Domesticated in Mexico several thousands of years ago, agaves have proven to be among the most versatile and useful of all plants on Earth. People have produced alcoholic beverages, medicines, sugar substitutes, a variety of foods, rope, paper, cloth, binding agents, and even building materials from them. An elderly Mexican farmer once explained, as we sipped fermented agave sap (pulque), that only our lack of imagination has prevented people from realizing all of the plant’s hidden uses.
While acknowledged widely for their material benefits, one under-appreciated aspect of agave cultivation has been its role in shaping the surface of the Earth. For millennia, Mexican farmers have cultivated agaves to anchor soils, provide structural support for agricultural landforms, break-up compact soils, and restore degraded lands to productive capacity. As pioneer species, agaves are often the first plants able to colonize marginal lands and, through a chain of ecological succession, condition the land for farming. Over time, strategic agave cultivation allows farmers to reduce their inputs of labor and capital into the agricultural system, which makes farming more efficient and sustainable.
In the second half of the twentieth century, environmental groups began to take note of these ancient cultivation practices and started imitating them. Through its agriculture ministry, the Mexican government promoted agave cultivation as a means of soil and water conservation, environmental restoration, and now, in the twenty-first century, ecosystem services provision. The result today is a mosaic of ancient and modern agave landscapes that form distinct spatial patterns. Here, we explore some of these patterns and the processes that drive their formation.
In Mexico, agave landscapes can be divided into two broad categories: landscapes of cultivation and landscapes of conservation. In cultivation, the obvious goal is to produce crops, and as noted, agaves are important crops. But agaves also support the structure and function of entire agro-ecosystems. On hill slopes agave root systems stabilize soils and assist farmers with developing land for agriculture, creating protected habitat for less-competitive plant species including most other crop plants. They serve as anchor points for landforms like agricultural terraces and as living barriers between fields. When intercropped with corn, beans, and squash, they are key components of polyculture agriculture across Mexico.
But while the primary goal of agriculture is to produce crops, an ideal corresponding goal is to protect natural resources. When farmers are able to produce crops in perpetuity—while protecting natural resources and minimizing degradation—we can say that agriculture is more sustainable. Sustainable agriculture is conservative in the sense that it minimizes resource consumption and waste by protecting resources for future use. In this context, sustainable, conservation-minded farming relates more to the microeconomics of soil and water management than it does to a farming ethos or ethic. This perspective on conservation differs from contemporary understandings of the term, which more often relate to environmental policies and institutions. We refer to agave landscapes of conservation in the latter context; namely, as focused efforts to protect the environment and its resources, rather than efforts to produce food. They typically occur at scales larger than the household level and serve a variety of conservation goals—from the provision of specific ecosystem services (e.g., water, soils, biodiversity) to comprehensive environmental repair (e.g., restoration, reclamation, rehabilitation).
In sum, while prudent agriculture may be conservative in how it utilizes resources, food production remains the primary goal. When the protection of resources becomes the primary goal, we enter the realm of conservation. Yet while distinct, agave landscapes of cultivation and conservation share some similarities in structure and function. One primary example relates to the use of cross-slope terraces, which, having first been developed by farmers over several millennia, are now widely built by environmental groups. The mimicry of agricultural terracing echoes a larger trend in the field of restoration ecology, where efforts to repair ecosystems increasingly borrow from the technologies of intensive agriculture (Whisenant 1999). Furthermore, agriculture and conservation practices are not mutually exclusive and may overlap in time and space, structure and function. In Mexico, agave cultivation serves both purposes, with one common thread being the role the plants play in stabilizing soils. This attribute provides the clever engineer, whether a farmer or a conservation official, with a low-cost, natural, and efficient means of shaping the surface of the Earth.
In the United States most farmers grow crops on flat land. The Great Plains, the gentle hills of the Upper Midwest, and the valley bottoms of California are prime examples. Differences in climate aside, farming these environments is often easier than farming sloped land as soils tend to be better and water management is more practical. Flat land distributes gravity’s effects evenly, which reduces the spatial complexity of managing soil and water.
In much of the world, however, farmers cultivate sloped land where soil and water management can be infinitely more challenging. Here, although gravity may assist with water distribution on slopes, water storage is problematic. Furthermore, groundwater is often inaccessible on slopes, which makes most irrigation impractical. Partly as a result, rainfed agriculture—cultivation that is dependent on rainfall—predominates on sloped land, providing most of the food produced and consumed in developing countries. Because of the physical challenges of managing soil and water on slopes, and the unpredictability of rainfed agriculture, farming slopes can be risky. One method to reduce this risk is to artificially flatten slopes by building terraces, which also deepens soils, improves moisture retention and water-use efficiency, and decreases the risk of runoff and erosion. Once fully developed, terraces can provide an efficient and effective way to grow crops in an otherwise marginal environment.
But one size does not fit all, and farmers must carefully tailor terrace designs according to different slope gradients, soil characteristics, vegetation patterns, and climates. Farmers must consider all of the environmental factors that scientists measure and model to understand Earth surface processes without having formal coursework in hydrology or geomorphology. For farmers, terrace cultivation is both a science and an art learned through trial and error, experience, feel, and intuition. Cumulative knowledge is passed from generation to generation, the result of millennia of trial and error in adaptive management. From these co-evolutionary processes, agave cultivation and terracing have merged to become one of the Mexican farmers’ most effective ways to manage agricultural slopes.
Although methods of terracing slopes differ, all involve the use of hillslope impediments, which are structures that check the downward movement of soil and water over the Earth’s surface that results from precipitation. By obstructing runoff, impediments allow soil and water to accumulate, forming flat perches of terrain immediately upslope from the impediment. Flatter terrain encourages water infiltration, which, in turn, reduces the potential for additional runoff to develop. If properly managed, slope impediments allow farmers to simultaneously mitigate runoff and erosion while increasing soil depth and water infiltration capacity. Through the incremental accretion of soil and water behind slope impediments, patient farmers may build terraces over many years with minimal inputs of capital and labor (LaFevor 2014a).
Farmers use several types of impediments to build terraces. Stone or rock walls are probably best known, although earthen berms are also common. Contour ditches are often positioned upslope from the berms, which capture and store runoff and allow it to slowly infiltrate into the subsurface, while the runoff sediment (soil moved by water) is left behind. Periodic cleaning of sediment from the ditches maintains their storage capacity, while the sediment itself serves as nutrient-rich fertilizer that can be redistributed either to the terrace planting surface or to the berms. In addition to the periodic cleaning of ditches, rock walls and earthen berms must be raised and strengthened to accommodate the accumulating soil behind them. By periodically maintaining and strengthening rock walls, earthen berms, and ditches, farmers mitigate the risk of runoff and erosion and incrementally develop the terrace system (Doolittle 1984).
Stone or rock-wall impediments usually accompany flat, step-like terraces commonly referred to as bench terraces. In contrast, ditches and berms usually accompany terraces that may change the natural slope gradient little or none at all. Geographers refer to these as sloping-field terraces (West 1970). But for reasons not completely understood by geographers or archaeologists, bench terraces are rare in Mexico while sloping-field terraces abound.
There are advantages and disadvantages of each form. Instead of abruptly flattening the slope into a bench-like form, sloping-field terraces work with the natural gradient and require less initial investment in labor and capital to build. But since much of the original gradient remains, runoff and erosion is more likely to occur. Therefore, maintenance of the ditches and support for the earthen berms of sloping-field terraces are critical. To provide this support, Mexican farmers strategically cultivate agaves.
In highland central Mexico, agave-lined, sloping-field terraces are often referred to as metepantles (me-te-pánt-les) (Wilken 1987). The term derives from Náhuatl, the primary indigenous language of the region, and is thought to be a combination of metl (agave) and tepantle (wall of fence) or nepantla (in the middle or between). However, as farmers sometimes explain, the term can have several meanings, referring either to the rows of agaves, to the planting surfaces between them, or to the entire terrace system (Figure 1).
Perched atop the berms of metepantle terraces, agaves serve several important functions. The first and foremost is soil stabilization. Their root systems (rhizomes) thrive in shallow soils and grow horizontally, capturing moisture and storing it in the plant’s succulent biomass. The root systems of adjacent plants often interlock and form a chain of reinforcement for the earthen berms (Figure 2). Mature rows of large, interlocked agaves also serve as windbreaks, limiting aeolian erosion and encouraging soil deposition. Their long, fleshy leaves (pencas) can grow to a length of three meters and are edged with sharp spines that inhibit larger animals from getting too close. This protects the immediate environment from overgrazing, provides shade topography, and encourages soil moisture retention around the plant (Figure 3). Atop the berms and downslope from the ditches, agaves are protected from runoff and erosion and competition from other plant species (Borejsza 2006).
After several years of growth, reproductive shoots called hijos (children) or hijuelos (pups) appear at the plant’s base (Figure 4). Diligent farmers replant the shoots in the spaces between the parent plants. After 10-25 years, depending on the pruning techniques applied to different species, parent plants send long masts (quiotes) into the air from the tissue at the center of the rosette of leaves (corona). The plants then flower and die soon thereafter. The quiotes come crashing down and the agaves begin to whither (Figure 5). Parent plants are then pruned, removed from the berm, and plowed back into the planting surface as mulch. But the shells of the main corona stems (mensontetes) are often left in place to harvest precipitation and to protect the nutrient-rich microenvironment for the next cohort of shoots (Figure 6). When the shoots are ready for planting, the decayed mensontetes are finally removed, exposing the moist, fertile soils that will serve as the beds for the young shoots. Before replanting, the shoots must first dry for a month or more so their existing roots will not cause rot (Figure 7). That agaves can survive for more than a month without soil and water is yet another testament to their hardiness and versatility.
The sequence and spacing of agave succession varies widely, although prevailing practice is to plant agave shoots immediately adjacent to their parent plants, forming a juvenile-adult-juvenile-adult (J-A-J-A) pattern as one moves horizontally down the berm. But patterns of agave succession also depend on the different arrangements of ditch-and-berm structures. For example, some farmers replant agave shoots downslope atop a second berm, on the rim of a ditch, or simply aligning with the parent plants (Figure 8). Others only anchor the endpoint of a berm with agaves, where accelerated runoff and erosion from fields and roads is greater (Figure 9). Still others may be less strategic or diligent with maintenance, simply allowing shoots to grow wherever they appear (Figure 10). Especially among younger generations of farmers, some even view agave berms as a nuisance and plow over them in order to increase the planting area for other crops, or to provide better access for plows to move vertically between fields (Figure 11). Despite these practices, most farmers nevertheless recognize the role that agave-lined berms play in limiting soil erosion; and regardless of age consider well manicured agaves, ditches, and berms to be a general sign of good farming (Figure 12).
Although the term metepantle is still in use in areas with large indigenous populations (e.g., Tlaxcala, Puebla, and Hidalgo states), farmers in other areas often use a more generic Spanish term, sistemas zanja-y-bordo (ditch-and-berm systems) to describe them. Conservation groups from around the country also have adopted the term, which they use to describe a wide variety of related earthworks.
In the middle of the twentieth century the Mexican government began promoting agave-lined terraces as a means of combating environmental problems. Initially, agro-development programs provided agaves to landowners to reduce soil erosion on farms. But these programs soon expanded and increased funding, technical assistance, and access to heavy machinery for groups willing to construct large plots of agave-lined, sloping-field terraces—not only on farms and ejidos (communal landholdings), but also on public lands. By the early 2000s, on the heels of the United Nation’s Millennium Ecosystem Assessment (MEA), the environmental focus of these groups had shifted to water conservation and how to improve freshwater supply in areas where groundwater extraction outpaced recharge. As a drought-tolerant, native form of vegetation, officials viewed planting agaves, and building the terrace forms with which they had become associated, as a natural, low-cost, and efficient means of harvesting rainwater. Now, well into the twenty-first century, these programs have grown in terms of funding, areal extent, and the diversity of agave-lined terrace forms they build.
