Daniel M. Ricciuto

Peatlands cover only ~3 % of Earth’s land surface yet store ~30 % of global soil carbon (C), making them critical components of the terrestrial C cycle and influential regulators of C-climate feedbacks. However, their responses to climate warming and elevated CO₂ remain highly uncertain, in part because Earth system models represent peatland processes with varying levels of complexity and realism, and because peat C pools turn over on centennial to millennial timescales that challenge model evaluation. Here, we present results from the SPRUCE (Spruce and Peatland Responses Under Changing Environments) Model Intercomparison Project (SPRUCE-MIP), which evaluates 15 terrestrial ecosystem models against observations from a long-term whole-ecosystem warming and CO₂-enrichment experiment at an ombrotrophic bog in northern Minnesota, USA. Models were driven by observed meteorology from in situ warming treatments spanning +0 to +9 °C, at either ambient or elevated CO₂ (+500 ppm). Simulated net ecosystem exchange (NEE), net primary productivity (NPP), heterotrophic respiration (HR), and methane (CH₄) fluxes were benchmarked against multi-year observations. Model predictions exhibit a large spread in both baseline C balance, ranging from strong sinks to strong sources under +0 °C warming, and temperature sensitivity, indicating substantial uncertainty in predicting peatland C responses to environmental forcing. Across the ensemble, models fall into four distinct functional response types: (i) models that simulate peatlands as net C sources across all warming and CO₂ conditions; (ii) models that transition from C sinks to sources under warming irrespective of CO₂ level; (iii) models that transition only under ambient CO₂ but remain C sinks under elevated CO₂, reflecting strong CO₂ fertilization effects; and (iv) models that maintain persistent sinks or near-neutral C balance even under extreme warming. While most models predict enhanced NPP under elevated CO₂ across all warming treatments, SPRUCE observations show little or no NPP enhancement under elevated CO₂ at the +0 and +2.25 °C warming levels, with a positive CO₂ fertilization effect emerging only under stronger warming. This discrepancy highlights persistent model biases in representing interactions between warming and CO₂ responses. Process-based analysis further indicates that divergence in modeled responses is associated with differences in vegetation structure (light competition and moss representation), nutrient cycling (nitrogen and phosphorus), and dynamic peat representation. These structural differences systematically influence ecosystem productivity, heterotrophic respiration, and net C balance across the model ensemble. An illustrative parameter sensitivity analysis using the SPRUCE-specific Energy Exascale Earth System Model (E3SM) Land Model (ELM-SPRUCE) further shows that parameter choices can also modulate the magnitude of simulated responses. Together, these results demonstrate that peatland responses to warming and elevated CO₂ are highly sensitive to model structure, parameterization, and process coupling, highlighting the need for improved representation of key peatland processes to reduce uncertainty in Earth system projections. The SPRUCE experimental framework provides a unique benchmark for evaluation and improving process representation and constraining near-term peatland response to environmental change, thereby strengthening confidence in longer-term C-climate projections.

Peatlands store one-third of the global soil carbon (C) on only ∼3% of Earth’s land surface but risk becoming C sources to accelerate climate change due to warming-induced C loss. Elevated CO ₂ (eCO ₂ ) may mitigate this loss by boosting plant C uptake. However, how eCO ₂ and climate warming interact to influence peatland C dynamics remains unclear. Here, we investigated peatland C fate at five warming levels (+0°, +2.25°, +4.5°, +6.75°, and + 9°C), each under ambient CO ₂ (aCO ₂ ) and eCO ₂ (+500 parts per million), by model-guided synthesis of 18 measured variables for each of the 10 treatments from an experiment. Our results showed that the peatland C loss increased with warming at a rate of 62.6 g C m ⁻² year ⁻¹ °C ⁻¹ , reaching 476.5 ± 104.7 g C m ⁻² year ⁻¹ at +9°C under aCO ₂ , primarily because of increased Sphagnum mortality and accelerated decomposition. eCO ₂ reduced the loss rate to 17.3 g C m ⁻² year ⁻¹ °C ⁻¹ , nearly halving the C loss at +9°C (269.4 ± 80.5 g C m ⁻² year ⁻¹ ), mainly due to enhanced tree and shrub growth. Methane emissions increased with warming, similarly under aCO ₂ and eCO ₂ . Our findings suggest that northern peatlands likely become major C sources under climate warming, although rising atmospheric CO ₂ concentration may partially slow down the C loss. Climate-driven shifts in plant community structure, rather than peat decomposition alone, play a critical role in regulating peatland C fate and may threaten the stability of peatland ecosystem functions and services in the future.

