We have reconstructed the altitudinal position of the upper forest line (UFL) during the last 6000 years. This boundary between montane forest and páramo (tropical alpine grasslands) has important ecological and societal relevance. It is suggested that human-induced fires and deforestation during the long occupation history of the Central Valley of Ecuador have caused a downslope shift of the UFL and have given way to a downslope expansion of páramo vegetation. More recently, montane forests and lower páramo have been replaced to a large extent by agricultural land. Pollen analysis of a 90 cm long sediment core G15-II from a small mire at 3400 m elevation, 200 m below the actual UFL in Guandera Biological Reserve (0°36'N, 77°42'E), shows the altitudinal position of the UFL during seven discrete intervals: (1) from 7150 to 6240 cal. yr BP the UFL was at c. 3100—3200 m and climatic conditions were cool; (2) from 6240 to 5320 cal. yr BP the UFL shifted to c. 3600 m and upper montane rainforest (UMRF) surrounded the mire; (3) from 5320 to 2160 cal. yr BP the UFL was at 3600—3650 m elevation and montane forest consisted mainly of Hedyosmum, Weinmannia , Melastomataceae, Ilex, Scrophulariaceae and Symplocos; (4) from 2160 to 910 cal. yr BP the UFL shifted downslope to 3350 m and the mire was located in the lowermost páramo; (5) from 910 to 520 cal. yr BP cooler climatic conditions prevailed and the UFL was at 3250—3300 m; (6) since c. 520 cal. yr BP the UFL has shifted upslope to 3600 m. During this period presence of agricultural weeds (Rumex) and evidence of draining and disturbance of the mire indicate that agricultural activities expanded and almost reached the reserve area; (7) during the last 150 cal. yr disturbance increased. We conclude that during the last 6000 years the UFL reached a maximum altitude of 3650 to 3700 m, indicating that páramo grasslands above this elevation represent a natural ecosystem. Under the Kyoto Protocol-driven reforestation activities, trees should be planted up to a maximum of 3700 m. Planting trees (exotic species in particular) above 3700 m would contribute to the degradation of the natural ecosystem.