碳量进行了估算.其中1995年和2000年森林火灾释放的黑碳总量分别是38.4、97.2 t,占整个浙江省黑碳释放总量中的比重分别是0.12%、0.17%,对于区域大气质量有一定的影响.     

Global characterization of fire activity: toward defining fire regimes from Earth observation data

There is interest in the global community on how fire regimes are changing as a function of changing demographics and climate. The ground-based data to monitor such trends in fire activity are inadequate at the global scale. Satellite observations provide a basis for such a monitoring system. In this study, a set of metrics were developed from 6 years of MODIS active fire data. The metrics were grouped into eight classes representing three axes of fire activity: density, season duration and interannual variability. These groups were compared with biophysical and human explanatory variables on a global scale. We found that more than 30% of the land surface has a significant fire frequency. The most extensive fire class exhibited high fire density, low duration and high variability and was found in boreal and tropical wet and dry environments. A high association was found between population distribution and fire persistence. Low GDP km⁻² was associated with fire classes with high interannual variability and low seasonal duration. In areas with more economic resources, fires tend to be more regular and last longer. High fire duration and low interannual variability were associated with croplands, but often with low fire density. The study was constrained by the limited length of satellite data record but is a first step toward developing a comprehensive global assessment of fire regimes. However, more attention is needed by the global observing systems to provide the underpinning socio-economic observations to better quantify and analyze the human characteristics of fire regimes.     

Impact of a drier Early-Mid-Holocene climate upon Amazonian forests

This paper uses a palaeoecological approach to examine the impact of drier climatic conditions of the Early-Mid-Holocene (ca 8000-4000 years ago) upon Amazonia's forests and their fire regimes. Palaeovegetation (pollen data) and palaeofire (charcoal) records are synthesized from 20 sites within the present tropical forest biome, and the underlying causes of any emergent patterns or changes are explored by reference to independent palaeoclimate data and present-day patterns of precipitation, forest cover and fire activity across Amazonia. During the Early-Mid-Holocene, Andean cloud forest taxa were replaced by lowland tree taxa as the cloud base rose while lowland ecotonal areas, which are presently covered by evergreen rainforest, were instead dominated by savannahs and/or semi-deciduous dry forests. Elsewhere in the Amazon Basin there is considerable spatial and temporal variation in patterns of vegetation disturbance and fire, which probably reflects the complex heterogeneous patterns in precipitation and seasonality across the basin, and the interactions between climate change, drought- and fire susceptibility of the forests, and Palaeo-Indian land use. Our analysis shows that the forest biome in most parts of Amazonia appears to have been remarkably resilient to climatic conditions significantly drier than those of today, despite widespread evidence of forest burning. Only in ecotonal areas is there evidence of biome replacement in the Holocene. From this palaeoecological perspective, we argue against the Amazon forest 'dieback' scenario simulated for the future.     

On multiple solutions of the atmosphere–vegetation system in present-day climate

An asynchronously coupled global atmosphere–biome model is used to assess the stability of the atmosphere–vegetation system under present-day conditions of solar irradiation and sea-surface temperatures. When initialized with different land-surface conditions (1, the continents, except for regions of inland ice, completely covered with forest; 2, with grassland; 3, with (dark) desert; and 4, with (bright) sand desert), the atmosphere–biome model finds two equilibrium solutions: the first solution yields the present-day distribution of subtropical deserts, the second reveals a moister climate in North Africa and Central East Asia and thereby a northward shift of vegetation particularly in the south-western Sahara. The first solution is obtained with initial condition 4, and the second with 1, 2, 3. When comparing these results with an earlier study of biogeophysical feedback in the African and Asian monsoon area, it can be concluded that North Africa is probably the region on Earth which is most sensitive considering bifurcations of the atmosphere–vegetation system at the global scale.     

