Effects of sink removal on transpiration at the treeline: Implications for the growth limitation hypothesis

In this paper, we tested if sink removal in treeline trees induces downscaling of the stomatal conductance in treeline trees. Körner [Körner, C., 1998. A re-assessment of high elevation treeline positions and their explanation. Oecologia115, 445-459.] hypothesized that treeline trees are particularly sink limited due to low temperatures during the growing season. This implies that they would have lower sapflow rates and their sapflow rates would be less sensitive to variations in temperature than the sapflow rates of control trees. We removed about 75% per year or all buds during 2 subsequent years. We did not find any differences in sapflow rates and their sensitivity to daily mean temperatures in our data. Therefore, our data indicate that the photosynthesis of trees at the treeline might not be sink limited. Furthermore, trees increased their branch extension growth in response to debudding and it seems probable that trees can compensate for a sink removal by increasing the growth rates of remaining meristems.