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) 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.