palm-size Texas horned lizard tries to stay cool in the shade of a creosote
Deserts aren't necessarily the product of outside forces like decreasing rainfall, they say. Rather, it's the internal ecology of the desert itself--its web of plants, animals, and soil--that drives its growth to maturity and stability. Nor does the transformation of a grassland to a desert necessarily mean the creation of a place where life is more scarce--only one where life is rearranged.
In North America alone an estimated 1.1 billion acres have been desertified
Desertification is sufficiently serious a threat that representatives of 87 countries have drafted a treaty to combat it; only two other environmental crises--ozone destruction and global warming--have earned such attention
Jornada is an exceptional place. It is arguably the best-studied desert in the world. As Kris Havstad, the director of the U.S. Department of Agriculture's Jornada Experimental Range
May Humphrey Stacy after crossing the basin in 1857. The whole extent, as far as vision reached ahead, was a level plain, covered thickly with the most luxurious grass.
early settlers could water their livestock only at springs in the bordering mountains, they raised rather few cattle. It wasn't until we brought the technology to drill wells in places like this, where the water is 400 feet down, that this land opened up to heavy grazing, says Havstad. After water was brought to the surface in the 1880s, there were 20,000 head of cattle out here. This place just got hammered.
in 1912 USDA officials fenced off 192,000 acres of the basin and set up the Experimental Range, where they tried to understand what was happening and how they could stop it.
early researchers did not consider themselves ecologists. They were range scientists, dedicated to figuring out how to make the Jornada grow food for cattle. They did everything they could to stop the basin's transformation. They cut the herd down to a few hundred head. They tore up mesquite, poisoned the shrubs, seeded grasses, and dug giant pits to help water penetrate the ground. They failed. The Jornada researchers estimate that in 1858 about 5 percent of the range was dominated by mesquite or creosote, 37 percent had a few shrubs in it, and the remaining 58 percent was shrub free. Just over 100 years later, in 1963, 64 percent of the range was dominated by mesquite and creosote and none of the remaining 36 percent could unqualifiedly be called grassland any longer. Now, Havstad estimates, about 80 percent is classic desert shrub land.
We can move livestock, impose droughts, burn strips--we can do almost anything you can imagine
1800s, when the basin was a grassland. Though its climate was precariously dry, the ecosystem had remained stable for millennia, thanks in part to its ability to create its own weather. Its spongy soil soaked up rain, and when the water evaporated back into the air, it formed clouds that then recycled the rain back to the basin.
grassland was also able to shut out competing plants. While a few creosote bushes and mesquite trees grew in the basin, they had a tenuous existence [waiting for optimal conditions to thrive]
the shallow, dense roots of the grass absorbed the rain before it could percolate down to the shrubs' deeper roots.
ecologists speculate that as the cattle trampled the ground to their favorite feeding spots, the ground they habitually walked on became less able to absorb water. Rain flowed over this soil rather than into it, forming channels. No longer lingering in the upper soil, where grass roots grew, the water instead either escaped downstream or percolated through the channel bottoms, down to where only the deep-rooted shrubs could get to it. Hoofprints gave rise to pools of water that infiltrated the soil, creating spots for a seedling to take root and thrive. Water was no longer evenly spread over the basin, but now concentrated in scattered places.
[organic matter builds up under the new shrubs from wind/water erosion/deposits of soil and detritus matter. Animals also nest under shrubs]
[even the insects changed from chewing and creating frass droppings to more suckers that don't produce as fibrous droppings]
When a shrub dies, the island it leaves will be a nursery for a new one, which will be protected from fire by the distance from one island to the next. Neither hard, bare soil nor shrub-dominated islands will offer any hope to colonizing grass. And since the soil will hold less and less water, rain recycling will stop, making the desert even drier.
[7 years later] Just about all the evidence we've collected so far supports it,
Laura Huenneke, for example, has been measuring the mass of the Jornada vegetation, and she finds that the central tenet of the model holds true: arid grassland and shrub-dominated desert contain about the same weight of plant material. It's just arranged differently.
basic process seems universal. [global w/ different species]
model predicts that the chemicals important to life get concentrated under vegetation, while unnecessary elements like lithium and bromine remain smoothly scattered.
The debate has a hell of a lot less to do with science than emotion, says Whitford. It's about economics, about whose ox is getting gored.
In 1982 the Jornada ecologists closed off some plots from the USDA's cattle. Over the following ten years many of the plots that still had some grass in them dramatically improved, compared with unprotected grass nearby; meanwhile, shrub-dominated plots saw no change. Still, some of the plots untouched by cows also died out, suggesting that drought too must play a role in the long-term survival of grassland.
New Mexico State University ecologist William Conley has shown that droughts may not have been so important in the Jornada. Grass is indeed susceptible to drought, but only when it strikes during the summer growing season. The USDA's 80-year record of rainfall in the basin shows that droughts this century have actually hit the Jornada more during the winter, the growing season of mesquite and creosote. If anything, they should have helped the grassland survive. Conley also made a statistical analysis of the rainfall record that shows that the droughts were not freakish; they had probably hit the Jornada every few decades for centuries. If they had the power to desertify, the Jornada should have become a desert long before Mexicans first passed through it.
1993, on the border between the two plant communities, USDA researchers set up 18 fenced enclosures, each about 750 feet square. In 9 enclosures they hacked down the mesquite and painted the stumps with herbicide; the other 9 sites were left untouched.
Before this winter is over, Havstad will bring two dozen head of cattle into 6 of the corrals and let them graze for 24 hours, in which time they should devour two-thirds of the grass and trample the ground. This summer he will let them loose in 6 others, while the 6 remaining will stay unmolested. In the next five years the researchers will measure how the soil, plants, and animals change in response. Two of the grazed plots will be burned and 2 others will be covered in rain-out shelters to see how fires and droughts enhance or reduce the effects of grazing.
Climatically, North Africa should be a grassland savanna, he says. You can go back to historical records and read about the trees on the mountains and lush grass, about how they were the breadbasket for Rome, providing grain and meat to the empire