Data from: 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.