Induced polarization applied to biogeophysics: recent advances and future prospects

This paper provides an update on the fast-evolving topic of the induced polarization (IP) method applied to biogeophysics. It emphasizes new understandings of the IP phenomena associated with biological activity, pointing out new developments and applications, and identifying existing knowledge gaps. The focus is on the use of IP as related to living organisms, including micro-organisms and plants (both roots and stems). We first discuss observed links between the IP signal and microbial cell structure, activity and biofilm formation. We provide an up-to-date conceptual model of the electrical behavior of the microbial cell and biofilm and examine the role of extracellular electron transfer mechanisms on the functionality and development of biofilms. We review the latest biogeophysical studies, including work on hydrocarbon biodegradation, contaminant sequestration, soil strengthening and peatland characterization. We then elaborate on the IP signature of the plant root zone, relying on a state-of-the-art conceptual model of the biogeophysical mechanisms of a plant root cell. The first laboratory surveys show that single roots and root system are highly polarizable. They also present encouraging results for imaging the root system embeded in a medium and gaining information on the mass density, the structure or the physiological characteristics of the root system. IP is also used to characterize wood and tree structures in the lab but also at the field scale, through tomography of the stem. Finally, we discuss up- and down-scaling between lab and field studies as well as joint interpretation. We emphasize the need for intermediate scale studies and the benefits of using IP as a time-lapse monitoring method. We conclude with the promising integration of IP in global mechanistic models to better understand and quantify subsurface biogeochemical processes

Interactions Among Fire, Insects and Pathogens in Coniferous Forests of the Interior Western United States and Canada

1 Natural and recurring disturbances caused by fire, native forest insects and pathogens have interacted for millennia to create and maintain forests dominated by seral or pioneering species of conifers in the interior regions of the western United States and Canada. 2 Changes in fire suppression and other factors in the last century have altered the species composition and increased the density of trees in many western forests, leading to concomitant changes in how these three disturbance agents interact. 3 Two- and three-way interactions are reviewed that involve fire, insects and pathogens in these forests, including fire-induced pathogen infection and insect attack, the effects of tree mortality from insects and diseases on fuel accumulation, and efforts to model these interactions. 4 The emerging concern is highlighted regarding how the amount and distribution of bark beetle-caused tree mortality will be affected by large-scale restoration of these fire-adapted forest ecosystems via prescribed fire. 5 The effects of fire on soil insects and pathogens, and on biodiversity of ground-dwelling arthropods, are examined. 6 The effects of fire suppression on forest susceptibility to insects and pathogens, are discussed, as is the use of prescribed fire to control forest pests