date: Thu, 24 Jul 2008 07:41:00 -0400
from: h.langenberg@nature.com
subject: Information for reviewing Nature Geoscience manuscript
to: K.Briffa@uea.ac.uk

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   B3.07; Q3.07) Date: Thu, 24 Jul 2008 07:41:00 -0400 Message-Id:
   <95121689966084@rhwww3.nature.com.nature.com> Dear Keith,
   Some time ago you kindly refereed for us the original version of the manuscript entitled
   "Sulphur deposition causes a large-scale growth decline in boreal forests in Eurasia"
   (manuscript number NGS-2008-02-00218B). The authors have now revised their manuscript in
   response to the reviewers' comments and our editorial requirements.
   The authors believe that they have been able to answer the criticisms raised, and have
   documented their responses in a letter that you will find with the manuscript. (However,
   please note that they appear to have exchanged the referee numbers).
   We should be most grateful if you would look at the revised manuscript and tell us whether
   you think publication in Nature Geoscience is now justified. Please also feel free to
   comment on the authors' responses to the other referee's criticisms.
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   ********************************************
   Dr Heike Langenberg
   Chief Editor
   Nature Geoscience
   [2]http://www.nature.com/ngeo/index.html
   Reviewer Reports:
   Reviewers Comments:
   Reviewer #1 (Remarks to the Author):
   Comments on Nature submission by Yuliya Savval and Frank Berninger .( NGS-2008-02-00218 )
   Sulphur Deposition causes a large scale growth decline in boreal forests in Eurasia.
   This paper is not suitable for publication in any journal in its present form. Even by
   Nature standards it can only be described as opaque. There is nothing like enough material
   or detail presented even in the supplementary material to allow a full balanced assessment
   of the general argument that Sulphur pollution is negatively impacting on pine growth in
   the western part of the Eurasian boreal forest.
   My first observation is that it is odd that this paper makes no reference at all to the
   widely cited paper by Briffa and colleagues pointing out the decline in boreal forest
   growth since the 1950's relative to the temperature trends that would be expected to
   produce increased growth. This issue and the implications for Paleoclimate studies are
   widely debated and given much provenance in the recent IPCC reports.
   Putting this to one side Savval and Berninger choose to adopt some dendroclimate
   techniques, the implications of which for their analysis they then fail to discuss, and
   choose not to use other dendroclimatic techniques or results of previous work, specifically
   the construction of site chronologies and the descriptions of the climate responses of
   trees in their regions.
   The findings of this paper depend critically on the methods used to account for the
   expected reduction in radical ring width as a function of stem expansion as the tree grows
   and in the validity of the statistical model used to identify and remove the influence of
   climate variability on ring width variation.
   The authors' use three forms to of the measurements they have extracted from the publicly
   available tree ring data bank. In the first they use the measurements directly. The early
   radial growth measurements over the period 1920-1940 cannot be directly compared to
   measurements from the same trees in later periods because of the well known thinning in the
   measurements as tree age, so the ratio approach (see figure 2A) is invalid.
   The use of negative exponential functions (figure 2B) to remove the effect is prone to end
   fitting problems and effective response (in a time-series filtering context) is
   unpredictable. Also it seems very unlikely that the residuals from these functions could
   produce data virtually identical to the original data as is implied in Figure 2. Similarly
   the use of 'blind' regression to remove the climate effect is undesirable (see earlier
   comments) and is not likely to have no effect on the data trends as is implied in figure
   2C. Then we come on to the analysis of the association between tree decline and N and S
   pollution. There is no clear statement of why the sites used were chosen. There is strong
   Finnish/Western Siberia bias with 5 or 6 spatial outliers.
   It is worrying that associations (illustrated in figure 3) between tree growth and
   pollutant loading are strongly influenced by 6 points showing high numbers of trees with
   growth differences (between earlier and later times) for low pollution loading. Without
   these points, then is no relationship. We are not informed where these points are? We also
   do not know what the interpretation of growth differences at each site means - is it a
   consequence of poor detrending? Is it statistically significant? To me figure 3 merely
   shows that high S generally means high N. In figure 3B, just as many sites show positive as
   show negative residuals for high N and S.
   Similarly, in figure 4 the data shows barely significant results and for the 'cold March'
   cases the result is highly leveraged by a single very high S deposition site. No
   information is provided about this site.
   Overall I find the study opaque and the conclusions not demonstrably supported by the
   analysis.
   I recommend rejection.
