The dynamic history of the upper forest line ecotone in the northern Andes

In the Andean cordilleras very conspicuous ecotones can be found. The transition from continuous upper montane forest to treeless herbaceous vegetation, regionally known as "páramo" (Cleef 1981; Luteyn 1999) is known as the "upper forest line" (UFL) or "timber line" (Holtmeier 2009). Above the UFL trees may occur forming small patches with diameters of ten to several hundreds of metres. The elevation where individual trees find their altitudinal limits is at significantly higher altitudes and this limit reflects the "upper tree line". In the Colombian Andes the upper tree line, most formed by dwarf trees of Polylepis, may be up to 800 m above the UFL. Therefore, it is relevant to differentiate between both ecotones. Across the Andes the altitudinal position of the UFL varies much depending latitude (Schmithüsen 1976) but in the area under consideration located between 0 and 11°N the altitudinal position of the UFL mostly varies between 3,000 and 3,800 m. In this paper we focus on the tropical Andes of northern Ecuador and Colombia. The spatial and temporal dynamics of the UFL ecotone, which has scientific and economic relevance is explored and discussed.