Nicholas Muhlenberg, 81, a University of Pennsylvania professor from 1963 to 1992, died of a heart attack last Tuesday at the Watermark at Logan Square, the retirement community where he lived.

Born in West Reading, Pa., he earned his bachelor's degree in forestry from the University of Michigan in 1949 and his master's in forestry there in 1952.

Following study on a Fulbright scholarship in New Zealand in the 1952-53 school year, his daughter Mimi said, he earned a master's in conservation from Yale University in 1957 and a doctorate in resource economics there in 1959.

An academic in his later years, Mr. Muhlenberg got his hands dirty early, working in West Coast forests.

After returning from New Zealand, Mr. Muhlenberg worked for Crown Zellerbach Corp., the paper manufacturer, from January 1954 to May 1956, first as a tree farm mapper in Seaside, Ore., then as a forester supervisor on a tree farm in Nehalem, Ore.

After earning his master's at Yale, he returned to Crown Zellerbach as a land and tax research analyst in Portland, Ore., from July 1957 to July 1960.

International experience was added when he went to Geneva, Switzerland, and wrote market reports on forest products with the United Nations' Food and Agriculture Organization and the Economic Commission for Europe from July 1960 to July 1961.

Mr. Muhlenberg began his academic career as a lecturer in forest economics at the Berkeley campus of the University of California in the 1961-62 school year.

In the 1962-63 school year, he was at the Pennsylvania State University School of Forestry on a Resources for the Future grant.

Mr. Muhlenberg's Penn career began in 1963. From 1966 until he retired in 1992, he was an associate professor of regional planning and landscape architecture at what is now the University of Pennsylvania School of Design.

Mr. Muhlenberg contributed to the 1968 work Via 1; Ecology in Design with Louis I. Kahn, Ruth Patrick, Ian McHarg and others.

He contributed papers to the publications Forest Science and Journal of Forestry.

Besides his daughter, he is survived by sons Kobi and Mattias; another daughter, Dela Muhlenberg-Bryan; a stepdaughter, Julie Galaudet; two brothers; a sister; his first wife, Erika Muhlenberg; and his companion, Barbara Simmons. His wife of 35 years, Virginia, died this year.

A memorial was planned for 2 p.m. Friday in the fourth floor auditorium at the Watermark at Logan Square, 2 Franklintown Blvd.