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The behaviour of directed aerial descent has been described for numerous taxa of wingless hexapods as they fall from the tropical rainforest canopy, but is not known in other terrestrial arthropods. Here, we describe similar controlled aerial behaviours for large arboreal spiders in the genus Selenops (Selenopidae). We dropped 59 such spiders from either canopy platforms or tree crowns in Panama and Peru; the majority (93%) directed their aerial trajectories towards and then landed upon nearby tree trunks. Following initial dorsoventral righting when necessary, falling spiders oriented themselves and then translated head-first towards targets; directional changes were correlated with bilaterally asymmetric motions of the anterolaterally extended forelegs. Aerial performance (i.e. the glide index) decreased with increasing body mass and wing loading, but not with projected surface area of the spider. Along with the occurrence of directed aerial descent in ants, jumping bristletails, and other wingless hexapods, this discovery of targeted gliding in selenopid spiders further indicates strong selective pressures against uncontrolled falls into the understory for arboreal taxa.

Lightning strikes millions of trees worldwide each year, yet structured lightning damage surveys are relatively rare. Estimates drawn from the literature suggest that lightning directly or indirectly kills up to 4% of large canopy trees in a stand annually. Lightning is a major cause of death for pines in southeastern US forests and for large cacti in some deserts, but its landscape-level effects exclusive of fire at higher latitudes are poorly known. We quantified damage to trees from lightning and other sources in hemlock-hardwood forests of the Huron Mountain Region of Michigan, USA. This region receives ca. 100 cloud-to-ground lightning flashes per year, with most occurring in May to August. We recorded abiotic and biotic damage on 309 trees distributed among nine transects, each >2 km long. None of the transect trees had lightning scars, and we observed only 14 clear cases of lightning damage among thousands of trees examined during associated meander surveys (each ca. 0.5 ha). This damage was more commonly associated with emergent stature (50% of struck trees) and higher rates of biotic damage (50%) than we observed in the 309 transect trees (22% emergent status and 16% incidence of biotic damage). Nearly all (93%) of the lightning damaged trees were conifers, suggesting that either susceptibility to, or response to, lightning strikes has a phylogenetic basis. These preliminary results provide a foundation for comparative studies in other forests. Accurate quantification of lightning-induced tree mortality will improve forest turnover models and facilitate predictions of future forest structure under conditions of increased lightning frequency. Key words: lightning, forest, Michigan, survey, review. La foudre frappe des millions d'arbres a l'echelle du globe chaque annee mais les inventaires structures des dommages causes par la foudre sont relativement rares. Les estimations tirees de la litterature indiquent que la foudre tue annuellement, directement ou indirectement, jusqu'il 4% des arbres de l'etage dominant dans les peuplements. La foudre est une cause majeure de mortalite chez les pins dans le sud-est des E.-U. et chez les gros cactus dans certains deserts mais ses effets a l'echelle du paysage, a l'exclusion du feu, aux latitudes plus elevees sont mal connus. Nous avons quantifie les dommages causes aux arbres par la foudre ou d'autres causes dans les prucheraies de la region des montagnes de Huron dans l'Etat du Michigan, aux E.-U. Cette region recoit environ 100 eclairs nuage-sol par annee, dont la plupart surviennent de mai a aout. Nous avons note les dommages abiotiques et biotiques sur 309 arbres repartis le long de neuf transects, chacun ayant plus de 2 km de long. Aucun des arbres presents dans les transects ne portait de cicatrices de foudre et nous avons observe seulement 14 cas evidents de dommages causes par la foudre parmi les milliers d'arbres examines durant des inventaires par cheminement associes (chacun ayant env. 0,5 ha). Ces dommages etaient le plus souvent associes a une stature emergente (50% des arbres frappes) et a des taux plus eleves de dommages biotiques (50%) que ceux que nous avons observes chez les 309 arbres dans les transects (22% avaient une stature emergente et 16% avaient subi des dommages biotiques). Presque tous (93%) les arbres endommages par la foudre etaient des coniferes, ce qui indique que soit la susceptibilite soit la reaction a la foudre aurait un fondement phylogenetique. Ces resultats preliminaires jettent les bases pour effectuer des etudes comparatives dans d'autres forets. La quantification precise de la mortalite des arbres causee par la foudre va ameliorer les modeles de renouvellement de la foret et faciliter les predictions concernant la structure future de la foret soumise a une augmentation de la frequence de la foudre. [Traduit par la Redaction] Mots-cles: foudre, foret, Michigan, inventaire, revision., Introduction Lightning is among the most destructive of atmospheric phenomena. Its dramatic but localized effects on natural systems--from killing plants to shattering rocks (Jones and Gilbert 1918; Knight and Grab [...]

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. Key words: lightning, Michigan, resistivity, Kentucky, moisture. Dans les regions forestieres les arbres constituent l'interface terrestre avec l'atmosphere. Les proprietes electriques des arbres peuvent influencer leur reaction aux conditions atmospheriques et aux phenomenes potentiellement letaux (p. ex. la foudre). Nous avons fait une revue de la litterature qui porte sur les proprietes electriques des arbres et nous presentons un tableau sommaire des methodes et objectifs de chaque etude. Nous formulons l'hypothese que la resistivite electrique varie uniformement parmi les especes et que des facteurs physiologiques ou anatomiques, autres que la teneur en humidite, font partie des mecanismes importants qui sous-tendent ces differences. Nous avons releve la resistivite de huit especes d'arbre et de trois especes de vigne au Michigan et au Kentucky et nous avons quantifie la resistivite de blocs de bois de quatre especes d'arbre en fonction d'un gradient d'humidite. La resistivite variait de facon significative selon l'espece et la forme de croissance. Plus particulierement, la resistivite des arbres etait environ 200 % plus elevee que celle de la vigne et la resistivite des coniferes etait 135 % plus elevee que celle des feuillus. La comparaison entre les regions n'a revele aucune difference de resistivite chez l'erable rouge (Acer rubrum L.) et l'erable a sucre (Acer saccharum Marsh.) entre le Michigan et le Kentucky. Ces resultats, combines aux differences interspecifiques observees parmi les blocs de bois, indiquent que la variation de la resistivite a un fondement phylogenetique qui reflete probablement des differences anatomiques et physiologiques. Notre revue de la litterature et notre releve empirique fournissent un cadre de travail pour etudier l'ecologie de la foudre dans le contexte des proprietes electriques des arbres. [Traduit par la Redaction] Mots-cles : foudre, Michigan, resistivite, Kentucky, humidite., Introduction Scientific interest in the electrical properties of plants spans more than a century (e.g., Stone 1903). We briefly summarize the literature concerning the biologically relevant electrical characteristics of plants [...]