Overview comments
This paper uses a large dataset of tree-ring chronologies from eastern North America to determines species growth rates and assess climatic suitability. The authors aimed to answer 3 questions: are spatial patterns of tree growth rates related to differences in climate suitability? Have tree growth rates changed through time? And are these changes due to changes in climate suitability? Overall, I greatly enjoyed reading and reviewing this manuscript. It was well written, thoroughly researched, and delivered clear messages. The methods in particular were detailed and appropriately reproducible. My issues with the work are minor.

I had some concerns with the authors hypotheses and use of growth rates as the sole indicator of fitness, without accounting for other measures of fitness such as reproduction. I understand with a study of the size and scope, it is impossible to address all possible caveats and assumptions, but there were some very serious caveats that warrant explanation and discussion. The hypothesis that trees should “grow faster” in areas with high climatic suitability seems too narrow – trees may grow faster/slower for a variety of reasons unrelated to climatic suitability. For example, Taxodium distichum had significantly negative relationships between growth and climatic suitability. This may be because yearly ring growth is not the best indicator of fitness in this species – it could be root growth, knee development, or simply their ability to live to extraordinary ages. All of these species have unique life histories. Some species had significantly positive relationships between growth and climate suitability – this is interesting and would be expected, not just because of what is known about tree growth, but also because of the significant bias within the ITRDB for climatically sensitive trees since so many collections are done with climatic reconstruction in mind. More “ecologically representative” trees are less often sampled for the ITRDB and thus conclusions based on this large dataset maintain this significant bias. The authors do address the bias in the discussion.

Additionally, trees that grow faster (within species) may be less dense than slow growing counterparts – this is in direct opposition to growth = fitness.
Additionally, there was some broad speculation in the introduction about how species growing in hotter regions should suffer from rising temperatures, while the opposite is true for those growing in colder areas. This is not always the case, and ignores the possibility of biotic interactions playing a role in limiting species distributions. A species is more likely to be constrained by biotic interactions than climate at the southern range extent due to increased competition for resources (e.g. see Paquette, A., & Hargreaves, A. L. (2021). Biotic interactions are more often important at species’ warm versus cool range edges. Ecology Letters, 24(11), 2427–2438. https://doi.org/10.1111/ele.13864)

The result and discussion being related to phylum needs a bit more set up in the introduction and objectives. The objectives were clear but should be expanded to include the question related to phyla. Lastly, I find that the author’s overall conclusion that “we should not count on increased growth of North American trees to help offset anthropogenic carbon emissions” are not particularly supported by the results of this manuscript. I agree with the sentiment, but without further discussion and presentation of evidence that explain increased growth rates = increased carbon uptake, there is not a direct link, because of what was previously mentioned: what about root growth? Secondary growth, e.g. branches? Leaf production? Reproduction? I think that the manuscript would be improved with further explanation and justification for this conclusion, in the introduction and the conclusion. It is a strong and worthwhile conclusion, especially with regards towards recent increases in inappropriate support for afforestation (such as in savannas).

Detailed comments

Introduction
L28-29 how does growth rate relate to fitness specifically?
L31-33 it is not always the case that these changes increase the risk of extinction/impede ecosystem services. I can think of numerous examples when the opposite is true – for example, with invasive species or with early CO2 fertilization. This could be improved by providing more specific examples of when this occurs, such as through cascading effects across trophic scales and in the context of preserving native “high-functioning” ecosystems.
L48-49 interesting point about numerous studies pointing to decreased growth rates over time. This should be supplemented with a discussion of scale – are these studies exploring species level or multi species? It just seems too broad to say “many trees are showing increases or decreases” since it is far more nuanced than that, and different species will respond in different ways. In addition, the same species may respond in different ways at its northern vs southern extent. I realize the authors need to provide sufficient background and that a study of this scale requires care in choosing what to include, but this is too broad.
L56-58 “should benefit” could be changed to “may benefit”. This is not true for all species, and species may be limited by biotic interactions at their warmer range limits. See above ref.
L68-72 Nice clear objectives. See above about more clearly adding in an objective that indicates you will be assessing differences across taxonomic groups (phyla). This is a major finding of the paper and should be introduced in the objectives.
L71-75 why call your hypotheses “a priori expectations” instead of hypotheses? Why is speed of growth used as a sole measure of fitness? Slow growth does not definitely mean less fit, if the tree is allocating resources towards reproduction/masting, taproot growth, or vertical/lateral growth (such as in flat-topped cypress or pines).
L76-77 here the authors explain their hypotheses re:phyla having an effect, but haven’t mentioned this anywhere else in the introduction or objectives. Need to discuss why different phyla would have differences in growth – such as differences in life histories or methods of reproduction, etc. Also needs to be included as an objective, possibly Q4? It seems out of place currently.

Data and Methods
L95-96 found this a bit confusing, “sites with > 100 tree rings per species”, I am unsure what this means. So if a it only have 2, 50 year old trees, it was included because it totaled 100 rings? I think I am just misunderstanding something here, so this could be clarified to improve the manuscript.
L153 the ordering of the bioclimatic variables for the stepwise selection being based on “a priori assessment” needs further justification. A few sentences describing why/how you determined what was more important. The current justification is not sufficient.

Results
L208-213 major result without enough set up in the introduction/discussion. Why do you think we are seeing these results?
L214-220 see discussion above about bias in the ITRDB data set. Yearly growth may not be the only indicator of species fitness. Some species are more climatically sensitive tan others, especially considering the bias in the ITRDB dataset. I keep wondering, what about within species? I think a within species analysis, comparing samples from the colder and warmer range limits would be more informative than comparing species against each other, which may show “fitness” in different ways due to unique life histories.

Discussion
L231-233 needs more explanation describing why the results would contrast so significantly from gymnosperm/angiosperm. Is this because of life histories? Reproductive strategies? Wood anatomy?
L258 citation for Pinus palustris having possible been impacted by pest/pathogens? This species is well known for being highly resistant to pests/pathogens
L295 remove “an in any case, historical climate estimates are inherently “coarse” “. Seems defensive and unnecessary
L304 hurricanes?
310-311 particularly looking within individual species northern vs. southern range limits.
318 as discussed above, I have a bit of a problem with the conclusion that these observed decreases in growth rates inherently suggest we cannot depend on trees for carbon uptake – while this is theoretically sound, there is more involved in carbon uptake. I think this can stay but should not be presented as a major conclusion from the results of your study, which did not look at carbon storage. You could discuss how biomass is a proxy for carbon storage and tie it into your use of basal area increment in the introduction and methods to reinforce this conclusion.