Fine root production, demography and turnover in northern hardwood forests

Data were collected from two sugar maple (Acer saccharum Marsh.) forests in 1989 and 1990. Direct observations of fine root production, development and mortality were used in conjunction with physical harvests of fine root biomass and nitrogen content to measure root growth and death, and to estimate the amount of carbon and nitrogen allocated to fine root production and subsequently returned to the soil via fine root turnover.; Analyses of cumulative survival distributions of contemporaneous 1989 and 1990 cohorts revealed that roots at the northern site consistently lived longer on average than roots born during the same periods of time at the southern site. The longer lifespan of roots in the northern forest was due to significantly lower first-season mortality rates; new roots were lost 64% faster at the southern forest (0.41 vs. 0.25 %{dollar}cdot{dollar}day{dollar}sp{lcub}-1{rcub},{dollar} p {dollar}{dollar} 0.5; 0.14 vs. 0.12 %{dollar}cdot{dollar}day{dollar}sp{lcub}-1{rcub}){dollar} among the sites. Patterns of fine root mortality at each site were the same for roots produced at all times of the year, and our results suggest that temporal differences in biomass "turnover" may be due to temporal variation in root production, not in root mortality.; Greater than 50% of annual length production occurred before mid-summer in both ecosystems, while the period of greatest mortality was from late summer through winter. About 1/3 of annual fine root production and mortality occurred simultaneously, with little observable change in total root length. Total fine root length observable in the minirhizotrons peaked in mid-summer in both ecosystems.; Annual production values of approximately 8000 and 7300 kg{dollar}cdot{dollar}ha{dollar}cdot{dollar}yr{dollar}sp{lcub}-1{rcub}{dollar} were calculated at the Southern and Northern sites, respectively, representing about 60% of total NPP in both forests. Corresponding biomass mortality (i.e. turnover) values were 6700 and 4800 kg{dollar}cdot{dollar}ha{dollar}cdot{dollar}yr{dollar}sp{lcub}-1{rcub},{dollar} and total nitrogen returns to the soil from fine root mortality were 72 kg{dollar}cdot{dollar}ha{dollar}cdot{dollar}yr{dollar}sp{lcub}-1{rcub}{dollar} at the Southern site and 54 kg{dollar}cdot{dollar}ha{dollar}cdot{dollar}yr{dollar}sp{lcub}-1{rcub}{dollar} at the Northern site. Fine roots dominated total biomass and N litter inputs to the soil in both ecosystems, accounting for over 55% of total biomass and nearly 50% of total N returns. In both ecosystems, roots {dollar}<{dollar}0.5 mm comprised the bulk of fine root biomass and N pools, and the contribution of these roots to northern hardwood ecosystem carbon and nitrogen budgets has probably been underestimated in the past.