Peatlands cover only ~3 % of Earth’s land surface yet store ~30 % of global soil carbon (C), making them critical components of the terrestrial C cycle and influential regulators of C-climate feedbacks. However, their responses to climate warming and elevated CO₂ remain highly uncertain, in part because Earth system models represent peatland processes with varying levels of complexity and realism, and because peat C pools turn over on centennial to millennial timescales that challenge model evaluation. Here, we present results from the SPRUCE (Spruce and Peatland Responses Under Changing Environments) Model Intercomparison Project (SPRUCE-MIP), which evaluates 15 terrestrial ecosystem models against observations from a long-term whole-ecosystem warming and CO₂-enrichment experiment at an ombrotrophic bog in northern Minnesota, USA. Models were driven by observed meteorology from in situ warming treatments spanning +0 to +9 °C, at either ambient or elevated CO₂ (+500 ppm). Simulated net ecosystem exchange (NEE), net primary productivity (NPP), heterotrophic respiration (HR), and methane (CH₄) fluxes were benchmarked against multi-year observations. Model predictions exhibit a large spread in both baseline C balance, ranging from strong sinks to strong sources under +0 °C warming, and temperature sensitivity, indicating substantial uncertainty in predicting peatland C responses to environmental forcing. Across the ensemble, models fall into four distinct functional response types: (i) models that simulate peatlands as net C sources across all warming and CO₂ conditions; (ii) models that transition from C sinks to sources under warming irrespective of CO₂ level; (iii) models that transition only under ambient CO₂ but remain C sinks under elevated CO₂, reflecting strong CO₂ fertilization effects; and (iv) models that maintain persistent sinks or near-neutral C balance even under extreme warming. While most models predict enhanced NPP under elevated CO₂ across all warming treatments, SPRUCE observations show little or no NPP enhancement under elevated CO₂ at the +0 and +2.25 °C warming levels, with a positive CO₂ fertilization effect emerging only under stronger warming. This discrepancy highlights persistent model biases in representing interactions between warming and CO₂ responses. Process-based analysis further indicates that divergence in modeled responses is associated with differences in vegetation structure (light competition and moss representation), nutrient cycling (nitrogen and phosphorus), and dynamic peat representation. These structural differences systematically influence ecosystem productivity, heterotrophic respiration, and net C balance across the model ensemble. An illustrative parameter sensitivity analysis using the SPRUCE-specific Energy Exascale Earth System Model (E3SM) Land Model (ELM-SPRUCE) further shows that parameter choices can also modulate the magnitude of simulated responses. Together, these results demonstrate that peatland responses to warming and elevated CO₂ are highly sensitive to model structure, parameterization, and process coupling, highlighting the need for improved representation of key peatland processes to reduce uncertainty in Earth system projections. The SPRUCE experimental framework provides a unique benchmark for evaluation and improving process representation and constraining near-term peatland response to environmental change, thereby strengthening confidence in longer-term C-climate projections.

