1. Aikman, J.M., Distribution and Structure of the Forests of Eastern Nebraska, Univ. Nebraska Stud., 1927, vol. 26, pp. 1–75.
2. Bertness, M.D. and Callaway, R.M., Positive Interactions in Communities, Trends Ecol. Evol., 1994, vol. 9, pp. 191–193.
3. Callaway, R.M., Effect of Shrubs on Recruitment of Quercus douglasii and Quercus lobata in California, Ecology, 1992, vol. 73, pp. 2118–2128.
4. Callaway, R.M. and Pugnaire, F.I., Facilitation in Plant Communities, in Pugnaire, F.I. and Valladares, F. (eds.), Handbook of Functional Plant Ecology. N.Y., US Marcel Dekker, New York, 1999, pp. 624–648.
5. Callaway, R.M., Nadkarni, N.M., and Mahall, B.E., Facilitation and Interference of Quercus douglasii on Understory Productivity in Central California, Ecology, 1991, vol. 72, pp. 1484–1499.