High altitude tropical regions have been heavily deforested and offer large areas for carbon offsets by secondary forest regeneration. However, a general assumption exists that productivity, and thus, carbon sequestration of high altitude forests is low, but the scarcity of data from high altitude tropical forests does not allow for a sound confirmation of this assumption. To determine the carbon sequestration potential of natural forest regeneration and reforestation efforts at high altitudes, aboveground biomass was quantified of several successional stages of two monospecific secondary forests in the high Ecuadorian Andes. The studied forests were, at 3200 m, an Alnus acuminata forest with 8-, 20-, 30- and 45-year-old stands and, at 3600 m, a Polylepis incana forest with 6-, 15- and 30-year-old stands and old growth forest without known age. Both trees are pioneer species. Additionally, a soil survey was conducted and leaf N/P ratios and N and P retranslocation rates were determined. Total aboveground biomass (TAGB) estimates of the oldest stages of the two forests were 241 Mg ha-1 for the Alnus and 366 Mg ha-1 for the Polylepis forest. Productivity, expressed as annual aboveground biomass accumulation (ABA) was in both forests highest in the establishment phase (14.2 and 15.0 Mg ha-1 per year at 8 and 6 years, respectively) due to high sapling numbers, and then slowed down significantly (5.9 and 6.9 Mg ha-1 per year, respectively at age 30). TAGB and ABA figures of the investigated forests are comparable to those of lowland tropical forests and do not indicate a significant decrease of these parameters at higher altitudes in the tropics. Analysis of site conditions, ecological requirements and ecophysiological properties of the tree species indicated that in both cases an optimal combination of these factors strongly favoured forest growth. It is not possible to verify whether our forests are representative for other high altitude tropical forests or if they should be treated as exceptional cases. The results do show, however, that high carbon offsets at high altitudes are possible.