Simulating Lightning-Induced Tree Mortality in the Dynamic Global Vegetation Model LPJ-GUESS.

Lightning is an important yet often overlooked disturbance agent in forest ecosystems. Recent research conducted in Panama suggests that lightning is a major cause of large tree mortality in tropical forests. However, lightning-induced tree mortality is not included in state-of-the-art ecosystem models. Here, we implement a general lightning mortality module in the dynamic global vegetation model LPJ-GUESS to explore the impacts of lightning on forests at local and global scales. Lightning mortality was implemented stochastically in dependency of local cloud-to-ground lightning density and simulated forest structure based on findings from the Panamanian forest. For this site, LPJ-GUESS adequately simulates the average number of trees of different size classes killed per lightning strike, with a total of 2.9 simulated versus 3.2 observed. The model also captures the estimated contribution of lightning to the overall mortality of large trees (21% simulated vs. 24% observed). Applying the new model version to other tropical and temperate forests for which observation-based estimates on lightning mortality exist, LPJ-GUESS reproduces estimated impacts in some forests but simulates substantially lower impacts for others. Global simulations driven by two alternative products of cloud-to-ground lightning densities suggest that lightning kills 301-340 million trees annually, thereby causing 0.21-0.30 GtC yr. of dead biomass (2.1%-2.9% of total killed biomass). The simulations also reveal that the global biomass would be 1.3%-1.7% higher in a world without lightning. Spatially, simulated lightning mortality is largest in the tropical forests of Africa. Although our simulations suggest an important role of lightning in forest ecosystems on a global scale, more data on lightning-induced tree mortality across different forest types would be desirable for more accurate model calibration and evaluation. Given the anticipated increase in future lightning activity, incorporating lightning mortality into ecosystem models is needed to obtain more reliable projections of terrestrial vegetation dynamics and carbon cycling.