| 06 Aug 2025
Photosynthesis of tropical Andean tree species: Insights from mechanistic modelling and sensitivity analysis
Abstract. Andean tropical montane forests are highly biodiverse ecosystems with a carbon storage capacity comparable to lowland forests. However, their response to climate change remains uncertain, as species photosynthesis depends on their thermal acclimation capacity. This study evaluates the variability of photosynthetic traits across montane and lowland tree species using a leaf level photosynthesis model and data from a transplant experiment across three elevations (14 °C, 22 °C, and 26 °C) in the tropical Andes. Six montane species and two lowland species were analyzed to assess photosynthetic responses to environmental conditions. We find that intraspecific variability in photosynthetic parameters, such as the apparent maximum carboxylation capacity (Vcmax) and the apparent maximum electron transport rate (Jmax), is key to accurately model photosynthesis in these ecosystems. Apparent Vcmax was identified as the primary determinant of diurnal variations in photosynthesis, especially under varying thermal environments. Additionally, stomatal conductance (g1) was highly variable and responded to vapor pressure deficit (VPD), suggesting that stomatal regulation is crucial for adaptation to environmental changes. Sensitivity analysis revealed that at higher altitudes (14 °C), photosynthetically active radiation (PAR) and temperature were the main limiting factors for photosynthesis, while at lower altitudes (22 °C), VPD was the dominant factor. Finally, the study demonstrates that the common use, within global vegetation models, of average parameters from lowland species to simulate montane forest is inadequate as such parameterizations tend to underestimate montane forest photosynthesis by up to 65 %. It is also recommended that vegetation models incorporate both intra- and interspecific variability to improve predictions of the carbon cycle in tropical Andean forests and their response to climate change.
González-Caro et al.
The authors seek to determine the sensitivity to climate as well as the appropriate parameterisation for photosynthesis of tropical Andean trees. They use trees from different elevations of origin transplanted across 2 different elevational regimes (elevations, not temperatures!) to determine photosynthetic model parameters for their saplings for 8 species. They intend to simulate leaf net C uptake and then determine physiological states contributing to these C fluxes and the environmental limitations for this uptake. They consider three environmental drivers (light/PAR, VPD and temperature) for their model, fit some of the parameters with a subset of the data, and estimate the relative contribution of these drivers to simulated net carbon uptake. Advantages of the study include that it pertains to a unique biological system of Andean forests, the deep extent of physiological modelling, and the partitioning of key environmental variables. It is a very valid point that tropical forest is taken by models to signify both montane and lowland tropical forests, and an outcome of the study should help identify relevant model parameters for montane tropical forests in South America.
There are a number of issues in the study for the authors to resolve, and I make 7 key points here about flaws in the manuscript, and raise others in the detailed comments. 1) Quite a few aspects of the study are glossed over and not presented in enough detail (see the many detailed comments below). Central to this is whether the temperature kinetics of Vcmax and Jmax were determined or not, given that this is how acclimation to temperature is often defined. There don’t appear to be any temperature kinetics in the modelling, or some default is assumed but not tested. 2) The target of assigning limitations is a set of diurnal measurements of gas exchange, which aren’t actually shown in the manuscript nor is how well the modelling recovers the actual net C uptake evaluated. So the assignment of limitations assessed in Figures 4 and 5 cannot be evaluated. We also don’t know anything about the environmental conditions on the particular days of measurements as this isn’t shown, nor do we know if these days were the same with simultaneous measurements at the two sites, or whether they were done on different days, and we don’t know if these measurements days were representative of the respective elevations. This seems like modelling on a blind target. 3) The writing uses good English but incomplete descriptions and explanations. 4) The sites and species are not described much if at all. The sites are improperly described by temperatures rather than the physical elevations where they occurred. 5) Much of the text in the Intro and Discussion is given over to modelling these ecosystems, yet all the data is from young trees. How in the Intro and Discussion are we to make inferences about mature trees from this? I know that mature trees cannot be transplanted, but did anyone look at differences from the juvenile (sapling) to mature phase? If not, then please do not include text about mature forest biomass and about modelling net CO2 uptake of Andean forests (most of which are mature, not planted). 6) there are no hypotheses grounding the study, and 7) the Intro section does not describe past efforts involving limitations to net CO2 uptake and modelling them, even though this is a major feature of the manuscript.
