Abstract
Amazon forests account for ~25% of global land biomass and tropical tree species. In these forests, windthrows (i.e., snapped and uprooted trees) are a major natural disturbance, but the rates and mechanisms of recovery are not known. To provide a predictive framework for understanding the effects of windthrows on forest structure and functional composition (DBH ≥10cm), we quantified biomass recovery as a function of windthrow severity (i.e., fraction of windthrow tree mortality on Landsat pixels, ranging from 0%-70%) and time since disturbance for terra-firme forests in the Central Amazon. Forest monitoring allowed insights into the processes and mechanisms driving the net biomass change (i.e., increment minus loss) and shifts in functional composition. Windthrown areas recovering for between 4-27years had biomass stocks as low as 65.2-91.7Mg/ha or 23%-38% of those in nearby undisturbed forests (~255.6Mg/ha, all sites). Even low windthrow severities (4%-20% tree mortality) caused decadal changes in biomass stocks and structure. While rates of biomass increment in recovering vegetation were nearly double (6.3±1.4Mgha-1 year-1 ) those of undisturbed forests (~3.7Mgha-1 year-1 ), biomass loss due to post-windthrow mortality was high (up to -7.5±8.7Mgha-1 year-1 , 8.5years since disturbance) and unpredictable. Consequently, recovery to 90% of "pre-disturbance" biomass takes up to 40years. Resprouting trees contributed little to biomass recovery. Instead, light-demanding, low-density genera (e.g., Cecropia, Inga, Miconia, Pourouma, Tachigali, and Tapirira) were favored, resulting in substantial post-windthrow species turnover. Shifts in functional composition demonstrate that windthrows affect the resilience of live tree biomass by favoring soft-wooded species with shorter life spans that are more vulnerable to future disturbances. As the time required for forests to recover biomass is likely similar to the recurrence interval of windthrows triggering succession, windthrows have the potential to control landscape biomass/carbon dynamics and functional composition in Amazon forests.
Highlights
Natural disturbances such as windthrows impact tropical forests worldwide (Burslem, Whitmore, & Brown, 2000; Everham & Brokaw, 1996; Lugo, 2008; Mitchell, 2013; Vandermeer, la Cerda, Boucher, Perfecto, & Ruiz, 2000)
Combining repeated inventories of windthrown forests, locally calibrated biomass estimation models, and functional trait data, we empirically modeled the dynamics of biomass and community mean wood density to investigate how the biomass trajectory of forest recovery and its components varied with windthrow severity
We address the following questions: (a) What is the influence of windthrow severity on the recovery of biomass stocks? (b) Which processes and associated mechanisms determine the successional dynamics of net biomass change? (c) What is the role of disturbance‐driven species turnover in net biomass change and forest resilience?
Summary
Natural disturbances such as windthrows (i.e., snapped and uprooted trees) impact tropical forests worldwide (Burslem, Whitmore, & Brown, 2000; Everham & Brokaw, 1996; Lugo, 2008; Mitchell, 2013; Vandermeer, la Cerda, Boucher, Perfecto, & Ruiz, 2000). In the Central and Western Amazon, there is growing evidence indicating that relatively small windthrows, ranging from few toppled trees (~400 m2) to intermediate‐sized events of several hectares, occur frequently and are a major mechanism of mortality of adult trees (Espírito‐Santo et al, 2010; Negrón‐Juárez et al, 2018, 2017 ; Nelson, Kapos, Adams, Oliveira, & Braun, 1994) They create a forest mosaic of differently sized patches reflecting the legacy of a disturbance regime that may trigger succession and cause spatial variation in forest structure and species composition (Chambers et al, 2013; Marra et al, 2014; Rifai et al, 2016).
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