Reciprocal root-shoot cooling and soil fertilization effects on the seasonal growth of two treeline conifer species

Background: We currently lack a conclusive explanation for the physiological mechanisms behind the ceasing of tree growth in cold climates. Experiments with controlled temperatures might help to fill this gap in our knowledge. Aim: To elucidate the interdependency of root and shoot growth from each other in cold climates, and to test if the cold limitation of tree growth is caused by insufficient soil nutrition. Methods:Larix decidua and Pinus uncinata seedlings were grown in a full-factorial rootshoot cooling experiment, with organs exposed to either 6 degrees C or12 degrees C, and half of the seedlings receiving soil fertilization. Results: Although severely restricted, seedlings still showed net-biomass increments at 6 degrees C. Partial cooling of either roots or shoots ameliorated growth compared with completely cooled seedlings, but cooling of one part also affected the productivity of the warmer tissues. Fertilization was ineffective on completely cooled seedlings, but doubled productivity of both species if roots and shoots were exposed to warmer temperatures simultaneously. Conclusion: Meristematic growth is interdependent among tree organs, leading to reduced growth of all parts of a tree, independent if roots or shoots are directly affected by cold temperatures. Cold-climate growth limitation of trees cannot be lifted by soil fertilization.