Impact of a tropical forest blowdown on aboveground carbon balance

Abstract

Field measurements demonstrate a carbon sink in the Amazon and Congo basins, but the cause of this sink is uncertain. One possibility is that forest landscapes are experiencing transient recovery from previous disturbance. Attributing the carbon sink to transient recovery or other processes is challenging because we do not understand the sensitivity of conventional remote sensing methods to changes in aboveground carbon density (ACD) caused by disturbance events. Here we use ultra-high-density drone lidar to quantify the impact of a blowdown disturbance on ACD in a lowland rain forest in Costa Rica. We show that the blowdown decreased ACD by at least 17.6%, increased the number of canopy gaps, and altered the gap size-frequency distribution. Analyses of a canopy-height transition matrix indicate departure from steady-state conditions. This event will initiate a transient sink requiring an estimated 24–49 years to recover pre-disturbance ACD. Our results suggest that blowdowns of this magnitude and extent can remain undetected by conventional satellite optical imagery but are likely to alter ACD decades after they occur.

Deciphering the fingerprint of disturbance on the three-dimensional structure of the world’s forests

“…First, it appears that at least in certain forest types GSFDs are not well captured by a power-law, which tends to overestimate the frequency of large gaps (Wedeux & Coomes, 2015). Even when a power-law is a good fit to the data, λ has been shown to vary considerably among different forest types (Goodbody et al, 2020), as well as within the same landscape due to differences in disturbance (Cushman et al, 2021;Reis et al, 2021), topography and soil fertility (Lobo & Dalling, 2013;Goulamoussene et al, 2017). For example, using ALS data from 650 sites across the Brazilian Amazon, Reis et al (2021) showed that λ ranges between 1.66 and 2.50 across the basin, primarily reflecting underlying gradients in tree mortality, canopy height and human disturbance.…”

“…Finally, growing access to repeat ALS surveys of the same location through time provide an opportunity to study gap dynamics in action, mapping when and where new gaps form, and how quickly they close (Silva et al, 2019;Silvério et al, 2019;Cushman et al, 2021). Using repeat ALS from five sites in the Amazon, Dalagnol et al (2021) showed that gap dynamics are closely correlated to rates of tree mortality derived from plot census data, reflecting broadscale gradients in forest dynamics across the region.…”

“…Using repeat ALS from five sites in the Amazon, Dalagnol et al (2021) showed that gap dynamics are closely correlated to rates of tree mortality derived from plot census data, reflecting broadscale gradients in forest dynamics across the region. Beyond ALS, high-resolution imagery from other remote sensing platforms such as drones and satellites can also be used to track gaps through time, resulting in much richer time-series of canopy dynamics (Dalagnol et al, 2019;Cushman et al, 2021). For instance, Araujo et al (2021) collected monthly drone imagery over 5 years at Barro Colorado Island in Panama and found that treefall events mostly occurred during periods of extreme rainfall accompanied by high winds and lightningresulting in gap dynamics that are highly spatially and temporally clustered (Fisher et al, 2008;Negrón-Juárez et al, 2010).…”

The shape of trees: Reimagining forest ecology in three dimensions with remote sensing

“…Moreover, work using repeat airborne LiDAR acquired before and after the global El Niño event of 2015–2016 has shown that these edge effects can amplify the impacts of drought, as do LiDAR‐detectable fine‐scale topographic features such as steep slopes and ridges (Leitold et al, 2018; Nunes et al, 2021). A similar before–after approach has been used to quantify the impact of tropical storms on forest biomass stocks and show how biomass losses vary predictably across landscapes with flood risk and exposure to high winds (Cushman et al, 2021; Hall et al, 2020). In the near future, repeat airborne surveys may provide a unique opportunity to study other important but historically overlooked drivers of forest disturbance; for example lightning strikes, which at Barro Colorado Island in Panama have been shown to cause 40% of large tree mortality and 20% of annual gap formation (Gora et al, 2021; Yanoviak et al, 2020).…”

Soils and topography control natural disturbance rates and thereby forest structure in a lowland tropical landscape

“…For example, canopy tree mortality can be observed from optical satellite platforms, but satellite images are afflicted by high cloud cover in the tropics and the difficulty of automated detection of mortality events (Asner, 2001; Clark et al, 2004). Three‐dimensional forest structure from airborne LiDAR allows detailed characterisation of forest structural change but airborne LiDAR collection is relatively expensive, often preventing repeat measurements needed to detect structural changes (Cushman et al, 2021; Dalagnol et al, 2021; Leitold et al, 2018). Much previous research has classified forest disturbance using the frequency and size structure of ‘standing’ canopy gaps—areas of forest below a threshold height at a single point in time (Brokaw, 1982a; Jucker et al, 2018; Kellner & Asner, 2009; Lobo & Dalling, 2013), but few studies have explored how standing forest structure relates to dynamic measures of new canopy disturbances over time (Dalagnol et al, 2021).…”