Global patterns of mobile carbon stores in trees at the high-elevation tree line

Aim Across all latitudes, high-elevation tree lines represent a drastic change in the dominant plant life-form, from upright trees to low-stature alpine plants. Although associated with low temperatures, the physiological mechanisms controlling this boundary are still not clear. The growth-limitation hypothesis assumes a direct low-temperature restriction of tissue formation at otherwise sufficient photoassimilation. In order to test this hypothesis, we present a global synthesis of previously published and new data on tree carbon supply status at high-elevation tree lines. Location Global; 13 regions between 68 degrees N and 45 degrees S. Methods Late-season concentrations of non-structural carbohydrates (NSC) in foliage and branch wood were measured at three elevations across the tree line ecotones, from upper tall forests (timber line) to the edges of aborescent tree growth (tree line). Year-round records of -10 cm soil temperatures were taken at the tree line. Results Despite large differences in elevation and season length, the mean growing season temperature at the tree line was similar (approximately 6.6 degrees C) between all sites. NSC concentrations were not depleted at any of the elevation gradients between timber line and tree line, indicating no shortage of C at the uppermost tree stands. Tested across all sites, NSC concentrations in fact significantly increased with elevation. On average, NSC increased by 18% in leaves and 26% in branch wood from the lowest to the highest stands, primarily due to higher starch concentrations rather than free sugars. Hence, these responses do not reflect osmotic adjustments to lower temperatures at the end of the growing season. Main conclusions This global data set contributes to a mechanistic understanding of tree line formation based on biological principles across climatic zones and tree genera. No evidence of C shortage was found at the high-elevation tree line in either seasonal and non-seasonal regions. The increasing trend of NSC concentrations with elevation is in line with the growth-limitation hypothesis.