Mexico's ministry of Agriculture, Livestock, Rural Development, and Fisheries (SAGARPA) has been the largest government agency involved in these programs. In 2001 it began its most ambitious terrace-building program to date: The Program for Sustainable Agriculture and the Rehabilitation of Lands with Recurring Drought (PIASRE). Lasting from 2001 through 2006, PIASRE spent more than three billion pesos (300 million USD) on projects covering about 1.7 million hectares (4.2 million acres) of land (LaFevor 2014b: 95). Although PIASRE constructed several types of ditch-and-berm (zanja y bordo) terraces, in areas where metepantles were an indigenous form of slope management, their projects were often indistinguishable from existing practices. In other realms, PIASRE built extensive, agave-lined terrace systems where before there had been none. To assist farmers, the program published manuals explaining how to design and build terraces and provided a technical agent (TA), usually a university-trained engineer, who assisted with designing and constructing each terrace. The TAs also approved the design of each project, allocated funding, and acquired the necessary heavy machinery and agaves for each project. Although repackaged and marketed by PIASRE as an innovative effort, PIASRE’s zanja-y-bordo terracing projects were simply variations on what good farmers had been doing for millennia.
PIASRE’s terraces were built in a dizzying array of combinations, all of them variations on the basic sloping-field terrace design. The positioning of the ditches and borders varied greatly, although most design templates position the ditches above the berms so as to harvest runoff and to protect new agave saplings from being washed away. Occasionally one can find the reverse pattern, especially on gentler slopes closer to the water table where ponding is more likely to occur and where farmers prioritize water drainage over water infiltration (Figure 13).
Occasionally, PIASRE also built sequences of small check dams (retenes) within the ditches of zanja-bordo terraces. Their function is to stop the lateral flow of water, allowing it to pond and slowly infiltrate into the soil. Once ponding of water exceeds the height of the dams it then overflows and drains laterally away from the field. Ponding also allows sediments to settle in the ditches, while the excess water drains away. Farmers rework these nutrient-rich sediments back into the berms or the adjacent field surface, thus strengthening the berm or leveling the terrace surface (Figure 14).
Retenes allow farmers to specify how much soil and water enters and leaves the system according to needs at a given time. For these reasons, technical field agents from the various government agencies often encourage those farmers who do not use retenes to build them. Among those farmers who were already using retenes in their zanja-bordo systems, many learned the practice from earlier generations. In fact, recent archaeological evidence reveals that farmers have been using retenes in agricultural slope management for thousands of years (López Corral 2000).
Among the other forms of zanja-bordo terraces built by PIASRE, terrazas zanja-trinchera (trench-ditch terraces) are similar to zanja-bordo terraces with retenes in how they harvest surface runoff and limit its lateral movement. In the zanja-trinchera design, however, ditches occur discontinuously across a slope, which prevents all of the harvested runoff from moving laterally. Given this design, water either infiltrates into the ditches or flows further downslope once full. One disadvantage of zanjas-trincheras is that cleaning the ditches of sediment is more difficult, as they are typically deeper and harder to access. As a result, the form is more often used in situations where water infiltration is prioritized over soil conservation (Figure 15).
Similar to zanjas-trinchera, tinas ciegas, which loosely translates as “blocked tubs, containers, or vats,” are also designed primarily for water capture and retention, but are spaced even further apart, with ditches positioned in an alternating vertical pattern so that all runoff will be intercepted. Tinas ciegas are commonly used in environments with hardened or degraded soils, where large-scale, continuous ditch digging may be impractical. They are also used in environmental restoration programs that focus on reforestation with native species. Because agaves qualify as native species, SAGARPA often plants them in the excavated earth of tinas ciegas as pioneer species that will multiply and improve ground cover. In the case of tinas ciegas, agaves serve less to stabilize the crusty mounds of earth excavated from the ditches, and more to improve groundcover over time.
