| 20 Oct 2026
Canopy Structure Amplifies the Role of Radiation in Vegetation Responses Under Stratospheric Aerosol Injection
Abstract. Stratospheric aerosol injection (SAI) is a climate intervention approach that would inject aerosols into the stratosphere, reflecting a fraction of incoming sunlight and reducing surface temperatures to mitigate impacts of climate change. While ecological studies of SAI have largely focused on temperature and precipitation, observations following volcanic eruptions suggest that radiation changes may be a primary driver of vegetation responses to atmospheric aerosols. Here, we leverage two global land surface models with different complexity to isolate the effects of radiation under SAI. The big-leaf model, which represents vegetation as a single bulk canopy, suggests that radiation has little influence on vegetation, whereas the cohort-based model, which resolves vegetation size structure and canopy stratification, finds that radiation changes are a major driver of plant responses. In the cohort-based model, radiation changes increase global net primary production by 3.9% for broadleaf trees, 9.7% for needleleaf trees, and 2.6% for C3 grasses, but reduce it by 1.3% for C4 grasses, while the big-leaf model shows little overall response to radiation. These findings suggest that vegetation and agricultural impact studies that do not explicitly account for changes in radiation, or rely on simplified big leaf models, may not capture the full response to SAI.
Clark and Visioni conducted a modelling study to investigate how canopy structure influences terrestrial productivity under a stratospheric aerosol injection scenario. While the topic is interesting, I am concerned that the authors fail to disentangle the underlying mechanisms due to an unclear model setup. The NPP differences between CLM and FATES may arise not only from canopy structure but also from differences in carbon allocation, competition for water and nutrients, vegetation dynamics, respiration, and forcing differences (N deposition, land cover change, etc.). However, the authors provide no convincing evidence that radiation is the primary driver of these differences rather than the factors mentioned above. Furthermore, the authors attribute the NPP increases to reduced leaf temperature and VPD resulting from aerosol-induced radiation changes (although the VPD differences are very small, <0.02 kPa, in Fig. S1). This attribution seems rather speculative without support from sensitivity experiments. Overall, I think the results only demonstrate that FATES exhibits different NPP responses from CLM under the stratospheric aerosol injection scenario, while the attribution of these differences to the explicit representation of canopy structure remains insufficiently supported.
I would also point out that although CLM does not explicitly represent light competition among cohorts, it still accounts for the vertical distribution of radiation within the canopy, with the highest photosynthetic rates generally occurring near the canopy top. I therefore question the robustness of the authors' fundamental argument that "big-leaf models", which typically distinguish sunlit and shaded leaves and account for canopy radiation extinction following Beer's law, fail to adequately represent canopy radiation processes.
The sensitivity experiments are very unclear. For example, what is the ratio ‘r’ indicating in L216? How did authors isolate the effects of leaf temperature changes that induced by atmospheric temperature or by weakened radiation? What’s the differences between the “total radiation” and “radiation combined” in Fig. 2? If FATES and CLM also exhibit substantial differences in their responses to temperature and precipitation (Fig. 3), how can the authors justify attributing the radiation-induced differences solely to canopy structure rather than to other differences between the models?