Given the importance of root dynamics for soil C storage, the aim of this study was to analyze first the seasonal dynamics of belowground productivity and then the short-term effects of grazing exclosure on root dynamics in mountain grasslands. Soil coring and root ingrowth cores were used to assess belowground biomass (BGB) and productivity in grazed and ungrazed (grazing exclosures) plots in two mountain grasslands. Annual belowground production ranged from 472 to 590 gm−2, representing from 14 to 22% of the maximum root biomass measured over the year. Spring was the most productive season, accounting for more than 50% of total annual production, indicating that factors besides temperature may affect seasonal root dynamics. Although belowground production was much higher in the top 5 cm compared to deeper, the relative productivity rate (production-to-BGB ratio) and renewal time was higher at the subsurface (5–15 cm) layer. The contribution of the subsurface layer to total belowground production increased in spring, possibly due to occasional freezing events at the uppermost layer in the early growing season. The stronger seasonality in subsurface relative productivity rates may reflect depth-dependent changes in root characteristics and lifespan. Excluding grazing increased belowground productivity in summer, but its effects on BGB showed great variability between sites.This study was funded by the Spanish Ministry of Science and Education (MEC) within the framework of the Carbopas (REN2002-04300-C02-02) project and by the European Comission under Carbomont project (EVK1- CT-2001-00125). This research was also supported by the Balangeis project (SUM2006-0030-CO2-02), Agroeco II (CGL2009-13497-CO2-02) and the Graccie project (Consolider Nº 22422) launched by the Spanish Ministry of Science and Technology.