The Intro contains a set of disjunct arguments around tree adaptations to climate focussed on photosynthesis. The arguments are not explained well and often not grounded in reality, so the basis for the work isn’t established very well. For example, the authors set up a strange disconnect between biomass per unit land area and photosynthesis per unit leaf area, without elaborating on the large conceptual distance between these two things and the number of processes that intervene between the two. The authors variously seem to argue that species that are temperate or tropical do not show local variation in photosynthetic characteristics with climate, or that local species photosynthetic traits are very important. In the Intro, the Cuni-Sanchez paper that is cited isn't about biomass at all. And in Duque et al. (2021) Table 1, there is a clear monotonic decline in biomass (Mg C ha-1) from 89 at 500-1200 metres to 59 at 2800-3600 metres, not what the authors say in terms of a lack of biomass differences with elevation. Please explain these properly.
Such logical disconnects throughout the Intro need to be reconciled in a uniform, stream-lined text. I would suggest eliminating l. 36 up to line 43 and stick to photosynthesis in the Intro, which after all is the actual topic of the manuscript.
Detailed comments
Imagine that photosynthesis is reduced by high temperatures. Is this a 'thermal limitations to photosynthesis'? What is 'extreme warming' and why does it is extreme when climate is hotter? Isn't climate just hotter on average?
I think I know the answers but my point is that these things are poorly if even slighly explained, yet underpin most of the study and don't seem well developed in this manuscript as yet.
The species are never stated in the text (they are listed in Table 1), and this seems like a must. There is no description of the species either, they could be herbs or trees, this is totally unclear.
Then these data, which are the target of the modelling, are never shown. So in the context of the assessment of the various limitations, one cannot evaluate what conditions the measurements were done at, whether they contrast between the two sites or not, nor how they encompass specific conditions or a range of conditions.
The authors are gaming that all 1000 individuals involve different compositions of lowland and mid-elevation species as far as I can tell, but this needs a great deal of further explanation. There is an underlying (and unwritten) assumption that only mid-elevation species are best-performing at their middle elevation, whereas actual species composition at the elevations they simulated are never stated. In fact, it isn’t even clear if the species they planted comprise common species at the different elevations they studied, so the target for the simulations is unclear.
For the Photosynthetic parameters in Table 2, the max and min here are from an absolute max and min of the fitted parameters to each set of observations, but this should be clear in the Table caption.
Why weren't the temperature kinetics done for Vcmax and Jmax?
Why arent the diurnal data shown? Why don't we see how well the modelled Anet performed for these diurnals against actual measurements? That is an important step to validation.
All we get is Fig. 3 with diel variation of the simulated parameters. The colour legend for Fig. 3 is mixed with the x-axis label, which looks confusing.
Fig. 3 shows 'simulated relative contribution of physiological parameters to simulated net carbon uptake'. But did the model actually recover measured net CO2 uptake? Is this leaf-level?
If we don't know how the model performed against the measured data, then the partitioning of net C uptake seems a bit like a futile exercise, or at least one without any context or grounding in reality.
Fig. 4 is clearer that these are leaf-level simulations, but equally unclear if the relative contributions are towards simulated leaf net C uptake or measured? Ditto for Fig. 5
Fig. 6 caption could be more clearly written. The part 'weighing their contribution accordingly to their abundance in a forest of 1000 individuals' isn't entirely clear, and if the Anet values are weighed then why is the Y-axis labelled 'Anet x 1000'?