The Natural History of the Present, Chapter 7

Chapter 7: Limits of Sustainability in the More or Less Edible Landscape; Varieties of Desert Agriculture

Some landscapes, like the northern European hardwood forests, or the humid prairies of North America, are more tolerant of human agricultural occupation; some are less so. (Both the former are part of the loess belts of thick, wind-deposited glacial silts, which make excellent, if easily erodible, agricultural soils.) Limits often appear more dramatically in arid landscapes. The Papago Indians (the so-called bean eaters: their proper name is the Tohono O’odham) inhabited the Sonoran Desert of Arizona and northern Mexico. They were flood-water farmers; or partly so: it is thought 20-25% of their calories came from cultivated foods. Among the 75% of their diet that came from hunted and gathered foods, 4 times as many calories came from plants as from animals. The Papago farmed the mouths of desert washes, where the flood waters from the July rains spread out. Low embankments, shallow ditches and brush weirs were used to divert and distribute the flow. A brush weir constructed across the flat slowed the rush of water, made it spread out and drop its load of leaves, small sticks, rodent droppings (all excellent fertiliser); slowing its flow helped the water sink into the ground. Corn, squash, beans and pumpkins were seeded in the damp ground. Fields used for many years developed their own associations of wild plants. Some of these were allowed to seed in and were in a sense cultivated. (So-called cultivated devil’s claw, a plant used for basketry, may have been among them, as was goosefoot, a weed harvested for greens; and cross-pollination with wild chilis growing around the edge of the garden added heat to next year’s planted chilis.) To mature well, the crops usually needed a second and even a third watering. This was provided by the later, gentler, female rains, though tepary beans needed less water than corn, and corn less than pumpkins. Yuma corn, grown on the floodplain of the Colorado River to the west, matured in 60 days in ground left saturated by the receding floodwaters. So this was a system dependent on natural regularity, in a very variable environment; that is, the amount of rain and its timing varied from year to year. The environment’s variability helps explain the low level of the Papago’s dependence on cultivated foods.

In all ways water is the desert dwellers’ problem. The Papago dug shallow reservoirs near their fields to catch additional run-off (which they might use for hand irrigation as well as for household water) and also dug wells in the beds of washes. These usually held water until October or November. By then the crops had been harvested, the agricultural year was over, and the deer dance was performed to mark the start of the winter season. The tribe moved from the drier mid-altitudes where its fields were located and where the desert fruits, the foods of spring and summer, grew, to its higher, better-watered perennial grasslands: the animal pastures. Flood-water fields were partly created; they might be leveled with earth dug from the reservoirs, and enriched with nitrogen-rich earth collected under mesquite trees or other nitrogen-fixing desert shrubs, and carried in baskets to the site. Their layers of silt were increased by the brush dams. The settling silt helped level them and also made them fertile; the yearly flood of silt and organic matter renewed the fertility. Older fields had over a hundred characteristic plants, besides crop plants, and a rich insect and invertebrate life. They attracted a more varied and concentrated collection of mammals, birds, reptiles and amphibians than the surrounding desert. The man-made concentration of water and nutrients turned the fields into little oases; such fields became centers of local biological diversity. (One hundred and ten species of birds are found about Papago fields). Some of this life, especially the competitors for the crops, like rabbits, would be eaten.

Many of the collected foods of the Papago were strongly seasonal; abundant for two weeks or a month, and then gone. Such foods included the fruit of the Sahuaro and Organ-pipe cactus, which ripened in June; cholla buds and fruit; mesquite beans (mesquite yields something like 150 pounds of pods per acre, and the beans don’t require cooking to be edible); the seeds of ironweed and paloverde, and of several desert grasses, out of which a nutrituous mush was made, higher in protein and fat than cooked wheat; various greens; and sandfood (a parasitic fungus that grows on the roots of a desert shrub). A family might also consume 12 to 15 black-tailed deer per hunter, as well as other game. If these figures are accurate, it puts Papago meat consumption from large animals far above that of Northeastern tribes like the Iroquois, whose take of deer and bear in the winter hunt is estimated at 40 pounds of dressed meat per person, or less than one animal each. (The Creeks of the Southeast reportedly used 25 to 30 deer per household annually for meat and skins; in the 1700s during the trade in deerskins for European goods, Creek hunters killed 100 deer per household per year.) With these numbers, each member of a family of 5 Papago is consuming 2 to 3 deer. The desert also provided black-tailed jackrabbits, an animal of cyclical, sometimes enormous abundance, which the Pueblo Indians killed in drives; white-tailed deer; desert bighorned sheep; Merriam’s elk (now extinct); pronghorn antelope; doves and quail and their eggs; and wild turkeys. Grasshoppers were gathered for a protein-rich mush, and the larvae of the lined sphinx moth, extremely abundant after the summer rains, were dried and stored.

