Abstract
We analysed 10 years (2008–2017) of continuous eddy covariance (EC) CO2 flux measurements of net ecosystem exchange (NEE) in a young pedunculate oak forest in Croatia. Measured NEE was gap-filled and partitioned into gross primary productivity (GPP) and ecosystem reparation (RECO) using the online tool by Max Planck Institute for Biogeochemistry in Jena, Germany. Annual NEE, GPP, and RECO were correlated with main environmental drivers. Net primary productivity was estimated from EC (NPPEC), as a sum of −NEE and Rh obtained using a constant Rh:RECO ratio, and from independent periodic biometric measurements (NPPBM). For comparing the NPP at the seasonal level, we propose a simple model that aimed at accounting for late-summer and autumn carbon storage in the non-structural carbohydrate pool. Over the study period, Jastrebarsko forest acted as a carbon sink, with an average (±std. dev.) annual NEE of −319 (±94) gC m−2 year−1, GPP of 1594 (±109) gC m−2 year−1, and RECO of 1275 (±94) gC m−2 year−1. Annual NEE showed high inter-annual variability and poor correlation with annual average global radiation, air temperature, and total precipitation, but significant (R2 = 0.501, p = 0.02) correlation with the change in soil water content between May and September. Comparison of annual NPPEC and NPPBM showed a good overall agreement (R2 = 0.463, p = 0.03), although in all years NPPBM was lower than NPPEC, with averages of 680 (±88) gC m−2 year−1 and 819 (±89) gC m−2 year−1, respectively. Lower values of NPPBM indicate that fine roots and grasses contributions to NPP, which were not measured in the study period, could have an important contribution to the overall ecosystem NPP. At a seasonal level, two NPP estimates showed differences in their dynamic, but the application of the proposed model greatly improved the agreement in the second part of the growing season. Further research is needed on the respiration partitioning and mechanisms of carbon allocation.
Highlights
IntroductionGlobal forests store approximately 30% of total anthropogenic CO2 emissions [1], the potential of forests to act as a carbon sink in the future is uncertain due to possible saturation effect [2], or negative impact of changed environmental conditions on forest productivity [3].The eddy covariance (EC) technique is a widely used, state-of-the-art method, and it has become a standard in the estimation and monitoring of high frequency (typically half-hourly) carbon and water fluxes within terrestrial ecosystems [4]
Global forests store approximately 30% of total anthropogenic CO2 emissions [1], the potential of forests to act as a carbon sink in the future is uncertain due to possible saturation effect [2], or negative impact of changed environmental conditions on forest productivity [3].The eddy covariance (EC) technique is a widely used, state-of-the-art method, and it has become a standard in the estimation and monitoring of high frequency carbon and water fluxes within terrestrial ecosystems [4]
Comparison of annual Net primary productivity was estimated from EC (NPPEC) and NPPBM showed a good overall agreement (R2 = 0.463, p = 0.03), in all years NPPBM was lower than NPPEC, with averages of 680 (±88) gC m−2 year−1 and 819 (±89) gC m−2 year−1, respectively
Summary
Global forests store approximately 30% of total anthropogenic CO2 emissions [1], the potential of forests to act as a carbon sink in the future is uncertain due to possible saturation effect [2], or negative impact of changed environmental conditions on forest productivity [3].The eddy covariance (EC) technique is a widely used, state-of-the-art method, and it has become a standard in the estimation and monitoring of high frequency (typically half-hourly) carbon and water fluxes within terrestrial ecosystems [4]. Global forests store approximately 30% of total anthropogenic CO2 emissions [1], the potential of forests to act as a carbon sink in the future is uncertain due to possible saturation effect [2], or negative impact of changed environmental conditions on forest productivity [3]. Long-term data series of net ecosystem carbon exchange (NEE). Net ecosystem productivity (NEP, NEP = −NEE), in combination with meteorological and biometric measurements, provide invaluable information on the response of forest ecosystem to environmental conditions and climate change [5,6,7,8]. There are 101 forest sites in the global network of micrometeorological tower sites (FLUXNET) 2015 database, out of which 44 datasets are ten or more years long, and only 13 of those are datasets for sites classified as Deciduous Broadleaf
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