Shulou(Shulou.com)11/24 Report--

Trees engrave environmental changes in rings, the weather is simple, using a tree core to record thousands of years of sunshine and cold, from a piece of broken wood to see the vicissitudes of life for hundreds of millions of years. This is a true story of one flower and one world, from the most loyal and straightforward historians in the world.

How is the ring of a tree formed? Mr. Shu has a heavy "gentleman's baggage" and always shows people in a well-dressed image, wrapped in a triple thick coat of xylem, cambium and phloem from the inside to the outside. The cambium cells in the middle are active and rely on the continuous division and differentiation of cambium cells to produce new xylem and phloem, and the trees become thicker day after day. The growth of the cambium will be affected by the external environment. After the spring gets warmer, the cells become particularly active, the summer weather gets cooler, and the cells become lazy in autumn. In the cold winter, the cambium may also enter dormancy and suspend activities. Wait for the recovery of the following spring.

Fig. 1 | the part of cells in which the stem structure of woody plants differentiates into xylem, because the changes of the four seasons show different appearance: xylem grows faster in spring and summer, and the cell gap is large, so the texture is loose, and this part of wood is called "early wood". In autumn and winter, trees grow slowly, and the xylem cells formed are compact and darker, so they are called "late wood". There is usually only a narrow circle in the tree species with rapid changes in rings (a clear demarcation between early and late wood within a year). The layers of circles with different widths overlap year after year and become what we see as "rings".

Figure 2: except for the marks left by the normal changes of the four seasons, the dry, wet, cold and warm differences between years will also be recorded by tree rings. In short, if the external conditions are suitable for growth, it is easy to form light and wide xylem, and vice versa.

In our two previous articles on climate and history (climate change has made "countless buildings shrouded in wind and rain"? Interpretation of history and climate in terms of environmental science: in those years, many of the cases mentioned in nature were climate reconstruction through trees, fossilized trees, or wood products. Compared with loess, ice cores, stalagmites and deep-sea sediments, Mr. Tree, who abides by the "one circle a year" rule, has several great advantages such as high resolution, high continuity and convenient dating. Years have carved traces on its body, and when these traces are discovered by environmentalists, they become important messages for the restoration of the ancient environment.

The password sealed in the tree ring 1. Tree ring width and density: cold and warm I know that through X-ray irradiation combined with optical densitometer scanning, we can get the detailed information of the width and density of early and late wood. This set of analytical methods has become a standard experimental method in tree ring climatology. Because the growth and division of cambium cells are closely related to temperature, precipitation and other conditions, the analysis of the formed rings can infer the basic climatic characteristics of each year during plant growth.

Fig. 3 | schematic diagram of tree ring density measured by X-ray analysis (1-5 represents maximum late wood density, minimum early wood density, early wood width, late wood width and whole ring width, respectively) temperature and precipitation have an impact on tree rings, so annual rings alone may not be able to accurately deduce the climate of the current year. In practical application, tree ring chronology and other indicators are usually combined to complete the reconstruction of paleoenvironment. Of course, there are general rules for reference: when the precipitation is low, the ring width is negatively related to temperature; when the precipitation is appropriate, the effect of temperature is not significant; when the precipitation is high, the ring width is positively correlated with temperature, but there is a certain limit.

The method of correlating temperature and humidity with ring width is more suitable in high latitudes and high elevations. In the temperate zone, the temperature and humidity conditions are moderate, the sensitivity of ring width to climate change is low, but there may be obvious differences in the change of tree ring density. It is confirmed that there is a significant positive correlation between the maximum late wood density of tree ring and summer temperature.

In short, Mr. Shu seriously fulfilled his duties as a "climate historian". The following two images are from research projects based on Central Europe (Poland and Slovakia) and Delingha, Qinghai Province, China, showing the effectiveness of extrapolating temperature and precipitation based on tree ring characteristics.

Figure 4 | comparison of temperature reconstructed based on ring characteristics (red line) with instrument temperature record (black line). An and B are respectively based on the prediction of ring width and maximum late wood density (from B ü ntgen, etc.)

Figure 5 | High correlation between precipitation based on Qilian cypress wheel width inversion and measured records (from Shao Xuemei et al.) 2. Secret invisible to the naked eye: isotope low latitudes are hot all the year round and the boundaries of the four seasons are not clear, making it impossible for many tree species to form clear rings. In this case, will the tree ring lose its research value?

In addition to width and density, stable isotopes such as D, 18O, 13C and 15N stored in tree rings can also be used as indicators of climate change, and their accuracy is no less than that of width and density. There are also seasonal periodic variations in δ 13C and δ 18O. With the help of isotope means, we hope to refine the paleoclimate reconstruction to the annual variation.

The 18O in the tree ring can reflect the changes of δ 18O in relative humidity, temperature and precipitation, which is further related to more environmental information of the year. For example, a study from Nagoya University revealed the correlation between δ 18O and ENSO (El Nino-Southern Oscillation) in the cellulose of cypress rings in Southeast Asia. It can be seen that during the half century from 1950 to 2000, the tree ring δ 18O value and MEI (the multivariate composite index reflecting ENSO) have a basically synchronous change trend.

However, using oxygen isotopes to record short-term environmental changes is not a tree ring's strength. Mr. Shu has taken corresponding remedial measures: the δ 18O of leaf water can well record the climate information on a short time scale. Tree rings are better at keeping records with carbon isotopes.