Jagüeyes are one of the most common of the agave-lined, ditch-and-berm earthworks built by government agencies, resembling wide, semi-circular ponds dug into the surface of hillslopes. In the United States, the equivalent is the farm or retention (detention) pond. The term jagüey appears to have origins in Taino, an indigenous language of the Caribbean, where Cuban farmers still use it to describe any pond-like feature. Its use on other islands appears limited. In Mexico, the term is popular in the southern and eastern states of Veracruz, Tabasco, Hidalgo, Puebla, and Tlaxcala, which, being closer to the Caribbean, may reflect an aspect of spatial diffusion. Similar to the term metepantle, however, many rural Mexicans are familiar with the word and its meaning, but more often use conventional Spanish terms to describe them such as large berms (bordos grandes), cattle or farm ponds (bordos de agua or de abrevadero), water-collecting pots or pans (ollas de agua), reservoirs (represas), and other colloquial phrases (Figure 16a and Figure 16b)
Entire cottage industries are devoted to digging jagüeyes and lining them with agaves for support. Backhoes and their operators sit by roadsides with signs advertising hourly rates, and many municipalities own at least one backhoe for assisting residents with their jagüey-digging projects (Figure 17). To build a jagüey, backhoes first excavate massive amounts of soil and position it downslope. Bulldozers then plow it into a semi-circular earthen bund that opens to the slope above to serve as an embankment for capturing and holding runoff. For the average farmer, the prospects of having an on-farm jagüey can be quite appealing. In theory, jagüeyes collect enough water during the rainy season either to provide supplemental irrigation during dry spells or to water livestock. Other, more ambitious farmers use jagüeyes for aquaculture, mostly carp and tilapia species, which provides another source of household protein.
Unfortunately, water captured by jagüeyes during the rainy season often evaporates or is lost to seepage before it can be used. Geo-membranes can prevent seepage, but are prohibitively expensive for most. Some farmers attempt to meld large, plastic political posters and advertisements together to serve as geo-membranes, but this rarely provides consistent results. Across the Mesa Central, slopes are dotted with empty jagüeyes that stand as testaments to these risks. Nevertheless, enough successful examples exist to inspire farmers to continue to request them.
In addition to their uses in agriculture and conservation, the creativity of individuals is expressed in other forms of agave cultivation. Property owners use agaves to grow living fencerows to discourage trespassing and grazing on both farms and residential properties (Figure 18). Others use agaves as makeshift drying stands and as dispensers for fodder (Figure 19). Ornamental agave species often adorn the medians of roadways, transportation ways and public lands, or the small lots of residential properties for no purpose other than decoration (Figure 20). At times, agaves are planted in carefully spaced, artistic patterns that deliberately break from the more traditional horizontal pattern. In this example from an eco-resort in Tlaxcala, which the lead author helped cultivate, the oblique rows of agaves were specifically designed to be recognizable on satellite images, a marker for those looking for the resort on Google Earth (Figure 21). Although some of the plants will be harvested for pulque, most will serve only as ornamental species—despite all of their other potential uses.
As climates continue to change, agaves will remain versatile tools for shaping the Earth’s surface—both at landscape and local levels. But like all technologies and tools for environmental management, persistent human involvement is essential. Farmers have been incorporating agaves into different forms of agricultural slope management for millennia, and in the case of agave-lined terraces, the technology has proven effective for managing soil and water. Over the last 50 years, conservation groups have taken note of this and have incorporated these agricultural practices into modern conservation contexts. But greater collaboration between farmers and officials is needed to insure the efforts are sustainable. Whether for cultivating crops or for producing ecosystem services, incentivizing farmers to stay engaged with the land is key. When fully vested in efforts to manage hillslopes, farmers are often the most qualified environmental engineers.
Research made possible by the Research Grants Committee (RGC) of the University of Alabama.
All photographs by Matthew LaFevor.