Dr. Tim Seastedt
Institute of Arctic and
Alpine Research
1460 60th Street
Campus Box 450
Boulder, CO 80309-0450
October 7 1998
Dear Dr. Seastedt,
I am returning two copies of
the revised manuscript "Accumulation and depletion of base cations in
forest floors in the Northeastern US" (MS 98-259). I have explained my responses to each of the
reviewer's comments below. I agree that
the reviews were very useful, and I have added their names to the
Acknowledgements.
Reviewer #1, Art Johnson
1. The reviewer was troubled by the lack of
repeatability in the Mg measurements. We
have since reanalyzed the 1970 samples, with much better agreement in Mg
concentrations. This comparison is now
reported in the methods: the figure is no longer important.
2. We have calculated the amount of change that
would be necessary to detect a significant difference between sampling dates
for each of the studies (added to each of the Results). I consider to be the most important improvement
made to the paper in revision. Yes, this
rate is an order of magnitude smaller than streamwater export, so we were not
looking for an impossible rate of change.
3. Yes, the acid-extractable Ca we report is
nearly equivalent to exchangeable Ca.
The point is important because non-exchangeable Ca could be assumed to
be less readily leached. The comparison
is now included in the methods section.
4. Johnson's student Andersen found Ca loss
mainly in forest floors with >25 cmol Ca/g soil. I would cite his study if our results
supported it, but they don't.
Comments on the ms.
p. 2 "Forest floors could both accumulate
base cations and be a source."
True. Changed to read "net
source."
p 3. "How does atmospheric dep compare with
biomass increment in such stands?"
He is correct, regrowth could be supported by atmospheric dep rather
than soil sources. I removed the
sentence "This regrowth implies that Ca is being obtained from soil
stores."
p 5. l 16. "Original" for
"earlier." Good.
p 8. There was a typo in the K value, which has been
fixed.
p 8. I added a reference to Table 2, which
identifies the significance of changes.
p 11. "The W-5 whole-tree harvest did not show
Ca loss in years 3 or 8." We added
discussion of the W-5 results to the discussion.
p 12. "or due to sampling in a different
place." I'm not sure what the
reviewer meant to suggest. Variation in
the calculated cation contents of the samples can be explained by variation in
the measured variables. The correlation
of changes in cation content with changes in cation concentration tells us
nothing about how comparable the sample sites were.
p 14. l 3. "How about the age of the forest when
cut?" There are many other sources
of variation between stands. I changed
the sentence to read "other sources of variation" instead of
"spatial variation."
p 14 l 22
"anomolously" low has been changed to "especially " low.
p 14. l 24. "These are comparisons over 6-12
years. Given the variability in space,
differences are nearly certain to represent sampling error." I meant these comparisons to illustrate the
danger in comparing only two points in time: sampling error is interpreted as
change. The preceeding sentence offers
an explanation in terms of sampling error (soil moisture affects the
deliniation of horizons).
p 15. "This cation capital was not lost from
the soil--it was captured in mineral horizons." I think he refers to the change in forest
floor pools at HBEF W-5. This paragraph
concerns the timing of nutrient changes in the forest floor, and doesn't
address the fate of the nutrients lost.
Rapid losses of nutrients in streamwater are cited to support the short
time frame of some processes contributing to nutrient loss. No change.
Reviewer #2, Kate Lajtha
1. We replaced the sentence in the abstract that
suggested that our focus on the forest floor was motivated primarily by its
ease of measurement. Cation declines
have been reported in other studies of the forest floor, and the forest floor
is important to nutrient cycling. The
introduction covers all these motivations.
2. The reviewer pointed out that our alternative
hypotheses are not mutually exclusive. A
third is that mineralizing slash could increase Ca in young stands. I changed the language in this paragraph.
3. I didn't mean to imply that any change in
ecosystem stores would be reflected in the forest floor. I meant that contents, not concentration, are
appropriate to assessing changes in storage. I removed "in the
ecosystem" from this sentence, which should leave the meaning more clear.
4. With the reanalysis of samples from 1982
this pattern has changed. The paragraph
has been removed.
5. Yes, forest floor organic matter content
might change over succession. But the
organic matter fraction is constrained by the definition of the forest
floor. The discussion of differences in
organic matter fraction in different collections has been moved to the
discussion. The definition of the forest
floor and its relation to organic matter fraction is now in the methods.
6. Unfourtunately, we do not have sufficient
information on stand age in this study to determine whether the aggrading
forest floors are in young stands.
7. The reviewer was not suprised that we found
increased organic matter in the forest
floor of young stands. She has not been
taught the dogma in northern hardwoods that the forest floor should decline to
50% of its mass in the first 20 years after clearcutting. This expectation is important to refute and
is the subject of another paper, currently in preparation, focusing on the
pattern of organic matter change in the chronosequence.
The figure suggested by the
reviewer is one we have used in other presentations. We do not use it here for two reasons. The main reason is that that there is a lot
of variation between stands that is unrelated to their age, so they do not
track along a neat line like the one she drew.
The other reason is that the information in such a graph is completely
redundant with Table 2: anybody who wants to see it can construct such a graph.
The final suggestion is that
the emphasis on the acid rain question be redirected to the simpler question of
changes in forest floor cations over time.
We believe that there is currently widespread interest in the question
of cation depletion by acid rain, and other studies have reported cation losses
from the forest floor and interpreted them as due to acid rain. Preliminary results of our own study have
been cited as evidence of cation depletion by acid rain.
In addition to the changes
requested by the reviewers, we added the 1997 results to the long-term record
from Hubbard Brook, and we revised those from 1982, which had a problem with
sample analysis (all the samples were reanalyzed). Finally, we are reporting values for the 1970
Hubbard Brook samples that are consistent with the published study, rather than
reporting our reanalysis of the stored samples.
None of these revisions has changed the interpretation of the results.
Thank you for a most
constructive review process.
Sincerely,
Ruth Yanai
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