Fire, fragmentation, and windstorms: A recipe for tropical forest degradation

Abstract

Widespread degradation of tropical forests is caused by a variety of disturbances that interact in ways that are not well understood. To explore potential synergies between edge effects, fire and windstorm damage as causes of Amazonian forest degradation, we quantified vegetation responses to a 30‐min, high‐intensity windstorm that in 2012, swept through a large‐scale fire experiment that borders an agricultural field. Our pre‐ and postwindstorm measurements include tree mortality rates and modes of death, above‐ground biomass, and airborne LiDAR‐based estimates of tree heights and canopy disturbance (i.e., number and size of gaps). The experimental area in the southeastern Amazonia includes three 50‐ha plots established in 2004 that were unburned (Control), burned annually (B1yr), or burned at 3‐year intervals (B3yr). The windstorm caused greater damage to trees (>10 cm DBH) in the burned plots (B1yr: 13 ± 9% of 785 trees; B3yr: 17 ± 13% of 433) than in the Control plot (8 ± 4% of 2,300; ± CI). It substantially reduced vegetation height by 14% in B1yr, 20% in B3yr and 12% in the Control plots, while it reduced above‐ground biomass by 18% of 77.7 Mg/ha (B1yr), 31% of 56.6 (B3yr), and 15% of 120 (Control). Tree damage was greatest near the agricultural field edge in all three plots, especially among large trees and in B3yr. Trunk snapping (70%) and uprooting (20%) were the most common modes of tree damage and mortality, with the height of trunk failure on the burned plots often corresponding with the height of historical fire scars. Of the windstorm‐damaged trees, 80% (B1yr), 90% (B3yr), and 57% (Control) were dead 4 years later. Trees that had crown damage experienced the least mortality (22%–60%), followed by those that were snapped (55%–94%) and uprooted (88%–94%). Synthesis. We demonstrate the synergistic effects of three kinds of disturbances on a tropical forest. Our results show that the effects of windstorms are exacerbated by prior degradation by fire and fragmentation. We highlight that understorey fires can produce long‐lasting effects on tropical forests not only by directly killing trees but also by increasing tree vulnerability to wind damage due to fire scars and a more open canopy.

Prolonged tropical forest degradation due to compounding disturbances: Implications for CO2 and H2O fluxes

“…A windstorm that impacted the experimental plots in 2012 punctuated this pattern. Fire-damaged trees had trunks that were more vulnerable to wind breakage and crowns that were more exposed to wind and associated uprooting (more details in Silvério et al, 2019). As large trees died, AGB biomass continued to drop across the burned plots, reaching their lowest levels by the end of the recovery period.…”

“…[B3yr] or unburned control) from 2004 to 2010, and impacted by a blowdown event in 2012 (Silvério et al, 2019; Figure S1). These highly degraded forest areas could follow different trajectories of recovery, leading to alternate states-either a degraded, derived-savanna environment or recovery of a closed-canopy forest ecosystem (e.g., low resilience).…”

“…In addition, increased postfire mortality of canopy trees creates niche opportunities for light-demanding pasture grasses and lianas that can outcompete woody native species (D'Antonio, Hughes, & Vitousek, 2001). Finally, windstorm-related tree snapping, uprooting, and canopy damage can elevate postfire mortality rates of large trees for several years and delay recovery (Barlow et al, 2003;Silvério et al, 2019).…”

“…Specifically, the fires weakened trunks and increased crown exposure of large trees to strong winds that impacted the experimental forest(Silvério et al, 2019). The LiDAR overflights were conducted in October 2014.…”

The mechanical stability of the world’s tallest broadleaf trees

“…Post-damage surveys have shown that tall trees are more likely to be damaged by wind than shorter trees (Rifai et al, 2016; Silvério et al, 2018). However, these data represent the first direct measurements of bending strains and mechanistic study of wind damage risk in a tall tropical forest.…”