Size, species, and fire behavior predict tree and liana mortality from experimental burns in the Brazilian Amazon

Anthropogenic understory fires have affected large areas of tropical forest in recent decades, particularly during severe droughts. Yet, the mechanisms that control fire-induced mortality of tropical trees and lianas remain ambiguous due to the challenges associated with documenting mortality given variation in fire behavior and forest heterogeneity. In a seasonally dry Amazon forest, we conducted a burn experiment to quantify how increasing understory fires alter patterns of stem mortality. From 2004 to 2007, tree and liana mortality was measured in adjacent 50-ha plots that were intact (B0 – control), burned once (B1), and burned annually for 3 years (B3). After 3 years, cumulative tree and liana mortality (≥1 cm dbh) in the B1 (5.8% yr−1) and B3 (7.0% yr−1) plots significantly exceeded mortality in the control (3.2% yr−1). However, these fire-induced mortality rates are substantially lower than those reported from more humid Amazonian forests. Small stems were highly vulnerable to fire-induced death, contrasting with drought-induced mortality (measured in other studies) that increases with tree size. For example, one low-intensity burn killed >50% of stems <10 cm within a year. Independent of stem size, species-specific mortality rates varied substantially from 0% to 17% yr−1 in the control, 0% to 26% yr−1 in B1, and 1% to 23% yr−1 in B3, with several species displaying high variation in their vulnerability to fire-induced mortality. Protium guianense (Burseraceae) exhibited the highest fire-induced mortality rates in B1 and B3, which were 10- and 9-fold greater than the baseline rate. In contrast, Aspidosperma excelsum (Apocynaceae), appeared relatively unaffected by fire (0.3% to 1.0% mortality yr−1 across plots), which may be explained by fenestration that protects the inner concave trunk portions from fire. For stems ≥10 cm, both char height (approximating fire intensity) and number of successive burns were significant predictors of fire-induced mortality, whereas only the number of consecutive annual burns was a strong predictor for stems <10 cm. Three years after the initial burn, 62 ± 26 Mg ha−1 (s.e.) of live biomass, predominantly stems <30 cm, was transferred to the dead biomass pool, compared with 8 ± 3 Mg ha−1 in the control. This biomass loss from fire represents ∼30% of this forest's aboveground live biomass (192 (±3) Mg ha−1; >1 cm DBH). Although forest transition to savanna has been predicted based on future climate scenarios, our results indicate that wildfires from agricultural expansion pose a more immediate threat to the current carbon stocks in Amazonian forests.

Methods
These files contain the abundance data of trees in two fire experiments. Both experiments are located in a transitional forest in Mato Grosso, Brazil, in the southern part of the Amazon Basin (13o04’S,52o23’W). In the first experiment, three 50 ha (50 x 1000 m) plots bordering a crop field were established in 2004 (“Big-plot” experiment from hereafter). In each plot, transects of 500 m in length and 20 m in width were created at 10, 30, 100, 250, 500, and 750 m from the forest edge and all trees with diameter at breast height (i.e., 1.3 m from the ground; dbh) greater than 20 cm were measured within these transects. One of these plots was left unburned (i.e., Control), one plot was burned thrice (2004, 2007, and 2010; hereafter “B3yr” treatment) and the remaining plot was burned yearly from 2004 to 2010, except in 2008 (hereafter “B1yr” treatment). Trees were measured in 2004, 2008, 2010, 2012 and 2016, always prior to the experimental fires. Additional details regarding this experiment are available in Balch et al. (2011). 

The second experiment evaluated the effect of fuel addition and fire frequency on fire intensity and tree mortality. This experiment followed a randomized block design, with a total of 6 blocks and 4 plots of 40 m x 40 m within each block (“Block” experiment from hereafter). All trees with dbh greater than 5 cm were measured within these plots. Treatments consisted of unburned plots (control area), plots burned once in 2016 under natural conditions (i.e., no fuel addition), plots burned twice (2013 and 2016) under natural conditions, and plots burned twice (2013 and 2016) with fuel addition (50% increase in fine fuel loads) only before the 2013 fire. In this experiment, trees were measured yearly from 2011 to 2018, except for 2017, always prior to the experimental fires. Additional details regarding this experiment are available in Brando et al. (2016).