Carbon Stocks of Hardwood Floodplain Forests along the Middle Elbe: The Influence of Forest Age, Structure, Species, and Hydrological Conditions

Round 1

Reviewer 1 Report

In this relatively short communication the authors document variation in carbon stocks in hardwood floodplain forest plots located along 100km of the Elbe River within a UNESCO biosphere reserve.  Hardwood floodplain forests exist only as relics of an important ecosystem type within Europe and as such this report adds to a very sparse literature on their structure and function.  The authors found, not surprisingly, that C stocks increased with stand age and, somewhat surprisingly, that among older stands stem density and position within the hydrologic landscape had no effect on the size of C stocks.  Methods of data collection and analysis were standard for the field and results are clearly presented and explained.  Overall, the manuscript is reasonably well prepared, with appropriate literature cited and the research objectives and justification well articulated.  Given the scarcity of robust published C stock estimates in this forest type, this contribution constitutes a valuable addition and should be of interest to those managing floodplain environments.  I have only a few concerns.

The distribution of sampling plots within the various age and habitat types is not clear.  Figure 3 shows 15 total plots, but line 157 mentions 30 plots.  Table 1 contains no sample size information at all!  The reference to Figure 2 in paragraph 2.2, apparently to illustrate the sampling design, is not helpful.  This needs to be clarified.

Related to this, the authors should make explicit the sample sizes of all means and standard deviations being reported in the tables and figures.  Only with this information will these data be useful in future quantitative syntheses. 

Line 194 – bolded text needs removal (careless editing before submission).

Line 294 – which ‘other plots’ are they referring to?  There is only one other 200 yr old plot – had DWD clearly been removed from that plot?  Was that true for the ~100-150 yr old plots as well?  This is an interesting point that bears elaborating on since the STD/DWD C stock component evidently can add up to a lot in these older plots. 

Author Response

Please see the attachment. 

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors present a quantification of carbon stocks in temperate floodplain forests on the Elbe River. The methods and analyses are rigorous. The results are presented clearly and the manuscript is well written. Perhaps the biggest criticism that one could make is the conclusion that stands of mature trees contain more carbon than young stands is proving the obvious. However that would be missing the point. The current interest in forest carbon storage is from the perspective of management decisions regarding the carbon sequestration potential of reforestation in different settings. To inform those decisions accurate quantification of representative reference forests is needed, and this study provides such an accurate estimate. To help put that estimate into context, the authors should add a paragraph to the discussion comparing the carbon storage in mature floodplain forests to previously published estimates the carbon storage in mature upland forests in the region. Such a comparison can help decision makers prioritize different topographic settings for reforestation for carbon sequestration. From that perspective, it would also be interesting to compare growth rates of floodplain trees with upland trees nearby. It has been shown previously that floodplain trees grow much more quickly than upland trees (e.g. Dybala, Matzek, Gardali, and Seavy 2019 Global Change Biology; Marks, Yellen, Wood and Nislow 2020 Wetlands). If floodplain plantations achieve old growth structure in half the time of upland tree plantations, that would be another argument in favor of floodplain reforestation from a carbon sequestration perspective. Adding some curves from previously published upland forest data to figure 3 could help make that argument. At the very least the authors should report growth rates in table 1 (calculated from the tree core data). 

Another way to improve the manuscript is to provide more information on the study sites in the introduction and methods sections. For example, the authors use the term "young plantations" which implies that these forest were planted. The authors should add a sentence saying that the "young plantations" refer to planted forests if they were indeed planted. They should also clarify if these plantings were done as part of silvicultural operations post harvest of an existing forest or if they were planted on abandoned agricultural floodplain fields or pastures. If they were not planted but rather arose through natural recruitment, the authors should not call these forests "plantations". Instead the authors should refer to those sites simply as "young forests" or "young stands". Moreover if stands arose naturally, the authors should clarify if the stands arose through natural recruitment on abandoned floodplain fields or on sandbars because the historical landcover (which can be determined form historical aerial photos) affects the species composition, initial stand density, etc. All of this matters because this research is valuable if it can inform management decisions. Therefore, I urge the authors to report more about the management of these research sites. The distinction between "dense forests" and "sparse forests" being due to past differences in selective harvesting and/or stand thinning is another important piece of information that the authors don't mention until the discussion, but should have been presented when the authors first use these terms in the introduction. 

Minor edits:

Line 194: an error occurred in the reference management software

Line 307-311: When comparing cleared land, dense young stands, and sparser mature stands, one would certainly expect substantial differences in roughness that affect flows, but I question whether the relatively small differences in density among sparse and dense mature stands reported in Table 1 would suffice to have a significant effect on flows. 

Author Response

Please see the attachment. 

Author Response File: Author Response.pdf