S. C. Davis, Institute for Genomic Biology, 1206 W. Gregory Dr. Urbana, IL 61801, USA. tel. +1 217 244 5952, e-mail: email@example.com
Large areas of the tropics and subtropics are too arid or degraded to support food crops, but Agave species may be suitable for biofuel production in these regions. We review the potential of Agave species as biofuel feedstocks in the context of ecophysiology, agronomy, and land availability for this genus globally. Reported dry biomass yields of Agave spp., when annualized, range from <1 to 34 Mg ha−1 yr−1 without irrigation, depending on species and location. Some of the most productive species have not yet been evaluated at a commercial scale. Approximately 0.6 Mha of land previously used to grow Agave for coarse fibers have fallen out of production, largely as a result of competition with synthetic fibers. Theoretically, this crop area alone could provide 6.1 billion L of ethanol if Agave were re-established as a bioenergy feedstock without causing indirect land use change. Almost one-fifth of the global land surface is semiarid, suggesting there may be large opportunities for expansion of Agave crops for feedstock, but more field trials are needed to determine tolerance boundaries for different Agave species.
Biomass feedstocks that grow on semiarid lands could be a sustainable answer to increasing demands for renewable fuels that do not conflict with food and feed production. If high yielding crops that require minimal inputs of water and nutrients can be grown on lands that are marginal for food crops, land use competition could be reduced. Plants that use the Crassulacean Acid Metabolism (CAM) pathway have low water requirements and are productive in semiarid regions because they assimilate carbon at night thereby decreasing the diffusive gradient of water out of leaves and improving water use efficiency (Nobel, 1994). Water use is decreased because potential evapotranspiration increases exponentially with temperature, and by opening stomata during cooler nighttime conditions, less water is lost per CO2 assimilated. It follows that the greater the diurnal variation in temperature, the greater the water use efficiency advantage of CAM. Because of this efficient resource use, CAM plants have recently been introduced as potential bioenergy crops (Smith, 2008; Borland et al., 2009).
In the context of biofuels, CAM physiology provides benefits beyond nutrient and water use efficiency. Lignin represents one of the major impediments to digestion of lignocelluloses to release sugars, a process that allows for the fermentation of cellulose and hemicelluloses to fuels. Current plant feedstocks can be up to 30% lignin (Somerville, 2007), a cross-linked polymer that gives the xylem adequate strength to withstand the tensions imposed during transpiration. However, in CAM plants, tension in the xylem is low because transpiration is low and thus less lignin is required. As a result, lower amounts of lignin are evident in the composition of CAM tissue. Agave leaves, for example, have 3–15% lignin by dry weight and up to 68% cellulose (Iñiguez-Covarrubias et al., 2001; Santiago et al., 2002, Vieira et al., 2002, Garcia-Reyes & Rangel-Mendez 2009). Another characteristic of CAM physiology that is advantageous for biofuel feedstock is the accumulation of soluble nonstructural carbohydrates in plant tissue to support PEPC-mediated carboxylation (Borland et al., 2009). The low lignin and high soluble carbohydrate reserves of CAM plants would require less energy for conversion to fuels and may therefore result in a higher quality feedstock (Smith, 2008; Borland et al., 2009).
The Agave genus is composed exclusively of obligate CAM plants and includes species that are grown commercially. Importantly, they thrive in conditions that are unsuited to major food crops and pasture grasses. The most widespread commercial uses of Agave spp. are for fibers and beverages, derived usually from the leaves and from the stem, respectively. At the peak of production in 1964, over 1 Mha of Agave were cultivated globally for sisal fibers (FAO, 2010), the majority of which were in Africa (Lock, 1969; FAO, 2010). Since then, the widespread production of synthetic fibers caused a decline in sisal production (Nobel, 1994) and there were <0.5 Mha of sisal planted by 2008 (FAO, 2010). In the 1990s, ∼70 000 ha of Agave, predominantly the species A. tequilana, were cultivated in México for the production of alcoholic beverages, along with 200 000 ha of A. fourcroydes, or henequen (sometimes called sisal), grown for fiber production (Nobel, 1994). Agave sisalana (sisal) is also native to México, but is now primarily grown in Brazil and Eastern Africa for fiber.
