He specialises in lichenology and phytogeography, including the uses of lichens as indicators of pollution and devising methods for web-based identification keys.

His father, Carlo, was an Alpine soldier who survived the Russian expedition of WW2, returned on foot, and became a prosperous baker; his mother, Matilde, was a popular teacher who taught whole generations of young people.

Nimis studied at the Liceo Classico Jacopo Stellini in Udine, after which he went to the University of Trieste, where he worked on a thesis on the thorny-cushions vegetation of the high Mediterranean mountains under the mentorship of Sandro Pignatti.

A post-doc research at University of Western Ontario (Canada), devoted to the vegetation of the Alaska Highway, tutored by László Orlóci, let him discover the world of lichens, whose study he later pursued with his Master and friend Josef Poelt.

[6] After the nuclear accident at Chernobyl in 1986 he led programmes to map and monitor levels of radioactive caesium in macrofungi, forest plants and mosses in Italy.

[2][10] Presently, Nimis is working on a computer-aided key to all lichens hitherto known from Italy and neighbouring countries, whose publication in paper-form is foreseen for 2026.