CAUSES Ol CHANGE 



If we have reverted to the climate 

 characteristic of the early 1800's, what 

 displacements in the world agricul- 

 tural pattern will occur in the next 

 decade? 



The answers to these and a host of 

 related questions will require a much 

 more sophisticated knowledge of cli- 

 mate and the man-environment sys- 

 tem than we now possess. Time is 



short and the challenge to science is 

 clear. 



Environmental Change in Arid America 



One of the great controversies in 

 ice-age paleoecology is how to explain 

 the virtually simultaneous coast-to- 

 coast extinction of large mammals in 

 North America around 11,000 years 

 ago. We know, for example, that ele- 

 phants once existed even in the pres- 

 ently arid lands of the West. Paleon- 

 tologists have commonly recovered 

 the bones of Mammuthus columbi in 

 arid America, along with bones of 

 other extinct large mammals, includ- 

 ing horses, camels of two extinct 

 genera, extinct bison, and ground 

 sloth. 



Did the climate change suddenly? 

 Fossil elephants and the like inevitably 

 provoke visions of a wetter climate 

 and a more productive ecosystem 

 than today's arid land will support. 

 But the fossil-pollen record has indi- 

 cated otherwise. 



Fossil Pollen and 

 Other Forms of Evidence 



The technique of fossil-pollen anal- 

 ysis has proved of unique value in 

 determining what the vegetation and, 

 by implication, the primary produc- 

 tivity of arid America must have been 

 during the period when this region, 

 along with the rest of the continent, 

 supported large numbers of native 

 large mammals. 



Pollen is a very popular fossil be- 

 cause it is produced in quantity by 

 certain plants and, thanks to its acid- 

 resistant outer wall or shell, is pre- 

 served in many types of sediments. 

 Unlike fossils of larger size, pollen is 

 usually dispersed evenly throughout 



a deposit rather than aggregated in 

 one or a few distinct beds. Under 

 relatively uniform sedimentation, as 

 determined by closely spaced radio- 

 carbon dates, one can estimate the 

 intensity of the local pollen rain 

 through time, as Davis has done in a 

 study of vegetation history at Rogers 

 Lake, Connecticut. Different vegeta- 

 tion zones shed different amounts of 

 pollen — a tundra much less than a 

 forest, for example. This is revealed 

 by the fossil pollen extracted through 

 hydrofluoric-acid treatment of lake 

 muds. 



In many deposits, especially in arid 

 lands, absolute values cannot be esti- 

 mated. The relative amounts of the 

 dominant pollen types in a deposit can 

 be compared with the pollen content 

 of sediments presently being deposited 

 in areas of natural vegetation. Literal 

 interpretation of the relative pollen 

 percentage cannot be made — i.e., 10 

 percent pine pollen does not mean 

 that 10 percent of the trees in the 

 stand were pines. But the pollen 

 spectrum of all types identified in a 

 fossil count can be matched, through 

 computer programs or simple direct 

 comparison, with the pollen rain of 

 modern natural communities. This 

 method works especially well in west- 

 ern United States, where there are 

 extensive areas of relatively undis- 

 turbed vegetation. In this way, any 

 major or increasing number of minor 

 changes in vegetation through time 

 can be detected. 



As opportunity allows, the fossil- 

 pollen record can be compared with 

 other forms of evidence. Macrofossil 

 remains of plants, including seeds and 



leaves, are found in certain lake muds. 

 They have been reported in remark- 

 able abundance in ancient wood-rat 

 middens of certain desert regions by 

 Wells. The oldest rat's nests studied 

 by Wells are over 30,000 years in age, 

 essentially older than can be deter- 

 mined by the radiocarbon method. 



The Climatic Record of 

 Western America 



The fossil record of radiocarbon- 

 dated deposits covering the last 30,000 

 years in western America indicates an 

 initial cool, dry period becoming 

 colder and wetter by 20,000 to 16,000 

 years ago. At this time, there were 

 ponderosa-pine parkland and pinyon- 

 juniper woodland at elevations about 

 3,300 feet below their present lower 

 limits on western mountains. The fate 

 of prairie, both short and tall grass- 

 land, is unknown. The present prairie 

 region was occupied by spruce in the 

 north and pine in the south. This 

 suggests that arid America, like other 

 regions, was affected by the late 

 Pleistocene cooling associated with ice 

 advance over Canada. 



Around 12,000 years ago the cli- 

 mate changed rather rapidly, becom- 

 ing warmer and drier, until conditions 

 were only slightly cooler and wetter 

 than now. Modern vegetation zones 

 have occupied their present positions, 

 with minor fluctuations, continuously 

 for the last 8,000 years. 



Thus, the record shows that the 

 environment of western America in- 

 habited by mammoth, camels, native 



73 



