The time interval between fire detections is not considered in this analysis, such that fires on consecutive and nonconsecutive days at the same ground location are treated equally. A 1-km radius is also consistent with fire spread rates of 200–5000 m h 1 for grass, grass/shrub, and deforestation fuel types , such that even slow-moving grassland fires would spread beyond the 1-km search limit on sequential days. Fires which burn on consecutive days at the same ground location can occur where fuel loads are very high, as is the case in deforestation fires when woody fuels that are piled together may smolder for several days.Specifically, the variety of days on which fires were detected was determined for each cell of the standard MODIS 250-m grid using a search radius of 1 km to interpret the center locations of all high-confidence fire detections for each year. This gridded product of fire days was then used to select those fire detections contributing to high-frequency fire activity and characterize fire frequency for recent deforestation events.To determine whether active fire detections associated with the conversion of forest to other land uses are unique in terms of fire frequency, we compared active fire detections from recently deforested areas with four additional types of fire management. In the following text, we describe the test datasets used to evaluate patterns in active fire detections for maintenance of cattle pastures, indigenous reserves in Cerrado savanna-woodland land cover, small properties associated with government settlement programs,blueberries in containers growing and sugarcane production regions. We used data on recent deforestation and land use following deforestation to identify and characterize active fire detections associated with forest conversion.Deforestation was mapped using highresolution Landsat Thematic Mapper or ChineseBrazilian Environmental Research Satellite data from approximately August of each year 2001–2005 .
We developed our approach for identifying deforestation fires with data for Mato Grosso state. For individual deforestation events 425 ha in size, we also evaluated differences in patterns of active fire detections for conversion of forest to pasture, forest to mechanized agriculture, and forest conversions not in agricultural production . The post clearing land use for each deforestation event was identified previously using phenological information from time series of MODIS data at 250 m resolution . Finally, we examined fire activity in the year before deforestation detection by PRODES, year of forest clearing, and for as many years post clearing as possible to characterize the nature of fire usage during the conversion process. These comparisons provide the timing, frequency, and degree of repeated burning detected by the MODIS sensors for forest conversion to different land uses. We selected annual deforestation from 2003– 2005 to utilize combined Terra and Aqua fire observations. Because few areas are deforested without the use of fire in Amazonia, deforestation events without any MODIS fire detections provide a measure of the extent of omission due to satellite observation and fire characteristics . We utilized data on historic deforestation and recent land use changes to identify maintenance fires on agricultural lands in Mato Grosso state. The dataset is derived from areas that were deforested before the initial year of PRODES digital data , buffered by 1 km from remaining forest edges to exclude fires from new deforestation. Next, we removed areas that underwent conversion from pasture to cropland during 2001–2004 and previously cleared areas that were identified as secondary forest . The resulting dataset isolates old deforestation not associated with forest edges, secondary forest, or recent conversion to cropland. To identify patterns of fire detections for extensive grassland fires in Cerrado regions, we selected 18 indigenous reserves in Mato Grosso and Tocantins states covering more than 42 000 km2 .
Fire is used during the dry season on some indigenous reserves to facilitate hunting, but extensive land cover change is rare . Small properties are an additional challenge for separating evidence of fire activity in the same location. To test the influence of property size on fire frequency, we considered a subset of the demarcated Instituto Nacional de Colonizac¸a˜o e Reforma Agra´ria land reform settlements in Mato Grosso without large deforestation events in either 2004 or 2005 . The typical lot size in these settlements is 100 ha, of which 20–50 ha may be cleared for agricultural use. Although some sugarcane is grown in the Amazon region, the majority of Brazil’s sugarcane industry is located in the southern and northeastern regions of the country. Sa˜o Paulo State had more than 3 million hectares planted in sugarcane in 2005. We evaluated active fire detections in 31 municipalities in Sa˜o Paulo state with 420 000 ha of sugarcane planted in 2005 to calculate the degree of high-frequency fire associated with sugarcane production.High-frequency fire activity is common in areas of recent deforestation but rare for other fire types in Amazonia . Deforestation in Mato Grosso state had more total fire detections than allother fire types in Table 1 combined and seven times the number of fires detected in the same location on 2 or more days during 1 year. High-frequency fire activity accounted for 27% of high-confidence MODIS detections associated with small producers in Mato Grosso 2004 and 2005, but the total number of detections was small , suggesting that property size is not the main component of the pattern of repeated fire usage associated with deforestation. Fires detected on 2 days at the same location are rare within indigenous reserves and agricultural areas of Mato Grosso state or sugarcane production municipalities in Sa˜o Paulo state; fires on 3 or more days are almost exclusively linked to deforestation. Mato Grosso had both the highest total fire activity and greatest fraction of high-frequency fire activity during 2003–2007 of any state in Brazilian Amazonia .
