Two approaches are generally used to provide chronological control for the wood megafossils. Ray tracheids absent and walls of ray cells conspicuously pitted – Abies. (e)Ĭross-field pits small and bordered – Picea. (b)Ĭross-field pits large with indistinct borders – Pinus. Wood with abrupt latewood–earlywood transition. An example of a simple key for some typical boreal gymnosperm genera is as follows: 1. The absence of vessels identifies the wood as a gymnosperm (Pinophyta). Identification of wood megafossils can typically be done to the level of genus using thin sections of wood to examine the characteristics of the xylem cells and associated structures ( Core et al., 1979 Hoadley, 1990). The use of geographic positioning systems (GPS) to provide exact coordinates of the finds, careful note taking, and photographic documentation of the finds are important components of the field work. Finally, newly exposed landscapes created by the recent melting of perennial snowbanks or glacier fronts can reveal megafossils from previous warm periods. Megafossils can be recovered from some lake or peat cores. The waters of lakes can also be surveyed to find material lying on top of the sediment. In some cases, systematic foot surveys and transects are done of the tundra surface or along stream channels and other exposures. In addition to being found on the surface, wood megafossils are commonly preserved in the waters and sediments of lakes, peatbogs, fluvial deposits, and colluvium ( Figure 4(b) and 4(c)).įield sampling strategy to collect megafossils varies. Human population densities are generally low in the tundra zone, limiting, although not completely restricting, the removal or use of wood by people. 4.įire frequency in sparsely wooded treeline and tundra sites is generally extremely low, limiting the chance that wood will be consumed by fire. The radial growth rates for northern trees are often very slow, producing narrow growth rings and very dense wood, which is more resistant to water infiltration and rot than fast growing, less dense wood. The relatively dry conditions of many Arctic and sub-Arctic sites further restrict decomposition rates. The cold temperatures and brevity of the warm season in the Arctic and northern sub-Arctic restrict microbiological and macrobiological decomposition rates. Early to mid-Holocene wood megafossils: (a) Larix log lying on the tundra surface on the Taimyr Peninsula, Russia, (b) Pinus stump in the shallow waters of a small lake on the Kola Peninsula, Russia, and (c) Larix stumps covered with colluvium and subsequently exposed by stream erosion near the mouth of the Lena River, Russia.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |