Dendroclimatology is the science of determining past climates Climates encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elements in a given region over long periods of time. Climate can be contrasted to weather, which is the present condition of these same elements and their variations over periods up to two weeks from trees A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to 6 m; some authors set a minimum of 10 cm trunk diameter (primarily properties of the annual tree rings). Tree rings are wider when conditions favor growth, narrower when times are difficult. Other properties of the annual rings, such as maximum latewood Wood is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression. In the strict sense wood is produced as secondary xylem in the stems of trees (and other woody plants). In a living tree it transfers water and nutrients to the leaves and other growing tissues, and has a support function, density (MXD) have been shown to be better proxies than simple ring width. Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous. By combining multiple tree-ring studies (sometimes with other climate proxy Climate proxies are devices that suggest the climate patterns of the past, even before those patterns were archived by humans. To produce the most precise results, systematic cross-verification between proxy indicators is necessary for accuracy in readings and record-keeping. The study of past climates is known as paleoclimatology. Examples of records), scientists have estimated past regional and global climates (see Temperature record of the past 1000 years The temperature record of the 2nd millennium describes the reconstruction of temperatures since 1000 CE on the Northern Hemisphere, later extended back to 1 CE and also to cover the southern hemisphere. A reconstruction is needed because a reliable surface temperature record exists only since about 1850. Studying past climate is of interest for).

Contents

Advantages

Tree rings are especially useful as climate proxies Climate proxies are devices that suggest the climate patterns of the past, even before those patterns were archived by humans. To produce the most precise results, systematic cross-verification between proxy indicators is necessary for accuracy in readings and record-keeping. The study of past climates is known as paleoclimatology. Examples of in that they can be well-dated (via matching of the rings from sample to sample, i.e. dendrochronology Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. This has three main areas of application: paleoecology, where it is used to determine certain aspects of past). 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 are clearly demarked in annual increments, as opposed to other proxy methods such as boreholes A borehole is the generalised term for any narrow shaft drilled in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases (such as natural gas), as part of a geotechnical investigation or environmental site assessment, for mineral. Furthermore, tree rings respond to multiple climatic effects (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 #Climate factors.

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 adjust for these challenges.

Confounding factors

There are multiple climate and non-climate factors as well as nonlinear In mathematics, a nonlinear system is a system which is not linear, that is, a system which does not satisfy the superposition principle, or whose output is not directly proportional to its input. Less technically, a nonlinear system is any problem where the variable to be solved for cannot be written as a linear combination of independent effects that impact tree ring width. Methods to isolate single factors (of interest) include botanical studies to calibrate growth influences and sampling of "limiting stands" (those expected to respond mostly to the variable of interest).

Climate factors

Climate factors that affect trees include temperature, precipitation, sunlight, and wind. 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 more affected by temperature variation (which is "limited") than precipitation variation (which is in excess). Conversely, lower elevation treelines are expected to be more affected by precipitation changes than temperature variation. This is not a perfect work-around as multiple factors 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 multiple climate factors. However, this method is rarely used.

Non-climate factors

Non-climate factors include soil, tree age, fire, tree-to-tree competition, genetic differences, logging or other human disturbance, herbivore impact (particularly sheep grazing), pest outbreaks, disease, and CO2 concentration. For factors which vary randomly over space (tree to tree or stand to stand), the best solution is to collect sufficient data (more samples) to compensate for confounding noise. Tree age is corrected for with various statistical methods: either fitting spline curves to the overall tree record or using similar aged trees for comparison over different periods. Careful examination and site selection helps to limit some confounding effects, for example picking sites undisturbed by modern man.

Non-linear effects

In general, climatologists assume a linear dependence 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. For instance, at the upper treeline, where the tree is "cold limited", it's unlikely that nonlinear effects of high temperature ("inverted quadratic") will have numerically significant impact on ring width over the course of a growing season.

Botanical inferences to correct for confounding factors

Botanical studies can help to estimate the impact of confounding variables and in some cases guide corrections for them. These experiments may be either ones where growth variables are all controlled (e.g. in a greenhouse A greenhouse is a structure with a glass or plastic roof and frequently glass or plastic walls; it heats up because incoming visible solar radiation from the sun is absorbed by plants, soil, and other things inside the building. Glass is transparent to this radiation. The warmed structures and plants inside the greenhouse re-radiate this energy in--add ref), partially controlled (e.g. FACE [Free Airborne Concentration Enhancement] experiments—add ref), or where conditions in nature are monitored. In any case, the important thing is that multiple growth factors are carefully recorded to determine what impacts growth. (Insert Fennoscandanavia paper reference). With this information, ring width response can be more accurately understood and inferences from historic (unmonitored) tree rings become more certain. In concept, this is like the limiting stand principle, but it is more quantitative—like a calibration.

