The net primary productivity of fine roots, NPPFineRoot, is estimated using root observation chambers called rhizotrons (Burke and Raynal, 1994). These observation chambers are framed on three sides using wood, the fourth side is a 6 mm thick pane of glass 40 cm deep and 35 cm wide. Each month, root increments are recorded from the rhizotron by tracing over visible roots with permanent markers on transparent A4 acetate sheets placed over the glass face. Information on root diameter class is recorded through colour coding (>1 mm = black, 1-2 mm = blue, 2-3 mm = red, < 4 mm = green) and the dates of root appearance and disappearance are recorded by numbering each root increment traced. Root intersections with the rhizotron screens are counted on each sheet and converted to biomass production per unit ground area using the method described by Bernier and Robitaille (2004). This method uses information on the number and root diameter of roots touching the rhizotron screen at each data collection session. These values permit us to calculate the total cross sectional surface area of the roots intersecting the rhizotron screen at each session (XSr, mm2) from root radius (r, mm) using the following equation (Bernier and Robitaille, 2004):
XSr = (π2 x ∑r2) / √ 2 (1)
Root production (NPPFineRoot, Mg ha-1) is estimated for each data collection session using the following equation (Bernier and Robitaille, 2004):
NPPFineRoot = 2 x 104 x Dr x (1 - Fc x XSr x ((sin α x cos γ) / W) (2)
Where Dr is the density of the root tissue (g mm−3), Fc is the percentage of coarse fraction of soil (i.e. soil particles > 2 mm), α is the angle of the rhizotron observation screen relative to the ground surface, γ is the ground surface angle relative to the horizontal, and W is the width of the rhizotron observation screen (mm), 104 converts mm2 ground area into one hectare, and grams into Mg. Finally, the equation multiplies this result by 2 because it is assumed that, if there was no empty space on the other side of the rhizotron screen and roots could grow freely across the screen, an equal amount of roots would intersect the screen from the other side. For this same reason, roots which branched after contact with the rhizotron observation screen are not counted as new roots at the time of branching. Dr is obtained by dividing the root volume by the root mass recorded from the ingrowth core data. A detailed description of this method is described in Bernier and Robitaille (2004) and Metcalfe et al. (2007) compares this method with three other existing methods to convert rhizotron root length measurements into estimates of root mass production per unit ground area.
