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Xeric Conifer Woodland, then and now
MB said something about Joshua trees and junipers in the same field of view a few days back, and I promised a post. It’s not actually all that uncommon to see the two species growing together, as the lower altitudinal limit for junipers in the Mojave is just about where the upper elevational limit of Joshua trees lies, with some overlap due to local microclimates and such. Sometimes a fault will run through the bedrock and provide a path for groundwater to approach the surface, and junipers — which in the Mojave are generally kept from growing lower than around 6,000 feet due to drought stress — will thrive a bit lower, their roots stuck in the water table. If you climb the Sierra Club trail up Teutonia Peak on Cima Dome and look back to the east from which you came, you’ll see a straight line of old junipers stretching a mile back to Sunrise Rock. They grow at just above 5,000 feet, and though they set abundant seed each year there are no other junipers nearby. You have to climb a few hundred feet up Teutonia to find junipers growing off the fault line. On the plain between Teutonia and Kessler peaks, the junipers form a line of perforation running through the thick Joshua trees, the only real source of shade in the forest.
Go to one of the junipers, look around for a while — mind the cholla stems — and you will likely find nearby a low mound of twigs and Joshua tree leaves and cactus skeletons, sometimes two feet high by five wide. This is the home of the desert woodrat, a.k.a the desert packrat, a.k.a. Neotoma lepida. Packrats are ubiquitous in the Joshua tree forest, and the wealth of berries a juniper provides each year prove a powerful inducement to settle nearby. The rats, which are the smallest of their genus at about eight inches from nose to tail tip, are mainly nocturnal: they come out of their nests — “middens” — at night and forage for plants to eat. On Cima Dome they seem especially fond of Joshua tree leaves. Or maybe that’s just what’s there. As you walk you’ll see trees where each leaf on one side of a particular branch has been sheared off evenly — nay, meticulously and methodically, as if the branch were a cob of corn being eaten by an ex-Navy man at a Fourth of July barbecue. This is the work of Neotoma lepida, and if you look around you will find a packrat midden within 50 feet: if not a pile of debris out in the open, then a comfy sheltered midden wedged between rocks or under overhangs, which is the setup the rats actually prefer.
Thus, two important facts about Neotoma lepida: they collect plant material and their range is about a hundred feet in diameter. Add to those two facts a third: a midden, once built, may house many generations of packrats. Many generations. Hundreds. Thousands. There’s a Pleistocene-era midden in the Colorado Rockies that was used more or less continuously from 950,000 years ago to 800,000 years ago: 150,000 years of habitation, a pile of sticks and leaves with a history compared to which the cathedral at Chartres is essentially a Quonset hut, Stonehenge a tilt-up strip mall. That, my friends, is tenancy.
Middens that are out in the open suffer the depredations of rain and snow, gales and determined badgers. The rock-fortified middens are more secure. Over the years the rats will bring in leaves and twigs, fruits and shiny stones and car keys, and then new layers of material will be brought in to put atop the old, and as the woodrats carry out their daily affairs in the midden they will urinate over it all over a period of years. Woodrat kidneys are desert kidneys, rather efficient at excreting salts without too much water to carry them, and the salty mess congeals into a hard, ochrous resin.
That resin — charmingly called “amberrat” by packratmiddenographers — preserves the plant material. In a cave, or under a nice big sheltering rock, away from the elements, the amberrat can preserve the material for thousands of years.
Thus an old packrat midden is a record of the vegetation of the immediate area over the span of time in which the midden has been used. And where you have thousands of years’ worth of well-preserved plant material collected from a precise location, there you have paleontologists. (Where the material is preserved by stanky rat piss, there you also have graduate students whose job it is to clean the stuff.) The plant material is identified and dated, and then we have a bit of a picture of how the vegetation in the area changed, if at all, over thousands of years. (Given the rats’ proclivity for collecting small pieces of bone, the middens occasionally provide a glimpse into animal life as well.)
And since packrats are thick on the ground in the southwest deserts, our recapture of information contaned in packrat middens is limited primarily by the supply of graduate students. There are lots and lots of data out there. And in the Mojave, one of the things those data tell us is this: the altitudinal ranges of both Joshua trees and junipers were once very different than they are today.
17,000 years ago the desert didn’t just smell like rain, it often felt like rain. Where there are now playas and salinas at the bottoms of Mojave valleys, there were broad freshwater lakes rich in wildlife. It was a wetter time, and though the countryside was not particularly lush, there was enough water in the soil that junipers could grow far down-slope. So could Joshua trees, for that matter, and even single-needle pines. In a number of places throughout the desert, the valley floors — when not flooded — were covered in what is sometimes called a Xeric Conifer Woodland, what modern-day desert rats of the two-legged variety call “P-J,” or Piñon-Juniper Woodland. Joshua trees were a minor component of that forest.
And then the dry came. Around 16,000 years ago the drought started, and it really got underway about 10-12,000 years ago. The lakes dried up, even the deepest one in the Mojave, Lake Manly, leaving its bed in Death Valley dry and crackling in the heat except for sometimes. The valley floors got too dry and hot for PJ with scattered Joshua trees. The first creosote bushes started to sprout on valley floors, and some of those plants are still alive. With their fruit dispersed by any number of birds and mammals, piñon and juniper gained a foothold in the (relatively) wetter mountains. Joshua tree didn’t rise quite as far. But deprived of competition with their former conifer neighbors, the Joshuas now dominate their part of the Mojave — at least visually.
In the Antelope Valley, 60 miles from the ocean, downstream from the well-watered San Gabriels and Tehachapis, the valley floor still hosts mixed forests of Joshua trees and junipers. Or at least it does in those few square miles, like in the photo above, not cleared for development. Flickers still cut their arcing paths between the trees, and woodrats collect sardine can keys and pull tabs to perplex the graduate students of 14007 AD.
Posted by: Chris Clarke
I wrote this late, and neglected to link to a painting of the Pluvial Xeric Conifer Woodland biome. My bad.
By: By Chris Clarke on 2007 05 06
Thanks, Chris. Now where do the Pinyon pines fit in? I’d never seen pines and Joshuas interspersed before that particular canyon. I’m sure it happens all the time, being a relative Mojave desert newbie, I was surprised.
By: By MBW on 2007 05 07
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