Net ecosystem production in a Spanish black pine forest after a low burn severity fire: Significance of different modelling approaches for estimating gross primary production

Many studies have examined post-fire net ecosystem production (NEP), however, the CO2 dynamics in Mediterranean mountain ecosystems after low bum-severity fires are poorly understood. To fill this gap, we used biometric and flux chamber-based methods to assess the NEP in a mature Spanish black pine (Pinus nigra Am. ssp. salzmannii) forest at an unburnt (UB) and a low bum-severity (LS) sites during the early (1.5-4.5 years) post fire stages. In this study, we also compared the significance of two different laborious modelling approaches for estimating the gross primary production (GPP) in order to deduce the NEP estimates: GPP(C) and GPP(M). The former is based on a carbon (C)-mass balance approach which is calculated as the sum of the aboveground net primary production, total belowground C flux, and aboveground autotrophic respiration. The latter is based on a whole-canopy photosynthesis-modelling approach and is obtained by combining an environmental-dependent non-rectangular hyperbolic light-response model applied to different pine needle age-cohorts and coupled to a two-leaf scaling-up strategy. Our results indicate that both sites were a consistent C sink between 2011 and 2013, with a mean respective annual NEPC of 2.43 and 2.09 Mg C ha(-1) year(-1) at the UB and LS sites, and a mean annual NEPM of 2.04 and 1.82 Mg C ha(-1) year(-1) at the UB and LS sites, respectively; all these figures are comparable in magnitude to other European forests. The difference between the NEPC and NEPM estimates within a given year varied by 6-25%, which, based on the fact that we used a single-ecosystem respiration (R-eco) data set, indicates a strong correlation between the GPP(C) and GPP(M) estimates. Our findings indicated that the low burn-severity fire did not substantially alter the annual GPP, R-eco, or NEP fluxes in this particular disturbed-forest ecosystem. Moreover, this work provides evidence for the suitability of either modelling approach to effectively simulate the GPP in future stand-scale C-cycling studies. Thus, our results are relevant not in that they directly evaluate and compare the approaches themselves, but also because they identify method-specific uncertainties that deserve exhaustive evaluation, which should be an important component in the future work of environmental researchers, especially those interested in ecological modelling.