1. Allen, S E, Grimshaw, H M, Parkinson, J A and Quarmby, C 1974 Chemical Analysis of Ecological Materials. Blackwell, London. 763p.
2. Beck, E, Senser, M, Scheibe, R, Steiger, H-M and Pongratz, P 1982 Frost avoidance and freezing tolerance in Afro alpine ?giant rosette? plants. Plant Cell Environ. 5, 215?222.
3. Bekker, R P and Cleef, A M 1985 La Vegetación del Páramo de la Laguna Verde (Municipio de Tausa, Cundinamarca). Analysis Geografcos 14, IGAC, Bogotá, Colombia. 193p.
4. Billings, W D 1973 Arctic and alpine vegetations: Similarities, differences and susceptability to disturbance. Bio Science 23, 697?704.
5. Binkley, D and Vitousek, P M 1989 Soil nutrient availability. In Plant Physiological Ecology. Field methods and instrumentation. Eds. P WPearcy, JEhlerirtger, H AMooney and P WRundel. pp 75?96. Chapman and Hall, London.