Fine Root Dynamics in a Northern Hardwood Forest Ecosystem, Hubbard Brook Experimental Forest, NH

Abstract

1 Patterns of fine root biomass, morphology, growth and longevity were examined in the northern hardwood zone of Hubbard Brook Experimental Forest to aid understanding of the role of roots in ecosystem function. 2 Fine root biomass in the mature hardwood forest was 471 gm-2 for < 2 mm roots in June 1987 and was concentrated in the surface soil, with 43% in the forest floor horizons. After clearcutting, fine root biomass accumulated rapidly in the regrowing forest, reaching 71 % of that in the mature forest after only four years of recovery. 3 Fine root diameter distributions and specific root length (SRL; length/mass) differed among species. SRL was higher in the forest floor than mineral soil horizons, and decreased with increasing root diameter. 4 Fine root production in the mature forest, measured with in-growth cores, averaged 254 g M-2 year1, but this method probably underestimated production. Rapid disappearance of fine roots was observed for roots growing through in situ screens, and these ephemeral roots are difficult to quantify. 5 The initiation of fine root growth in the forest floor was coincident with leaf expansion in the forest canopy; root growth in the mineral soil began 1-2 weeks later. Root growth was most rapid in early summer (mid-June to early July), and the lifespan of these early season roots averaged about 8-10 months across three years of study. This estimate of longevity was consistent with that obtained from the ratio of fine root biomass to production, after correcting the production value for the observed root disappearance from in situ screens (about 50% of fine roots disappeared from screens within an annual cycle). These longevity estimates also appeared to be consistent with an analysis of the soil C budget based upon soil and fine root respiration and total root allocation. Fine root production was apparently nearly twice as high as leaf production in this ecosystem. 6 These fine root production and turnover estimates are not consistent with results from previous studies of fine root decomposition, and we suggest that fine root decay has been underestimated because existing methods inhibit the saprotrophic activity of rhizosphere organisms.