During great part of the present century, the northern sector of the Nahuelbuta mountain range has suffered one of the most important environmental modifications produced since the arrival of Spanish settlers: the replacement of native vegetation by forest plantations. In spite of this, at present the evolutive characteristics of this processes are unknown. The main characteristics of the vegetation replacement process were analyzed in a small lacustrine watershed, located in the Nahuelbuta mountain range, through the use of GIS and aerial photographs. Through time, the reduction of native vegetation cover was closely tied with the increase in reforested areas. Geographically, the reforested areas advanced towards the south of the watershed, over steeper slopes displacing the remnant native forest. Finally, a conceptual model of the replacement vegetation processes is proposed.