The reason isn't clear, but physical differences between cultivars may play a part.

This research was funded by a grant from the Jessie Smith Noyes Foundation, New York. The authors thank Dr. A. P. Gutierrez for assistance in the analysis and interpretation of the data, as well as M. Liebman, M. Cornelius, J. Steggall, T. Sha-nower, N. Davidson, R. Barbehenn, S. Richards, and C. Gold for their valuable help in data collection and plot maintenance in the field.