The neighbors of the Papago, the Pima (Akimel O’odham), who lived along the Gila, were floodplain irrigators. Where the Pima lived, the Gila floodplain was four miles wide, much of it floodplain forest. The oval fields of the Pima were surrounded by planted hedgerows of mesquite, willow and aspen. Sticks were woven among the trunks; the fence helped slow and control the irrigation waters. Mesquite was common on the floodplain, and its beans were harvested for food. Pima canals were up to 10 feet deep and 4 to 6 feet wide. The water was forced into the canals by diversion dams. Pima crops would mature with one heavy, pre-planting irrigation. The muddy floodwaters, a product of the winter’s snowmelt, also fertilised and helped level the fields. The spring flood of the Gila was caused by mountain snows, which were more reliable than summer rains, but would fail once every 5 years. Additional irrigation during the growing season produced more abundant crops and the Pima also constructed temporary embankments and dikes on the flats beyond the floodplain to divert sheet and arroyo runoff from the summer rains onto their fields. Such structures may have helped them garden in years when the spring flood failed. It is thought the Pima got 50-60% of their calories from cultivated crops. Their additional manipulations of the landscape somewhat reduced their dependence on climate variability. Instead of being dependent on a cloudburst flooding the drainage of a wash, they were dependent on a season’s snowfall over a large mountain drainage, and, to a lesser extent, on the summer rains that fed their other water traps. Since they cast a wider net, the likelihood of failure was reduced, and they could be more dependent on agriculture. In a good year the Pima could grow 2 crops (one with the spring flood, one with the July rains), and so have harvests in both July and October. Many of the Pima’s other foods (such as cactus fruit and mesquite beans) were similar to those of the Papago. In bad years they ate more collected foods. For this reason they may have encouraged the growth of mesquite trees on the floodplain. They also ate fish. Before Euro-American settlement, the Gila, like many southwestern rivers, was a slow-flowing, low-banked river, with canopy forests of cottonwood, willow, mesquite and walnut, grassy meadows, and rich marshlands with waterfowl and amphibians (the cienagas). Fish from the Gila were a staple (the Spanish claimed the Pima supplemented their diet of fish with corn and beans), especially the humpbacked sucker, which was netted in commercial quantities from the Salt River until the 1940s. (The humpbacked sucker is now extinct in the Gila. Fish were also important for the tribes along the lower Colorado.) Beaver were eaten. Beaver were plentiful along the tributaries of the Gila until trapped out by the Americans, another indication that much or most of the near-river floodplain remained densely wooded.

