date: Tue, 2 Sep 2003 12:34 -0400
subject: Review Received by Geophysical Research Letters
Dear Dr. Briffa:
Thank you for your review of "Large-scale warming triggers Siberian treeline advances<" by Jan Esper, Fritz Schweingruber [Paper #2003GL018177], which we have safely received. A copy of this review is attached below for your reference.
Thank you for your time and effort!
Geophysical Research Letters
Science Category: Science Category 4
Presentation Category: Presentation Category C
Annotated Manuscript: Yes
This paper provides some new evidence of recent germination of trees near to tree line in northern Russia. Taken together with earlier published information it provides support that mid to late 20th century warming is initiating a potential advance of tree line, but the evidence shows (though the authors do not discuss) that this advance is variable in magnitude and timing. The information is, of itself, interesting and worthy of publication, but it is described in a cursory way here and it is not clear that this manuscript is appropriate for Geophysical Research Letters. My overall opinion is that the authors should develop the discussion further and submit it to a more ecologically orientated journal.
The paper sets out to �test the assumption that large-scale temperature forcing has an effect on treeline dynamics� and to �compare the recent treeline positions with historic ones by documenting in situ remnants of relict stumps and logs�. I feel that the paper goes some way to achieving the first aim but is too limited in its discussion of the evidence and makes little, if any, useful contribution to achieving the second aim. Without a more quantified analysis of the regional extent of treeline response, in terms of tree recruitment and migration, to absolute temperature change, and some explicit discussion of the implications for interpreting treeline changes with respect to large-scale temperature (or other climate) changes, the relevance to a geophysical journal is far less obvious than its relevance to the more general ecological literature.
The paper points, in its introduction, to the numerous factors that exert some control over treeline dynamics, but the subsequent description does not discuss their relevance here, other than showing a Figure of germination dates and growth releases side by side with annual temperatures. There is mention of �correlation� between these, and it is stated that this is higher than for �summer� temperatures, but the discussion is very vague and no actual quantitative relationships are provided or discussed.
Similarly, the sampling strategy is said to be �a qualitative stratification of treeline forests by structure and tree form�. This is a good idea, but unfortunately the results are not clearly differentiated with regard to the classes of forest, and it is not clear whether we are seeing latitudinal or elevational changes in tree line or how these are differentiated in the different types of ecotone. Again, though a �longitudinal and latitudinal structure� is referred to across the network, it is not clear what the climate controls (or specific temperature) influence is on this and how the germination or survival of new trees is influenced in the different parts of this structure.
The results summarised in Figure 4 are certainly interesting, but they suggest several questions not addressed by the authors. Why do the recruitment pulses commence prior to the major onset of warm periods (such as in 1939, 1949 and the early 1970s)? Similarly, why are there major episodes of abrupt growth increases (in the mid 1970s and mid 1980s) that do not correspond directly either to strong warming or germination phases? The period of strong warmth in the mid 1940s (and the years of extreme warmth ~1944, 1962, 1968, 1981, 1984), are not synchronous with germination peaks. Does this imply a requirement for cumulative warmth over several years or is there a confounding effect e.g. snow cover? More importantly, do these average results mask regional (or different ecotone) variability in response to warmth as is implied by Figure 3? This shows homogeneous responses in category B sites but significant differences in A and C. It is very interesting that URA1 shows no germination prior to the very recent period, while other regions (especially in C) show significant germination prior to 1950, but none in the most recent period! The question of mortality is crucial. No reference is made to this, yet the interpretation of past tree lines must be viewed in the light of the survival of new seedlings. Did the authors see any evidence of recently dead seedlings?
The information shown here, with a cursory inspection, clearly implies some response to mid 20th century and more recent warming, but the unprecedented accuracy in the timing of tree growth germination should allow a more detailed comparison of the influence of short-term temperature changes, and seems to show a complex relationship as regards germination and growth release in trees. The latter appear to be non-linearly related and the implication for dendroclimatic studies of such trees might usefully be mentioned.
The existence of large fossil remains is important but the authors do not give sufficient detail of the dates (or growth rates) of these samples to provide useful insight into the likely absolute temperatures that accompanied their growth. Did the authors also explore the possibility of absolute thresholds in the regional responses to local temperatures that may be more significant than the relative changes shown in their Figure 4 (especially given that their sites cover a range of elevations and mean climates)?
These data are without doubt of value and interest to the scientific community, but to publish these results, in this form, in this publication, is not the most appropriate way of presenting them. Finally, I do not find the brief allusion to the workings of the carbon cycle and implied importance for climate change, at all convincing.
Indeed, it is far more likely that changes in local albedo caused by treeline changes are likely to exert a dominating positive influence on rates of high latitude warming.