Assessing temporal variation of primary and ecosystem production in two Mediterranean forests using a modified 3-PG model

Context: Forest ecosystem carbon uptake is heavily affected by increasing drought in the Mediterranean region. Aims: The objectives of this study were to assess the capacity of a modified 3-PG model to capture temporal variation in gross primary productivity (GPP), and ecosystem net carbon uptake (NEE) in two Mediterranean forest types. Methods: The model was upgraded from a monthly (3-PG) to a daily time step (3-PGday), and a soil water balance routine was included to better represent soil water availability. The model was evaluated against seasonal GPP and NEE dynamics from eddy covariance measurements. Results: Simulated and measured soil water content values were congruent throughout the study period for both forest types. 3-PGday effectively described the following: GPP and NEE seasonal patterns; the transition of forest ecosystems from carbon sink to carbon source; however, the model overestimated diurnal ecosystem respiration values and failed to predict ecosystem respiration peaks. Conclusions: The model served as a rather effective tool to represent seasonal variation in gross primary productivity, and ecosystem net carbon uptake under Mediterranean drought-prone conditions. However, its semi-empirical nature and the simplicity inherent in the original model formulation are obstacles preventing the model working well for short-term daily predictions