Advantages of long-term measurement of fine root demographics with a minirhizotron at two balsam fir sites

We used 15 site-years of minirhizotron observations (1998-2006 at one site; 1998-2000 and 2004-2006 at second site) from two mature balsam fir (Abies balsamea (L.) Mill.) sites to quantify interannual variability in fine root demography and assess the accuracy of estimates from early years of observation. Annual production varied fourfold at Foret Montmorency (FM) (5.8-26.5 roots . 100 cm(-2)) and twofold at Green River (GR) (7.2-14.2 roots . 100 cm(-2)). Annual mortality varied more than 30-fold at the two sites (FM: 0.7-23.2 roots . 100 cm(-2); GR: 0.3-10.9 roots . 100 cm(-2)), year-end standing crops varied two-to eight-fold (FM: 3.6-28.4 roots . 100 cm(-2); GR 8.5-18.6 roots . 100 cm(-2)), and median life-span of annual cohorts varied from 180 to 540 days at FM and from 350 to 577 days at GR. This variation illustrates that root demography estimates from short-term studies may differ widely from long-term means. Annual mortality and standing crops were lowest in the first year of observation and tended to increase for two or more years at both sites, whereas these trends were not observed for annual production. Our results indicate that minirhizotron tubes must be in place for more than 2 years to accurately estimate fine root demography at balsam fir sites.