This book investigates radiocesium movement in all major components of forest ecosystems, e.g. the plants, animals, insects, microorganisms, and soils, during the initial stage of contamination after the incident at Fukushima Daiichi Nuclear Power Plant. Most of the work was conducted at a common research site. More specifically, the book examines the contribution of surface uptake by trees in the dynamics of radiocesium during the initial contamination stage; the movement of radiocesium in the form of small organic fragments that are essential to the radiocesium dynamics in forest ecosystems; and the upward movement of radiocesium due to microorganism activity, which promotes the effective decontamination of the forest floor. Lastly, it explains why spiders could be a valuable indicator of the contamination level in forest ecosystems.