Short sampling intervals reveal very rapid root turnover in a temperate grassland

Although root growth and mortality play critical regulatory roles in terrestrial ecosystems, little is known about the temporal scale of these dynamics. In temperate grasslands, root dynamics may be particularly rapid because of the high proportion of production allocated to very fine root biomass. In this study, we used minirhizotron tubes to estimate root growth and mortality in an upland grassland in Yellowstone National Park that was grazed by migratory herds of ungulates. Monthly rates of root growth and mortality were estimated from May to September 2005, by measuring the elongation (growth) and disappearance (mortality) of roots at 3-day intervals. Average daily growth (millimeters of root length) was approximately 5 times greater in May and June than in July, August, and September. Average daily mortality (millimeters of root length) did not differ among months. A comparison of the June-September rates of root growth and mortality derived from sampling at short (3-day) and long (1-month) time intervals indicated that the long sampling intervals underestimated both growth and mortality by approximately 60% relative to the short intervals. These results suggest that estimates of grassland root dynamics from minirhizotrons are influenced significantly by sampling interval length, and that rapid root turnover may play a more critical role in regulating energy and nutrient fluxes in temperate grasslands than has previously been recognized.