Land models typically represent the entire fine-root system as a single homogeneous pool, collapsing structural and functional complexity into one set of parameters. This simplification weakens the belowground feedbacks that balance source-driven carbon dynamics. Here, we integrate the TAM (Transport and Absorptive fine roots with Mycorrhizal fungi) framework into the E3SM Land Model (ELM), replacing the single fine-root pool with three pools — transport roots, absorptive roots, and mycorrhizal fungi — defined by their roles in resource conduction, nutrient acquisition, and symbiotic exchange, each governed by pool-specific C : N ratios, longevity, allocation coefficients, and litter chemistry (21 new parameters). Sobol sensitivity analysis at 13 FLUXNET sites spanning 13 natural PFTs in ELM reveals a two-strategy parameterization dichotomy: deciduous and grass plant functional types (PFTs) concentrate variance in 2–3 allocation parameters, while evergreen PFTs distribute sensitivity across 5–6 parameters spanning chemistry, longevity, and allocation. Surrogate-assisted Bayesian calibration produces emergent fine-root properties that diverge from ELM defaults, with system-level nitrogen demand increasing at 10 of 14 PFTs (up to 4.3 ×) and absorptive-root longevity spanning 0.5–3.5 yr. A three-tier validation framework—monthly evaluation, temporal out-of-sample, and spatial out-of-sample—demonstrates that these changes yield transferable improvements: improved seasonal amplitude fidelity (mean GPP amplitude ratio 0.99 vs. 1.38 for the baseline), near-elimination of ER bias at a deciduous site (MBE from + 515 to + 33 gC m⁻² yr⁻¹), and cross-continental parameter transfer for C4 grasses (81 % RMSE reduction). Degradation at evergreen sites where the baseline overestimates productivity identifies a scope boundary where a belowground-only improvement cannot remedy upstream source-side bias. The function-oriented design provides natural coupling points for future extensions, including mechanistic root–water interactions and depth-resolved root profiles. ELM-TAM demonstrates that resolving belowground structural heterogeneity strengthens sink-based controls on the carbon cycle without overparameterization, establishing a foundation for these extensions and for global benchmarking in Earth System land models.

Peatlands cover only ~3 % of Earth’s land surface yet store ~30 % of global soil carbon (C), making them critical components of the terrestrial C cycle and influential regulators of C-climate feedbacks. However, their responses to climate warming and elevated CO₂ remain highly uncertain, in part because Earth system models represent peatland processes with varying levels of complexity and realism, and because peat C pools turn over on centennial to millennial timescales that challenge model evaluation. Here, we present results from the SPRUCE (Spruce and Peatland Responses Under Changing Environments) Model Intercomparison Project (SPRUCE-MIP), which evaluates 15 terrestrial ecosystem models against observations from a long-term whole-ecosystem warming and CO₂-enrichment experiment at an ombrotrophic bog in northern Minnesota, USA. Models were driven by observed meteorology from in situ warming treatments spanning +0 to +9 °C, at either ambient or elevated CO₂ (+500 ppm). Simulated net ecosystem exchange (NEE), net primary productivity (NPP), heterotrophic respiration (HR), and methane (CH₄) fluxes were benchmarked against multi-year observations. Model predictions exhibit a large spread in both baseline C balance, ranging from strong sinks to strong sources under +0 °C warming, and temperature sensitivity, indicating substantial uncertainty in predicting peatland C responses to environmental forcing. Across the ensemble, models fall into four distinct functional response types: (i) models that simulate peatlands as net C sources across all warming and CO₂ conditions; (ii) models that transition from C sinks to sources under warming irrespective of CO₂ level; (iii) models that transition only under ambient CO₂ but remain C sinks under elevated CO₂, reflecting strong CO₂ fertilization effects; and (iv) models that maintain persistent sinks or near-neutral C balance even under extreme warming. While most models predict enhanced NPP under elevated CO₂ across all warming treatments, SPRUCE observations show little or no NPP enhancement under elevated CO₂ at the +0 and +2.25 °C warming levels, with a positive CO₂ fertilization effect emerging only under stronger warming. This discrepancy highlights persistent model biases in representing interactions between warming and CO₂ responses. Process-based analysis further indicates that divergence in modeled responses is associated with differences in vegetation structure (light competition and moss representation), nutrient cycling (nitrogen and phosphorus), and dynamic peat representation. These structural differences systematically influence ecosystem productivity, heterotrophic respiration, and net C balance across the model ensemble. An illustrative parameter sensitivity analysis using the SPRUCE-specific Energy Exascale Earth System Model (E3SM) Land Model (ELM-SPRUCE) further shows that parameter choices can also modulate the magnitude of simulated responses. Together, these results demonstrate that peatland responses to warming and elevated CO2 are highly sensitive to model structure, parameterization, and process coupling, highlighting the need for improved representation of key peatland processes to reduce uncertainty in Earth system projections. The SPRUCE experimental framework provides a unique benchmark for evaluation and improving process representation and constraining near-term peatland response to environmental change, thereby strengthening confidence in longer-term C-climate projections.

