THE DYNAMICS OF FINE-ROOT LENGTH, BIOMASS, AND NITROGEN-CONTENT IN 2 NORTHERN HARDWOOD ECOSYSTEMS

The dynamics of fine (<2.0 mm) roots were measured in two sugar maple (Acer saccharum Marsh.) dominated ecosystems (northern and southern sites) during 1989 and 1990 using a combination of minirhizotrons and destructive harvests of fine root biomass and N content. Greater than 50% of annual length production occurred before midsummer in both ecosystems, while the period of greatest mortality was from late summer through winter. About one third of annual fine root production and mortality occur simultaneously, with little observable change in total root length pools. Using fine root length dynamics to derive biomass production and mortality, we calculated annual biomass production values of approximately 8000 and 7300 kg.ha(-1).year(-1), respectively, at the southern and northern sites. Corresponding biomass mortality (i.e., turnover) values were 6700 and 4800 kg.ha(-1).year(-1) and total nitrogen returns to the soil from fine root mortality were 72 kg.ha(-1).year(-1) at the southern site and 54 kg.ha(-1).year(-1) 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 <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 may have been underestimated in the past.