Growth, demography and carbon relations ofPolylepistrees at the world's highest treeline

Summary Growth, reproductive success and non‐structural carbon pools inPolylepis tarapacanaPhilippi 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 mPolylepisis 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.