Land models typically represent the entire fine-root system as a single homogeneous pool, collapsing structural and functional complexity into one set of parameters. This simplification weakens the belowground feedbacks that balance source-driven carbon dynamics. Here, we integrate the TAM (Transport and Absorptive fine roots with Mycorrhizal fungi) framework into the E3SM Land Model (ELM), replacing the single fine-root pool with three pools — transport roots, absorptive roots, and mycorrhizal fungi — defined by their roles in resource conduction, nutrient acquisition, and symbiotic exchange, each governed by pool-specific C : N ratios, longevity, allocation coefficients, and litter chemistry (21 new parameters). Sobol sensitivity analysis at 13 FLUXNET sites spanning 13 natural PFTs in ELM reveals a two-strategy parameterization dichotomy: deciduous and grass plant functional types (PFTs) concentrate variance in 2–3 allocation parameters, while evergreen PFTs distribute sensitivity across 5–6 parameters spanning chemistry, longevity, and allocation. Surrogate-assisted Bayesian calibration produces emergent fine-root properties that diverge from ELM defaults, with system-level nitrogen demand increasing at 10 of 14 PFTs (up to 4.3 ×) and absorptive-root longevity spanning 0.5–3.5 yr. A three-tier validation framework—monthly evaluation, temporal out-of-sample, and spatial out-of-sample—demonstrates that these changes yield transferable improvements: improved seasonal amplitude fidelity (mean GPP amplitude ratio 0.99 vs. 1.38 for the baseline), near-elimination of ER bias at a deciduous site (MBE from + 515 to + 33 gC m⁻² yr⁻¹), and cross-continental parameter transfer for C4 grasses (81 % RMSE reduction). Degradation at evergreen sites where the baseline overestimates productivity identifies a scope boundary where a belowground-only improvement cannot remedy upstream source-side bias. The function-oriented design provides natural coupling points for future extensions, including mechanistic root–water interactions and depth-resolved root profiles. ELM-TAM demonstrates that resolving belowground structural heterogeneity strengthens sink-based controls on the carbon cycle without overparameterization, establishing a foundation for these extensions and for global benchmarking in Earth System land models.

Land models typically represent the entire fine-root system as a single homogeneous pool, collapsing structural and functional complexity into one set of parameters. This simplification weakens the belowground feedbacks that balance source-driven carbon dynamics. Here, we integrate the TAM (Transport and Absorptive fine roots with Mycorrhizal fungi) framework into the E3SM Land Model (ELM), replacing the single fine-root pool with three pools — transport roots, absorptive roots, and mycorrhizal fungi — defined by their roles in resource conduction, nutrient acquisition, and symbiotic exchange, each governed by pool-specific C : N ratios, longevity, allocation coefficients, and litter chemistry (21 new parameters). Sobol sensitivity analysis at 13 FLUXNET sites spanning 13 natural PFTs in ELM reveals a two-strategy parameterization dichotomy: deciduous and grass plant functional types (PFTs) concentrate variance in 2–3 allocation parameters, while evergreen PFTs distribute sensitivity across 5–6 parameters spanning chemistry, longevity, and allocation. Surrogate-assisted Bayesian calibration produces emergent fine-root properties that diverge from ELM defaults, with system-level nitrogen demand increasing at 10 of 14 PFTs (up to 4.3 ×) and absorptive-root longevity spanning 0.5–3.5 yr. A three-tier validation framework—monthly evaluation, temporal out-of-sample, and spatial out-of-sample—demonstrates that these changes yield transferable improvements: improved seasonal amplitude fidelity (mean GPP amplitude ratio 0.99 vs. 1.38 for the baseline), near-elimination of ER bias at a deciduous site (MBE from + 515 to + 33 gC m⁻² yr⁻¹), and cross-continental parameter transfer for C4 grasses (81 % RMSE reduction). Degradation at evergreen sites where the baseline overestimates productivity identifies a scope boundary where a belowground-only improvement cannot remedy upstream source-side bias. The function-oriented design provides natural coupling points for future extensions, including mechanistic root–water interactions and depth-resolved root profiles. ELM-TAM demonstrates that resolving belowground structural heterogeneity strengthens sink-based controls on the carbon cycle without overparameterization, establishing a foundation for these extensions and for global benchmarking in Earth System land models.