Like many New World tribes, the Pima practised no soil fertilization, except through the yearly flows of muddy water. As I have pointed out, North America had no large domesticable wild animals, whose domestication would encourage the use of pasture crops, which could be rotated with grain, and whose fertilising manure could be spread on the fields. A Hidatsa woman, whose people gardened in the Missouri floodplain in the mid-nineteenth century, remembered carrying horse droppings out of her gardens: they were a source of weeds. Pima soils remained productive, though it was said new land produced better, and the wheat and barley introduced by the Spanish, and adopted by the Pima as winter crops, tended to be soil-depleting. The three main native crops (the Pima also grew cotton and tobacco, both notoriously hard on soils) were corn, beans and squash, planted together in the same field. Up to a point, these were soil maintaining. Corn, the soil-depleting crop, formed a standard for the climbing, and nitrogen-fixing, beans; squash, another nitrogen user, spread its trailing vines with their large leaves out between the corn hills and sheltered the ground from sun and erosive rain. Where floodwaters did not fertilize the ground, as with the light upland soils favored by Native Americans in New England, New York State, and southern Canada, fields were only cultivated for 10 years or so, then the field, and often the village, moved. Free-living, nitrogen-fixing bacteria will provide a yearly pulse of nitrogen to continuously cultivated fields, but growing cultivated crops year after year, without a rotation into sod or forest (the poplar plantations among the cornfields of Italy and France, the plantations of pine pulpwood on the Minnesota prairie), tremendously slows the mobilization of nutrients from the soil. So productivity falls, to a low, constant level. The fields of the Narragansetts of Rhode Island were unusual in that they were apparently capable of bearing continuous crops of corn, though some accounts indicate the fields were left fallow in alternate years. (There is now doubt that the New England Indians used spring-run herring for fertiliser: it has been suggested that the use of fish was an English innovation; one writer pointed out that the Narragansetts would have been better off eating the fish.)

Landscapes like those of the Pima are easily over-exploited. All rivers are subject to cycles of downcutting and channel migration, as their floodplains, built up by layers of springtime silt, mature and grow above the reach of the spreading floodwaters. This cycle is more rapid in the young, rapidly eroding landscape of the American Southwest. For some combination of reasons (a decrease in vegetation, perhaps from drought; an increase in rainfall; tree cutting or grazing that lets storms produce more runoff and more silt) a flood begins cutting a deeper channel. This leaves the old floodplain out of reach of the floodwaters. More water is confined to the channel, which increases its erosive power. The deepening channel migrates inexorably upstream, sometimes dramatically so, in the form of a receding waterfall. Floods on the Colorado River in 1905 and 1906 led to its taking over an irrigation canal leading to the Salton Sea, a natural depression in the desert, as its main channel. According to Indian legends, this was not the first time the river abandoned its delta in the Gulf of California. The rush of water created a headwall recession in the form of a waterfall 40 to 80 feet high and 1000 feet wide, that receded upstream at a rate of several thousand feet a day. People came out from Los Angeles to see it. After an episode of downcutting, parts of the old floodplain are undercut and washed away by the river, which finally adopts a shallow and braided (rather than a deep, single) channel. Trees, or other perennial vegetation, which might stabilize the channel, have trouble establishing themselves because of the annual floods. Few fish survive in the shallow, warmer water and beaver cannot colonize the river, and help stabilize it with their dams, without trees. In time, given a sequence of favorable events (two or three good years for aspen seedling survival, for instance; or colonization by the invasive tamarisk), a single channel will form again, and a floodplain reform.

Something like this seems to have happened to Chaco Canyon in north-central New Mexico in the 1100s. The canyon had been occupied by the Anasazi people for 600 years and was densely settled. Wide ceremonial highways connected massive public buildings, and carefully laid out roads, for transporting pottery and grain in backpacks, connected Chaco with other Anasazi settlements. The Anasazi of Chaco were a state trading culture, exchanging pottery and other goods for grain among their various settlements. Thus they redistributed grain in a region of variable climate. The Chaco River was a shallow seasonal stream with a floodplain several hundred feet wide. Chaco Canyon caught runoff from a wide area and had a high rate of soil renewal and a high alluvial water table. Its relatively low elevation gave it a long growing season. The early Anasazi grew crops in the damp alluvial soils and diverted sheet runoff over the canyon bottom into irrigation channels for further irrigation. They hunted deer and collected wild grass seeds. Later they also grew crops on the mesa tops, under a mulch of stones. The stone mulch broke the force of a hard rain, letting water seep in; reduced evaporative losses from the soil; reduced erosion; reduced the temperature variation between day and night; and condensed dew. (In modern reconstructions, rock-mulched soils have double the soil moisture and four times the average yield of unmulched soils. Rock-mulched gardens were especially common in the Southwest during the droughts of the 1200s and 1300s.) The nearby watershed of the canyon, including the mesa tops, was originally occupied by a pinion-juniper woodland. The Anasazi collected pinion nuts for food and cut the trees for building material and for fuel for cooking and firing pottery. Regrowth is slow in the Southwest and as the population grew, the nut and deer harvest declined, people ate more rabbits and mice, as well as domestic turkeys raised on corn, and cutting overtook the rate of regrowth. Southwestern pinyon-juniper woodlands are a mix of bare ground, grasses and trees. Approximately 20% canopy cover by the trees constitutes dominance (the roots go into the openings and compete with grasses for moisture); herbaceous cover amounts to 15-20%; the rest is bare ground. When too many trees are removed by cutting, or when the forest occupies too much of the ground because of fire suppression, or when the grasses are overgrazed by cattle or sheep, the patches of bare ground connect and erosion is inevitable; net soil loss has been measured at 0.5 inch per decade, or 5 inches per century. While the stone mulches on their upland gardens should have prevented erosion, erosion probably explains why little now grows on the mesas about Chaco.

