Evaluating minirhizotron estimates of fine root longevity and production in the forest floor of a temperate broadleaf forest

The minirhizotron technique (MR) for in situ measurement of fine root dynam ics offers the opportunity to obtain accurate and unbiased estimates of roo t production in perennial vegetation only if MR tubes do not affect the lon gevity of fine roots. Assuming fine root biomass is near steady-state, fine root production (g m(-2) yr(-1)) can be estimated as the ratio of fine roo t biomass (g m(-2)) to median fine root longevity (yr). This study evaluate s the critical question of whether MR access tubes affect the longevity of fine roots, by comparing fine root survivorship obtained using MR with thos e from a non-intrusive in situ screen method in the forest floor horizons o f a northern hardwood forest in New Hampshire, USA. Fine root survivorship was measured in 380 root screens during 1993-1997 and in six horizontal min irhizotron tubes during 1996-1997. No statistically significant difference was found between estimates of survivorship of fine roots (<1 mm dia.) at t his site from MR versus from in situ screens, suggesting that MR tubes do n ot substantially affect fine root longevity in the forest floor of this nor thern hardwood forest and providing greater confidence in measurements of f ine root production using the MR technique. Furthermore, the methodology fo r estimating fine root production from MR longevity data was evaluated by c omparison of fine root longevity and production estimates made using single vs. multiple root cohorts, and using root-number, root-length, and root-ma ss weighted methods. Our results indicate that fine root-length longevity e stimates based on multiple root cohorts throughout the year can be used to approximate fine root biomass production. Using this method, we estimated f ine root longevity and production in the forest floor at this site to be 31 4 days (or 0.86 yr) and 303 g m(-2) yr(-)1, respectively. Fine root product ion in this northern hardwood forest is approximately equivalent to standin g biomass and was previously underestimated by root in-growth cores. We con clude that the use of MR to estimate fine root longevity and production as outlined here may result in improved estimates of fine root production in p erennial vegetation.