Divergence problem

Main article: divergence problem

The divergence problem is the disagreement between the temperatures measured by the thermometers A thermometer (from the Greek θερμός meaning "warm" and meter, "to measure") is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs (instrumental temperatures) on one side and the temperatures reconstructed from the widths of tree rings on the other side, in the northern forests.

While the thermometer records indicate a substantial warming trend, many tree rings do not display a corresponding change in their width.[2] A temperature trend extracted from tree rings alone would not show any substantial warming. The temperature graphs calculated in these two ways thus "diverge" from one another since the 1950s, which is the origin of the term.

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 it can be shown that the parameter of interest (temperature, precipitation, etc.) varies similarly from area to area, for example by looking at patterns in the instrumental record. Then one is justified in extending the dendroclimatology inferences to 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 ring width is used to infer the overall climate change during the corresponding year (an approximation). Another problem is "memory" or autocorrelation Autocorrelation is the cross-correlation of a signal with itself. Informally, it is the similarity between observations as a function of the time separation between them. It is a mathematical tool for finding repeating patterns, such as the presence of a periodic signal which has been buried under noise, or identifying the missing fundamental. 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.

Collection difficulties

Tree rings must be obtained from nature, frequently from remote regions. This means that special efforts are needed to map sites properly. In addition, samples must be collected in difficult (often sloping terrain) conditions. Generally, tree rings are collected using a hand-held borer device, that requires skill to get a good sample. The best samples come from felling a tree and sectioning it. However, this requires more danger and does damage to the forest. It may not be allowed in certain areas, particularly with the oldest trees in undisturbed sites (which are the most interesting scientifically). As with all experimentalists, dendroclimatologists must, at times, decide to make the best of imperfect data, rather than resample. 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 data gathering is not as simple or cheap as conventional laboratory science. However, they also give the field's practitioners much enjoyment, working out of doors, with hands on trees and tools.

Other measurements

Initial work focused on measuring the tree ring Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. This has three main areas of application: paleoecology, where it is used to determine certain aspects of past 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 (MXD) is another metric used for estimating environmental variables [1]. 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, most studies are still based on ring widths at limiting stands.

Measuring radiocarbon Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method to date archaeological, geological, and hydrogeological samples. Carbon-14 was discovered on 27 February 1940, by Martin Kamen and Sam Ruben at the concentrations in tree rings has proven to be useful in recreating past sunspot Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. Although they are at temperatures of roughly 3,000–4,500 K , the contrast with the surrounding activity, with data now extending back over 11,000 years.[3]

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 to a high-profile field. The field has benefitted from increased funding and participation by more researchers. However, the field has also been impacted by the acrimony of the popular debates around global warming.

Dendroclimatology is a young science—improvements in methods are being made to squeeze the most insight from tree-ring evidence. While the number of factors affecting growth of a tree ring may seem daunting, there is much good information tied up in tree ring records. Current inferences from tree rings (even if imperfect) are better than knowing nothing about previous climate.

Notes

  1. ^ IPAE RAS Dendrochronology group research results summary
  2. ^ D'Arrigo, Rosanne; Wilson, Rob; Liepert, Beate; Cherubini, Paolo (2008). "On the ‘Divergence Problem’ in Northern Forests: A review of the tree-ring evidence and possible causes". Global and Planetary Change (Elsevier) 60: 289–305. doi A digital object identifier is a character string used to uniquely identify an electronic document or other object. Metadata about the object is stored in association with the DOI name and this metadata may include a location, such as a URL, where the object can be found. The DOI for a document is permanent, whereas its location and other metadata:10.1016/j.gloplacha.2007.03.004. http://www.ldeo.columbia.edu/~liepert/pdf/DArrigo_etal.pdf.
  3. ^ Solanki, S.K.; Usoskin, I.G., Kromer, B., Schüssler, M. and Beer, J. (2004). "Unusual activity of the Sun during recent decades compared to the previous 11,000 years". Nature 431 (7012): 1084–1087. doi A digital object identifier is a character string used to uniquely identify an electronic document or other object. Metadata about the object is stored in association with the DOI name and this metadata may include a location, such as a URL, where the object can be found. The DOI for a document is permanent, whereas its location and other metadata:10.1038/nature02995. PMID A PMID is a unique number assigned to each PubMed citation of life sciences and biomedical scientific journal articles. The related Pubmed Central archive may additionally assign a separate number, a PMCID (PubMed Central Identifier), normally written with a PMC prefix 15510145. http://www.ncdc.noaa.gov/paleo/pubs/solanki2004/solanki2004.html.

See also

References

External links

Categories: Paleoclimatology | Dendrology

 

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