The buildings of Chaco Canyon also took tens of thousands of ponderosa pine timbers, carried from up to 30 miles away. In the century that preceded abandonment, 75,000 to 100,000 large pines may have been cut in the Chaco River’s watershed (some writers say 200,000 trees; not a lot of trees, really, for a population of some tens of thousands of people over a century). Trading pottery (and to a lesser extent worked turquoise) for grain let the Anasazi maintain large populations in an uncertain climate. Agricultural settlements surrounded the canyon. Land yielded less and less under continuous corn and new fields were constantly opened up; rainfall also varied from place to place and year to year. Trade was a means of redistributing grain, and thus of evening out agricultural production over a large area. Grain distribution was controlled by the people in the Big House, and depended on a ready source of fuel to manufacture pottery. The Anasazi elite were about two inches taller than the farmers who lived in more simple dwellings, whose teeth show signs of episodic starvation. By 1000 AD the pinion-juniper woodland was gone. (It has not regrown 1000 years later.) The main flourishing of the Chaco people occurred from 950 to 1150, during a period of high rainfall. About 900 AD the irrigation channels on the canyon floor began to downcut below the level of the fields. Clearing of vegetation, in this case the pinion-juniper woodland as well as the upland Ponderosa forests, increases the speed and amount of overland flow from rainfall. Downcutting could have begun with a heavy rain; or with a drought, which would further reduce the vegetation about the canyon, followed by lighter than normal rainfall. Under these conditions a light rain would produce heavier then normal runoff. But rain was relatively abundant and to compensate for the downcutting, the Anasazi dammed streams in the side canyons and used the stored water for irrigation. They also captured the water that came off the cliff on the north side of the canyon and built rock dams across the river itself, to raise its level. The population of the canyon remained high until a long-term drought began about 1140 AD. By then, the population in the canyon had been a net importer of corn for 200 years. A more general drought meant crops would fail all over the area and the trading system would have too little grain to distribute. Corn can be stored for 2 to 3 years, but no culture stores more than 2 years of grain. (Thus the saying that 3 years of drought kills everyone.) The natural foods of the area (the nuts, animals, wild grasses) were no longer sufficient to feed the population and were reduced by the drought. The Anasazi at this time (like the Maya at a time of high population) showed signs of nutritional stress from their restricted diets: severe dental caries, tooth erosion from the stone grit in cornmeal, periodontal disease, arthritic diseases, osteoporosis. Child mortality among the elite was 9.5%, among the farmers 26-45%. The earlier diet of corn, beans and squash had been supplemented by significant amounts of wild game and wild plant foods but such a diet was only possible when population densities were relatively low. Some of the people of the canyon dispersed to areas of higher elevation, which have more rain, but a worse drought late in the 1200s would destroy the Anasazi as a culture. Imports of food, and of luxury items such as pottery, stone, turquoise, macaws, shell jewelry and copper bells, into Chaco Canyon continued until near the end, which was rather sudden. Six hundred years later, in 1849, Chaco stream was clear and continuous with no signs of gullying. By 1871 the stream had cut an arroyo 16 feet deep and 40 to 60 feet wide; by 1930 the arroyo was 25 to 30 feet deep and 300 feet wide and the stream had become intermittent. Once it begins, downcutting is irreversible. Heavy machinery can be used to stabilize braided floodplains, although at considerable expense (hundreds of thousands of dollars per mile).

