We report tree seedling (mostly Picea engelmannii, some Abies lasiocarpa, very infrequent Pinus contorta) invasion into meadows at upper timberline in the Snowy Range, Wyoming, from 1994 to 1996. We used gradient analysis to relate this to environmental patterns, particularly plant community structure (as aggregates of plant life-forms) and persistence of snowpack in 1995 and 1996. Tree seedlings established best at sites where snow melted earliest; the site with the shortest growing season had fewest seedlings. Microsites dominated by vascular plants admitted few or no seedlings; seedlings were most likely to be found where cryptogams dominated. These findings have implications for forest advance under some climate scenarios and successional hypotheses.