Regional fire dynamics and its contributions to carbon flux variability in South Asia

The decline in aboveground wood production after canopy closure in even‐aged forest stands is a common pattern in forests, but clear evidence for the mechanism causing the decline is lacking. The problem is fundamental to forest biology, commercial forestry (the decline sets the rotation age), and to carbon storage in forests. We tested three hypotheses about mechanisms causing the decline in wood growth by quantifying the complete carbon budget of developing stands for over six years (a full rotation) in replicated plantations ofEucalyptus salignanear Pepeekeo, Hawaii. Our first hypothesis was that gross primary production (GPP) does not decline with stand age, and that the decline in wood growth results from a shift in partitioning from wood production to respiration (as tree biomass accumulates), total belowground carbon allocation (as a result of declining soil nutrient supply), or some combination of these or other sinks. An alternative hypothesis was that GPP declines with stand age and that the decline in aboveground wood production is proportional to the decline in GPP. A decline in GPP could be driven by reduced canopy leaf area and photosynthetic capacity resulting from increasing nutrient limitation, increased abrasion between tree canopies, lower turgor pressure to drive foliar expansion, or hydraulic limitation of water flux as tree height increases. A final hypothesis was a combination of the first two: GPP declines, but the decline in wood production is disproportionately larger because partitioning shifts as well.We measured the entire annual carbon budget (aboveground production and respiration, total belowground carbon allocation [TBCA], and GPP) from 0.5 years after seedling planting through 6½ years (when trees were ∼25 m tall). The replicated plots included two densities of trees (1111 trees/ha and 10 000 trees/ha) to vary the ratio of canopy leaf mass to wood mass in the individual trees, and three fertilization regimes (minimal, intensive, and minimal followed by intensive after three years) to assess the role of nutrition in shaping the decline in GPP and aboveground wood production.The forest closed its canopy in 1–2 years, with peak aboveground wood production, coinciding with canopy closure, of 1.2–1.8 kg C·m−2·yr−1. Aboveground wood production declined from 1.4 kg C·m−2·yr−1at age 2 to 0.60 kg C·m−2·yr−1at age 6. Hypothesis 1 failed: GPP declined from 5.0 kg C·m−2·yr−1at age 2 to 3.2 kg C·m−2·yr−1at age 6. Aboveground woody respiration declined from 0.66 kg C·m−2·yr−1at age 2 to 0.22 kg C·m−2·yr−1at age 6 and TBCA declined from 1.9 kg C·m−2·yr−1at age 2 to 1.4 kg C·m−2·yr−1at age 6. Our data supported hypothesis 3: the decline in aboveground wood production (42% of peak) was proportionally greater than the decline in canopy photosynthesis (64% of peak). The fraction of GPP partitioned to belowground allocation and foliar respiration increased with stand age and contributed to the decline in aboveground wood production. The decline in GPP was not caused by nutrient limitation, a decline in leaf area or in photosynthetic capacity, or (from a related study on the same site) by hydraulic limitation. Nutrition did interact with the decline in GPP and aboveground wood production, because treatments with high nutrient availability declined more slowly than did our control treatment, which was fertilized only during stand establishment.

ABSTRACTWe investigated the use of a recently developed satellite-based vegetation optical depth (VOD) data set to estimate fire severity and carbon emission over Australian tropical savannahs. VOD is sensitive to the dynamics of all aboveground vegetation and available nearly every two days. For areas burned during 2003–2010, we calculated the VOD change (ΔVOD) pre- and post-fire and the associated loss in the above ground biomass carbon. ΔVOD agreed well with the Normalized Burn Ratio change (ΔNBR) which is the metric used to estimate fire severity and carbon loss compared well with modelled emissions from the Global Fire Emissions Database (GFED). We found that the ΔVOD and ΔNBR are generally linearly related. The Pearson correlation coefficients (r) between VOD- and GFED-based fire carbon emissions for monthly and annual total estimates are very high, 0.92 and 0.96, respectively. A key feature of fire carbon emissions is the strong inter-annual variation, ranging from 21.1 Mt in 2010 to 84.3 Mt in 2004. This study demonstrates that a reasonable estimate of fire severity and carbon emissions can be achieved in a timely manner based on multiple satellite observations over Australian tropical savannahs, which can be complementary to the currently used approaches.

