Estimating canopy gross primary production by combining phloem stable isotopes with canopy and mesophyll conductances.

Antoine Vernay,Annikki Mäkelä,Matthias Peichl,John D Marshall,Pantana Tor‐Ngern,Ram Oren,Zsofia R Stangl,Xianglin Tian,Sune Linder,Jinshu Chi

doi:10.1111/pce.13835

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 modelling 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 parameterized with EC data. The comparisons were performed in a highly instrumented, boreal Scots pine forest in northern Sweden, including a nitrogen fertilized and a reference plot. The resulting annual and daily GPPiso/SF estimates agreed well with PRELES, in the fertilized 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.

Highlights

Gross primary production (GPP) represents a key flux in the carbon (C) budget of a forest ecosystem
Using the daily data corrected for autocorrelation, we found a significant increase in the F plot; GPPiso/SF was higher by 8% and GPPPRELES was higher by 16% (Table 2 and see Figure S6)
We compared GPPiso/SF estimates from PRELES, a semi-empirical model parameterized with eddy covariance (EC) data

Summary

Introduction

Gross primary production (GPP) represents a key flux in the carbon (C) budget of a forest ecosystem. One of the advantages of PRELES is that it estimates ecosystem fluxes (GPP and evapotranspiration) by using routinely measured weather data It means that GPPPRELES can be estimated everywhere with no additional measurement than weather conditions (Tian, Minunno, Cao, Kalliokoski, & Mäkelä, 2020). Previous studies that compared between biometric/component fluxes and GPP from EC (GPPEC) data have found that the GPP trends agreed reasonably well over several years, but often failed to find the same absolute values at annual scales (Curtis et al, 2002; Ehman et al, 2002; Peichl et al, 2010). A previous study compared EC and dendrometric data and found a good correlation, but the dendrometric data do not provide flux estimates and require the development of site specific correlations (Zweifel et al, 2010)

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Discussion

Conclusion