The potential of CAM plants for biofuel feedstocks has been recently reviewed (Borland et al.,. 2009), yet there is very little information about biofuel production from CAM plants at a commercial scale. There is also no systematic record of field-scale dry matter productivity, which is crucial information for establishing the economic and logistic viability of a biofuel enterprise. Agave species cultivated for fiber have quite different production systems from those cultivated for beverages, but both are examples of commercially scaled agriculture that would be relevant to a bioenergy production chain. This paper will synthesize information from existing literature to evaluate the potential of Agave production systems for near-term sustainable biofuel feedstocks. We review the form, diversity, and ecology of Agave, the agronomy of the major crop species, and finally the land availability for biofuel production from this group of plants.
Review of Agave form, diversity and ecology
Agave is a genus of some 200–300 species, within the family Agavaceae, although it is sometimes placed in the Liliaceae or Amarylidaceae (Purseglove, 1972; Colunga-García Marín et al., 2007). Gentry (1982) recognizes two subgenera, Agave and Littaea, in North America. The natural distribution of the Agave genus is limited to the Americas, with the greatest diversity of species in México, although many species have been distributed across mediterranean and dry tropical regions for agricultural, hedgerow and horticultural uses (Purseglove, 1972). All species of the genus that have been analyzed use CAM and it is assumed that the genus as a whole uses CAM physiology (Szarek & Ting, 1977; Szarek, 1979).
Agave plants are perennial evergreen xerophytes, ranging in size from several centimeters to 4 m and producing large flowering stalks 2–12 m tall after 5–15 years (Gentry, 1982; Valenzuela-Zapata & Nablan, 2004). They have a basal rosette of large stiff, lanceolate, succulent and persistent leaves, often with a terminal spine, and sometimes with spiny margins. In transverse cross-section, the leaves are crescent shaped. The epidermis is highly cuticularized and the stomata are deeply sunken at the base of hypostomatal cavities. The mesophyll includes elongated water storage cells as well as idioblasts, which contain calcium oxalate crystals (Blunden et al., 1973). Stems are short and thick, or basal with leaves formed around the terminal meristem that they encircle. Shoots are typically monocarpic, i.e. they die after flowering, however side shoots may allow the plant to persist. Flowers are usually formed on a massive spike, sometimes termed a pole, and have a paniculate inflorescence (Gentry, 1982). Many Agave cultivars do not flower or are sterile, and propagation is either by suckers or from bulbils produced on the flowering stem (Purseglove, 1972).
Agave plants typically form a spreading fibrous root system that arises adventitiously from the stem base and, although shallow, may spread some distance. For example, in the case of A. sisalana, anchor roots of 2–4 mm in diameter may spread up to 5 m from the stem base, but vertically remain within the top 40 cm of soil. These roots become suberized, but produce smaller feeder roots along their length, that can bare numerous root hairs (Purseglove, 1972).
Agave spp. often grow on rocky soils of poor quality in regions with extreme drought and elevated temperatures. Optimum growth can be achieved with high annual rainfall amounts of 102–127 cm, but high production has also been observed in Agave spp. that grow in regions with only 25–38 cm of annual rainfall (Kirby, 1963). When considering drought, it is important to note that total precipitation is not the only determinant. The pattern of precipitation, i.e. a few high intensity events vs. an even distribution of smaller events will have a profound effect, the former being most typical of the semiarid tropics and subtropics. Water vapor pressure deficit, radiation and windspeed are the determinants of potential evapotranspiration, and all three can be high in the semiarid tropics and subtropics. Soil porosity and topography determine drainage and can also cause frequent droughts in otherwise high precipitation zones. For example, the major growing region of henequen (A. fourcroydes) in the Yucatán, México, is characterized by shallow calcareous gravel soils overlaying porous limestone. Even though precipitation reaches 760 mm yr−1, the land is unsuitable for other crops (Purseglove 1972; Colunga-García Marín et al., 2007).