Abstract. Landscape fires during the 21st century are expected to change in response to multiple agents of global change. Important controlling factors include climate controls on the length and intensity of the fire season, fuel availability, and fire management, which are already anthropogenically perturbed today and are predicted to change further in the future. An improved understanding of future fires will contribute to an improved ability to project future anthropogenic climate change, as changes in fire activity will in turn impact climate. In the present study we used a coupled-carbon-fire model to investigate how changes in climate, demography, and land use may alter fire emissions. We used climate projections following the SRES A1B scenario from two different climate models (ECHAM5/MPI-OM and CCSM) and changes in population. Land use and harvest rates were prescribed according to the RCP 45 scenario. In response to the combined effect of all these drivers, our model estimated, depending on our choice of climate projection, an increase in future (2075–2099) fire carbon emissions by 17 and 62% compared to present day (1985–2009). The largest increase in fire emissions was predicted for Southern Hemisphere South America for both climate projections. For Northern Hemisphere Africa, a region that contributed significantly to the global total fire carbon emissions, the response varied between a decrease and an increase depending on the climate projection. We disentangled the contribution of the single forcing factors to the overall response by conducting an additional set of simulations in which each factor was individually held constant at pre-industrial levels. The two different projections of future climate change evaluated in this study led to increases in global fire carbon emissions by 22% (CCSM) and 66% (ECHAM5/MPI-OM). The RCP 45 projection of harvest and land use led to a decrease in fire carbon emissions by −5%. The RCP 26 and RCP 60 harvest and landuse projections caused decreases around −20%. Changes in human ignition led to an increase of 20%. When we also included changes in fire management efforts to suppress fires in densely populated areas, global fire carbon emission decreased by −6% in response to changes in population density. We concluded from this study that changes in fire emissions in the future are controlled by multiple interacting factors. Although changes in climate led to an increase in future fire emissions this could be globally counterbalanced by coupled changes in land use, harvest, and demography.

Fire carbon emissions over Equatorial Asia (EQAS) play a critical role in the global carbon cycle. Most regional fire emissions (89.0%) occur in the dry season, but how changes in the dry-season length affect the fire emissions remains poorly understood. Here we show that, the length of the EQAS dry season has decreased significantly during 1979–2021, and the delayed dry season onset (5.4 ± 1.6 (± one standard error) days decade−1) due to increased precipitation (36.4 ± 9.1 mm decade−1) in the early dry season is the main reason. The dry season length is strongly correlated with the length of the fire season. Increased precipitation during the early dry season led to a significant reduction (May: −0.7 ± 0.4 Tg C decade−1; August: −12.9 ± 6.7 Tg C decade−1) in fire carbon emissions during the early and peak fire season. Climate models from the Coupled Model Intercomparison Project Phase 6 project a continued decline in future dry season length in EQAS under medium and high-emission scenarios, implying further reductions in fire carbon emissions.

Fire is an important disturbance in earth’s system. In the arctic region, fire can release huge amount of carbon as the arctic peatland stored almost one third of carbon storage in the earth’s soil. The gaseous carbon compounds are usually greenhouse gases, which can enhance the greenhouse effect, and warm the arctic region. Fire risk in the arctic region is positively correlated to the warmer climate. There is a potential positive feedback between the climate and the carbon loss from wildfire in the arctic region, but the relation between fire carbon emission and climate change in the arctic largely remains unknow. This study aimed to analyse the correlation between the climate factors and the fire carbon emission. This study used Multiple Linear Regression and ANCOVA to analyse the correlation of the carbon emission from 2001 to 2018 in Alaska. The findings indicated that the fire carbon emission in Alaska fluctuated significantly within those years, and the high carbon emission was associated with the large fire events, which burned massive amounts of areas. The carbon emission was mostly affected by the temperature and humidity, but the correlation was negative. As the fire carbon emission was not covaried with the warming climate, the future fire carbon emission might be depending on the interactions among various climate factors.

<p>Fire plays an important role in the earth system. While some aspects of fire including burnt area and fire frequencies have been extensively studied; fire carbon emissions, which could exert significant influence on the carbon cycle and a wide range of geophysical processes relating to ecosystem services and human well-being, are relatively understudied in terms of its global trends and drivers. We investigated fire emission trends from 2001 to 2019 at global and regional scales using total carbon emission data from the fourth generation Global Fire Emission Database (GFED4s). We identified geophysical and anthropogenic drivers for fire emission trends for regions defined by geographical regions and biomes with a causal model; and quantified driver importance with machine learning models by estimating their impact on fire emissions. We observed an insignificant global fire emission trend; mainly caused by conflicting fire emission trends in tropical savanna/grasslands and boreal forests. The two biomes were the largest sources for global fire emissions. Tropical savanna/grasslands contributed 60% to global fire emissions and showed a decreasing fire emission trend at a rate of -9.7±1.4 ×10<sup>12</sup> gC/year; boreal forests contributed around 8% and increased at a rate of 7.4±2.2 × 10<sup>12 </sup>gC/year (rates estimated by Huber robust regression). At the regional scale, we found that fire emission trends were driven by geophysical factors for all regions. Anthropogenic interventions only caused changes in fire emissions in limited regions, including all biomes in Africa, and some biomes in Boreal Asia, Central Asia and North America. Decreasing fire emission trends in tropical savanna/grasslands mainly occurred in Africa; and the most dominant drivers were anthropogenic interventions, namely agriculture expansion and the subsequent declines in vegetation. Increasing fire emissions from boreal forests largely came from Boreal Asia, where anthropogenic interventions were also important drivers, and climatic drivers relating to moisture, drought, and temperature played a vital role as well, especially moisture. Vegetation indices were also identified as drivers for this region but were the least important ones. Our results suggested future fire emission trend for boreal forests in Boreal Asia could be highly vulnerable to climate change. It is possible that fire emissions in this region continue to increase if the climate becomes drier since drivers relating to moisture were highly important. On the other hand, further decrease for fire emissions in African savanna/grasslands is limited by the already shrunk vegetation. Therefore, at the global scale, risks of increasing fire carbon emissions are rather high. Increasing carbon emissions and the slow recovery of carbon sink capacity in burnt forests imply long-term net carbon source from boreal forests, which could be challenging for climate mitigation.</p>

Abstract. Fires have influenced atmospheric composition and climate since the rise of vascular plants, and satellite data have shown the overall global extent of fires. Our knowledge of historic fire emissions has progressively improved over the past decades due mostly to the development of new proxies and the improvement of fire models. Currently, there is a suite of proxies including sedimentary charcoal records, measurements of fire-emitted trace gases and black carbon stored in ice and firn, and visibility observations. These proxies provide opportunities to extrapolate emission estimates back in time based on satellite data starting in 1997, but each proxy has strengths and weaknesses regarding, for example, the spatial and temporal extents over which they are representative. We developed a new historic biomass burning emissions dataset starting in 1750 that merges the satellite record with several existing proxies and uses the average of six models from the Fire Model Intercomparison Project (FireMIP) protocol to estimate emissions when the available proxies had limited coverage. According to our approach, global biomass burning emissions were relatively constant, with 10-year averages varying between 1.8 and 2.3 Pg C yr−1. Carbon emissions increased only slightly over the full time period and peaked during the 1990s after which they decreased gradually. There is substantial uncertainty in these estimates, and patterns varied depending on choices regarding data representation, especially on regional scales. The observed pattern in fire carbon emissions is for a large part driven by African fires, which accounted for 58 % of global fire carbon emissions. African fire emissions declined since about 1950 due to conversion of savanna to cropland, and this decrease is partially compensated for by increasing emissions in deforestation zones of South America and Asia. These global fire emission estimates are mostly suited for global analyses and will be used in the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations.

