Relationships between fine root dynamics and nitrogen availability in Michigan northern hardwood forests.

Minirhizotrons were used to observe fine root (≤1 mm) production, mortality, and longevity over 2 years in four sugar-maple-dominated northern hardwood forests located along a latitudinal temperature gradient. The sites also differed in N availability, allowing us to assess the relative influences of soil temperature and N availability in controlling fine root lifespans. Root production and mortality occurred throughout the year, with most production occurring in the early portion of the growing season (by mid-July). Mortality was distributed much more evenly throughout the year. For surface fine roots (0-10 cm deep), significant differences in root longevity existed among the sites, with median root lifespans for root cohorts produced in 1994 ranging from 405 to 540 days. Estimates of fine root turnover, based on the average of annual root production and mortality as a proportion of standing crop, ranged from 0.50 to 0.68 year-1 for roots in the upper 30 cm of soil. The patterns across sites in root longevity and turnover did not follow the north to south temperature gradient, but rather corresponded to site differences in N availability, with longer average root lifespans and lower root turnover occurring where N availability was greater. This suggests the possibility that roots are maintained as long as the benefit (nutrients) they provide outweighs the C cost of keeping them alive. Root N concentrations and respiration rates (at a given temperature) were also higher at sites where N availability was greater. It is proposed that greater metabolic activity for roots in nitrogen-rich zones leads to greater carbohydrate allocation to those roots, and that a reduction in root C sink strength when local nutrients are depleted provides a mechanism through which root lifespan is regulated in these forests.