In the appendix table, George 2003 reports the range of root respiration rates, converted to
and standard units:
In the original publication Allen (1969), root respiration was measured at
C.
The values can be found in table 3 and figure 2.
The data include a minimum (Group 2 Brunswick, NJ plants) and a maximum (Group 3 Newbery, South Carolina), which I assume are the ones used by George 2003: