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. Theconcentration–carbon feedback ( β ), also called the CO2 fertilization effect, is a key unknown in climate–carbon-cycle projections.A better understanding of model mechanisms that govern terrestrial ecosystemresponses to elevated CO2 is urgently needed to enable a moreaccurate prediction of future terrestrial carbon sink. We conducted C-only,carbon–nitrogen (C–N) and carbon–nitrogen–phosphorus (C–N–P)simulations of the Community Atmosphere Biosphere Land Exchange model (CABLE)from 1901 to 2100 with fixed climate to identify the most critical modelprocess that causes divergence in β . We calculated CO2 fertilization effects at various hierarchical levels from leaf biochemicalreaction and leaf photosynthesis to canopy gross primary production (GPP),net primary production (NPP), and ecosystem carbon storage (cpool) forseven C3 plant functional types (PFTs) in response to increasing CO2 under the RCP 8.5 scenario.Our results show that β values at biochemical and leaf photosynthesislevels vary little across the seven PFTs, but greatly diverge at canopy andecosystem levels in all simulations. The low variation of the leaf-level β is consistent with a theoretical analysis that leaf photosyntheticsensitivity to increasing CO2 concentration is almost an invariantfunction. In the CABLE model, the major jump in variation of β valuesfrom leaf levels to canopy and ecosystem levels results from divergence inmodeled leaf area index (LAI) within and among PFTs. The correlation of βGPP , βNPP , or βcpool each with βLAI is very high in all simulations. Overall,our results indicate that modeled LAI is a key factor causing the divergencein β in the CABLE model. It is therefore urgent to constrain processesthat regulate LAI dynamics in order to better represent the response ofecosystem productivity to increasing CO2 in Earth system models.

Leaf area index identified as a major source of variability in modeled CO&lt;sub&gt;2&lt;/sub&gt; fertilization

Leaf area index identified as a major source of variability in modeled CO<sub>2</sub> fertilization