Fig. 6 | Tree ring δ 18O and MEI most of the carbon in trees comes from CO2 absorbed by photosynthesis, and some C atoms migrate and are retained in tree rings. Photosynthesis consists of a series of enzymatic reactions (chemical reactions participated in by enzymes as catalysts). The activity of biological enzymes changes with the change of temperature, and the change of saturated water vapor pressure affects the stomatal conductance of leaves. So the tree ring δ 13C can reflect the temperature information. In addition, environmental humidity, wind speed and direction, soil nutrients and salinity also affect the value of δ 13C. Stable carbon isotopes can reconstruct paleo-atmospheric CO2 and paleoenvironmental parameters, and even record extreme weather events and air pollution.

Fig. 7 | comparison of δ 13C values between the pollution site and the control site (from Martin, etc.) Air pollution increases stomatal resistance, decreases its conductance and reduces CO2 absorption in the air. Because there is a mass difference between 12C and 13C, plants are easier to absorb and utilize the lighter 12C, so the C atoms in plants tend to be lighter than CO2 in the atmosphere, that is, the value of δ 13C is lower. However, the pollution hindered the normal work of the "takeout boy"-the stomata, and Mr. Shu, who could not receive the fresh 12C takeout, had to make use of the stored "old stock" to meet an emergency. Therefore, the δ 13C value of tree ring in the polluted period is higher than that in the non-polluted period.

3. "born in distress" pseudo-wheel: Mr. Tree will occasionally do something out of line in the face of adversity, such as working in the opposite direction, violating the rule of "one circle a year" and growing two or more rings in a year. Some of these tree species are naturally so wild and unruly, such as citrus. For most species, the N-circle of a year is indeed an abnormal phenomenon, so it is called "pseudo-ring" or "false ring" (False Ring).

If you read the formation process of the real rings before the civilization, then you can probably understand how the false rings are formed: after the trees enter the growing period, if they unfortunately encounter serious disasters such as drought, frost, hail, diseases and insect pests, and the growth of the cambium is hindered, at this time, the growth of the cambium is similar to that of the normal rings, and the trees grow quickly again after the disaster, until the severe cold of autumn and winter comes, and then the real latewood is formed.

In most regions of temperate and sub-cold zones, the formation of false rings usually indicates the emergence of extreme weather, including unusually low precipitation in spring and summer, extremely high temperatures and so on. Combined with the width and density of annual rings in the current year, abnormal climate events in local history can be inferred. In the Mediterranean climate region, there is less rain in the high temperature period and more rain in the low temperature period, this uncoordinated combination of rain and heat is disadvantageous to the growth of trees, and the low temperature or lack of water is more likely to affect the cambium activity and increase the possibility of pseudo-rings.

Figure 8 | Special vegetation type in Mediterranean climate: subtropical evergreen hard-leaved forest. The plant in this picture is an olive tree. Pseudo-rings are not the only abnormal structure in tree rings. These anomalies also include missing rings formed by long-term interruption of cambium activity, shallow wheels formed by the reduction of late wood belts, frost wheels formed by sudden drop in temperature, fire scars caused by combustion, and traumatic resin channels formed by trees actively resisting external damage. They represent special events in a historical period, including climate anomalies and sudden disasters, such as frosts, forest fires, long-term low temperatures caused by volcanic eruptions, and so on.

Figure 9 | impact of climate change on tree ring characteristics (from Li Caijuan et al.) 4. Eccentricity and plate rotation: more than 100 million years of vicissitudes of life in the primary school science class teacher taught us: the direction of dense tree rings is in the north. In the middle and high latitudes of the northern hemisphere, the light conditions on the south side are better than those on the north side, and the south side of the trunk grows faster, so the rings are sparse. The phototropism of the ring enables it to preserve some information related to the direction. A joint team from the Chinese Academy of Geological Sciences and Nanjing Institute of Geology and Palaeontology used tree rings to reveal the location changes of the North China block over a period of 100 million years. They measured the eccentricity (eccentricity) of some newly felled tree rings in Beijing and Jilin Province, and compared the eccentricity of fossil tree rings preserved since the Jurassic (eccentricity is the direction of the longest distance from the trunk to the outermost growth ring), thus discovering the secret of the earth.

Fig. 10 | the eccentricity of tree rings of living trees and fossilized trees is 219 °±5 °in the southwest. In the fossilized trees from the Middle and late Jurassic, the eccentricity of the rings is 237 °and 233.5 °in the southwest, respectively. The reason why the direction is not close to due south may be that trees grow faster when the sun is in the southwest. In other words, compared with the Jurassic period, trees now rotate about 16 °counterclockwise in the direction in which they are most exposed to light. The "absolute position" of the light source is unchanged, and the change of the relative position between the light and the ground reference means that the North China block has rotated clockwise since the Jurassic. After correction by means of paleomagnetism, the rotation angle of the block is estimated to be 26.5 °.

Summing up rings records more than just the lifespan of trees. The width and direction of the ring, the density and isotope content of wood, and the scars left by the wind and frost are the strongest evidence of climate and environmental change. A tree can live for more than a hundred years, and the trees will last forever. Write a chronicle that belongs exclusively to nature.

The author wrote a poem for Mr. Shu:

Chanting trees

I have seen withered glory for thousands of years, and cloth leaves hold Yuheng in the sky.

If you want to know how many frosts and rains, how many times will it be sunny when you are old?

Note: Yu Heng, an ancient celestial instrument. Here, "Zhi Yuheng" is used as a metaphor to record the climatic characteristics.

reference

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[13] https://www.163.com/dy/article/GHCJ596T05527ZAB.html

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This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), written by Jiang Anyuan, proofread: Xingsi, Ravens, edited by Qiga cc

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