Land models typically represent the entire fine-root system as a single homogeneous pool, collapsing structural and functional complexity into one set of parameters. This simplification weakens the belowground feedbacks that balance source-driven carbon dynamics. Here, we integrate the TAM (Transport and Absorptive fine roots with Mycorrhizal fungi) framework into the E3SM Land Model (ELM), replacing the single fine-root pool with three pools — transport roots, absorptive roots, and mycorrhizal fungi — defined by their roles in resource conduction, nutrient acquisition, and symbiotic exchange, each governed by pool-specific C : N ratios, longevity, allocation coefficients, and litter chemistry (21 new parameters). Sobol sensitivity analysis at 13 FLUXNET sites spanning 13 natural PFTs in ELM reveals a two-strategy parameterization dichotomy: deciduous and grass plant functional types (PFTs) concentrate variance in 2–3 allocation parameters, while evergreen PFTs distribute sensitivity across 5–6 parameters spanning chemistry, longevity, and allocation. Surrogate-assisted Bayesian calibration produces emergent fine-root properties that diverge from ELM defaults, with system-level nitrogen demand increasing at 10 of 14 PFTs (up to 4.3 ×) and absorptive-root longevity spanning 0.5–3.5 yr. A three-tier validation framework—monthly evaluation, temporal out-of-sample, and spatial out-of-sample—demonstrates that these changes yield transferable improvements: improved seasonal amplitude fidelity (mean GPP amplitude ratio 0.99 vs. 1.38 for the baseline), near-elimination of ER bias at a deciduous site (MBE from + 515 to + 33 gC m⁻² yr⁻¹), and cross-continental parameter transfer for C4 grasses (81 % RMSE reduction). Degradation at evergreen sites where the baseline overestimates productivity identifies a scope boundary where a belowground-only improvement cannot remedy upstream source-side bias. The function-oriented design provides natural coupling points for future extensions, including mechanistic root–water interactions and depth-resolved root profiles. ELM-TAM demonstrates that resolving belowground structural heterogeneity strengthens sink-based controls on the carbon cycle without overparameterization, establishing a foundation for these extensions and for global benchmarking in Earth System land models.

Boreal peatlands store 13 %–32 % of the global soil carbon (C) stock, a service dependent on plant-mycorrhizal fungi associations. In these nutrient poor systems, ectomycorrhizal and ericoid mycorrhizal fungi supply up to >80 % of the nutrient requirements of their plant hosts, partly with mined nitrogen (N) and phosphorus (P) from soil organic matter that are otherwise inaccessible to plants. Despite the ecological significance, mycorrhizal associations are only represented in a few land surface or ecosystem models. We modify the peatland branch of version 2 of the Energy Exascale Earth System Land Model (ELMv2-SPRUCE) to replace the default photosynthesis-driven inorganic N and P (NP) uptake process with a more realistic representation of the process via three pathways: (1) direct inorganic NP uptake by uncolonized fine roots, (2) indirect inorganic NP acquisition and (3) indirect NP acquisition from organic sources by mycorrhizal roots. We systematically evaluated the performance of the default and modified models with field observations from a whole ecosystem warming and carbon dioxide fertilization experimental site: Spruce and Peatland Responses Under Changing Environment (SPRUCE), in northern Minnesota, USA. The modified model reduces the underestimation of the growth response of shrubs in the default model to warming from 40 %–80 % to 17 %–35 % and reduces the overall relative absolute error on C fluxes from 1.61 to 1.54 in calibration. Improvements on modeled shrub growths and shrub-moss community net ecosystem exchanges are also seen in validation. The improved growth response of shrubs to warming is accompanied by several-fold increase in direct inorganic NP uptake and decrease in fungal colonization rate. The modified model simulates a smaller magnitude of transition of the ecosystem from C sink to C source under warming due to alleviation of plant nutrient limitation. Equifinality analysis shows the newly added parameters in the modified model can be constrained by the observed C fluxes. Sensitivity analysis shows the newly added parameters have stronger statistical interactions than the preexisting parameters in the default model. Overall, the modified model is an improvement over the default ELMv2-SPRUCE and will be a useful tool for understanding boreal peatland change.