Modelling decadal trends and the impact of extreme events on carbon fluxes in a deciduous temperate forest using the QUINCY model

Abstract. Changing climatic conditions pose a challenge to accurately estimate the carbon sequestration potential of terrestrial vegetation, which is often mediated by Nitrogen availability. The close coupling between the Nitrogen and Carbon cycles controls plant productivity and shapes the structure and functional dynamics of ecosystems. However, how carbon and nitrogen interactions affect both carbon fluxes and plant functional traits in dynamic ecotones, which are experiencing disturbance and species compositional shifts remains unclear. In this work, we use in-situ measurements of leaf chlorophyll content (Chl_Leaf, years 2013–2016) and leaf area index (LAI, years 1998–2018) to parameterise the seasonal dynamics of the QUINCY ('QUantifying Interactions between terrestrial Nutrient CYcles and the climate system') terrestrial biosphere model (TBM) to simulate the carbon fluxes at the Borden Forest Research Station flux tower site, Ontario, Canada, over 22 years from 1996–2018. QUINCY was able to simulate leaf-level maximum carboxylation capacity (V_c(max),25), Chl_Leaf  and leaf nitrogen quite consistent with observations. The improved model captured observed daily gross primary production (GPP) well (r² = 0.80). Nevertheless, we found that although observed GPP increased significantly during the study period, and NEE shifted towards a stronger sink, these trends were not captured in the model. Instead, QUINCY showed a significant increasing trend for total ecosystem respiration (TER), that was not present in the observations. The severe drought in 2007 affected observed carbon fluxes strongly, lowering both GPP and TER also in the following year. QUINCY was able to capture some of the decrease in GPP and TER in 2007. However, the legacy effect of the drought in 2008 was not captured by the model. These results call for further work on representing legacy effects in TBMs, as these can have long-lasting impacts on ecosystem functioning.