Ahti started developing an interest in botany at the age of 15, when he worked on a class project involving collecting 100 species of plants. His attention turned to
lichens when a classmate who had worked for
Veli Räsänen pointed them out during a
birdwatching excursion in
Helsinki. His interest was further fuelled when a couple of years later, he had to pass a test on identification of forest floor lichens and
bryophytes as part of an application for work at the
Finnish Forest Research Institute. He honed his identification skills during another summer job a few years later inventorying reindeer in
Lapland. His
Arctic lichen research expeditions have taken him to locations such as the
Murmansk Coast, the northeast coast of
Iceland, and the
Sakha Republic (
Russian Arctic). One of his favourite exotic locations was the
tepui mountains of the
Venezuelan Guayana, reachable only by helicopter. As of 2017, Ahti had more than 280 publications dealing with lichens,
mosses,
fungi, and
phytogeography. Known as a specialist of the
Cladoniaceae, he wrote a
monograph on this subject for the journal series
Flora Neotropica, which reviewer
William Culberson called "the long-awaited fulfillment of an old promise by one of the world's master taxonomists." In the monograph, Ahti accepted 184 species of Cladoniaceae from the
Neotropical realm, including 29 new
taxa. Ahti made the numerous publications of
William Nylander generally accessible through a five-volume
reprint edition. He has also made the subject of botany and lichens more popular and accessible to the general public through his work with the
Nordic Lichen Flora, a series of books describing all lichens found in
Nordic countries. ==Personal life==