Tree root dynamics in montane and sub-alpine mixed forest patches

Belowground processes in plant communities drive the sequestration of carbon into soil, but how these processes are modified by climate remains largely unexplained. Elevational gradients are unique tools for studying the responses of communities to climatic variability in situ. We installed rhizotrons in contrasting forest communities at 1400 m, 1700 m and 2000 m. We measured the number of growing roots and elongation every month over 4years. Results showed that mean daily root elongation rate (RER) was driven by soil temperature between 0-8°C, outside which extreme soil temperatures perturbed growth. RER peaked in springtime, and a smaller peak was sometimes observed in autumn. Cumulated root length was not significantly different between altitudes, although the growing season was significantly shorter at 2000 m. However, when winter snow cover was > 6 months, root growth was severely limited the following year. Root longevity was dependent on altitude and the season in which roots were initiated; roots emitted in the autumn lived significantly longer than those initiated in springtime.Root diameter was a significant factor explaining much of the variability. Few differences in root demography between contrasting plant communities within each altitude were found, and were explained by air or soil temperature.We conclude that at high altitudes and when extreme abiotic events are not restrictive to carbon supply, more resources are invested belowground in a shorter period of time, revealing a plastic response to climatic variables within a communit