The mortality rates of large trees are critical to determining carbon stocks in tropical forests, but the mechanisms of tropical tree mortality remain poorly understood. Lightning strikes thousands of tropical trees every day, but is commonly assumed to be a minor agent of tree mortality in most tropical forests.We use the first systematic quantification of lightning-caused mortality to show that lightning is a major cause of death for the largest trees in an old-growth lowland forest in Panama. A novel lightning strike location system together with field surveys of strike sites revealed that, on average, each strike directly kills 3.5 trees (> 10 cm diameter) and damages 11.4 more.Given lightning frequency data from the Earth Networks Total Lightning Network and historical total tree mortality rates for this site, we conclude that lightning accounts for 40.5% of the mortality of large trees (> 60 cm diameter) in the short term and probably contributes to an additional 9.0% of large tree deaths over the long term.Any changes in cloud-to-ground lightning frequency due to climatic change will alter tree mortality rates; projected 25-50% increases in lightning frequency would increase large tree mortality rates in this forest by 9-18%. The results of this study indicate that lightning plays a critical and previously underestimated role in tropical forest dynamics and carbon cycling.
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.
Woody debris (WD) stocks and fluxes are important components of forest carbon budgets, and yet remain understudied, particularly in tropical forests. Here we present the most comprehensive assessment of WD stocks and fluxes yet conducted in a tropical forest, including one of the first tropical estimates of suspended WD. We rely on data collected over 8 years in an old-growth moist tropical forest in Panama to quantify spatiotemporal variability and estimate minimum sample sizes for different components. Downed WD constituted the majority of total WD mass (78%), standing WD contributed a substantial minority (21%), and suspended WD was the smallest component (1%). However, when considering sections of downed WD that are elevated above the soil, the majority of WD inputs and approximately 50% of WD stocks were disconnected from the forest floor. Branchfall and liana wood accounted for 17% and 2% of downed WD, respectively. Residence times averaged 1.9 years for standing coarse WD (CWD; >20 cm diameter) and 3.6 years for downed CWD. WD stocks and inputs were highly spatially variable, such that the sampling efforts necessary to estimate true values within 10% with 95%
Trees form the terrestrial interface with the atmosphere in forested regions. The electrical properties of trees may influence their response to atmospheric conditions and potentially lethal phenomena (e.g., lightning). We review the literature describing electrical properties of trees and provide a tabular summary of the methods and goals of each study. We hypothesized that electrical resistivity varies consistently among species and between growth forms. We surveyed resistivity of eight tree and three vine species in Michigan and Kentucky, and we quantified resistivity over a moisture gradient for wood blocks of four tree species. Resistivity varied predictably with stem diameter and differed among species and growth forms. Specifically, resistivity of trees was approximately 200% higher than resistivity of vines, and resistivity of conifers was 135% higher than that of hardwoods. The regional comparison showed no difference in resistivity of red maple (Acer rubrum L.) and sugar maple (Acer saccharum Marsh.) between Michigan and Kentucky. These results, in combination with interspecific differences observed among wood blocks, suggest that there is a phylogenetic basis for variation in resistivity that reflects differences in anatomy and physiology. Our review and empirical survey provide a framework for studying the ecological effects of lightning in the context of the electrical properties of trees.
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