Combined with fires in neighboring Para´ and Rondoˆnia states, these three states contributed 83% of the fires that burn on 2 or more days and 74% of the total fire activity in the Brazilian portion of the Amazon Basin during this period. Inter annual variability in thetotal number of fires highlights drought conditions in Roraima state during 2003 and widespread drought in 2005 affecting Rondoˆnia, Acre, and Amazonas states. The fraction of total fire activity from burning on 2 or more days also increased during drought years in these states. Fire detections were highest in 2005 for Para´ and Amapa´ states, although these regions were less affected by drought conditions; the fraction of repeated fire activity did not increase in 2005 compared with other years. After a decrease in the fire activity in Brazilian Amazonia during 2006, fires in 2007 returned to a similar level as seen in 2004 and 2005, led by increased fire activity in southeastern Amazonia. Major contributions to this increase in 2007 were from low-frequency fires in Tocantins and Maranha˜o states and additional high-frequency fires in Mato Grosso and Para´. Overall, fires on 2 or more days during the same dry season accounted for 36–47% of the annual fire activity in Brazilian Amazonia during 2003–2007,planting blueberries in containers with greater contributions from repeated fires in years with highest fire activity. At the national scale, fire activity in Brazil and Bolivia accounted for 98% of all fire detections in the Amazon Basin during 2003–2007 . High frequency fires contribute a large fraction of MODIS detections in both countries, with peak repeated fire activity during 2004 in Brazil and 2007 in Bolivia. Small contributions to overall fire activity from other Amazon countries are primarily low-frequency fires, with the notable exceptions of 2004 and 2007 in Colombia, 2003 in Guyana and Suriname, and 2003 and 2007 in Venezuela. Spatial patterns of high-frequency fire activity in 2004 and 2005 highlight active deforestation frontiers in Mato Grosso, Rondoˆnia, and Para´ states in Brazil and in southeastern Bolivia . Isolated locations of high-frequency fire activity can also be seen across other portions of the Amazon Basin, but these areas have low total fire detections. Differences in the total fire activity and high-frequency fire detections between 2004 and 2005 highlight the influence of drought conditions in western Amazonia on fire frequency. Total fire detections in central Mato Grosso decreased slightly between 2004 and 2005, while fire detections in drought-stricken northern Rondoˆnia, southern Amazonas, and eastern Acre states in Brazil show higher total fire activity in 2005 than in 2004. The number of 0.251 cells with 450% of fire activity occurring on 2 or more days is similar during 2004 and 2005 , but the spatial distribution is broader in 2005 than in 2004, as fires associated with deforestation activity in Mato Grosso, Para´, and southern Rondoˆnia spread west into northern Rondoˆnia, Acre, and southern Amazonas states. In addition to deforestation-linked fires, slow-moving forest fires and contagion of other accidental burning events may also have contributed to the higher fraction of repeated fire activity in these regions.Among deforested areas in Mato Grosso, the intensity of fire usage varies according to post clearing land use . Forest conversion for cropland exhibits the most frequent fire usage; more than 50% of the 2004 cropland deforestation events had fire detections on 3 or more days during the 2004 dry season and 14% burned on 10 or more days.
Over 70% of the forest clearings with fires on more than 5 days were subsequently used for cropland. Because of more frequent fire usage in preparation for mechanized agriculture, few areas deforested for cropland in 2004 had no high-confidence fire detections during 2004 . Deforestation for pasture averaged less than half as many fire days as deforestation for cropland, measured as either the maximum or mean days of fire detection per clearing. Even among very large clearings , fire usage was significantly higher for cropland deforestation than forest clearing for pasture . Only 13% of all deforestation events for pasture 4200 ha averaged 3 or more fire days in any year, suggesting that mechanized forest clearing and high-frequency burning are more related to post clearing land use than clearing size. For both pasture and cropland deforestation, polygons in which the conversion occurs within 1 year have a greater number of fire days in the year that the deforestation was detected than conversions occurring over 2 or more years , consistent with the expectation that higher fire frequency leads to higher combustion completeness. For those areas that showed no clear pasture or cropland phenology in the years following deforestation, fire activity was minimal. Nearly 50% of the areas described as NIP showed no high-confidence fires in 2004, and only 22% of these deforestation events exhibited fires on 2 or more days typical of other deforestation events. The timing of fire use during the dry season also differed for cropland and pasture deforestation . Deforestation fire activity may begin during the late dry season in the year before the deforestation is mapped and continue for several years post clearing as the initial forest biomass is gradually depleted to the desired conditions for cropland or pasture use. September was the most common month of fire activity for all types of deforestation in Mato Grosso in 2004. More than 70% of the fires associated with 2004 deforestation for pasture during 2003–2005 occurred during the late dry season . In contrast, fire activity for conversion to cropland was more evenly distributed through the dry season, with 45% of fire detections occurring in May–July. Burning activities initiated in the early dry season for both pasture and cropland deforestation continue to burn in subsequent months. The highest percentage of fires without detections on additional days occurred during the late dry season; approximately 30% of the fires for conversion to pasture during September and October were the first fire detection for those deforestation events compared with 11% of all fires for cropland conversion during this period. High-frequency fire activity may last for several years following initial forest clearing, further increasing the expected combustion completeness of the deforestation process . Forty percent of the areas deforested for cropland during 2003–2005 had 2 or more years during 2002–2006 with 3 1 fire days. The duration of clearing for pasture was more variable. Most areas cleared for pasture had 0–1 years of high-frequency fire usage, although a small portion had frequent fire detections over 2–3 years typical of mechanized forest clearing.