In this chapter the authors establish that the forests of North America take up more carbon than they release (i.e., are a sink), and this sink is quite variable spatially but resides primarily in the U.S.. Forest regrowth along with nitrogen deposition and elevated CO2 contribute to the strength of this sink. Working against this sink are timber harvest and escalating disturbance regimes, leading to the prediction that the capacity of North American forests to provide a net uptake of carbon will diminish in the future.
A major new result in this report is the conclusion that Mexican forests now are considered a carbon sink. In the previous assessment (SOCCR1) conducted about 10 years ago, it was reported that forest harvesting in Mexico contributed about 9 Tg C per year to the atmosphere. The more complete accounting presented in this report indicates these forests sequester approximately 41 Tg C per year. This is an important result and it would benefit from further support. Specifically, the authors should address what new process or fluxes have been included, or what values were modified to switch Mexican forests from a source to a sink for atmospheric carbon.
The definition of forest articulated on the first page (line 23) is a bit perplexing. Here forest is defined as having a land area as small as 0.5 ha and a canopy cover of as little as 10%, which seems to be a very “low bar” for defining a forest. Consider alternative definitions of forests, and whether this might affect the major findings of this chapter. (See additional related comment below).
Clarifying the temporal dynamics of carbon fluxes in forests would make this chapter more approachable. One of the challenges in understanding carbon cycling in forests is the time lag caused by the long-term storage of carbon in wood. When a tree is cut down and burned, it causes an immediate release of carbon to the atmosphere. This carbon was removed from the atmosphere over the past one or two hundred years, depending on the age of the tree. This displacement between carbon uptake and release poses a challenge when considering trees for bioenergy. For this reason it is worth considering spatial integration instead of temporal integration to evaluate carbon-budget consequences— i.e., is there enough forest area in recovery right now to offset the carbon release from the areas being harvested or disturbed?
Statement of Task Questions
- Does the report accurately reflect the scientific literature? Are there any critical content areas missing from the report?
For the most part, this chapter accurately reflects the scientific literature, but see specific comments below.
- Are the findings documented in a consistent, transparent and credible way?
Yes, the findings are documented in a consistent, transparent and credible way.
- Are the report’s key messages and graphics clear and appropriate? Specifically, do they reflect supporting evidence, include an assessment of likelihood, and communicate effectively?
The embedded text in Figure 9.1 is illegible (too small), and the inclusion of “product partnerships” on the right side of the figure is cluttered and distracting. Similarly, Figure 9.5 is not readable.
The authors should define the meaning of positive and negative signs in Table 9.3.
- Are the research needs identified in the report appropriate?
The authors have done an admirable job of identifying knowledge gaps and research needs in 9.8.2. This section would benefit from also discussing the shortcomings of current modeling and inventory approaches because they have large uncertainty in simulations and substantial spread among models.
- Are the data and analyses handled in a competent manner? Are statistical methods applied appropriately?
The numbers for the magnitudes of carbon sinks/sources and stocks are presented with multiple significant digits and are not associated with uncertainty estimates. It is fairly important to provide some kind of uncertainty estimates or data ranges.
- Are the document’s presentation, level of technicality, and organization effective? Are the questions outlined in the prospectus addressed and communicated in a manner that is appropriate and accessible for the intended audience?
Yes, the document’s presentation, level of technicality, and organization are effective.
Section 9.4: Need to clarify: what are the indicators? what are the feedbacks? are they feedbacks to the climate system? It appears that this section does not contain indicators or feedbacks as its title indicates. These components should be added; otherwise, the title of the section should be modified accordingly.
- What other significant improvements, if any, might be made in the document?
It would be helpful if the numbers for magnitudes of carbon sinks/sources and stocks could be compared to those reported in SOCCR1, which can illustrate how the numbers have changed because of various factors (e.g., changes in carbon fluxes/stocks, changes in methods used, differences in components considered).
The second paragraph of the Introduction provides a definition of forests used in this chapter: land greater than 0.5 ha with trees greater than 5 m and canopy cover of > 10%. This is quite different from the definition of forests in widely-used classification schemes for satellite-derived land-cover maps. For classification schemes like IGBP, forests are defined as areas with >60% of tree cover with >2m tree height. Most modeling studies are based on land-cover maps with classification schemes like IGBP and therefore use a different definition of forest compared to this chapter. How were differences in forest definition and the resulting discrepancies in forest area and carbon fluxes/stocks among different approaches and studies reconciled in in this chapter? This should be appropriately addressed.
