Publication

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 <10cm. Three years after the initial burn, 62+/-26 Mg ha(-1) (s.e.) of live biomass, predominantly stems <30cm, was transferred to the dead biomass pool, compared with 8+/-3 Mg ha(-1) in the control. This biomass loss from fire represents similar to 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. (C) 2010 Elsevier B.V. All rights reserved.