the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jul 2024
Modelling Herbivory Impacts on Vegetation Structure and Productivity
Abstract. Animal herbivory can have large and diverse impacts on vegetation and hence on the state and function of ecosystems. Despite this, quantitative understanding of vegetation responses to consumption of green leaf tissue by herbivores is currently lacking and presents a critical gap. More and more species are becoming endangered or extinct, whereas ecosystem restoration and rewilding are also increasingly moving into the focus of the scientific community. The large-scale impacts of changes in herbivore abundance on ecosystem function have yet to be investigated. Process-based modelling can help to quantify how animals affect important processes, such as ecosystem carbon cycling. To do so, we linked the dynamic global vegetation model LPJ-GUESS with the Madingley model, a model of multi-trophic functional diversity. This implementation allows us to simulate feedbacks between the availability of green vegetation biomass, herbivory and the whole trophic chain and vice versa. In the coupled model system, we see an overall reduction in ecosystem productivity, leaf area index and carbon mass, compared to the stand-alone version of LPJ-GUESS. The impact of herbivory is most prominently visible in the boreal ecosystems. We evaluated LPJ-GUESS output against remote sensing datasets and flux measurements and find that the coupled LPJ-GUESS/Madingley model preserves LPJ-GUESS’s ability to predict realistic biome distributions and carbon pools.
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RC1: 'Comment on egusphere-2024-1646', Anonymous Referee #1, 27 Oct 2024
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This study is about the coupling of a DGVM with a trophic model and the simulation tests of the foraging effects of animals on vegetation productivity and biomass. It is very interesting study because the effects of animal foraging are largely ignored in DGVMs, though it has been well known that vegetation composition, structure and productivity can be significantly affected by animal foraging.
As the first try, I understand the complexity (and difficulty) of coupling two well-established models, and why the authors chose to use the data files to exchange the information between these two models and the interactions between vegetation biomass and herbivores. However, I still want to see more details of the model mechanisms and processes that drive the vegetation and trophic dynamics through animal foraging.
As I can get from the method section, the browsing of herbivores from the Madingley model acts as a disturbance to vegetation leaf biomass. So, I think it would be necessary for the authors to give more details how the demographic and growth processes respond to the reduction of leaves. For example, do they have a “compensation” mechanism to grow new leaves? Are all the trees affected by animal foraging samely (i.e., the same leaf reduction ratio) or these effects are size-dependent? (because the taller trees have low probability of being eaten by animals compared to the seedlings). Also, how is the size structure and regeneration of vegetation are affected?
In the model, there should have at least two types of key parameters, consumption coefficients and regrowth rates, that are used to describe browsing effects and vegetation responses. More details are needed.
The authors analyzed the vegetation productivity and biomass changes in response to animal foraging. For the trophic model (Madingley), it should have the biomass (or density) of herbivores, omnivores, and carnivores. Is it possible to present their biomass/density as well? I think it would be informative for understanding how the coupled models work and to justify if the results are realistic, because I think the effects of herbivores on forest ecosystems are overestimated in this model. It like surface fire in savanna, once the tree seedlings escape the “browsing trap”, they will not be threatened by animals.
So, I think it needs a comprehensive uncertainty analysis in the mode-data evaluation with a ensemble runs at different key parameters.
Detailed comments:
- Lines 67~82, about the Madingley model, it would be easier for the readers if the consumption rate and energy flow ratio from vegetation to carnivores are described.
- Lines 98 ~ 122, LPJ-GUESS model, I’d like to see how the demographic processes and cohort dynamics are simulated in LPJ-GUESS, because they can be affected by herbivore foraging.
- Lines 140~141, why the evergreen leaves have longer damage effects than deciduous leaves? Why is it related to the leaf lifespan? New leave can grow out anyway.
- Lines 145~150: I am confused here. In the previous section, the authors talked about the damages to leaves. However, here, they talked about the biomass. Does it only include “leaves”? Or, the branches and stems are included?
- Section 2.5 Study setup. I think it would be helpful if the authors conduct a set of ensemble runs with different parameter values at site level and then report the changes in the biomass at different trophic levels. The large spatial scale runs cannot give such details.
- Line 215, Figure 3. The legend of the vegetation types can be the full name. there are enough spaces for that.
- Lines 229~231, Also, I think it is extremely high for the browsing effects on boreal forests. That is why we need to conduct site-level runs with different parameter values. Or, the setting of eatable evergreen leaves needs to be examined carefully.
- Line 238 “no shifts in vegetation compositions”. Savanna can be affected by browsing. Are the PFTs and different sized trees are uniformly affected by animals?
- In Discuss section, I think the interactions between demographic processes and the herbivore browsing should be explained.
- Section 325~340: The discussion should link their results with model processes and assumptions.
Citation: https://doi.org/10.5194/egusphere-2024-1646-RC1
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