Estimation of European Terrestrial Ecosystem NEP Based on an Improved CASA Model

Net ecosystem productivity (NEP) is a key indicator to describe terrestrial ecosystem functions and carbon sinks. The CASA model was improved by optimizing the parameters optimum temperature and maximum light use efficiency (
${\varepsilon }_{\max }$
), and the NEP value of the European terrestrial ecosystem was calculated by combining the soil respiration model. The results showed that when using vegetation classification data to optimize parameter
${\varepsilon }_{\max }$
, the
R
²
of NEP between estimates and observations increased from 0.252 to 0.403, and the RMSE decreased from 84.557 to 64.466 gC·m
⁻²
·month
⁻¹
. After further optimizing the optimum temperature,
R
²
increased to 0.428, and the RMSE decreased to 63.720 gC·m
⁻²
·month
⁻¹
. It indicated that the CASA model could be improved by optimizing
${\varepsilon }_{\max }$
as well as optimum temperature, which was a good approach to improve the NEP estimations. Based on this, the NEP spatiotemporal changes in various regions of Europe were analyzed using the optimization results. The NEP values of European terrestrial ecosystem has regional differences, showing a pattern of western region > southern region > central region > eastern region > northern region. The monthly change of NEP in each region is a single peak curve with high in summer and low in winter, and the annual overall value is positive (i.e., it shows a carbon sink). The research can enable us to obtain the carbon source/sink distribution information in Europe more accurately and provide a scientific reference for carbon balance policy formulation in the region.