Pinus cembra L. (arolla pine), a common tree in the inner French Alps since the early Holocene and above the present tree line: a synthesis based on charcoal data from soils and travertines

Aim In this study, charcoal‐based data for Pinus cembra L. (arolla pine) were gathered from soil and travertine sequences in order to reconstruct its historical biogeography at the landscape level in the inner western Alps during the Holocene. Location The study sites are located between 1700 and 2990 m a.s.l., in the southern (Queyras Massif and Ubaye Valley) and the northern (Maurienne Valley) parts of the inner French Alps. Methods Charcoal fragments were extracted from sediments by water sieving, using meshes of 5, 2, 0.8 and 0.4 mm. The charcoal mass of P. cembra was determined in each charcoal assemblage. Accelerator mass spectrometry and conventional ¹⁴C measurements were used to date the fragments. Results Supported by 40 ¹⁴C datings, the fragments show that, over 2000 m a.s.l., P. cembra accounts for around 40% (mean value) of identified fragments. Data reveal that arolla pine once extended between 260 and 375 m above the present‐day local tree lines. It was established in the southern and the northern French Alps from at least c. 9000 and 6000 cal yr bp , respectively. Main conclusions While present‐day populations of P. cembra are very fragmented in the inner French Alps, charcoal records indicate large past occurrences of this tree since the early Holocene. Human disturbance since the Neolithic seems to be the main reason for the regression of the arolla pine woodlands. On the south‐facing slopes of the study sites, currently deforested, this species extended up to 2800 m a.s.l. In the northern areas, charcoal records of the P. cembra expansion are consistent with the regional pollen archives, but in the southern massifs charcoal records indicate its presence c. 2600 years earlier than other palaeobotanical studies suggest. This discrepancy highlights the necessity to crosscheck data using several different proxies in order to assess the validity of conclusions regarding tree development in space and time.     

Contributions of Fire Use Study to Land Use/Cover Change Frameworks: Understanding Landscape Change in Agricultural Frontiers

The impact of fire use and hazard in frontier settlement is a critical environmental concern that has been historically overshadowed by deforestation issues- and thus underexamined at local and regional scales by social scientists. Consequently, conceptual frameworks of LUCC change consider fire use as an outcome of land use decisions and neglect the capacity of burning choices to influence these decisions and subsequent land cover change. This paper examines the relationship of settlement, land use, and fire use. It considers recent LUCC frameworks, and then uses household surveys on fire use practices to discuss how the study of fire use can contribute to understanding frontier landscape change. Planting decisions, settlement history, location desires, and burning logistics work in combination to influence burning choices and thus LUCC.     

Biomass carbon stocks and their changes in northern China’s grasslands during 1982–2006

Grassland covers approximately one-third of the area of China and plays an important role in the global terrestrial carbon (C) cycle. However, little is known about biomass C stocks and dynamics in these grasslands. During 2001–2005, we conducted five consecutive field sampling campaigns to investigate above-and below-ground biomass for northern China’s grasslands. Using measurements obtained from 341 sampling sites, together with a NDVI (normalized difference vegetation index) time series dataset over 1982–2006, we examined changes in biomass C stock during the past 25 years. Our results showed that biomass C stock in northern China’s grasslands was estimated at 557.5 Tg C (1 Tg=10¹² g), with a mean density of 39.5 g C m⁻² for above-ground biomass and 244.6 g C m⁻² for below-ground biomass. An increasing rate of 0.2 Tg C yr⁻¹ has been observed over the past 25 years, but grassland biomass has not experienced a significant change since the late 1980s. Seasonal rainfall (January–July) was the dominant factor driving temporal dynamics in biomass C stock; however, the responses of grassland biomass to climate variables differed among various grassland types. Biomass in arid grasslands (i.e., desert steppe and typical steppe) was significantly associated with precipitation, while biomass in humid grasslands (i.e., alpine meadow) was positively correlated with mean January-July temperatures. These results suggest that different grassland ecosystems in China may show diverse responses to future climate changes.