Consideration of radiation absorption by stems in forests for microclimate modeling

Béland, Martin; Bonan, Gordon; Kobayashi, Hideki; Baldocchi, Dennis

doi:10.5194/egusphere-2026-2201

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https://doi.org/10.5194/egusphere-2026-2201

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08 May 2026

| 08 May 2026

Status: this preprint is open for discussion and under review for Geoscientific Model Development (GMD).

Consideration of radiation absorption by stems in forests for microclimate modeling

Martin Béland, Gordon Bonan, Hideki Kobayashi, and Dennis Baldocchi

Abstract. Forest canopy models are used to simulate biosphere–atmosphere coupling in global climate models, as well as the microclimate influences of forests at the stand scale. It has recently been shown that wood structures store significant heat following radiation absorption and impact air temperature diurnal patterns inside canopies. Yet, radiation absorption by woody stems is not fully considered in current models. Here we modify the radiative transfer component of the CanVeg2 multilayer canopy model to include radiation absorption by woody stems. We evaluate the model modifications by comparing estimates against a validated 3D ray tracing radiative transfer model parametrized using ground lidar measurements, and against tower observations of albedo in four broadleaf forests. We found a very good agreement between the 1D and 3D models, and a good agreement between models and observations. Our approach provides a tractable and computationally efficient implementation of radiation absorption by woody stems to calculate biomass heat storage in canopy models.

Received: 16 Apr 2026 – Discussion started: 08 May 2026

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Martin Béland, Gordon Bonan, Hideki Kobayashi, and Dennis Baldocchi

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RC1:
'Comment on egusphere-2026-2201', Anonymous Referee #1, 16 May 2026 reply
General Comments: Béland et al. developed the CanVeg2 model, which explicitly incorporates within-canopy woody structures and their storage effects on the forest microclimate. By correcting sub-canopy light and heat conditions, this model holds great potential for improving the predictive capabilities of Land Surface Models (LSMs). While the core scientific question is highly relevant, I believe the current version of the manuscript exhibits two major deficiencies that should be addressed before publication.
Major Concerns:
#1.Context-Specific Efficacy and Model Boundary Conditions: Although this study utilizes four distinct sites to demonstrate the superiority of CanVeg2 in simulating microclimate dynamics, the manuscript fails to clarify when and where the inclusion of woody structures is most critical. The authors should explicitly define the environmental or structural thresholds under which CanVeg2 provides the greatest improvement. For instance, if a site is characterized by a high leaf area index (LAI) but a low wood area index (WAI) or low woody biomass density, does CanVeg2 still outperform traditional models? This implies that CanVeg2's advantages may not be universal, but rather highly dependent on the relative dominance of woody structures in regulating the microclimate. A clearer justification of its contextual applicability is needed.
#2.Impact on Carbon-Water Coupling Processes: The authors should conduct sensitivity experiments or diagnostic simulations to evaluate how the inclusion of woody structure parameters alters carbon and water coupling processes within CanVeg2. Specifically, under conditions of high woody biomass density, what is the quantitative difference in gross primary productivity (GPP) and evapotranspiration (ET) when accounting for stem-level radiation absorption versus ignoring it? Investigating these cascading effects on downstream ecohydrological fluxes would substantially strengthen the physical mechanism and value of the model.

Minor comments:
Abstract: Why is it mentioned that the 1D and 3D models are similar? If they are indeed similar, what is the rationale or necessity for constructing a 3D model? Furthermore, are there any effective approaches or solutions to address this issue?

Site Description: Could you provide a general description of the forest density across different sites? Forest density is highly likely to influence branch and stem distribution, which in turn could impact the PAR simulation.

Figure 2: If the fraction of diffuse radiation increases, would it exert a significant impact on the structure? Currently, the proportion of direct radiation appears to be relatively high.

Figure 5: Although the proposed method shows improvements, a discrepancy still exists between the new broadband method and the ray-tracing method. From what perspectives or aspects can this discrepancy be mitigated or eliminated?

Results and Discussions: I would like to see more extensive discussions under varying scenarios incorporated into the manuscript. For instance, when the biomass/volume of branches and stems increases, in what specific aspects does the improved method enhance simulation accuracy? Alternatively, under different atmospheric conditions—such as varying diffuse radiation fractions, different amounts of incident light, and across different seasons—does the new method yield improvements in simulating PAR and its corresponding vertical profiles?

In summary, additional discussions and results regarding the improvement of model performance should be included in the paper to better demonstrate the advancement and superiority of the proposed model.

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Citation: https://doi.org/10.5194/egusphere-2026-2201-RC1

Martin Béland, Gordon Bonan, Hideki Kobayashi, and Dennis Baldocchi

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https://doi.org/10.5194/egusphere-2026-2201-supplement

Martin Béland, Gordon Bonan, Hideki Kobayashi, and Dennis Baldocchi

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Short summary

Computer models used to simulate forest canopy microclimates rely on radiative transfer simulations. These models commonly only consider tree leaves and exclude wood structures. Here we present modifications to a commonly used model to consider stem absorption. We compared model estimates to detailed simulations using 3D data from ground lidar at four deciduous forest sites. We also compared simulations against field observations of sunlight reflected by canopies.


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