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1 The Pleistocene and Holocene Epochs Prehuman Context 3 For most of the past two million years the prehistoric landscapes of the Great Basin were in the grip of the Pleistocene “Ice Age.”1 Through these millennia, vegetation responded to glacial and interglacial cycles of frigid and warm periods , and a variety of large mammals roamed the landscape (appendix 1 summarizes these events). Paleoenvironmental studies have expanded our understanding of the late Pleistocene epoch in recent years. Core samples containing pollen from ponds, bogs, and lakes, and from sediments deposited in caves have allowed scientists to develop a virtually continuous sequential record of the plants of the region, many of which can be identified to genus or species. Studies of fossilized plant materials preserved in wood rat den middens have advanced our knowledge of climate and allowed the determination of vegetation trends going back tens of thousands of years.2 Studies of growth rings using living and dead trees provide a definitive record of growing conditions.3 The Pleistocene vegetation of the Panamint Range bordering Death Valley, reflecting colder conditions 30,000–11,000 years ago,contrasts sharply with the vegetation we see in what is now one of the hottest and driest places on earth. Today’s vegetation is largely creosote bush and white bursage. The vegetation 20,000 years ago was dominated by shadscale, Joshua trees, Utah juniper, and Whipple yucca. In the latter stages of the Pleistocene, Joshua trees grew at elevations down to 1,400 feet; today they are found no lower than about 4,000 feet.4 To the northeast, in the Snake Range, pollen samples from a cave spanning 40,000 years indicate that bristlecone pine was a predominant plant both in the mountains and in the lowlands through the late stages of the Pleistocene. Pinyon pine and juniper woodlands and desert species now cover most of the lower-elevation late glacial sites that once supported bristlecone pine, which now grows 3,000 feet higher.5 Pleistocene pollen samples from Hidden Cave in northwestern Nevada indicate that yellow pines grew at much lower elevations 4 | n e v a d a ’ s c h a n g i n g w i l d l i f e h a b i t a t than today. Saltbushes, such an important vegetal component today, were at best rare during the latter stages of the Pleistocene.6 In northwestern Nevada, wood rat middens as much as 30,000 years old provide evidence of the plants that characterized Pleistocene vegetation. Juniper woodland covered much of the area at intermediate elevations. Whitebark pine and limber pine, now absent, grew as low as 4,530 feet in the Virginia Mountains near Reno. During this cooler period, curlleaf mountain-mahogany occurred 2,625 feet lower than it does today.7 Studies in northern Nevada suggest that the vegetation was of an alpine and subalpine character during the late glacial period. Along the Snake River Plains–Great Basin divide, subalpine coniferous forests including subalpine fir, whitebark pine, and lodgepole pines predominated. These species grow at much higher elevations today. During the last full glacial episode, the Wisconsin, mountain glaciers were present in sixteen of Nevada’s mountain ranges—the most impressive being in Lamoille and Rattlesnake canyons on the western slope of the Ruby Mountains southeast of Elko. Calculations derived from Late Pleistocene vegetation and glaciers suggest the annual mean temperature was 9˚F–13˚F lower than today. Glaciers were already waning when pluvial lakes (shallow runoff lakes) in the valleys were hitting their high-water mark some 14,200 years ago.8 The largest of these, Lake Lahontan, covered 8,665 square miles in western and northwestern Nevada and had a shoreline approximately 10,000 miles long. Pyramid and Walker lakes remain as remnants of this once vast freshwater body. Lake expansion from melting of the glaciers occurred under a warmer, wetter climate . Rapid wasting of glaciers, shrinking of Lake Lahontan and other shallower lakes, and catastrophic flooding accompanied postglacial conditions, marking the onset of the warmer Holocene epoch about 10,000 years ago. The interglacial Holocene epoch, which continues today, has featured periods of distinctly different climates. Fluctuating climates have resulted in elevation and latitudinal shifts in vegetation, falling and rising lake levels, and buildup and melting of glaciers. The climate in the Early Holocene, 10,000–7,500 years ago, was initially moist. During this period the Ruby Marsh was a freshwater lake deeper than today...

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