Because droughts are frequent in semi-arid regions, viable perennial crops must not only be efficient in their use of water, but also capable of surviving without any accessible water between rainfall events. Agave spp. achieve this not by tolerance to low water potential, but by hydraulic isolation. During a dry-down event the roots shrink, leaving an air gap between the soil and root surface. This prevents dehydration due to water moving from the plant to soil as the soil water potential declines. In parallel, the thick cuticle and closed sunken stomata prevent water loss to the atmosphere and maintain high plant water potential. Even though roots of Agave are susceptible to cavitation, the high stem water potential limits the occurrence of cavitation during prolonged droughts (Linton & Nobel, 1999).
From 14 different studies that report annual Agave productivities, it is clear that eco-physiological responses of species to different environments will affect the success of new commercial plantations. There are substantial differences in the reported productivities of Agave spp. (Fig. 1), but these differences are confounded by regional variation (Fig. 2). Climate is likely to be a major determinant of yields within and among species. For example, A. lechuguilla produces only 3.8 t ha−1 yr−1 but occurs in areas with moderate annual rainfall (427 mm) while A. mapisaga produces 32 t ha−1 yr−1 in regions with 848 mm of annual rainfall (Fig. 3). Consider as another example that the yield of A. salmiana ranges from a mean of 10 t ha−1 in an arid but relatively cool region to 34 t ha−1 in a semiarid region with moderately warm daytime temperatures (Fig. 3). Much of the data on dry matter yield is based on extrapolation from measurements on single plants, rather than yields measured over large areas, which adds uncertainty to the true yields that might be obtained in large-scale plantings.
The relationship between nighttime temperature and carbon assimilation has been described for A. tequilana (Luiz Corral, 2007), but is not necessarily consistent for the majority of other Agave species. Using minimum temperature data for the four locations shown in Fig. 3, and assuming 8 h of active CO2 assimilation daily, the range in estimated CO2 uptake is less than actual observations. We estimated that the theoretical CO2 assimilation of A. tequilana only ranges from 39 to 42 Mg CO2 yr−1 (or about 21–23 t biomass ha−1 yr−1) at sites with mean nighttime temperatures that range from 9.6 to 20.6 °C. The productivity of other Agave species are far outside of this range, suggesting that the physiological assumptions for A. tequilana, at least in response to nighttime temperatures, may not be directly transferable to other Agave spp.
Biodiversity of Agave, at the species and genetic levels, has been affected by land management choices in the last century. There has been steady selection pressure on the genetic diversity of Agave species that are grown for tequila and fiber (Vargas-Ponce et al., 2009). There has also been a decline in the husbandry practices that historically promoted populations of many species for food, fiber, and forage (Valenzuela-Zapata & Nablan, 2004). A. fourcroydes and A. sisalana were selected for leaf length to provide long fibers and A. tequilana was selected for the high sugar content of the piñas (Fig. 4). Piñas are the swollen and nonstructural carbohydrate rich stem bases that are harvested as the fermentation feedstock for the production of tequila (Fig. 4). There are other varieties of A. tequilana that may produce greater biomass, but less sugar, that could be a better choice of feedstock for cellulosic fuels. Published productivities of species vary spatially, and there is no one species that appears to have the greatest productivity across all regions of México or elsewhere. The Agave species with the greatest reported annual yields (e.g. A. mapisaga; Fig. 1) have not been thoroughly evaluated for commercial-scale plantation agriculture.
