Bianca Zoletto

African tropical forests lie within regions of high atmospheric electrical activity, yet the ecological consequences of lightning disturbances remain poorly understood. This thesis investigates the role of atmospheric electrical phenomena in shaping forest structure, species composition, and tree-level responses across multiple spatial scales in Central African tropical forests. Chapter 1, the general introduction, provides an overview of current knowledge in lightning ecology and summarizes what is known about the impacts of lightning on trees globally. It highlights the potential, but largely unexplored, significance of atmospheric electrical phenomena in forest ecosystems and introduces the conceptual framework of this thesis. Chapter 2 presents a new methodological approach for identifying lightning strikes based on field observations from Bwindi Impenetrable National Park, a montane tropical forest in Uganda. Lightning damage in tropical forests has traditionally been identified through flashover effects among neighbouring trees, while lightning scars on trunks have been considered rare or absent. Our observations show that such scars are in fact common and diagnostically useful. We propose criteria for distinguishing lightning scars from other types of damage and demonstrate how their inclusion improves the detection of small-scale lightning disturbances and provides direct evidence of interactions between high electrical currents and woody tissues. Chapters 3 and 4 investigate vulnerability to lightning at different scales, distinguishing between (i) the probability of being struck and (ii) the severity of damage once a strike occurs. Because direct estimation of strike probability is not feasible, this thesis uses the probability of exhibiting lightning-caused damage as a proxy for strike probability. Using this approach, Chapter 3 examines how distribution of lightning-caused damage varies across landscapes and among trees. At the landscape scale, the analyses examine how elevation and topography influence the spatial distribution of lightning-caused damage. At the stand scale, the study investigates how tree-level traits such as height relative to neighbouring trees, canopy exposure, and species identity affect the likelihood of exhibiting lightning damage. The results show that trees located on ridges display evidence of lightning damage more frequently than trees in valleys. At the stand level, trees that are taller than their neighbours or possess relatively larger crowns have a higher probability of exhibiting lightning damage, although the strength of these effects varies between montane and lowland forests and depends on local topographic conditions. Tree species also differ in their probability of exhibiting lightning damage, independent of tree size. Chapter 4 examines the drivers of variation in lightning-damage severity once a strike has occurred. Building on the exposure patterns identified in Chapter 3, this chapter tests whether trees exposed to higher lightning risk—for example due to their landscape position or structural characteristics—experience reduced damage severity, which could indicate adaptation to lightning at the community or individual level. Although the severity of lightning damage varied widely across sites and landscapes, no evidence was found that forests experiencing higher lightning exposure show reduced damage severity overall. Instead, patterns emerged primarily at finer scales. Larger trees tended to experience less severe damage than smaller trees, and species differed in their susceptibility to lightning damage, with some species consistently experiencing above- or below-average levels of damage. These findings suggest that although community-level adaptation to lightning is not evident, individual- and species-level differences may still influence longer-term patterns of forest structure and composition. In Chapter 5, the focus shifts to whether the atmospheric electric field influences internal electrical activity within trees. Using measurements of trunk electrical potential, we show that days with thunderstorms are associated with increased voltage variability inside trees. This finding reveals a previously undocumented connection between atmospheric electricity and plant internal electrical dynamics, with potential implications for plant physiology and lightning susceptibility. The general discussion (Chapter 6) synthesizes the results of the thesis and places them within the broader context of lightning ecology. I explore key questions arising from the observed variability in lightning damage, particularly the occurrence and characteristics of lightning scars, and discusses possible mechanisms that could explain these patterns. In this chapter I also evaluate the use of visible lightning damage as a proxy for strike probability in field studies. Finally, I consider what the findings of this thesis, together with existing knowledge from other tropical forests, suggest about the potential influence of lightning on forest structure and evolution. While several intriguing hypotheses emerge and are discussed, this chapter highlights that empirical data remain limited and that further studies, particularly those comparing lightning with other disturbance processes, are needed. Overall, this thesis advances the field of lightning ecology by developing improved field-based methods for detecting lightning damage, quantifying lightning effects across spatial scales in Central African tropical forests and introducing a new physiological perspective on tree–atmosphere electrical interactions.