Accurate quantification of agents of tree mortality is essential for the development of robust forest dynamics models. Lightning causes a large fraction of tree deaths in some forests, and ecologists consistently list lightning as an agent of tropical tree mortality, yet supporting data are few and largely anecdotal. Resolving this problem is increasingly important to understanding the role of tropical forests in ecosystem processes, as cloud-to-ground (CG) lightning frequency is expected to increase in a warmer climate.
We are using a camera- and sensor-based network to locate and characterize lightning strikes in real-time on Barro Colorado Island (BCI), Panama. Tree damage and death data from repeated surveys of known strike locations over the past three wet seasons were compared with long-term mortality rates for different tree size classes in the 50 ha forest dynamics plot on BCI. We also used satellite-based lightning frequency data to estimate the fraction of lightning-caused annual tree mortality given various predictions of increased lightning frequency.
Results/Conclusions
Each lightning strike on BCI kills an average of 4 trees (range = 0-16) and damages 14 others (1-32). However, the death of damaged trees is slow in most cases, and accurate estimates of total mortality require longer post-strike surveys (>2 years) than typically available in our dataset (<13 months). Lightning is the single most important agent of large (> 60 cm DBH) tree death in this forest; it results in ca. 35% of large tree mortality (CI: 23-48%) over the short term (up to 13 months post-strike), and ca. 53% (CI: 40-67%) over the long term assuming 20% of damaged trees die due to indirect effects. Given these estimates, a 25% increase in CG lightning frequency will increase the fraction of lightning-caused large tree mortality to ca. 66% (CI: 52-82). Consequently, lightning has important consequences for tree population dynamics, forest community structure, and carbon turnover rates by reducing the canopy residence times of the largest trees by 3-6 decades. This project also is providing the first data regarding the characteristics of individual lightning flashes, enabling us to associate the amount of tree damage for a given strike with the intensity and duration of a given flash.