5 | Conclusions
The GPPiso/SF method provides an alternative empirical
method to estimate forest stand GPP that is independent of eddy
covariance (EC). We compared GPPiso/SF estimates from
PRELES, a semi-empirical model parameterised with EC data. When compared
across two years, the GPP estimates from the two methods were not
significantly different. Moreover, when fertilised plot was compared to
the reference plot using annual means, no differences were detected.
However, when compared using daily estimates, the fertilized plot was
8% higher than the reference plot. The annual conclusion agrees with
previous estimates on this site, the daily conclusion does not.
Adjusting GPPiso/SF for gm was
necessary; we explored three alternatives for doing so. The inclusion of
mesophyll conductance provides an empirical/mechanistic means of
connecting isotopic measurements to gas-exchange measurements and
GPPiso/SF provides a means of scaling from individual
trees to tree stands and canopies. Finally, a critical advantage of the
sap flux/isotope based method for estimating GPP is that its
requirements for the terrain and atmospheric conditions are less
restrictive than for EC measurements. It can be applied in complex
terrain, complex canopy structure, and non-turbulent atmospheres.