Growth and carbon relations of tree forming conifers at constant vs. variable low temperatures

1. Natural climatic driven treeline positions follow a mean growing season temperature of around 6 °C in seasonal as well as non-seasonal climates. The influence of constant vs. variable temperature regimes on growth was explored by experimentally exposing two-year-old seedlings of two montane conifer species (Larix decidua and Pinus mugo) to either constant temperatures of 6 °C or 12 °C, or seasonally and diurnally changing temperatures that varied around the same means (covering a 13 K amplitude across the season). 2. In both species, exposure to 6 °C during a 20 weeks growing season reduced growth dramatically in constant as well as variable temperature treatments. Nevertheless, new biomass production was still measurable at 6 °C, indicating that a complete cessation of growth occurs at a lower temperature. 3. The biomass increment by the end of the season was hardly influenced by the presence or absence of temperature variation for both, the 6 °C and 12 °C mean treatments. While seasonal and diurnal variable temperatures had a slightly positive effect on growth compared to a full season at constant temperatures in the deciduous Larix, biomass increments of the evergreen Pinus were the same at constant and variable temperature treatments. 4. End of season sugar and starch concentrations in different tissues revealed higher concentrations in cold treated seedlings at constant and varying temperatures. In line with previous field data, low temperatures did not lead to carbon depletion (carbon limitation). We thus assume a stronger impact of cool temperatures on meristematic processes (sink activity) than on photosynthesis (source activity). 5. Synthesis. The similar growth rates of trees in experimentally constant vs. variable growing season temperatures match field observations. Although metabolic responses to temperature are nonlinear, this implies an overall 'quasi-linear' response of growth to temperature in trees at their lower temperature limit, which awaits further explanation. The higher concentrations of non-structural carbohydrates in cold treated seedlings support the sink-limitation hypothesis of plant growth in cold climates.