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Dendroclimatology is the science of extracting past climate information from information in trees (primarily tree rings). Tree rings are wider when conditions favor growth, narrower when times are hard. Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous (insert some example refs). Combining multiple tree-ring studies (or using tree ring studies with other proxy records), scientists have estimated past regional and global histry (e.g. MBH98, Moberg05). For other uses, see Tree (disambiguation). ...
The growth rings of an unknown tree species, at Bristol Zoo, England Pinus taeda Cross section showing annual rings, Cheraw, South Carolina Pine stump showing growth rings Dendrochronology or tree-ring dating is the method of scientific dating based on the analysis of tree-ring growth patterns. ...
Advantages
Tree rings are especially useful as climate proxies in that they can be well-dated via "wiggle-matching" of the rings from sample to sample (dendrochronology). This allows extension backwards in time using deceased tree samples, even using samples from buildings or from archeological digs. Another advantage of tree rings is that they give abundant data, clearly demarked in year increments, as opposed to other proxy methods such as boreholes. Furthermore, tree rings respond to multiple climatic efffects (temperature, moisture, cloudiness) so that various aspects of climate (not just temperature) can be studied. However, this can be a double-edged sword as discussed in Limitations. A proxy server is a computer network service which allows clients to make indirect network connections to other network services. ...
The growth rings of an unknown tree species, at Bristol Zoo, England Pinus taeda Cross section showing annual rings, Cheraw, South Carolina Pine stump showing growth rings Dendrochronology or tree-ring dating is the method of scientific dating based on the analysis of tree-ring growth patterns. ...
Limitations Along with the advantages of dendroclimatology are some limitations: confounding factors, geographic coverage, annular resolution, and collection difficulties. The field has developed various methods to partially correct for these difficulties.
Confounding factors There are many climate and non-climate factors as well as nonlinear effects that affect tree ring width.
Climate factors Climate factors include temperature, precipitation, and sunlight. To differentiate among these factors, scientists collect information from "limiting stands". An example of a limiting stand is the upper elevation treeline: here, trees are expected to be most limited by temperature rather than precipitation. Conversely, lower elevation treelines are expected to be most influenced by precipitation rather than temperature. This is not a perfect work-around as multiple factors can still impact trees even at the "limiting stand", but it helps. In theory, collection of samples from nearby limiting stands of different types (e.g. upper and lower treelines on the same mountain) should allow mathematical solution for each confounding variable. However, this method is rarely used.
Non-climate factors Non-climate* factors include soil, insect outbreaks, fire, competition, genetic differences, human or animal (particularly grazing) disturbance, and CO2 concentration. For factors which vary from over space (tree to tree or stand to stand), the best solution is to collect sufficient data (more samples) to compensate for random noise. In some cases, examination of the site helps to avoid confounding effects: e.g. looking to see that no human disturbance has occurred.
Non-linear effects In general, climatologists assume a linear dependance of ring width on the variable of interest (e.g. moisture). However, if the variable changes enough, response may level off or even turn opposite. The home gardner knows that one can underwater or overwater a house plant. In addition, it is possible that interaction effects may occur (for example "temperature times precipitation" may affect growth as well as temperature and precipitation on their own. Here, also, the "limiting stand" helps somewhat to isolate the variable of interest (e.g. if you are at the upper treeline, it's unlikely that nonlinear effects of temperature ("inverted quadratic") will have significant impact on RW over a growing season).
Botanical inferences to correct for confounding factors For all growth factors, botanical studies can help to estimate the extent of impact of probable confounding variables. These experiments may be either ones where growth variables are all controlled (e.g. in a greenhouse), partially controlled (e.g. FACE exeriments to look at CO2 effects--add ref), or where conditions in nature are monitored. In any case, the important thing is that all growth factors are carefully recorded to determine what impacts growth most and how. (insert Fennoscandanavia reference). With this information, ring width response can be more accurately understood and inferences from historic (unmonitored) tree rings can be more reliable. In concept, this is like the limiting stand principle, but it is more quantitative--like a "calibrated" limiting stand. A greenhouse in Saint Paul, Minnesota. ...
