He has conducted over thirty-five years of research on the biological and physical processes governing the dynamics of natural forests and on the creation of their agroforestry analogs. His long-term research concentrates on tropical and temperate forests of the Asian and American realms. His field sites within these regions were selected specifically to allow comparison of growth, adaptation, and plasticity within and among close assemblages of species that have evolved within forest climates with differing degrees of seasonality. The results of his research have been applied to the development and testing of silvicultural techniques for restoration of degraded lands. He is the author of over 160 peer reviewed journal papers; an author of two field guides to tropical forest trees; an author of the primary silviculture textbook used throughout North America; and an editor or author to twelve other monographs and books concerning the management of forests for a variety of social values concerning agroforestry, watershed management and climate mitigation. Ashton has been recognized by fourteen university awards for his teaching and advising, the David M. Smith Award for Silvicultural Research by the Society of American Foresters, and the UNESCO Sultan Quaboos Award for tropical forest conservation.