Christensen's research focuses on the effects of disturbance on structure and function of populations, communities and ecosystems. On going studies include an analysis of patterns of forest development following cropland abandonment as these are affected by environment, stand history and plant demographic patterns. He and his students are pursuing comparative studies of ecosystem responses to varying fire regimes across temperate North America. He is conducting research on the utilization of remote sensing systems such as synthetic aperture radar to evaluate long-term changes in forest ecosystems. In addition to these interests in basic ecological science, Christensen has written widely on the importance of natural disturbance in the management of forests, shrublands, and wetlands. He is interested in the application of basic ecological theory and models to management, and has collaborated with others in the development of the concept of ecosystem management.