Abrupt increases in Amazonian tree mortality due to drought-fire interactions

Abstract

Interactions between climate and land-use change may drive widespread degradation of Amazonian forests. High-intensity fires associated with extreme weather events could accelerate this degradation by abruptly increasing tree mortality, but this process remains poorly understood. Here we present, to our knowledge, the first field-based evidence of a tipping point in Amazon forests due to altered fire regimes. Based on results of a large-scale, long-term experiment with annual and triennial burn regimes (B1yr and B3yr, respectively) in the Amazon, we found abrupt increases in fire-induced tree mortality (226 and 462%) during a severe drought event, when fuel loads and air temperatures were substantially higher and relative humidity was lower than long-term averages. This threshold mortality response had a cascading effect, causing sharp declines in canopy cover (23 and 31%) and aboveground live biomass (12 and 30%) and favoring widespread invasion by flammable grasses across the forest edge area (80 and 63%), where fires were most intense (e.g., 220 and 820 kW ⋅ m(-1)). During the droughts of 2007 and 2010, regional forest fires burned 12 and 5% of southeastern Amazon forests, respectively, compared with <1% in nondrought years. These results show that a few extreme drought events, coupled with forest fragmentation and anthropogenic ignition sources, are already causing widespread fire-induced tree mortality and forest degradation across southeastern Amazon forests. Future projections of vegetation responses to climate change across drier portions of the Amazon require more than simulation of global climate forcing alone and must also include interactions of extreme weather events, fire, and land-use change.

Keywords: MODIS; fire mapping; fireline intensity; forest dieback; stable states.

Fig. 1.

High-resolution image (i.e., 1.85 m) of the experimental area in 2011 captured with the sensor Worldview-2. The dashed line represents the border between the North–South forest edge (0–100 m) and the forest interior (100–1,000 m). The North–South edge of the plots is bordered by a road and open agricultural fields, and the other plot boundaries are in contiguous forest. The control represents an unburned area, and B1yr and B3yr areas that were burned annually and every 3 y, respectively, from 2004 to 2010 (with the exception of 2008).

Fig. 2.

(Left) Annual MCWD between 2000 and 2010 for the Upper Xingu Basin (solid circles) and the experimental field site (Fazenda Tanguro, solid triangles). The shaded area represents the SD of the mean and accounts for the spatial variability in MCWD across the Upper Xingu Basin. (Right) Average dry-season length (i.e., number of months with precipitation ≤100 mm) and the locations of both the Upper Xingu Basin (in gray) and the fire experiment (triangle). MCWD and monthly precipitation were derived from the TRMM.

Fig. 3.

Air temperatures (Upper) and relative humidities (Lower) during the experimental fires between 1000 and 1600 hours. These two variables were measured at a meteorological station 4 km from the experimental area.

Fig. 4.

Fine fuel loads measured in 2007 (Left) and in other fire years (2004, 2005, 2006) (Right). Litter represents only leaves, whereas 1-h fuels represent twigs ≤0.6 cm in diameter. These fuels were measured in the experimental area using the Brown’s planar transect technique (43) along 27 transects per plot.

Fig. 5.

Annualized tree mortality rates for 2004–2010 in the edge zone and forest interiors for three stem diameter (dbh) size classes: (A) 10–20 cm, (B) 20–40 cm, and (C) ≥40 cm. B1yr was burned in 2004, 2005, 2006, 2007, and 2009, and B3yr was burned in 2004 and 2007. Mortality rates were calculated using methods described in Balch et al. (26). In 2008 we did not conduct the experimental fires.

Fig. 6.

Temporal patterns in (A) cumulative tree mortality for trees ≥10 cm dbh, (B) aboveground standing live biomass, (C) LAI, and (D) forest understory VPD. Symbols in red denote when a given plot was experimentally burned (B1yr: 2004, 2005, 2006, 2009; B3yr: 2004 and 2007). Note that these values refer to postfire measurements within a given year. In 2008 we did not conduct the experimental fires (*).

Fig. 7.

Relationships between annualized tree mortality rates and fire intensity for three classes of diameter at breast height: 10–20 cm; 20–40 cm; and ≥40 cm. Each point represents an average for the forest interior or edge of B1yr or B3yr. These data were available for 2004 and 2007.