He became interested in how plants in the tropics, which experience limited seasonal temperature variation, respond to seasons of drought in terms of growth and flowering. At KU, he continued publishing in this area, and carried out field work in Costa Rica. He led undergraduate KU Study Abroad programs in Bonn, Germany, and San Jose, Costa Rica, and spent sabbaticals in Massachusetts and Darmstadt (Germany). He also spent a sabbatical at the field station of Harvard University, Harvard Forest, where he received a Bullard Fellowship. Rolf collaborated with many colleagues from tropical countries in Latin America and Asia, analyzing large phenological data sets, and was a reviewer for numerous research papers on phenology (periodic plant and animal life cycle events and how these are influenced by seasons and habitat).