BG1-2: Proportional allocation to fine root production declines with increasing total NPP both within and among forest types.

Fine Root Production and Mortality

Fine root production and mortality are measured using minirhizotrons (Ruess et al. 1998, 2003). At each site, a 30 X 30 m plot was established and five minirhizotron tubes (2 m in length, 5.6 cm OD) were installed randomly throughout the plot at angles varying from 21° in black spruce sites to 40° in early successional sites. Each tube is permanently scribed with 120, 1.4 X 1.2 cm frames every 1.2 cm along the tube, which was sealed permanently at the bottom. Each tube extends approximately 20 cm out of the soil, where it was painted black and then repainted white, and capped between measurements. Images of each frame are taken at approximately monthly intervals throughout the growing season beginning the summer following installation using a Bartz color micro-video camera (Bartz Technology, Santa Barbara, CA) and stored on Hi8 video tape. Images are then digitized using a PC-based interactive image analysis program (ROOTS) (Michigan State University, East Lansing, MI), and production and mortality of fine roots calculated as extension growth of live roots, and live to dead or missing roots, respectively (Hendrick and Pregitzer 1992, 1993, Ruess et al. 1998, 2003). Programs used for production and mortality computations can be downloaded from the author’s webpage (http://mercury.bio.uaf.edu/~roger_ruess/). Monthly production and mortality throughout the growing season are typically reported as mm fine root per cm² minirhizotron tube per day (mm cm^-2 d^-1). These values are based on 201.6 cm² of viewing image per tube. We assume that fine root production and mortality between the fall and spring sampling periods occur sometime during fall or over winter, and for estimations of annual production, include these over-winter values as previous year’s growth. Annual values for fine root production and mortality are simply the sum of growth and mortality across all actual sampling intervals throughout the year. This characterization is somewhat arbitrary, because we don’t know how much growth and mortality actually occurred during late fall vs. early spring. In order to translate annual production values to a ground area basis, we assumed a depth of view of 2 mm for the minirhizotron camera (Merrill and Upchurch 1994). We use specific root length values reported for first-order roots by Pregitzer et al. (2002) taken from Tanana River floodplain stands to calculate production for each minirhizotron tube separately, to account for the fact that tube angle, and thus tube depth, varied slightly among tubes.