Quantification and identification of lightning damage in tropical forests

Abstract

Accurate estimates of tree mortality are essential for the development of mechanistic forest dynamics models, and for estimating carbon storage and cycling. However, identifying agents of tree mortality is difficult and imprecise. Although lightning kills thousands of trees each year and is an important agent of mortality in some forests, the frequency and distribution of lightning-caused tree death remain unknown for most forests. Moreover, because all evidence regarding the effects of lightning on trees is necessarily anecdotal and post hoc, rigorous tests of hypotheses regarding the ecological effects of lightning are impossible. We developed a combined electronic sensor/ camera-based system for the location and characterization of lightning strikes to the forest canopy in near real time and tested the system in the forest of Barro Colorado Island, Panama. Cameras mounted on towers provided continuous video recordings of the forest canopy that were analyzed to determine the locations of lightning strikes.We used a preliminary version of this system to record and locate 18 lightning strikes to the forest over a 3-year period. Data from field surveys of known lightning strike locations (obtained from the camera system) enabled us to develop a protocol for reliable, ground-based identification of suspected lightning damage to tropical trees. In all cases, lightning damage was relatively inconspicuous; it would have been overlooked by ground-based observers having no knowledge of the event. We identified three types of evidence that can be used to consistently identify lightning strike damage in tropical forests: (1) localized and directionally biased branch mortality associated with flashover among tree and sapling crowns, (2) mortality of lianas or saplings near lianas, and (3) scorched or wilting epiphytic and hemiepiphytic plants. The longitudinal trunk scars that are typical of lightning-damaged temperate trees were never observed in this study. Given the prevalence of communications towers worldwide, the lightning detection system described here could be implemented in diverse forest types. Data from multiple systems would provide an outstanding opportunity for comparative research on the ecological effects of lightning. Such comparative data are increasingly important given expected increases in lightning frequency with climatic change.

A mechanistic and empirically supported lightning risk model for forest trees

“…We recorded lightning-caused tree damage resulting from 19 lightning strikes in the 50-ha plot from 2015 to 2018. These strikes were located using a camera-based lightning monitoring system (Yanoviak et al, 2017). Any gaps in our monitoring period were short in duration and applied consistently across the 50-ha plot.…”

“…Lightning attachment typically is limited to a single tree (hereafter the directly struck tree) even in dense forest; however, the electric current distributed from a single attachment point commonly damages or kills additional trees up to 45 m from the directly struck tree (hereafter secondary lightning damage; Anderson, 1964;Furtado, 1935;Magnusson et al, 1996). Secondary lightning damage apparently occurs infrequently in temperate forests (Murray, 1958;Taylor, 1974), potentially because of the higher electrical resistivity of temperate trees (Gora, Bitzer, Burchfield, Schnitzer, & Yanoviak, 2017), but is characteristic of lightning damage in tropical forests (Anderson, 1964;Furtado, 1935;Magnusson et al, 1996;Yanoviak, Gora, Burchfield, Bitzer, & Detto, 2017).…”

“…After lightning attaches to one tree, secondary damage to surrounding trees primarily occurs via flashover-the jumping of electric current from one object to another across an air gap (Darveniza, 1992;Furtado, 1935;Yanoviak et al, 2017). Field observations indicate that lightning jumps from the directly struck tree F I G U R E 1 Hypothetical crowns of canopy trees viewed from above (Panel a) and in profile (Panel b) depicting key variables used in the models: exposed crown area as A f , idealized crown overlap as O c , idealized inter-crown distance as d c , trunk distance as d t and canopy position (subcanopy, canopy, emergent) [Colour figure can be viewed at wileyonlinelibrary.com] to the nearest branch of a neighbouring tree, regardless of groundlevel distance to that neighbour (Anderson, 1964;Furtado, 1935;Yanoviak et al, 2017). This is consistent with the principles of electromagnetism; the electrical resistance of air is c. 13 orders of magnitude greater than that of tree tissues (Defandorf, 1955), thus the most important factor determining whether a tree experiences flashover presumably is the proximity of its branches to the directly struck tree.…”

“…This is consistent with the principles of electromagnetism; the electrical resistance of air is c. 13 orders of magnitude greater than that of tree tissues (Defandorf, 1955), thus the most important factor determining whether a tree experiences flashover presumably is the proximity of its branches to the directly struck tree. Although less common, secondary damage also occurs when the branches of neighbouring trees are in direct contact or are connected by lianas (Yanoviak et al, 2017). Consequently, factors that increase proximity to neighbouring crowns, such as large crown area or the presence of neighbouring trees with expansive crowns, should increase the likelihood of secondary lightning damage (Anderson, 1964).…”

Effects of lightning on trees: A predictive model based on in situ electrical resistivity

“…We combined Equation and Ohm's Law to quantify the potential for lianas to function as natural lightning rods. Lianas were conspicuously damaged by electric current in >90% of the lightning strikes on BCI (Yanoviak, Gora, Burchfield, Bitzer, & Detto, ) in a separate study, demonstrating that electric current flows through both trees and their resident lianas during a strike. Consequently, we assume that the electrical potential (voltage) across all main stems in a liana‐tree complex is the same during a lightning discharge.…”