An Agave plant creates a microhabitat that hosts unique bacteria, fungi and invertebrates. A genetically diverse bacterium, Zymomonas mobilis, originally discovered on Agave leaves, has been the subject of biotechnological research because of its potential as a fermentative organism with high ethanol tolerance, but the role of micro-organisms in the nutrient budget of Agave is unknown. Better understood are a number of parasitic organisms that benefit from Agave (Virgen Calleros, 2010), most notably the weevil Schyphohorus acupunctatus which burrows into Agave and the fungus Fusarium spp. which causes severe necrosis in the xylem tissue (González Hernández et al., 2007). Another damaging parasite is the rhinoceros beetle, Strategus spp. that can kill a young Agave plant in 24 h by eating the root system (González Hernández et al., 2007). Resistance to these pests will most likely be maintained by increasing genetic diversity in crops (Valenzuela-Zapata & Nablan, 2004) or selecting for new resistant clones.
Review of Agave agronomic practices
Agave species that are cultivated in commercial plantations do not produce seed because the plants are harvested before flowering occurs. The seeds that are produced may not be viable for commercial production because seeded plantlets are more sensitive to the dry periods that are common in semiarid climates, and therefore vegetative propagation has proven necessary for the commercial production of Agave spp. (Kirby, 1963; Purseglove, 1972; Nobel, 1994). Because of the high degree of heterozygosity in fertile Agave, seed can also be problematic because it produces highly variable stands, which will be challenging for management and harvest. In addition, establishment from seed takes longer than from bulbils or suckers. Bulbils are young plantlets which occur on the inflorescence of mature plants. In sisal production, bulbils are used for vegetative propagation and planting (Nobel, 1994). A single sisal pole may produce 2000 bulbils, which can be harvested from the ground after they have fallen or are shaken from the pole. They are far more numerous than suckers and have been found to give a more uniform crop (Purseglove, 1972).
Although A. tequilana plants can also produce bulbils, commercial propagation is most commonly by removal of offset rhizome shoots from mother plants. These are harvested annually. The offsets are grown in nurseries until they reach >750 g of fresh weight, and are then transplanted to commercial fields (Valenzuela-Zapata, 1985). Before new Agave individuals are planted, the roots are often sterilized with a 10% formaldehyde solution and left to callus for over a month. Increasingly, micropropagation is being used to clone and propagate plantlets that are then grown in nurseries before planting in the field (Nikam et al., 2003; Ramirez & del Real Laborde, 2007; Ramirez-Malagon et al., 2008). Typically, 2 years of care in a nursery are required before planting micropropagated individuals in the field (del Real Laborde, personal communication).
Nutrient requirements vary throughout the life of an Agave individual. Nursery plants are established in peat pots that are supplemented with slow release calcium and magnesium. In the field, a supplemental nutrient solution is applied along with a compost mixture that includes bagasse, a byproduct from tequila processing that often increases soil acidity. Under these conditions, a mixture of nitrogen, phosphorus (P2O5), potassium (K2O), boron, copper, and zinc are typically applied at a ratio of 40 : 120 : 40 : 4 : 2.5 : 16 in the first year and 40 : 80 : 80 : 4 : 2.5: 16 in the second year (Uvalle Bueno & Velez Gutierrez, 2007). Then nutrient amendments are limited to nitrogen, phosphorus, and potassium in the third year at a ratio of 60 : 40 : 80, and only nitrogen is required in subsequent years (Uvalle Bueno & Velez Gutierrez, 2007).
Before the establishment of an Agave plantation, the land is usually cleared by burning, bulldozing, or digging to remove existing vegetation and obstructions. Mulching of any vegetation covering the land may be a better method, since it will boost soil organic matter and help preserve any residual soil moisture. In present practice, the land is usually plowed twice to a depth no greater than 15–20 cm and then harrowed (Lock, 1962). However, given the wide spacing, Agave would appear a good candidate for no-till planting. In places where Agave spp. are planted in exceptionally rocky areas known as ‘ceboruco,’ the soil is worked by hand and cannot be tilled mechanically (Gobeille et al., 2006).
Young Agave plants are transplanted from a nursery into 30 cm deep furrows. Planting density varies with topography, soil quality, and polycultures. A. sisalana can be planted with 4900–9900 individuals per ha in optimal conditions (Kirby, 1963). The density of planting for A. tequilana ranges up to 6200–7400 plants per ha on steep slopes and 7400–11 000 plants per ha on flat lands (Valenzuela-Zapata, 1985). Row spacing for A. tequilana is sometimes as wide as ∼3 m with plants within a row ∼1.8 m apart (Valenzuela-Zapata, 1985), allowing only 1852 plants ha−1.
Cultivation techniques affect planting densities. Mechanization facilitates planting∼1500 individuals per hour (del Real Laborde, personal communication), a process that is far more time efficient than hand planting but full mechanization requires wider row spacing and possibly more land to achieve the same supply of biomass. Mechanization will also affect employment in rural communities. If managed mechanically, rows must be wide enough for tractors to access plants after maturity (3–4 m minimum distance). If manually pruned and harvested, a minimum distance between rows can be reduced to ≤2 m (Kirby, 1963). The economic and social trade-offs that result from replacing hand labor with mechanization are important considerations in the development of new Agave plantations.
The timing of plant maturity can be very specific to individuals, even within a species, and microclimates. At high altitudes and in poor soil conditions, individuals develop more slowly than their lowland relatives. Mature leaves are harvested from A. sisalana and A. fourcroydes for fiber using a sickle-like knife with a curved end called a ‘coa de jima’ (Valenzuela-Zapata & Nablan, 2004). Such harvests can continue for 15 years before replanting is necessary. In the case of A. tequilana that is used to make tequila, only the piña (stem or head) is harvested after at least 5 years of maturation, and then new plants must be established.
Leaves from A. tequilana are typically left on the field to return nutrients to the soil, but very rarely the leaves are used for co-combustion to generate energy at a distillation plant. The use of leaves, for nutrients, co-combustion, or biofuel feedstock, is an important consideration for life-cycle analyses of an Agave-based biofuel production system. In the short term, the use of leaves in cellulosic conversion processes or co-combustion could add to the efficiency of Agave biofuels, but little is known about the effects of leaf removal on soil organic matter and the long-term sustainability of this agronomic practice.
Mature canopies of crops grown for tequila (piñas) intercept less photosynthetically active radiation than the canopies of crops grown for fiber. Thus, the system used for fiber production has an inherently higher production potential. Agave tequilana will barely cover 5% of the ground at the time of planting and covers only about half of the ground surface at harvest. Thus, on average A. tequilana will only be able to intercept <25% of available solar radiation over the 5 years from planting to harvest. Henequen (A. fourcroydes) and sisal (A. sisalana) by contrast have more full canopies that persists for ∼15 years with annual leaf harvests and can therefore intercept more radiation.
A. mapisaga and A. salmiana produced the greatest yields observed in 14 studies of Agave species. With careful management that includes pruning, fertilization, irrigation, weeding and pest control, A. mapisaga yielded 38 Mg ha−1 yr−1 and A. salmiana yielded 42 Mg ha−1 yr−1 (Nobel et al., 1992). These yields far exceed corn, soybean, sorghum, and wheat productivities under intensive management. Even without irrigation, yields of A. mapisaga, A. salmiana, and A. tequilana can reach 25–26 Mg ha−1 yr−1 in semiarid conditions of west central México (Nobel, 1991). At the low end of productivity, in arid conditions of the southwestern US (<200 mm yr−1), yields of A. utahensis and A. vilmoriniana are <1 Mg ha−1 yr−1 (Clary & Jameson, 1981; Idso & Kimball, 1995). In the Yucatán peninsula of México, where there is greater rainfall (981 mm yr−1) but more porous soil, A. fourcroydes has a productivity of ∼16 Mg ha−1 yr−1 without irrigation (Nobel & Valenzuela-Zapata, 1987).
It is uncertain how yields of the more productive species of Agave would be reduced in more arid conditions. In fact, side-by-side replicated plot trials that would be necessary for any rigorous comparison of species or cultivars appear to be largely lacking. Most yields have been assessed for individual experimental plants rather than production fields, where yields are likely to be lower.
The composition of harvested Agave biomass varies and different methods are required for processing leaves and the piña after harvest. Leaves have greater cellulose and lignin, and the piña has greater amounts of soluble carbohydrates (Table 1). In a regional evaluation of crops in México, it was discovered that the composition of carbohydrates extracted from the same species differed according to the location of source crops that were subject to different climates (Mancilla-Margalli & López, 2006). For several species that were grown in the same region (including A. angustifolia, A. potatorum, and A. cantala), the carbohydrate profiles were similar among species. This underscores the importance of site selection for optimizing biofuel yields.
Table 1. Weight-based percentages of compounds in Agave leaves
The tequila industry provides a model for biofuel production because the plantations and the initial fermentation at a distillery are similar to what would be necessary for fuel ethanol production. Tequila Sauza, one of the largest tequila companies in México, processes approximately 400 Mg of piñas per day into alcohol at a single distillation plant. Approximately 1 L of 40% ethanol is produced for every 5.5 kg of dry biomass. The piña is ≥52% sugar (Mancilla-Margalli & López, 2006), and can have a greater concentration of soluble carbohydrates than corn grain. If one assumes the current ethanol conversion efficiency for corn grain (Perrin et al., 2009), then an ethanol plant that processes 400 Mg day−1 of piñas could theoretically produce 61 million L yr−1 of 100% ethanol for fuel (equal to ∼417 L t−1). This scale of production (400 t day−1) is similar to that of a tequila distillery, and does not include conversion of the cellulosic parts of Agave plants.
The biomass from the leaves left after piña processing could add substantially to the amount of ethanol that can be produced. The leaves of A. tequilana that are unused in the process of tequila production equate to ∼38% of the total plant biomass (Cedeño, 1995; Iñiguez-Covarrubias et al., 2001), and could serve as cellulosic feedstock to produce another 28 million L ethanol yr−1. In total, from piñas and leaves, the production capacity of an ethanol plant that processes Agave from the same amount of land used by a single tequila distillery could theoretically produce 89 million L yr−1 of ethanol (equal to ∼378 L t−1). Assuming crop yields of 24 Mg ha−1 yr−1 (Nobel & Valenzuela-Zapata, 1987), then 6083 ha would be harvested annually to support this production volume.
There are both solid and liquid byproducts after converting Agave to tequila or ethanol. Liquid waste (vinasse) from the initial processing of piñas can be treated (Méndez-Acosta, 2010), but is often recycled to fields without treatment resulting in degradation of water quality. Methane is produced as a byproduct from the water treatment process (Méndez-Acosta, 2010) and could be used for energy. Bagasse is the solid byproduct that is typically composted and reapplied to Agave fields, returning nutrients but also contributing to acidification of the soil. Other uses could be for co-combustion to produce heat and power or for additional cellulosic feedstock for liquid fuel (see bagasse composition in Table 1). If the bagasse, which is usually ∼40% of piña weight (Cedeño, 1995; Iñiguez-Covarrubias et al., 2001), were also converted to ethanol, an additional 22 million L yr−1 of ethanol could be produced. Uses of both vinasse and bagasse would thus be important considerations in a life-cycle analysis of an Agave-based bioenergy production system.
Land available for Agave bioenergy
Based on only the area of abandoned land previously used to grow Agave for fiber (FAO, 2010), there is an opportunity to reinstate Agave agriculture on 1.87 × 105 ha in Mexico, 3.43 × 105 ha in Africa, and 0.67 × 105 ha in other parts of the world. The Agave plantations in Australia pre-date FAO records, but there are regions on the continent that could be allocated to Agave feedstock crops (Holtum et al., 2011). Conversion of abandoned plantations back to Agave would result in minimal indirect land use change because of the low productivity that occurs in these regions. Actual opportunities within and beyond these abandoned lands depend on local politics, economics, and climate change. In total, semiarid regions that could support Agave agriculture comprise 18% of the global land surface (Hoekstra & Shachak, 1999).
