Growth Rings

① Growth rings, also known as tree rings or annual rings, are rings visible in horizontal cross-sections of the trunks of trees, in climates that vary by season or by year. They are the result of new growth in the vascular cambium, a layer of cells near the bark. This growth in diameter is also known as secondary growth; changes in secondary growth in vascular plants are responsible for the formation of growth rings in the trunks of trees. Changing seasonal conditions, particularly temperature and sunlight, lead to differences in tree growth that can be traced by examining growth rings. In areas with annual seasonal cycles, the tree rings can be counted to determine a tree’s age in years, while in areas without particularly distinct seasons, a tree’s rings may be indistinct or entirely absent.

② Each growth ring tends to be made up of two different parts, based on the growth conditions. Spring wood, more commonly referred to as early wood, grows quickly in the growing season, usually in the spring or early summer when the cambium produces numerous large cells with thin walls. Then, towards the end of the summer, growth slows down. The cells manufactured at this time of year are small, with thick walls. They form the summerwood, or latewood, which tends to be considerably denser than early wood and is generally darker in color. One year of growth is therefore represented by a ring consisting of a light part and a dark part. The darker wood is not formed in the winter, as some people believe, because the cambium is completely inactive in the winter.

③ Trees from the same region will tend to develop the same patterns of ring widths for a given period. These patterns can be compared and matched ring for ring with trees growing in the same geographical zone and under similar climatic conditions. Following these treering patterns of living trees back through time, chronologies or cross-dating can be built up both for entire regions and for sub-regions of the world. Thus, wood from ancient structures can be matched to known chronologies, whereby the age of the wood can be determined precisely. Cross-dating was originally done by visual inspection until computers were harnessed to do the statistical matching.

④ In fact, a cross section of a tree shows much more than its age. Diameter growth is particularly sensitive to fluctuations in the environment: moisture in the soil and air,temperature, and sunlight. Therefore, growth rings, particularly annual growth rings, can actually provide valuable records of the climate conditions during a tree’s lifetime. The study of climate change, as recorded by tree growth rings, is called Dendrochronology. In certain cases, trees can live for many hundreds of years, and in an extraordinary case, like the bristlecone pine, thousands of years. Each year, trees add growth rings, which can indicate what sort of growing season the tree experienced. With the ability of receiving sunlight limited in extremely hot and dry years, the rings tend to be smaller while with plenty of sunlight and precipitation in temperate conditions, the rings tend to be wider. Sometimes, multiple rings may form in a single year when excellent growing conditions alternate with distinctly bad conditions. There are almost no conditions in which a tree will not form a distinct ring during a year—one rarely recorded instance of a missing ring in oak trees occurred in the year 1816,also known as the Year without a Summer.

⑤ There are also limitations to this dendrochronological research. Trees in the temperate zone only record the growing season, so the winter season, no matter how dramatic,will not be seen in the ring record. Interestingly, trees in tropical regions grow year round and, therefore, show no real, obvious annual growth rings. Therefore, climate data from equatorial areas is difficult to piece out and use. The record is limited geographically in another way, too. Trees do not grow in all places on Earth, therefore we don’t have a tree ring record of climate change for each region and ecologic niche globally.

——2011年05月28日北美考试机经

According to Paragraph 2, all the following are differences between early wood and late wood EXCEPT:

A. Early wood normally has larger cells.

B. Early wood grows quicker than late wood.

C. Early wood is generally denser than late wood.

D. Early wood is lighter in color.

核心词汇：

词汇练习：

阅读下列句子，用所给单词（或词组）的正确形式填空：

equatorial temperate annual fluctuatio trace vary visual indistinct sensitive niche horizontal precipitation match

1. Rainfall was limited, and what water there was rushed across the plain in the _____ flood of melted snow. (TPO-26: Sumer and the First Cities of the Ancient Near East )

2. As a method of spanning space, the arch can support greater weight than a _____beam. (TPO-3: Architecture )

3. We need therefore to know how much the climate can _____ of its own accord in order to interpret with confidence the extent to which recent changes are natural as opposed to being the result of human activities. (TPO-10: Variations in the Climate )

4. The mediocre design of many contemporary buildings can be _____ to both clients and architects. (TPO-3: Architecture )

5. A color or pattern that is relatively _____ in one kind of light may be quite conspicuous in another. (TPO-17: Animal Signals in the Rain Forest )

6. When the planetarium sky _____ the sky outside, the birds fluttered in the direction of their normal migration. (TPO-11: Orientation and Navigation )

7. Of all the _____ arts, architecture affects our lives most directly for it determines the character of the human environment in major ways. (TPO-3: Architecture )

8. Wind power is a virtually unlimited source of energy at favorable sites, and even excluding environmentally _____ areas, the global potential of wind power is much higher than the current world electricity use. (Online Test: Electricity from Wind )

9. To build up a better picture of _____ appreciably further back in time requires us to use proxy records. (TPO-10: Variations in the Climate )

10. Succession is influenced by many factors: the nature of the soil, exposure to sun and wind, regularity of _____, chance colonizations, and many other random processes.(TPO-19: Succession, Climax, and Ecosystems )

11. In contrast, a complex climax community, such as a ____ forest, will tolerate considerable damage from weather to pests. (TPO-3: The Long-Term Stability of Ecosystems )

12. In the ____ mountains the low growth form is less prevalent. (TPO-1: Timberline Vegetation on Mountains )

13. Even if the new population is of a different species, it can approximately fill the ____vacated by the extinct population and keep the food web intact. (TPO-3: The Long-Term Stability of Ecosystems )

参考答案：

1. annual 2. horizontal 3. vary 4. traced 5. indistinct 6. matched 7. visual 8. sensitive 9. fluctuation 10. precipitation 11. temperate 12. equatorial 13. niche OpD232IyK1Hpoyt7Or+U9923Sj74wrWSzlNIFNR/DV2REKkqlDZgzG9xTd6Mp9xM