Abstract. Fire is an integral Earth System process that interacts with climate in multiple ways. Here we assessed the parametrization of fires in the Community Land Model (CLM-CN) and improved the ability of the model to reproduce contemporary global patterns of burned areas and fire emissions. In addition to wildfires we extended CLM-CN to account for fires related to deforestation. We compared contemporary fire carbon emissions predicted by the model to satellite-based estimates in terms of magnitude and spatial extent as well as interannual and seasonal variability. Long-term trends during the 20th century were compared with historical estimates. Overall we found the best agreement between simulation and observations for the fire parametrization based on the work by Arora and Boer (2005). We obtained substantial improvement when we explicitly considered human caused ignition and fire suppression as a function of population density. Simulated fire carbon emissions ranged between 2.0 and 2.4 Pg C/year for the period 1997–2004. Regionally the simulations had a low bias over Africa and a high bias over South America when compared to satellite-based products. The net terrestrial carbon source due to land use change for the 1990s was 1.2 Pg C/year with 11% stemming from deforestation fires. During 2000–2004 this flux decreased to 0.85 Pg C/year with a similar relative contribution from deforestation fires. Between 1900 and 1960 we predicted a slight downward trend in global fire emissions caused by reduced fuels as a consequence of wood harvesting and also by increases in fire suppression. The model predicted an upward trend during the last three decades of the 20th century as a result of climate variations and large burning events associated with ENSO-induced drought conditions.

<p>Tropical savannas are the biome with the highest fire occurrences worldwide and play a key role in fire carbon emissions dynamics at regional to global scales. During the past decades, however, climate change and land use management have altered their fire regimes via fire suppression or ignition related to conservation and agricultural practices, and extreme weather conditions, among others. In particular, the ongoing COVID-19 pandemic has modified human activities in both urban and rural environments, and thus provides an opportunity to study the interactions between socio-economic and biophysical drivers of fires. Using satellite-based observations, we analyze the spatio-temporal patterns of active fires (AF, from MODIS-MCD14ML) in the Llanos ecoregion (northern South America between Colombia and Venezuela) during the COVID-19 lockdown period (mid-March to December 2020). We also examine fire carbon emissions (from GFED4s) as well as monthly precipitation (from CHIRPS), maximum temperature, and vapor pressure deficit (VPD, from TerraClimate). Our results show that 2020 was the year with the highest number of AF (>60%) and fire carbon emissions (>50%) compared to the 2001 to 2019 average. We found that these increases occur mainly during the peak of the fire season (March and April), which corresponds to the beginning of the lockdown period in Venezuela (March 17) and Colombia (March 20). Pixels (at 0.05° resolution) with significant positive AF anomalies (p<0.05) occur primarily in Venezuela and over grassland and agricultural land covers. A large proportion of these pixels interact with significant positive anomalies (p<0.05) in VPD (>70% of pixels) and maximum temperature (>50%) in March and April. Furthermore, our results highlight that the increase of AF could be associated not only with potential changes in land use management but also with weather patterns anomalies during the lockdown period in the Llanos ecoregion. </p>

Abstract. Modeling fire as an integral part of an Earth system model (ESM) is vital for quantifying and understanding fire–climate–vegetation interactions on a global scale and from an Earth system perspective. In this study, we introduce to the Community Earth System Model (CESM) the new global fire parameterization proposed by Li et al. (2012a, b), now with a more realistic representation of the anthropogenic impacts on fires, with a parameterization of peat fires, and with other minor modifications. The improved representation of the anthropogenic dimension includes the first attempt to parameterize agricultural fires, the economic influence on fire occurrence, and the socioeconomic influence on fire spread in a global fire model – also an alternative scheme for deforestation fires. The global fire parameterization has been tested in CESM1's land component model CLM4 in a 1850–2004 transient simulation, and evaluated against the satellite-based Global Fire Emission Database version 3 (GFED3) for 1997–2004. The simulated 1997–2004 average global totals for the burned area and fire carbon emissions in the new fire scheme are 338 Mha yr−1 and 2.1 Pg C yr−1. Its simulations on multi-year average burned area, fire seasonality, fire interannual variability, and fire carbon emissions are reasonable, and show better agreement with GFED3 than the current fire scheme in CESM1 and modified CTEM-FIRE. Moreover, the new fire scheme also estimates the contributions of global fire carbon emissions from different sources. During 1997–2004, the contributions are 8% from agricultural biomass burning, 24% from tropical deforestation and degradation fires, 6% from global peat fires (3.8% from tropical peat fires), and 62% from other fires, which are close to previous assessments based on satellite data, government statistics, or other information sources. In addition, we investigate the importance of direct anthropogenic influence (anthropogenic ignitions and fire suppression) on global fire regimes during 1850–2004, using CESM1 with the new fire scheme. Results show that the direct anthropogenic impact is the main driver for the long-term trend of global burned area, but hardly contributes to the long-term trend of the global total of fire carbon emissions.

Wildfire is a crucial factor in influencing the earth system. Wildfire activities in the Arctic‐boreal region have received increasing attention particularly with increasing frequency and intensity under rapid climate warming in recent years. In this study, the historical simulation and future projection of the Arctic‐boreal fire carbon emissions and associated surface climate conditions (surface air temperature and precipitation) are examined using 17 CMIP6 Earth System Models. For the historical period, more than half (11 out of 17) of the models underestimate ∼40% of the observed annual mean fire carbon emissions in the Arctic‐boreal region (0.20 PgC/yr) due to a wetter bias in the Arctic‐boreal regions. Spatially, there is common underestimation of the fire centers in the eastern part of Eurasian Continent and overestimation of that in Europe. With respect to the model spread, it mainly shows large spread for fire carbon emissions (surface air temperature) in Europe (high‐latitudes). For the future projection, the fire carbon emissions in the Arctic‐boreal region is projected to exceed 100% (0.5 PgC/yr until the end of the 21st century compared with ∼0.2 PgC/yr at present). The projected precipitation and temperature in the Arctic‐boreal region land also show an upward trend during the 21st century (∼31% for annual mean precipitation and ∼10°C for surface air temperature). There are considerable bias and intra‐model spread among different models in both historical simulation and future projection.

Vegetation fires release a large fraction of light-absorbing components, which can contribute to the melting of snowpack and alpine glaciers. However, the relationship between variability in fire emissions and alpine glacier melting on the Third Pole (TP) remains poorly understood. This study provides evidence that carbon emissions from windward vegetation fires play a crucial role in comprehending glacier melting on the TP, particularly during the months of intense vegetation fires from March to May for monsoon-dominated glaciers and from June to October for westerlies-dominated glaciers. Furthermore, robust positive correlations (with p < 0.05) have been observed since 1997 across the TP between variations in glacier melting and fire carbon emissions during both annual periods and those intense fire months. In addition to climate warming, intensified fire carbon emissions could potentially accelerate the melting and mass loss of TP glaciers, especially during those traditionally nonmelting months. An urgent reassessment of the impact of fire carbon emissions on changes in TP glaciers is necessary, given that meltwater during traditionally nonmelting months can reshape freshwater resource supply patterns, and the projected increased wildfire risk in high mountainous regions in a rapidly warming climate.

Estimating the dynamics and driving factors of gross primary productivity over the Chinese Loess Plateau by the modified vegetation photosynthesis model

How the enhanced East Asian summer monsoon regulates total gross primary production in eastern China

Historical and future fire occurrence (1850 to 2100) simulated in CMIP5 Earth System Models