Read Online The Next Species: The Future of Evolution in the Aftermath of Man by Michael Tennesen - Free Book Online
Authors: Michael Tennesen
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control woody vegetation.
To get a look at how shrubs outcompete grasses, I followed Rob Jackson, a professor of environmental earth system science at Stanford University, on a warm afternoon, down a tall ladderinto the underground caverns of Powell’s Cave, about 150 miles (240 kilometers) west of Austin, Texas. We entered a world of stalactites and stalagmites in the porous limestone bedrock of the Edwards Plateau in west-central Texas. I scurried after Jackson through a maze of caverns and tight crawl spaces, over slippery pathways, and into rooms filled with glistening multicolored limestone structures, all carved by nature. We arrived at a point about sixty feet (eighteen meters) below the ground, where an underground stream gushes from the rock.
Like the British geologists who went underground to look for evidence of glaciers, Jackson went deep to try to explain how native juniper trees on the Edwards Plateau have invaded grasslands and are taking over. He showed me several thick tree roots that appeared to burst from the limestone walls, reach down into the stream, and suck water out of it. He explained to me: “A single taproot can provide a third or more of the tree’s water during a drought.”
Junipers put out roots along the full depths of their root systems so that they can get water from deeper roots in times of drought, and from shallower roots in times of rain. This gives them an advantage over grasses, which can pull only from their shallow roots no matter what the weather.
Woody shrubs and trees like junipers have invaded arid and semiarid grasslands and savannas in the US. Their presence limits the grass available for land managers, ranchers, and wildlife. Studies show that increased shrub and tree growth can rob from one-third to two-thirds of the stream water.
Juniper, mesquite, creosote, and Chinese tallow are problem plants in different parts of the US, particularly in the southern regions—the Great Plains, the Southwest, and the Gulf Coast. These plants existed here before, but overgrazing, fire suppression, and climate change have allowed plant populations to explode. Their increased presence has led tothickets that don’t allow enough light or room for other plants or grasses. “Thicketization” was what Steve Archer, a professor at the School of Natural Resources at the University of Arizona, Tucson, called it when I met him at an Ecological Society of America conference in Austin, Texas. Archer studies the ecology, management, and restoration of rangelands, which are any extensive area of land occupied by native vegetation that is grazed by domestic or wild animals that eat plants.
When huge herds of cattle were introduced to the western United States in the late 1800s, they devastated the grasses. This reduced the fuel for grass fires, and in their absence woody plants got better established. In earlier times, Indians regularly burned the grassy meadowshere to clear brush and trees and open them up for hunting. Today fire suppression is one of the problems promoting the juniper invasion. Without grass fires, woody plants spread unabated.
But it’s more than a recent problem. It goes all the way back fifteen thousand years ago to Ice Age hunters, who wiped out the large animals that once ate the woody plants in the grasslands of North America. In East Africa, they still have elephants that control woody plants, one of their natural foods. But the US no longer has wildlife populations that can do the job, so woody vegetation grows uncontrolled.
Woody plants in the US can be bad for ranchers, farmers, and wildlife. Black-capped vireos and golden-cheeked warblers, both considered endangered in Texas, are two species that need a mixed landscape of forest and open grassland to thrive. Woody vegetation suppresses the grasses and the open space that comes with them.
An example of the woody plant problem can be found at theTallgrass Prairie Preserve near Pawhuska, Oklahoma, the largest
To get a look at how shrubs outcompete grasses, I followed Rob Jackson, a professor of environmental earth system science at Stanford University, on a warm afternoon, down a tall ladderinto the underground caverns of Powell’s Cave, about 150 miles (240 kilometers) west of Austin, Texas. We entered a world of stalactites and stalagmites in the porous limestone bedrock of the Edwards Plateau in west-central Texas. I scurried after Jackson through a maze of caverns and tight crawl spaces, over slippery pathways, and into rooms filled with glistening multicolored limestone structures, all carved by nature. We arrived at a point about sixty feet (eighteen meters) below the ground, where an underground stream gushes from the rock.
Like the British geologists who went underground to look for evidence of glaciers, Jackson went deep to try to explain how native juniper trees on the Edwards Plateau have invaded grasslands and are taking over. He showed me several thick tree roots that appeared to burst from the limestone walls, reach down into the stream, and suck water out of it. He explained to me: “A single taproot can provide a third or more of the tree’s water during a drought.”
Junipers put out roots along the full depths of their root systems so that they can get water from deeper roots in times of drought, and from shallower roots in times of rain. This gives them an advantage over grasses, which can pull only from their shallow roots no matter what the weather.
Woody shrubs and trees like junipers have invaded arid and semiarid grasslands and savannas in the US. Their presence limits the grass available for land managers, ranchers, and wildlife. Studies show that increased shrub and tree growth can rob from one-third to two-thirds of the stream water.
Juniper, mesquite, creosote, and Chinese tallow are problem plants in different parts of the US, particularly in the southern regions—the Great Plains, the Southwest, and the Gulf Coast. These plants existed here before, but overgrazing, fire suppression, and climate change have allowed plant populations to explode. Their increased presence has led tothickets that don’t allow enough light or room for other plants or grasses. “Thicketization” was what Steve Archer, a professor at the School of Natural Resources at the University of Arizona, Tucson, called it when I met him at an Ecological Society of America conference in Austin, Texas. Archer studies the ecology, management, and restoration of rangelands, which are any extensive area of land occupied by native vegetation that is grazed by domestic or wild animals that eat plants.
When huge herds of cattle were introduced to the western United States in the late 1800s, they devastated the grasses. This reduced the fuel for grass fires, and in their absence woody plants got better established. In earlier times, Indians regularly burned the grassy meadowshere to clear brush and trees and open them up for hunting. Today fire suppression is one of the problems promoting the juniper invasion. Without grass fires, woody plants spread unabated.
But it’s more than a recent problem. It goes all the way back fifteen thousand years ago to Ice Age hunters, who wiped out the large animals that once ate the woody plants in the grasslands of North America. In East Africa, they still have elephants that control woody plants, one of their natural foods. But the US no longer has wildlife populations that can do the job, so woody vegetation grows uncontrolled.
Woody plants in the US can be bad for ranchers, farmers, and wildlife. Black-capped vireos and golden-cheeked warblers, both considered endangered in Texas, are two species that need a mixed landscape of forest and open grassland to thrive. Woody vegetation suppresses the grasses and the open space that comes with them.
An example of the woody plant problem can be found at theTallgrass Prairie Preserve near Pawhuska, Oklahoma, the largest
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