Modelling mature temperate forest responses to elevated CO2 and changing climatic conditions: insights from the BIFoR FACE experiment

Climate change has been occurring at a rapid rate and is being exacerbated by anthropogenic activities that increase global temperatures and atmospheric concentrations of greenhouse gases such as CO2. This greatly impacts ecosystems worldwide, resulting in more frequent and intense extreme weather events such as heat waves and drought. Understanding how ecosystems respond to elevated CO2 is critical for predicting the impacts of climate change on ecosystem processes, such as their ability to sequester carbon. Temperate ecosystems, in particular, are important in mitigating climate change, holding around 20% of the global plant biomass and approximately 10% of the global terrestrial carbon (Bonan, 2008). However, the capacity of these ecosystems to continue sequestering additional carbon dioxide in the future is uncertain when predicted using current terrestrial biosphere models (TBMs). To address this, improved mechanistic representations of ecosystem states and processes under changing climatic conditions are crucial, as well as the initialisation of the models using real-world observations. In this regard, ecosystem-scale experiments, such as Free-air CO2 enrichment (FACE) experiments, are extremely useful and powerful tools for improving model predictions and have frequently been used for model-data synthesis and ecosystem analysis (Walker et al, 2015). 

In this study, we examined the responses of mature temperate forests to rising atmospheric CO2 and changing climatic conditions using the Ecosystem Demography model (ED2), which is a cohort-based terrestrial biosphere model (TBM). We parameterised the model with data collected from the Birmingham Institute of Forest Research, Free-air CO2 Enrichment (BIFoR FACE) experiment site. As the first study using a TBM at BIFoR, this study analysed the model’s capacity to simulate ecosystem responses to elevated CO2 (+150 ppm above ambient) and extreme weather events such as the European drought of 2022 (Gharun et al, 2024). We ran two simulations and compared model outputs against field measurements of key eco-physiological measurements such as maximum rate of carboxylation, soil moisture, and Net Primary Production (NPP). This study demonstrates the capability and the limitations of the TBM to simulate the responses of a mature temperate forest to elevated CO2 conditions under changing and extreme climatic conditions.