At European contact, many Southwestern rivers flowed through wide floodplains with meadows, cottonwoods, willows and emergent marshes. The floodwaters distributed seeds, soil, organic matter and other nutrients; their slow recession let cottonwoods and other trees germinate (the seedlings’ roots followed the water down). Throughout the Southwest and Great Basin, beaver, with their frequent dams on smaller watercourses, and their maintainance of a dense riparian forest of poplar and willow, are thought to have been a major factor in maintaining floodplains. The streamside vegetation slows the movement of water in floods, making it spread out and drop its fertilising sediment on the floodplain terrace. With less sediment and less water, the flow descending from the tributary streams is less erosive. The Pima ate beaver, but apparently, like the Cree, they took a limited number. The Pima also farmed only a part of the floodplain. Why is uncertain, but that is a problem only for people like us, inhabitants of a market society, for whom growth is life. The river was important to them for fish and the uncultivated floodplain for other foods. Unlike the Anasazi, they never developed a state culture that demanded a surplus for its support. Perhaps only parts of the floodplain were within reach of their irrigation techniques. Most likely, they also saw the limitations of their environment and controlled their population.

Anglo-American settlement of the Southwest (the final and by far the most environmentally demanding settlement to date) was preceded by the trapping out of the beaver. Timber, mostly mesquite, which then was restricted to river floodplains, was cut for mine timbers, building material, fuel to process ores, and for cooking and heating. Cattle were introduced from Texas. (The cattle, by depositing the seeds from the nutritious mesquite pods in fertile pats of manure, helped mesquite make its move from stream bottoms to the uplands, where its takeover of former grasslands has tormented cattlemen ever since.) A railroad in 1881 made it possible to ship cattle and cattle products, and cattle in the Arizona Territory increased from 5000 in 1870 to more than a million in 1890. The grazing pressure on the river floodplains was tremendous. The 1700s and 1800s were mostly dry in the Southwest. It was wet with floods from 1881 to 1884, then dry from the 1920s to the 1980s. A flood in 1891 altered the channels of the Salt and the Gila rivers, destroying irrigation systems and canals along the rivers. A drought from 1891 to 1893 reduced the cattle herds by half. Then a period of somewhat above normal rainfall continued the downcutting begun by the 1891 floods. Most of the major river floodplains were converted to shallow, braided channels. Streams downcut deeply. Native floodplain forests, deprived of moisture, declined, and were invaded by the deep-rooted saltcedars (Tamarix ramosissima or T. chinensis), alien plants introduced as ornamentals to the Southwest (some were planted by government crews in the 1930s to control erosion), which now cover 1.2 million acres of southwestern river bottoms. The bunch grasses of the American Southwest and Great Basin are not adapted to continuous grazing. The native herbivores, such as black-tailed deer, bighorn sheep, Merriam’s elk, and pronghorn antelope, are browsers that sometimes eat grass. Elk and sheep eat more of it, elk largely in the winter when it does no harm to the plants. While it is thought that over-grazing by cattle helped initiate the downcutting of the rivers, cycles of downcutting and floodplain aggradation are endemic in the Southwest. The area had been grazed (relatively lightly) by domestic sheep for two centuries before the Texans came. One can see the effects of the influx of cattle in the levels of sediment deposited in southwestern alpine lakes, which rose by 5 times in the middle of the nineteenth century and are still 3 to 4 times above pre-nineteenth century levels. The hooves of the cattle broke the crusts on the arid soils (crusts of lichen or wind blown gravel) and let them erode and blow away. In this case, a flood, followed by a period of light rainfall, which reduced the vegetative cover, followed by a period of heavy rain started a cycle of downcutting. Heavy grazing by cattle, tree cutting, the removal of the beaver made the downcutting more likely, more extensive, and more severe. Such practices provide more sediment, which increase the erosive power of rivers.

