Читать книгу Reconstructing Earth's Climate History - Kristen St. John - Страница 19

Trees grow both up and “out” each year, getting taller and expanding the trunk diameter. In locations with strong seasonality, this growth is marked by a pattern of rings inside the tree trunk. Each ring typically includes a lighter colored portion that results from tree growth during the spring and early summer, and a narrower and darker colored portion that results from slower tree growth during the fall; together each light and dark couplet marks a year of tree growth. The tree ring pattern is influenced by the climate conditions in the growing area. For example, a drought can limit growth, resulting in a narrower ring. You will work with tree ring data from SW North America in Chapter 14.

1 Re‐examine Figure 1.4b showing the global distribution of tree ring sample locations. How might tree ring development differ in tropical versus temperate latitudes?

2 Go to the supplemental resources to watch a video on coring living trees for paleoclimate research.What are two factors to consider when selecting a live tree for coring?Why is it important to go at least half‐way into the center of a tree?

3 While the oldest living trees are nearly 10 000 yr old, most trees do not live this long. For example, the average lifespan of a paper birch is 100 yr, and the average lifespan of a post oak is 250 yr. Extending the historical climate record from living trees further into the past requires correlating recognizable ring patterns in overlapping samples from living trees to dead trees (e.g. tree stumps and wooden beams in archeological sites) of the same species in the same geographic region. Why do you think it is important to use overlapping samples from trees of the same species to reconstruct past changes in regional climate?

Correlating ring patterns in overlapping samples not only extends the paleoclimate record that tree ring data can provide, but also confirms the dendrochronology (i.e. the tree ring timeline) – it helps ensure accurate ages are assigned to each layer by identifying the common pattern and flagging individual irregularities (e.g. incomplete rings, or missing rings). In addition, radiometric dating (in this case ¹⁴C dating) is commonly used in conjunction with ring counting techniques to ensure good age control. You will learn more about radiometric dating in Chapter 3.