Tingey, D.T., D.L. Phillips, M.G. Johnson, P.T. Rygiewicz, P.A. Beedlow, and W.E. Hogsett. 2005. Estimates of Douglas-fir fine root production and mortality from minirhizotrons. For. Ecol. & Mgt. 204:359-370. WED-04-156
Minirhizotrons were used to assess the influence of soil resources on fine root (diameter ≤ 2 mm) production, mortality, and standing crop over a 2-year period. Two study sites were located, along an elevational transect, in the Oregon Cascade Mountains in mature (>100 years old), closed-canopy Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands growing on soils with differing resource levels. The low resource site (LRS) had lower soil N (0.15% total N) and lower available water capacity (83 mm) than the high resource site (HRS), where soil N was 0.25% total N and available water capacity was 145 mm. Minirhizotron tubes were installed at each site during August 1994. To allow time for recovery from tube installation, the first root images were collected in late fall 1995 and then at ~4 week intervals thereafter, except when snow prevented sample collection at the LRS. Root data from soil cores collected near the minirhizotron tubes were used to convert the minirhizotron data to the units of g m -2 (to a depth of 60 cm). Tree bole growth, at the HRS, was about twice that at the LRS, while fine root biomass, at the HRS, was only a third of that at the LRS. Seasonally, fine root production started before the onset of bole growth. Fine root standing crop, production, and mortality were consistently higher at the LRS than at the HRS. These findings support the concept that plants allocate more C to fine roots when soil resources are low. Our estimates of fine root production and mortality tend to be larger than previous reports for Douglas-fir stands. These differences can be attributed, in part, to the failure of the previous studies to account for fine root production and mortality between sampling events.