|This revised version pertains some of the points I had suggested to be adjusted. While valuing such a modelling attempt, it seems imperative that one makes a clear distinction between empirical facts and suggestions derived from a model. The title reads as if N controls treeline in this region, while this is, at best, a suggestion the authors deduce from a model that they configurated in a way that N plays a key role. I accept the point that all models need to assume certain input variables and a certain hierarchy of causalities. I had hoped the authors write a brief paragraph in which they explain the main assumptions that drive this model. These could include that they assume that C is a limiting resource for growth (hence a high priority in the chain of causalities), and because photosynthesis (A) requires N, this is causing growth to be N controlled etc.). Once there are a few sentences of this kind, every reader understands the rationale behind this model and can make up his/her own mind about the results. |
Since the results contradict what we know from observational data about treeline formation, the discussion would best explain why the authors think the model is still correct. By observational data, I mean the evidence that the treeline position tracks an isotherm globally, irrespective of local soil fertility. One could for instance argue that this correlation has an error term and within that variance, there is space for a nutrient effect. The authors could further argue why the site for which they model future treeline position is special in terms of tree nutrition, thus opening space for what the model outcome suggests. I do think readers have a right to read the authors' explanation of how the modelling framework employed (the hierarchy of drivers in particular) led to the main conclusion. I think this is within the tradition of modelling that model results are discussed with reference empirical data (which can be critisized as well, of course).
I am aware of the common jargon, but still find it needs more caution when the word 'test' is employed. We tested N deposition..., we tested CO2 effects... would better read: we performed test runs with the model to explore its sensitivity to...
On meristems: these are just one of the components it needs to build a tree. Mycorrhiza is another one. LPG assumes A drives meristems. Fine, why not say so? Others found it is the other way round. This is merely an issue of open debate and the need of an attempt to justify why the authors think A drives meristems (sensu growth).
On plant competition: sure this matters, in the seedling stage. So if there are good reasons to assume a recruitment limitation, a few words would be welcome on whether the model assumes a dominance of symmetric (soil-root) or assymmetric (light, LAI) competition, and for which life stage such interactions are assumed to be critical. A seed limitation can be excluded in mountain birch, given the billions of light weight seeds produced ever autumn. What matters is (a) the seedling establishment process within the aerodynamic boundary layer close to the ground and (b) the emergence of saplings from that layer. These are two entirely different steps in establishing a young tree.
Bioclimatic envelope: it would be good to read which thermal envelope the model assumes if any. I mean, all organisms have a thermal limit somewhere and mountain birch is no exception. Whatever envelope the model employs and how this envelope is assumed to act upon trees should be explained (it could be an influence on A, on respiration, on meristems, on freezing tolerance, on microbial soil activity... or a combination of all; we simply need to be told, how the range limit is defined in this LPG variant).
In summary, this papers requires more explicit statements on the drivers of tree responses assumed. It cannot be left to the reader to find this out in the LPG script.