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.