Strong temporal variation in treefall and branchfall rates in a tropical forest is explained by rainfall: results from five years of monthly drone data for a 50-ha plot

Abstract

Abstract. A mechanistic understanding of how tropical tree mortality responds to climate variation is urgently needed to predict how tropical forest carbon pools will respond to anthropogenic global change, which is altering the frequency and intensity of storms, droughts, and other climate extremes in tropical forests. We used five years of approximately monthly drone-acquired RGB imagery for 50 ha of mature tropical forest on Barro Colorado Island, Panama, to quantify spatial structure, temporal variation, and climate correlates of canopy disturbances, i.e., sudden and major drops in canopy height due to treefalls, branchfalls, or collapse of standing dead trees. Treefalls accounted for 77 % of the total area and 60 % of the total number of canopy disturbances in treefalls and branchfalls combined. The size distribution of canopy disturbances was close to a power function for sizes above 25 m2, and best fit by a Weibull function overall. Canopy disturbance rates varied strongly over time and were higher in the wet season, even though windspeeds were lower in the wet season. The strongest correlate of temporal variation in canopy disturbance rates was the frequency of 1-hour rainfall events above the 99.4th percentile (here 35.7 mm hour−1, r = 0.67). We hypothesize that extreme high rainfall is associated with both saturated soils, increasing risk of uprooting, and with gusts having high horizontal and vertical windspeeds that increase stresses on tree crowns. These results demonstrate the utility of repeat drone-acquired data for quantifying forest canopy disturbance rates over large spatial scales at fine temporal and spatial resolution, thereby enabling strong tests of linkages to drivers. Future studies should include high frequency measurements of vertical and horizontal windspeeds and soil moisture to better capture proximate drivers, and incorporate additional image analyses to quantify standing dead trees in addition to treefalls.

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

“…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). As our remote sensing toolbox continues to expand, so too will our ability to detect the fingerprint of different disturbance agents on the structure of forests (Milodowski et al, 2021;Nunes et al, 2021).…”

Tree crown damage and its effects on forest carbon cycling in a tropical forest

“…Disturbances are critical for explaining forest size distributions (Farrior et al, 2016) and have important implications for successional dynamics, as increased light levels in the understory favor fast‐growing light‐demanding species (Brokaw, 1987). Given that a significant proportion of canopy turnover is attributed to disturbance‐driven crown damage (Araujo et al, 2021; Chambers et al, 2001), representing this process is important for correctly estimating size distributions, and the impact of canopy gaps on recruitment and succession. Further, periodic, severe disturbances can cause high levels of defoliation and branch loss (Liu et al, 2018) with impacts that can last for months (Lodge et al, 1991).…”

“…We compared the canopy area damaged each year, and the ratio of mortality to damage canopy turnover from simulations with observations of branch fall from repeated drone measurements over BCI (from Araujo et al (2021)). Tree size distributions were also compared with the full BCI census data, census interval 2010–2015 (Condit et al, 2019).…”

“…In all simulations the number of very small and very large trees is underestimated relative to observations. Araujo et al (2021) analyzed monthly drone images over the 50 ha plot at BCI between 2017 and 2019 and found that of the total disturbed area of 23,289 m 2 (155.3 m 2 yr -1 ha -1 ), 23% was attributed to branch fall rather than mortality (35.7 m 2 yr -1 ha -1 ). In our damage plus mortality simulation (s4) 28% of total disturbed area is due to canopy damage, and the total disturbed area is overestimated at 177.2 m 2 yr -1 ha -1 .…”

“…Aside from the impact on mortality, crown damage itself accounts for a significant portion of total canopy turnover. In the 50‐ha plot on Barro Colorado Island (BCI), Panama, a drone‐based study of canopy disturbances found that branchfall accounted for 23% of the total area disturbed (Araujo et al, 2021). A ground‐based study estimated that branchfall accounted for 17% of the total volume of woody debris (Gora et al, 2019).…”