DUNG HAPPENS

     Peter Wigand is a brave man when it comes to looking for evidence about Southern Nevada's climate history.
      "I have both a fear of heights and a fear of snakes, and look what I do for a living," the Desert Research Institute scientist said one cloudless morning last week as he prepared to scale a steep, rocky slope on Mount Potosi about 20 miles southwest of Las Vegas.
      That's where clues about the climate have been preserved for 10,000 years in a pack rat's nest. The nest was wedged in a crevice and protected from the sun, wind and rain by a limestone ledge.
      Braced against the underside of this ledge, Wigand reached into the darkness and pulled out a hunk of a pack rat midden, the refuse pile of an ancient rodent. Wigand described it as a "weird-looking mass of crystallized plants and dung."
      For thousands of years, generations of pack rats, in this case bushy-tailed wood rats, hoarded twigs and pine needles, forming layers of debris and droppings that are sealed with urine.
      Urine may sound unpleasant, but Wigand said it kept air out and killed bacteria, preserving middens that otherwise would have decayed. Vegetation in the pack rats' nests, critical to providing information on the plant life of the past, also would have decayed, Wigand said.
      After 10,000 years, gone was the stink of dung. Instead, a tarlike pine scent emanated from the midden Wigand held with an outstretched arm.
      "Pack rat nests tell us when these plants arrived here," said Wigand, an ancient-ecology expert.
      By looking at the different types of vegetation found in the nests and age-dating them with a radiocarbon technique, scientists can deduce what Southern Nevada's terrain was like some 18,000 years ago when the planet began to thaw from the Ice Age.
      Scientists can re-create images of the region's landscape more than 10,000 years ago by comparing plant life found in the nests around Nevada to those found from similar dates throughout the Great Basin and as far north as eastern Washington and southern Idaho.
      "Today we get the Mojave Desert and desert shrub vegetation," Wigand said, referring to the valley formed by Red Rock Canyon, west of Las Vegas, where a cave containing middens overlooks the outskirts of Blue Diamond. "A lot of these plants are so-called recent invaders, as late as 9,000 years ago."
      Wigand said the change is due to the amount of rainfall and what time of year it came.
      "Ten thousand years ago, we didn't have much summer rainfall," he said. "What we had was winter rainfall that was about 2 1/2 times what we have (annually) today."
      Instead of the 4 inches of annual rainfall that Las Vegas now receives, the area was getting as much as 11 inches. At an elevation of 5,000 feet above sea level, that total was about 23 inches of rain a year.
      "What that means is you had a lot more rainfall at lower elevations, enough to recharge ground water in places," he said.
      In the Mount Potosi cave Wigand explored last week, other researchers have found evidence that juniper and pinyon trees thrived 15,000 years ago. That means this area back then would have featured open woodland with plenty of space between trees. Today, the terrain is dominated by white burr sage and creosote, both of which require summer rain. White burr sage didn't start growing in the area until about 8,500 years ago, Wigand said.
      Only Joshua trees survived the climate variations over the millennia.
      "They've been here for a long time," Wigand said. "We think it's well-drained soil that keeps them here."
      The natural time capsules preserved by pack rat nests were first discovered by scientists 30 years ago while conducting an inventory of plants on Frenchman Flat at the Nevada Test Site.
      "They just stumbled into these overhangs that had these crystallized masses," Wigand said. Curious about what they were, the scientists learned from age-dating that some were 16,000 years old.
      Next, the scientists -- including one Washington State University professor who would later teach Wigand -- branched out to other locations around Nevada and found that the wetter climate condition was true throughout Southern Nevada based on a wide range of middens of varying ages.
      Thus where desert now exists, timber pines, bristlecone pines and white fir trees once stood. These trees now are found primarily in higher locations.
      With different vegetation and more rainfall, Southern Nevada also was home to a different animal life.
      "You've got Colombian mammoths, ground sloths, two kinds of horses, American camels and giant condors," Wigand said. "We have a pack rat nest from east of Ash Meadows (near Death Valley) that had a condor leg bone with cartilage that was 18,000 years old. The Las Vegas Valley was wet enough that you had marshes with otters in them and lots of waterfowl."