Flux-gradient relationships based upon similarity theory have been rep
orted to severely underestimate scalar fluxes in the roughness sublaye
r above forests, as compared to independent flux estimates (for exampl
e, eddy covariance or energy balance measurements). This paper present
s the results of a unique three-month investigation into the validity
of similarity theory in the roughness sublayer above forests. Eddy cov
ariance and flux-gradient measurements of carbon dioxide (CO2) exchang
e were compared above a mixed deciduous forest at Camp Borden, Ontario
, both before and after leaf senescence. The eddy covariance measureme
nts used a Li-Cor infrared gas analyzer, and the flux-gradient (simila
rity theory) measurements featured a tunable diode laser Trace Gas Ana
lysis System (TGAS). The TGAS resolved the CO2 concentration differenc
e to 300 parts per trillion by volume (ppt) based upon a half-hour sam
pling period. The measured enhancement factor gamma (the ratio of inde
pendent flux estimates, in this case eddy covariance, to similarity th
eory fluxes) was smaller and occurred closer to the canopy than in mos
t previous investigations of similarity theory. Very good agreement be
tween the eddy covariance and similarity theory fluxes was found betwe
en 1.9 and 2.2 canopy heights (h(c)), and the mean enhancement factors
measured before and after leaf senescence were 1.10 +/- 0.06 and 1.24
+/- 0.07, respectively. Larger discrepancies were measured closer to
the canopy (1.2 to 1.4 h(c)), and mean enhancement factors of 1.60 +/-
0.10 and 1.82 +/- 0.11 were measured before and after leaf senescence
, respectively. Overall, the Borden results suggest that similarity th
eory can be used within the roughness sublayer with a greater confiden
ce than previously has been believed.