An Integrative Approach to Study How Driving Factors Control Biomass Carbon Density for Natural Mountain Forests, in China’s Loess Plateau

Round 1

Reviewer 1 Report

General comments

While I agree that it is important to understand the factors that contribute to controlling  biomass stores of carbon in forests, I have to wonder if this type of analysis provides as many insights as the authors indicate.  It has been known for some time that the age of forests influences their store of carbon.  I suppose it is reassuring to see that this result was confirmed, but it hardly comes as a surprise. 

The analysis is largely correlational in nature, and while it is good that certain relationships were confirmed, in many ways it represents a step backwards. This is largely because there is almost no attention paid to mechanisms and processes.  In the one section that seems to invoke mechanisms to explain the effect of age the authors fail to mention a key process, mortality, that explains a great deal of the temporal pattern.  Also there is scant attention paid to the field of plant physiology in terms of explanations. 

Although the analysis method reveals the structure of the data, I have to wonder if the relationships are particularly robust.  For example, if the distribution of age classes differed, or if the abundance of forest types changed, etc.  Given the number of potential confounding effects I have to wonder what the practical value of the models would be.

Finally, there seems to be no consideration of the management implications of the results. For example, if age is one of the most important variables explaining biomass  carbon stores in forests, then what does that imply about the management of forests?

Specific comments (page number)

1 The word “the” is not needed in the title

It is not clear how the average of natural conifer forests could be lower than all the values for all the age intervals of these forests. 

Does the comparison of hotspots indicate that broad leaf forests had their highest value in lower elevation and latitude as well as higher temperature and precipitation?  As written the comparison is not  clear.

“in order” is not needed in any sentence. Please remove here and elsewhere.

2 Latin binomials need to be underlined or put in italics

3 more Latin binomials not in italics. Also no authorities for the species listed.

5 more Latin binomials not in italics.

6 Some of these variables are likely to correlated with each other. For example, temperature and precipitation is likely to be correlated with elevation and latitude.  How was this correlation addressed?

8 Table 2.  Why was broad-leafed in bold but coniferous was not?

9 do the authors mean to write “the mean BCD of the former was almost lower than that of the latter for all age intervals”?

13-68 This discussion misses an extremely important point.  As forests age the rate of mortality increases and this also contributes to the slowing of biomass/carbon accumulation in trees.  As written this slowing seems only due to a decrease in NPP, which is not the case.  This is a major conceptual mistake that needs to be corrected.

14-127 The problem with this discussion is that precipitation and to some extent temperature are not what trees are actually responding to.  That is they respond to water availability.  If the temperature decreases, then for a given level of precipitation, water becomes more available because less is evaporated (or transpiration losses are reduced).

15 I find it very odd that Table 1 is after Table 2 and 3. Why are the tables inversely numbered?

15-151 I don’t understand why this analysis was needed to show that elevation and latitude have indirect effects on biomass stores. This was known in the 1950’s or earlier.

16-197 What does it mean that something was almost lower? Either it was lower or it was not lower.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

My comments are as follows

1. What is the main importance of this presentation.

2. What is the main portfolio to control the pollution from the forest.

3. How we control the fire in the forest.

4. What is the role of biomass conversion to carbon.

5. Authors will improve the abstract and introduction part.

6. Authors will improve the conclusion part.

7. If possible authors will add some figures about this manuscript.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

General comments

The authors have addressed most of my comments and concerns.  However, there is still  one section in the discussion that needs significant revision.  See below.

Lines 71-81

This section needs to be revised.  The age structure of a forest can’t be high or low.  I suppose it could be young or old, but actually the age structure is the abundance of age classes.  So more properly an age structure can be dominated by younger or older age classes, but the structure can’t be old or young.

Another problem is that the authors write that the absorption capacity of the forest dies when forests reach a certain age.  Absorption capacity can increase or decrease, I don’t see how it can die. 

It is not clear what the aging stage would be.  Forest age so they are all in the aging stage.  It makes no sense as written.

When trees die in a forest, it is not necessarily true that the forests become sources of carbon to the atmosphere.  This might occur if a great deal of mortality occurs, but it does not have to be so if a small fraction of the trees die.  So this statement is not correct or incomplete at best.

The last sentence literally says to understand the effect of age on forest carbon absorption one needs to study aging.  That is just  circular logic.  What one would have to understand is how the input (via NPP) and the output (via heterotrophic respiration) changes with forest age.  That would be a mechanistic explanation. 

 Author Response

Please see the attachment

Author Response File: Author Response.docx