Much remaking of the desert landscape was deliberate. The valley of the Salt River, where the Hohokum had grown irrigated crops for 1500 years (they had been replaced about 1500 by the less state-like Pima culture) was converted into a major producer of irrigated grapefruit, dates, cotton, lettuce, melons and salt tolerant winter grains. The river was dammed to provide water. The dams dried up much of the channel and the floodplain forests (when not removed for fields) collapsed from the falling water tables. The Hohokum had dealt with the salty groundwater of the Salt Valley by rotating their fields every ten years; only 9-10,000 of the 100,000 hectares they cultivated were under cultivation at any one time. The Hohokum also grew agave on the uplands and ate fish from the river. Unlike the Anasazi, who were dispersed by drought and replaced as a culture by the less environmentally demanding Pueblo, the Hohokum seemed to deal with the environmental stresses they faced. They were probably eliminated by European diseases before actual contact with Europeans. The modern irrigators dealt with the salty groundwater by digging drains. Irrigation works along the Salt were destroyed by the floods of 1891 but were rebuilt.

Continued heavy grazing in the twentieth century has speeded up a shift in the vegetation of the Southwest from grasses to shrubs. This change in vegetation results in a change in the distribution of soil nutrients from even to patchy, as nutrients are concentrated under the shrubs. Like the downcutting of streams, this shift in vegetation seems irreversible. Why is unclear. Many desert shrubs use carbon more efficiently in photosynthesis than desert grasses. The shrubs thus need to allow less carbon dioxide to filter into their leaves from the atmosphere (through openings called stomata) and correspondingly to let less water out; so they are also more efficient users of water. A shift from plants less efficient at using carbon dioxide to plants more efficient at using carbon dioxide has been going on under limited rainfall regimes worldwide for several million years. It is probably part of the long-term adaptation of terrestrial vegetation to the tremendous decrease in the carbon dioxide content in the atmosphere since the Carboniferous, when carbon began to be locked up in fossil fuels. Since carbon dioxide uptake by plants is related to water use, drier conditions favor plants that can take in more carbon dioxide with less transpiration of water. Shrubs are also deeper rooted; the individual plant draws water from a deeper soil profile and essentially depletes the water available for any grasses growing in between them. Nutrients from fallen leaves, from fine root turnover, from small animal droppings then accumulate under the shrubs, creating the so-called patchy distribution of nutrients, and making it harder for grasses to establish themselves on the bare ground in between.

Agricultural systems like those of the Papago, Pima, and Anasazi were used in dry environments worldwide. The Nabataens of the Negev Desert of modern Israel and Jordan, whose capital, Petra, was carved out of the walls of a canyon, lived on what was 2000 years ago a caravan route from the Red Sea to the Mediterranean. From the first century BC to the third century AD the route connected India, Africa and China with Egypt and Greece through ports in Gaza, handling trade in cinnamon, cassia, myrrh, frankincense, cotton, silk and ivory. The Nabataens fed a population of 60,000 people on grain grown with run-off irrigation. Rainfall then is thought to have been very similar to that now: from 2 to 15 inches a year (that is, highly variable on an annual basis), coming in showers over a 2 week period. The Nabataens dammed some wadis (canyons) in their upper reaches and led the captured water through canals to fields below. They dammed the whole courses of other wadis with low stone dams (so-called rubble masonry, stone dams with an earth core, the dams up to 2 meters high), the crest of one dam level with the base of the next. These dams converted the wadi bottom into a series of stepped fields, that were filled with silt and partly leveled by the run-off of silt and water that flowed with the rains down the wadi floor. Crops had to mature with this single, pre-planting irrigation. The density of dams was great: near Ovdat, another ancient city of the Negev, 17,000 have been found within 50 square miles, a square a little over 7 miles on a side.

