The trade-off between cold resistance and growth determines the Nothofagus pumilio treeline

The upper and poleward limit of tree distribution are usually determined by abiotic factors such as low temperature and strong winds. Thus, cold resistance is a key element for survival in high altitudes and latitudes where conditions can reduce plant growth. A trade-off between resource allocation to cold resistance and growth could emerge in populations frequently exposed to low temperatures like those in the treeline zone. We studied annual height growth and ice nucleation temperature in Nothofagus pumilio (Nothofagaceae) populations growing in its extremes of altitudinal distribution and in 3 sites situated on a latitudinal gradient in the Chilean Andes. Additionally, gas exchange, water and nitrogen use efficiency and total soluble sugar (TSS) were also measured as possible mechanisms for survival in high altitudes. Individuals from the treeline populations showed lower annual height growth and lower ice nucleation temperatures compared with those from lower populations. In the same way, individuals from more poleward populations showed lower annual height growth and lower ice nucleation temperatures. Gas exchange, water and nitrogen use efficiency and TSS were also higher in the high altitude populations. The results obtained support the hypothesis of trade-off, because the upper and poleward populations showed more cold resistance but a lower height growth. Additionally, we show that cold resistance mechanisms do not impact the physiological performance, suggesting possible adaptation of the high altitude populations. Low temperatures may be affecting cellular growth instead of photosynthesis, creating a pool of carbohydrates that could participate in cold tolerance. Other abiotic and biotic factors should be also assessed to fully understand the distributional range of Nothofagus species.