Estimating canopy gross primary production by combining phloem stable
isotopes with canopy and mesophyll conductances
Abstract
Gross primary production (GPP) is a key component of the forest carbon
cycle. However, our knowledge of GPP at the stand scale remains
uncertain because estimates derived from eddy covariance (EC) and EC
itself rely on semi-empirical modeling and the assumptions of the EC
technique are sometimes not fully met. We propose using the sap
flux/isotope method as an alternative way to estimate canopy GPP, termed
GPPiso/SF, at the stand scale and at
daily resolution. It is based on canopy conductance inferred from sap
flux and intrinsic water-use efficiency estimated from the stable carbon
isotope composition of phloem contents. The GPPiso/SF
estimate was further corrected for seasonal variations in photosynthetic
capacity and mesophyll conductance. We compared our estimate of
GPPiso/SF to the GPP derived from PRELES, a model
parameterised with EC data. The comparisons were performed in a highly
instrumented, boreal Scots pine forest in northern Sweden, including a
nitrogen fertilised and a reference plot. The resulting annual and daily
GPPiso/SF estimates agreed well with PRELES, in the fertilised plot and
the reference plot. We discuss the GPPiso/SF method as
an alternative which can be widely applied without terrain restrictions,
where the assumptions of EC are not met.