Herbarium Fellow, The New York Botanical Garden (1978–1981); Profesor de Botánica and Director del Herbario, P. Universidad Católica del Ecuador, Quito (1981–1983); Associate Curator, The New York Botanical Garden (1983–1984); Associate Professor (lektor) Aarhus University (1984–1989); Senior Associate Professor (docent) Aarhus University (1989–1997); Professor (1997–present) Aarhus University. Adjunct professor (2013–present), Kasetsart University.
The Royal Danish Academy of Sciences and Letter; Fellow of the Linnean Society, London; Danmarks Naturvidenskabelige Akademi; Association for Tropical Biology; International Palm Society; American Society of Plant Taxonomists; Society for Economic Botany; Commission of the Organization for Flora Neotropica.
Member of Editorial Boards of Anales del Real JardÃn Botánica de Madrid (2003–2009). Caldasia, Acta Botánica Mexicana, Ecotropica, Les Annales de Botanique de l’Afrique de l’Ouest, and Biodiversity and Conservation; Outside referee for American Journal of Botany, Brittonia, Opera Botanica, Symb. Bot. Upps., Biotropica, Bull. Inst. Franc. d'Etudes Andines, Ann. Missouri Bot. Gard., Willdenowia, Ann. Bot. Fennicii, Journal of Tropical Ecology, Journal of Vegetation Science, Flora Neotropica, Aquatic Botany, Journal of the Torrey Botanical Society, Kew Bulletin, Taxon and Ecology. Editor of AAU Reports, 1989–1994 and ENRECA-NEWSLETTER, 1990–1994; Editor of Tropical Forests: Botanical Dynamics, Speciation, and Diversity, Academic Press, London, 1989; Editor of Páramo: An Andean Ecosystem under Human Influence, Academic Press, London, 1992. Co-editor of Biodiversity and Conservation of Neotropical Montane Forests, NYBG, New York, 1995. Co-editor of Estudios Sobre diversidad y ecologia de plantas, PUCE, Quito, 1997. Co-editor of Botanica Austroecuatoriana – Estudios sobre los recursos vegetales en las provincias de El Oro, Loja y Zamora-Chinchipe. Abya-Yala, Quito, 2002.