Growth, demography and carbon relations of Polylepis trees at the world's highest treeline

Summary Growth, reproductive success and non-structural carbon pools in Polylepis tarapacana Philippi trees were examined across a transect between 4360 and 4810 m altitude on Nevado Sajama, Bolivia. The mean −10-cm soil temperature of 5·4 °C under trees at the treeline during the 265-day growing season matched the threshold temperature found at other subtropical and tropical treelines. Beyond 4400 m Polylepis is restricted to the warmer and drier equator-facing slopes, suggesting a direct thermal limitation of tree growth. Maximum tree height, annual shoot increment and mean tree-ring width decreased with altitude. Trees near the upper range limit reached a maximum tree height of 3·3 m and a maximum stem diameter of 34 cm. The smallest tree-height classes dominated populations at all altitudes, and the uppermost site revealed the highest proportion of seedlings. Tree-size demography indicates a critical phase for tree establishment during the sapling stage, when trees emerge from sheltered niches near the ground. No evidence of a depletion of mobile C stores (sugars, starch and lipids) was found in any tissue type with increasing elevation, suggesting a limitation of C investment (growth) rather than C acquisition (photosynthesis) at treeline.