Dirt. David R. Montgomery
Читать онлайн книгу.
a foot every century.
By the 1920s the surface of the river towered thirty feet above the flood-plain during the high-water season. This guaranteed that any flood that breached the levees was devastating. Floodwaters released from the confines of the levees roared down onto the floodplain, submerging farms, towns, and sometimes even whole cities beneath a temporary lake. In 1852 the river jumped its dikes and flowed north, flooding cities and villages and killing millions of people before draining out hundreds of miles to the north. More than two million people drowned or died in the resulting famine when the river breached its southern dike and submerged the province of Henan during the flood of 1887-89. With the river flowing high above its floodplain, levee breaches are always catastrophic.
Soil erosion in northern China grabbed international attention when a withering drought killed half a million people in 1920—21. Some twenty million people were reduced to eating literally anything that grew from the soil. In some areas starving people stripped the landscape down to bare dirt. The ensuing erosion triggered mass migrations when fields blew away. But this was not unusual. A 1920s famine-relief study documented that famine had occurred in some part of China during each of the previous two thousand years.
In 1922 forester and Rhodes scholar Walter Lowdermilk took a job at the University of Nanking to work on famine prevention in China. Touring the country, he deduced how soil abuse had influenced Chinese society. The experience impressed upon him the fact that soil erosion could cripple civilizations. Years later, after traveling widely to study soil erosion in Asia, the Middle East, and Europe, Lowdermilk described his profession as reading “the record which farmers, nations, and civilizations have written in the land.”2
Approaching the site where the Yellow River broke through its dikes in 1852, Lowdermilk described how a huge flat-topped hill rose fifty feet above the alluvial plain to dominate the horizon. Climbing up to this elevated plain inside the river's outer levee, Lowdermilk's party traversed seven miles of raised land before coming to the inner dike and then the river itself. Over thousands of years, millions of farmers armed with baskets full of dirt walled in and gradually raised four hundred miles of the river above its floodplain and delta. Seeing the muddy yellowish water, Lowdermilk realized that the heavy load of silt eroded from the highlands began to settle out when the river's slope dropped to less than one foot per mile. The more silt built up the riverbed, the faster farmers raised the dikes. There was no winning this game.
Determined to find the source of the dirt filling in the river, Lowdermilk traveled upstream to the province of Shansi (Shanxi), the cradle of Chinese civilization. There in northwestern China he found a landscape deeply incised by gullies, where intensive cultivation after clearing of forests from steep, highly erodible slopes was sending the soil downstream. Lowdermilk was convinced that deforestation alone would not cause catastrophic erosion—shrubs and then trees simply grew back too fast. Instead, farmers cultivating steep slopes left the soil vulnerable to erosion during intense summer downpours. “Erosion is only indirectly related to the destruction of the former extensive forests, but is directly related to the cultivation of the slope lands for the production of food crops.”
Rather than the axe, the plow had shaped the region's fate, as Lowder-milk observed. “Man has no control over topography and little over the type of rainfall which descends on the land. He can, however, control the soil layer, and can, in mountainous areas, determine quite definitely what will become of it.”3 Lowdermilk surmised how the early inhabitants of the province cleared the forest from easily tilled valley bottoms. Farms spread higher up the slopes as the population grew; Lowdermilk even found evidence for abandoned fields on the summits of high mountains. Viewing the effects of farming the region's steep slopes, he concluded that summer rains could strip fertile soil from bare, plowed slopes in just a decade or two. Finding abundant evidence for abandoned farms on slopes throughout the region, he concluded that the whole region had been cultivated at some time in the past. The contrast of a sparse population and extensive abandoned irrigation systems told of better days gone by.
Lowdermilk had first recognized the impact of people on the lands of northern China at a virtually abandoned walled city in the upper Fen River valley. Surveying the surrounding land, he discerned how the first inhabitants occupied a forested landscape blanketed by fertile soil. As the population prospered and the town grew into a city, the forest was cleared and fields spread from the fertile valley bottoms up the steep valley walls. Top-soil ran off the newly cleared farms pushing up the mountain sides. Eventually, goats and sheep grazing on the abandoned fields stripped the remaining soil from the slopes. Soil erosion so undercut agricultural productivity that the people either starved or moved, abandoning the city.
Lowdermilk estimated than a foot of topsoil had been lost from hundreds of millions of acres of northern China. He found exceptions where Buddhist temples protected forests from clearing and cultivation; there the exceptionally fertile forest soil was deep black, rich in humus. Lowdermilk described how farmers were clearing the remaining unprotected forest to farm this rich dirt, breaking up sloping ground with mattocks to disrupt tree roots and allow plowing. At first, plowing smoothed over new rills and gullies, but every few years erosion pushed farmers farther into the forest in search of fresh soil. Seeing how colonizing herbs and shrubs shielded the ground as soon as fields were abandoned, Lowdermilk blamed the loss of the soil on intensive plowing followed by overgrazing. He concluded that the region's inhabitants were responsible for impoverishing themselves—just too slowly for them to notice.
Over the next three years, Lowdermilk measured erosion rates from protected groves of trees, on farm fields, and from fields abandoned because of erosion. He found that runoff and soil erosion on cultivated fields were many times greater than under the native forest. Farmers in the headwaters of the Yellow River were increasing the river's naturally high sediment load, exacerbating flooding problems for people living downstream.
Today the cradle of Chinese civilization is an impoverished backwater lacking fertile topsoil, just like Mesopotamia and the Zagros Mountains. Both of these ancient civilizations started off farming slopes that lost soil, and then blossomed when agriculture spread downstream to floodplains that could produce abundant food if cultivated.
Another commonality among agricultural societies is that the majority of the population lives harvest-to-harvest with little to no hedge against crop failure. Throughout history, our growing numbers kept pace with agricultural production. Good harvests tended to set population size, making a squeeze inevitable during bad years. Until relatively recently in the agricultural age, this combination kept whole societies on the verge of starvation.
For over 99 percent of the last two million years, our ancestors lived off the land in small, mobile groups. While certain foods were likely to be in short supply at times, it appears that some food was available virtually all the time. Typically, hunting and gathering societies considered food to belong to all, readily shared what they had, and did not store or hoard—egalitarian behavior indicating that shortages were rare. If more food was needed, more was found. There was plenty of time to look. Anthropologists generally contend that most hunting and gathering societies had relatively large amounts of leisure time, a problem few of us are plagued with today.
Farming's limitation to floodplains established an annual rhythm to early agricultural civilizations. A poor harvest meant death for many and hunger for most. Though most of us in developed countries are no longer as directly dependent on good weather, we are still vulnerable to the slowly accumulating effects of soil degradation that set the stage for the decline of once-great societies as populations grew to exceed the productive capacity of floodplains and agriculture spread to the surrounding slopes, initiating cycles of soil mining that undermined civilization after civilization.
FOUR
Graveyard of Empires
To Protect Your Rivers, Protect Your Mountains
EMPEROR YU (CHINA)
IN THE EARLY