At the mouths of the wadis, natural terraces at the bottom of moderate slopes were used for another form of run-off agriculture. These fields used runoff funnelled down from the slopes above them. In modern reconstructions, the farms require about 20 acres of water catchment for 1 acre of cultivated ground. The degree of slope and the soil cover of the catchment are important. Steeper slopes tend to lose more water to rock fissures and depression storage. Moderate slopes yield about 20% of a light rainfall to the fields. The yield rises with a heavier rain. All slopes in the Negev are essentially devegetated. The system works precisely because the Negev is such a destroyed landscape (probably destroyed several thousand years before the Nabataens by people cutting wood and grazing goats, together with a slowly drying climate). The impervious soils, moderate slopes, and low rainfall lead to a rate of run-off sufficient to irrigate the terraces. In some cases, stones on the slopes of the ancient catchments have been collected into regularly spaced piles. Stone clearance increases run-off under very light rainfall regimes. (While it seems unlikely the stone piles create runoff from dew, such structures can also function as dew-catchers. Stone towers 30 feet high and 100 feet across at the base condensed water from the air to supply the ancient Crimean city of Feodosia; the average measured modern yield is 35 gallons per tower per day.)

The Negev receives about 100,000 gallons of water per acre per year. In modern times, experiments have been made with individual water catchments on the desert floor. This modern innovation involves much soil movement, but these catchments also work. The ideal size of the catchment varies with the location, but in general pomegranates will grow in 250 to 500 square meter catchments, which corresponds to a planting of 8 to 16 trees per acre (20 to 40 trees per hectare). By comparison, standard apple trees in humid regions are planted at 100 to 200 trees per acre. Grapes can be planted at 32 to 40 vines per acre (80 to 100 vines per hectare). The creation of such microcatchments also increases the density of forage plants like saltbush 20 to 30 times (they grow around the rim of the catchment and are used as forage by goats), and over time the water filtering down will clear the accumulated salts from the catchment’s soil. In such a ruined or severely altered landscape, such manipulation seems a definite improvement from a human point of view; and perhaps biologically. Catchments, colonized on their edges by forage shrubs (which help prevent soil movement), last more than a century, so the cost of construction has a long pay-off period. Regenerative agriculture in the Negev may mean encouraging the nitrogen-fixing cryptogramic crust typical of deserts (which foot traffic and bulldozing will destroy); and the rock-eating snails that are now the chief agents of soil formation in the Negev. The snails rasp into rocks to get at the lichens that grow under their surfaces; the rock comes out in the snail’s faeces converted to soil. Such biological weathering amounts to about a ton of material per hectare annually, and makes the snails critical agents in soil formation and nitrogen cycling, processes necessary for the establishment of higher plants.

Vinyards and pomegranate orchards would accumulate carbon in the soils under them, and so help remove carbon dioxide from the atmosphere. Millions of square miles of land world-wide that have been ruined by agriculture, grazing, logging, or mining, much of it dryland, including perhaps 30% of former agricultural lands, are candidates for such innovative, marginally profitable, carbon-removing schemes. Unlike modern commodity agriculture (much of it also marginally profitable), such schemes usually involve much hand work; that is, they also produce employment. One could not in modern times grow grain in the wadis of the Negev, or on the terraces of run-off farms, as the Nabataens did (the relative price of grain is now much, much lower than then) but one could grow other crops. Pomegranates or grapes in the Negev; salicornia, a halophyte that can be irrigated with seawater, whose yields are similar to alfalfa, and which makes a good livestock feed, in sea-side Middle Eastern or Indian deserts, alongside greenhouses that use cold seawater to condense moisture out of the desert air, at a gallon a day per square foot of glass (the water also cools the greenhouse, making the plants more efficient users of water); jojoba, a desert shrub whose seeds yield a useful oil, in Arizonan or Mexican fields that now grow irrigated alfalfa or cotton; Christmas trees, pulpwood or horticultural evergreens in the strip-mined hills of humid West Virginia, Pennsylvania and Ohio; deep-rooted native grasses and large herbivores underneath the towers of electricity-generating windmills, on the newly shaped, strip-mined hills of Wyoming or the Navaho Reservation in Arizona. Sunny landscapes can hold photo-voltaic collectors; or solar concentrators to make electricity from sun-heated steam. Such regenerating landscapes can be useful in many ways.