As indicated by the report, there are still differences in the methods used for estimating carbon stocks and their changes among the three countries. Were these differences explicitly considered and reconciled in this chapter? If yes, how?
P342, Line 35
Finding 3 is not written very clearly. In order for harvest to offset a part of the net carbon sink in forests the managed forestry must be considered separate from other forest land. Is that what is meant?
P343, Line 4
In point 4, natural disturbance and land conversion are considered together. Conceptually it may be more instructive to keep them separate. A natural disturbance will often be followed by some phase of regeneration, and an appropriate way to think about the disturbance-associated budget term would be as disturbance return interval, relative fraction of land under disturbance, and regrowth rates relative to the rate during the last cycle. Except for fire management intended to reduce fire severity or frequency, there are few opportunities to alter this budget component. Anthropogenic land conversions are different because they are permanent and directly related to management decisions.
P344, Line 14-35
The summary of the forest carbon cycle doesn’t explicitly note the live respiration term but focuses on photosynthesis and death, which partitions into debris, soil organic, and decomposition. Photosynthesis could be described as having multiple allocation pathways, either to new growth, or supporting respiration.
P344, Line 34
In stating which regions store the most carbon, consider distinguishing regions that have large stock because the area is large and those that have high carbon density. This statement would be enhanced by having a figure to indicate the regions, or reference the map in Figure ES1 if that is how regions are defined in this chapter.
P345, Line 2
In this chapter as in others, the units for carbon stocks are used inconsistently.
P345, Line 23
Should the loss of carbon from forest conversion to settlements be cross-checked against the sink for urban trees, to be sure there are no offsets or double counting?
P348, Line 2
The phrase “major contributor to net reductions in atmospheric CO2” is incorrect. CO2 is indeed increasing. Please rephrase.
P349, Line 8
When discussing the nitrogen deposition influence on forests the trends in deposition should be mentioned. National Atmospheric Deposition Program (NADP) trends (a good proxy), show declining NO3 in wet deposition and increasing NH4–thus indicating a complicated national trend with spatial variations.
P349, Line 14-29
“Tree” and “plant” are used interchangeably in this chapter. It is recommended that the authors stay with “tree” in this chapter.
P349, Line 30-39
The paragraph discussing SOC would benefit from some mention of the role of soil warming. See Melillo et al., 2017.
P350, Line 11-33
Discussion of how disturbance affects forest carbon budget ought to consider whether the overall carbon budget across a landscape or region is or is not at steady state, where steady state can be achieved with a small area being disturbed and offset by larger area that is recovering. Critical considerations are size of disturbed area relative to undisturbed regrowing area, and return intervals. Parts of this paragraph are not adequately quantitative. What are the regrowth rates after disturbance; how delayed is the decomposition? If these numbers are not well understood, then this should be spelled out as a research need.
P351, Line 15
Suggesting that forest sink strength will decline as forests age relies on dogma that old forests aren’t strong carbon sinks, but that is not supported by observations from the oldest stands available to study where carbon gain is still strong (Luyssaert et al., 2008).
P352, Line 13
Isn’t SCC typically expressed per ton?
P353, Line 13
The consequence of bioenergy doesn’t seem to be treated consistently in this chapter. On line 15 it is counted against strategies to reduce fire, but on line 26-27, substituting biofuels for fossil fuel is given as a strategy for reducing carbon emissions.
P354, Line 14-20
It would be useful to discussion whether forest harvesting is compensated by regrowth, considering regional balance as well as temporal balance. One can evaluate the balance by considering a unit of forest area that is harvested and recovers (or not) over many decades (as is done here); or one can take a regional approach where some patches are being harvested each year and the remainder is left alone. Is there enough land in regrowth to compensate for the harvest loss? The point that accelerated disturbances are reducing the carbon sink now should be paired with discussion about whether the land areas recently disturbed are likely to recover and become a large carbon sink in the near future.
P355, Line 28
The authors suggest that a priority for future research is creating a full climate impact assessment for forests, including albedo and methane and nitrous oxide fluxes. It may be prudent to cite Anderson-Teixeira et al. (2012), which provides a computation framework for integrated quantification of the climate regulating value of forests and other ecosystems.
P357, Line 34
The authors state, “Although the reclassification of land from non-forest to forest… does not… involve emissions or removals of atmospheric carbon, the processes underlying such reclassifications invariable do.” It is unclear why the authors feel that it necessary to draw this distinction.
P379, Table 9.3.
Is “2.Net due to forest land gain and Loss” meant to say “Net FLUX due to….”? (Without clarifying, one might mistakenly interpret the numbers to be areas).