Low-temperature resistance in Polylepis tarapacana, a tree growing at the highest altitudes in the world

The Polylepis tarapacana forests found in Bolivia are unique with respect t o their altitudinal distribution (4200-5200 m). Given the extreme environme ntal conditions that characterize these altitudes, this species has to rely on distinct mechanisms to survive stressful temperatures. The purpose of t his study was to determine low-temperature resistance mechanisms in P. tara pacana. Tissue was sampled for carbohydrate and proline contents and micro- climatic measurements were made at two altitudes, 4300 and 4850 m, during b oth the dry cold and wet warm seasons. Supercooling capacity (-3 to -6 degr eesC for the cold dry and -7 to -9 degreesC for the wet warm season) and in jury temperatures (-18 to -23 degreesC for both seasons), determined in the laboratory, indicate that P. tarapacana is a frost-tolerant species. On th e other hand, an increase in supercooling capacity, as the result of signif icant increase in total soluble sugar and proline contents, occurs during t he wet warm season as a consequence of higher metabolic activity. Hence, P. tarapacana, a frost-tolerant species during the colder unfavourable season , is able to avoid freezing during the more favourable season when minimum night-time temperatures are not as extreme.