Lightning-caused disturbance frequency and severity varies with topography in an Afromontane forest

Abstract

1. Lightning is an important agent of tropical tree mortality, but field data on lightning-caused disturbances is largely restricted to one lowland tropical forest in Panama. 2. Here we quantified variation in the frequency and severity of visually detectable lightning-caused disturbances across topography in the montane forests of Nyungwe National Park, Rwanda. This was the first systematic assessment of lightning-caused disturbance in any African tropical forest and a preliminary exploration of the association between lightning disturbance frequency and forest tolerance to lightning. 3. Lightning-caused disturbances were observed six times more frequently on ridges (19 strikes across 4.5 km of transects) than in valleys (2 strikes across 2.9 km), but lightning-caused disturbances in valleys tended to cause more tree-level damage than those on ridges (and also kill a higher proportion of the trees affected within each lightning disturbance, although the rarity of disturbances in valleys meant the latter effect was not statistically significant). Overall, lightning disturbances tended to kill a lower proportion of trees in this Afromontane forest than in previously documented lightning disturbances in tropical America. 4. We also observed less tree-level damage to ridge-dominant Syzygium spp. (Myrtaceae) compared to the community-wide average, providing support for taxonomic differences in lightning tolerance. 5. Synthesis. Our results indicate that lightning disturbance severity differs within and among sites, potentially mediated by differences in lightning tolerance among tree species and provide preliminary support for the hypothesis that disturbance severity declines with disturbance frequency.