Geographic coverage Trees do not cover the Earth. Polar and oceanic climates can not be estimated from tree rings. In tropical regions, the trees grow all year round and don't show clear annual rings. In some forest areas, the tree growth is too much influenced by multiple factors (no "limiting stand") to allow clear climate reconstruction. The coverage difficulty is dealt with by acknowledging it and by using other proxies (e.g. ice cores, corals) in difficult areas. In some cases, if it can be shown that the parameter of interest (temperature, precipitation, etc.) varies similarly (e.g. by looking at patterns in recent years of instrumental data), one may be justified in extending the dendroclimatology inferences to nearby areas where no suitable tree ring samples are obtainable.
Annular resolution Tree rings show the impact on growth over an entire growing season. Climate changes deep in the dormant season (winter) will not be recorded. In addition, different times of the growing season may be more important than others (i.e. May versus September) for ring width. However, in general the RW is used to infer the overall climate change during the corresponding year. Another problem is "memory" or autocorrelation. A stressed tree may take a year or two to recover from a hard season. This problem can be dealt with by more complex modeling (a "lag" term in the regression) or by reducing the skill estimates of chronologies. A plot showing 100 random numbers with a hidden sine function, and an autocorrelation of the series on the bottom. ...
Collection difficulties Tree rings must be obtained from nature. Conditions outdoors are not perfectly controlled, so efforts must be made to carefully map locations, to collect samples in difficult (often sloping) terrain. In general, tree rings are collected using a borer device, which while hand-operated and light, still requires skill to use properly to get a good sample. The best samples come from felling a tree and sectioning it. However, this requires more work and danger and does damage to the forest. It may not be allowed in certain areas, particularly with the oldest trees. As with all experimentalists, dendroclimatologists must at times, make the decision to make the best of imperfect data, rather than bear the expense and time loss of resampling. This tradeoff is made more difficult, because sample collection (in the field) and analysis (in the lab) may be separated significantly in time and space. These collection challenges mean that sample collection is not as simple or cheap as conventional laboratory science. However, they also give the field's practioners much enjoyment, working out of doors, with hands on tools and plants.
New measurements Initial work focused on measuring the tree ring width - this is simple to measure and can be related to climate parameters. But the annual growth of the tree leaves other traces. In particular maximum latewood density is another metric used for estimating temperature. It is, however, harder to measure. Other properties (e.g. isotope or chemical trace analysis) have also been tried. In theory, multiple measurements on the same ring will allow differentiation of confounding factors (e.g. precipitation and temperature). However, this part of the field has not yet borne out its promise, so to date most chronologies are based on ring width at limiting stands. Pinus taeda Cross section showing annual rings Cheraw, South Carolina Dendrochronology or tree-ring dating is the method of scientific dating based on the analysis of tree ring patterns. ...
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Relationship to global warming study Tree rings hold the promise of telling us whether 20th century warming is precedented or unprecedented (in last 1000 or so years). The importance of understanding posited global warming from man-made CO2, has moved dendroclimatology from a "sleepy science" started by Douglass to a high profile field with papers in Nature. The field has benefitted from increased funding and participation by good researchers. However, the field has also been impacted by the acrimonious nature of the popular debates around global warming.
While the number of things affecting growth of a tree ring may seem daunting, dendroclimatolgy is a new science. Improvements in basic methods are being made to get the most out of tree ring evidence. In addition, the inferences from tree rings (even if imperfect) are better than knowing nothing about previous climate.
See also The growth rings of an unknown tree species, at Bristol Zoo, England Pinus taeda Cross section showing annual rings, Cheraw, South Carolina Pine stump showing growth rings Dendrochronology or tree-ring dating is the method of scientific dating based on the analysis of tree-ring growth patterns. ...
Paleoclimatology is the study of climate change taken on the scale of the entire history of the Earth. ...
The temperature record of the past 1000 years describes the reconstruction of temperature for the last 1000 years on the Northern Hemisphere. ...
A varve is an annual layer of sediment or sedimentary rock. ...
External links - Trees as Indicators of Past Climate
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