Most alpine treelines occur at mean growing season temperatures around 6 °C, irrespective of the presents or absence of seasonality. Altitudinal increasing carbon reserve pools of trees along treeline ecotones suggested no C-limitation, but a direct growth-, or sink-limitation as the driving force for treeline formation in all climatic zones. In two greenhouse experiments, we tested the sink-limitation hypothesis, the significance of seasonality and the influence of differential below and aboveground temperatures on growth of saplings of three temperate conifer species. 1. Confirming our previous findings from field surveys, saplings that grew significantly less at cold temperatures (6°C) had significantly higher carbon storage concentrations than trees growing at warmer (12°C) conditions. 2. Surprisingly, biomass increments at the end of the 20 weeks growing season were very similar between seasonally constant and seasonally variable temperature treatments, indicating only a minor effect of seasonality on growth. 3. The extensive growth constraints at 6 °C were similar if either belowground or aboveground tissues were cooled separately, while the other half of the sapling was exposed to warmer (12 °C) temperatures. 4. Since the reduction of growth was still stronger when entire saplings were cooled, the warmer tissues in partially cooled saplings may, to a certain extent, ameliorate the negative effect of the remaining cooled parts. 5. Soil nutrient addition did not stimulate growth of completely cooled, but partly or completely warmed saplings. It therefore seems that soil nutrient uptake did not limit growth at the lower threshold temperature of tree existence.