   Reviewer #2 (Remarks to the Author):
   I. General
   This paper examines 20th century trends in tree ring width of Scots pine in northernmost
   (60-70 degN) Eurasia. The authors find that age-adjusted tree growth decreased, and that
   decrease follows the spatial pattern of anthropogenic sulfur deposition, partly offset by
   anthropogenic nitrogen deposition (which is taken to have a fertilizing effect). While the
   impacts of sulfur and nitrogen pollution on forests and on agriculture, both in temperate
   and boreal zones and in China and India, have been the topic of much study, I believe that
   the this paper is important in that it documents the impact of sulfur deposition on a wide
   swath of boreal forest, and would be of interest to a broad earth science community. I find
   the statistical arguments used for a significant reduction in tree growth linked with
   sulfur deposition adequately convincing, with some qualifications as outlined below.
   II. Broader scientific issues
   I would recommend publication in Nature Geoscience, but suggest that the paper would
   attract more interest if the authors more explicitly address the implications of their
   findings for current questions about the carbon cycle, atmospheric chemistry, and boreal
   climate change. In particular:
   1) Discussion of the negative impact of anthropogenic sulfate (and nitrate!) in temperate
   USA/Canada and Europe has focused on acid rain and resultant cation leaching as a mechanism
   for forest decline, whereas in this paper the authors focus on direct damage to leaves by
   SO2. Why would the latter mechanism be expected to be more important than the former in
   boreal forests? Can the effects of the two mechanisms be distinguished with this data set?
   2) An earlier paper of mine (NY Krakauer, JT Randerson [2003], Do volcanic eruptions
   enhance or
   diminish net primary production? Evidence from tree rings, Global Biogeochemical Cycles,
   17[4], 1118, doi: 10.1029/2003GB002076) used ITRDB series to show that narrow tree rings
   are found in the years following volcanic eruptions that release large amounts of sulfur
   into the upper atmosphere, with the effect being particularly pronounced in coniferous
   trees, including pines, in Eurasia north of 60 degN. I attributed this response to summer
   cooling and shorter growing seasons caused by sulfate aerosol scattering of sunlight,
   rather than to direct impacts of sulfur on trees. Comparing with the current paper, the
   following questions arise:
   a) Could the negative response of tree growth to volcanic eruptions be directly due to
   sulfur deposition, rather than to the climate impacts of sulfate in the atmosphere, as I'd
   hypothesized? How do levels of sulfate deposition at high latitudes following volcanic
   eruptions compare to those found by the authors to impact tree growth in the 20th century?
   b) Conversely, could the negative association between sulfur deposition and tree growth
   found by the authors be due to local cooling induced by pollution haze rather than to the
   direct effects of sulfur on leaves? What evidence is there for the climate impact of
   industrial pollution in this zone from e.g. weather stations?
   3) An increasing trend in satellite-measured surface greenness (NDVI), roughly
   corresponding to leaf area, has been found since 1981 in boreal Eurasia, as well as
   elsewhere. To what extent could this increasing greenness represent recovery from past
   pollution damage, as compared with the conventional attributions to longer growing season,
   CO2 fertilization, and N deposition (or perhaps a more complex synergy - e.g., because
   springs are warmer, trees are less vulnerable to sulfur damage)? What basis is there for
   generalizing from the current results, in Scots Pine, to the Eurasian boreal forest or
   parts thereof?
   III. Specific methodological and presentation comments
   1) Please clarify the map symbols used in Figure 1.
   2) Please show as a supplementary map the distribution of S and N deposition from the
   datasets used, allowing readers to visualize the size of the area in which tree growth has
   been hampered by pollution.
   3) Ring width will, in general, change systematically with tree age, interfering with the
   detection of climate or pollution influence. The authors detrend their ring series with
   exponential-decay fits, but if tree growth systematically deviates somewhat from an
   exponential decay, a residual trend will remain. The authors also try to detrend by the
   site mean age-width relationship, but because most sites tend to be almost even-aged this
   cannot be fully achieved. Supplementary Fig. 1 suggests that the (residual?) age effect is
   small compared to the observed 20th century growth suppression, but another possible test
   would be to apply the same procedures to 19th century tree rings (in 20-year periods and so
   on) and show that, because of the absence of pollution, no significant decadal trend is
   found.
   4) "Using gridded maps of oxidized (wet and dry) sulphur and nitrogen depositions across
   Europe and Eastern Russia, we revealed a close relationship between percentage of trees per
   site that increased growth in the period between 1970 and 1990 relative to the period 1940
   to 1960 and logarithms of oxidized nitrogen and oxidized sulphur depositions (R= 0.43, p =
   0.02) (Fig. 3A)." (p. 3) Are R and p for the multiple regression? Please clarify.
   5) Are the error bars in Figure 2 standard errors across the 40 sites? Please clarify.
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