the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Synchrony of African rainforest solar induced chlorophyll fluorescence and environmental factors
Abstract. Global atmospheric carbon dioxide concentrations are largely driven by terrestrial photosynthesis, of which tropical forests account for one third. Relative to other tropical regions, less is known about the seasonality of African tropical forest productivity and its synchrony with environmental factors due to a lack of in situ carbon flux data. To help fill this knowledge gap, we use spaceborne solar-induced chlorophyll fluorescence (SIF), vegetation indices, and climate data to investigate the seasonality and synchrony of photosynthesis in Africa’s tropical forest ecoregions. We find West African SIF to increase during the dry season and peak prior to precipitation, as has been observed in the Amazon. In Central Africa, we find a continental-scale bimodal seasonality in SIF, the minimum of which is synchronous with precipitation, but its maximum is likely less related to environmental drivers.
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RC1: 'Comment on egusphere-2023-3022', Anonymous Referee #1, 07 Jun 2024
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Main comments
The authors aim to link seasonality, minima and maxima of SIF and vegetation indices to environmental factors (precipitation, PAR and VPD). This is I think very valuable, but the manuscript and data analysis should be improved considerably. Firstly the abstract and the title only refers to SIF, while in the text also EVI and NDVI was mentioned. However, EVI is discussed and NDVI not or much less. In the M&M the pro’s and con’s of EVI, NDVI, NIRv and LSWI vegetation indices are mentioned, but in the Results it is not fully justified why EVI mosty is used and not the other three.
Discussion of the data is generally vague and often speculative. As a consequence, the relation (i.e. synchrony) with other environmental drivers (VPD, PAR) cannot be claimed/justified. This results a bit in overselling the paper.
It is also surprising that the discussion section is focused one comparison with the Amazon, which I thought was not the aim of the manuscript.
Finally, I think also more detailed and rigorous data analyses can be done, e.g. by, zero order and partial correlations. As such, as example, the response of precipitation controlled for PAR and/or VPD can be examined. I think this will add much more information to the discussion. This will allow a more rigorous and better structured discussion. The discussion is now very descriptive, mostly vague and sometimes speculative discussion.
In addition, most of the ecoregions studied have 1700 mm or less of rainfall, this should also be discussed, as this is at low end for an evergreen tropical forest.
Specific comments
Line 12: I think also fossil fuel emissions drive CO3 concentrations.
Line 22: the northern hemispheric biosphere is the key driver for intra-annual variation in CO2 concentrations.
Line 24: Carbon store? Write “carbon stock”.
Line 25, 26: It is rather the regional water cycle for each tropical basin. Please add some more recent references here.
Line 34: I think the main conclusion is rather constant carbon gain for African intact tropical forest, which diverting from the Amazon.
Line 40: is this really the case? I think you need to down turn this statement a bit as we don’t have the complete evidence for this.
Line 41-43: What is “Congolian”? Write Congo basin.
Line 51: What about mortality? Was this not observed?
Line 53: What you mean with “coastal forests”?
Line 56: SIF is I think an indirect observation and hence still a proxy for productivity. Can you elaborate here.
Line 59: Leaf physiology: can you be more specific.
Line 66: VPD and temperature are linked; please make clear in the text
Line 82 can you reformulate this hypothesis as not al forest in Africa you test are “moist”, see Table S1.
Line 107: is 735-758 correct, i.e. it is not 743-758 nm?
Line 111: in the Results you also present soil moisture data. It is not clear how this is calculated from the Materials and Methods section.
Line 127: you indicate 4 vegetation indices and there pro’ s and con’s. But in the Results you focus on EVI and a little bit on NDVI. Please explain/justify better your choice.
Line 147: why this selection of African forest types and not the one proposed by Réjou-Méchain et al. 2021, Nature ?
Line 152-154 : Where do we see this ? Some of the text here is also redundant.
Line 155: I though the classification in 8 11 regions was done a priori and not because of the seasonality.
Line 157: Elaborate Beter how were seasonality can be seen, i.e. by better referring to figure 1. But the legend and caption in figure 1 is not clear and needs to be improved.
Line 170 etc. In this section moments of the year are given. However, I think this should be done more precise be giving months of the year and not expressions like “mid-year”, ect.
Figure 2: The caption is not complete as also VPD is shown here. Justify why only “TROPOMI SIF” is shown (also valid for Figure 3).
Line 194 etc. In this section a mostly qualitative description is given of synchrony with precipitation, PAR and VPD. I think more efforts can be done to make these relationships more quantitative. Hence my suggestion on top for zero order and partial correlations,… And I am sure other techniques exists.
Figure 4 and related text: I do not understand why a difference in correlations is proposed here; why VPD + Precip is given, etc.
Figure 5: can you explain why these periods are chosen?
Figure 6: the caption of this figure needs to be improved to read the figure independently.
Line 269: It is surprising to see that the discussion is now mostly geared towards a comparison with the Amazon. Is this realy the scope of the paper? And than suddenly the discussion is on PAR and VPD, while this was not emphasised in the introduction of the paper or the results section.
Line 305, but where are the physiology and phenology data? You refer here to the vegetation indices? This is not clear.
Citation: https://doi.org/10.5194/egusphere-2023-3022-RC1
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