Land that is marginal, abandoned or of low-productivity represents one of the most appropriate opportunities for biofuel feedstocks because this will minimize indirect land use change (Somerville et al., 2010). While the sisal production in Brazil dwarfs the production of Agave fiber in other countries (Fig. 5), there are less abandoned land opportunities than there are in other regions of the world where sisal production has declined. The land area devoted to sisal production in Brazil is similar to what it was in 1964, at the peak of global production. In cultivation, Agave is not limited to semiarid lands, since it is also productive in areas with high rainfall and good soil water retention. As a result, not all sisal production is on semiarid land. Most of Brazil receives more than 1000 mm of precipitation annually (NOAA, 1991), and is a very large agricultural producer. In considering Agave as a biofuel feedstock, it will be important that plans are limited to areas where moisture prevents the viable production of food crops. CAM plants, because of their low stomatal conductivities, will be inherently less efficient than other crops in high moisture environments.
Arid and semiarid regions often intersect with lower economic opportunities because of the inherent limitations on land-based resources. For example, there are large tracts of semiarid to arid environments in Africa that support very few food crops. This may be in part a cause for political and economic instability in some of this region (Blench, 1996; Oketch, 2006). The theoretical minimum amount of water required for CAM plants with an average production of 43 Mg ha−1 yr−1 is 258 mm yr−1 (based on Borland et al., 2009), less than the precipitation at sites where Agave is grown commercially. While the theoretic production of Agave represents an opportunity for enhancing land-based resources in arid regions, rigorous replicated field trials that verify such remarkably high yields are lacking. Still, feedstocks that grow where food resources do not could stimulate local economies without displacing existing commodities, in effect minimizing indirect land use change.
Australia has the largest proportion of semiarid land of any continent. The greatest challenge for initiating Agave biomass crops in this region may be regulations against the use of exotic species. Although Agave spp. are typically not invasive, Australian regulations that govern importation of non-native species are restrictive relative to most other regions of the world. This is because of the unique biogeography of the continent and a history of biodiversity loss due to introductions of well-intended but disastrously aggressive exotic species. For example, Opuntia stricta, another CAM species, rapidly expanded onto 25 Mha of farmland after being introduced in central eastern Australia (Osmond et al., 2008). While this introduction shows the ability of CAM plants to thrive in this dry environment, it also leaves a historical scar that would likely result in opposition to another CAM plant introduction. Ironically, there was no evidence of invasion from Agave plantations that were established, but later abandoned in the early 1900s, in Australia. Policy position and support for Agave introductions may be a key factor in determining the success of bioenergy production from this genus in Australia.
The diversity of Agave spp. native to México that could serve as bioenergy feedstocks are currently not recognized internationally. Fiber produced from Agave is listed as a crop commodity by the Food and Agriculture Organization of the United Nations (FAO) under two categories: ‘Sisal’ and ‘Agave Fibres Nes.’ The official FAO definition of ‘Sisal’ is limited to A. sisalana, but the sisal reported for Mexico is actually from A. fourcroydes (henequen). According to FAO, the ‘Agave Fibres Nes’ category includes Haiti hemp (A. foetida), henequen (A. fourcroydes), ixtle, tampico (A. lechuguilla), maguey (A. cantala), pita (A. americana), and Salvador hemp (A. letonae). Clarification of these categories, inclusion of other known species, and bioenergy designations will need to be adopted before a global market for Agave bioenergy feedstock can be realized.
Bioenergy in a future climate
With respect to future climate change, Agave spp. may have an advantage over other crops because CAM physiology is adapted to extreme high temperatures and drought. Interestingly, the areas of the world that we have identified as the most suitable for Agave feedstock plantations (Mexico, Australia, and Africa) are also areas where interannual variation in temperature is relatively low (Sun et al., 2010). Although there is a general trend of warming in these regions (Sun et al., 2010), less sporadic fluctuations in climatic conditions will translate into lower risk for plantations given that they require at least 5 years for establishment. Long-term projections indicate there will be a decline in precipitation in the southwestern US, México, Australia, and northern Africa (IPCC, 2007). The trend toward less plant-available water underscores the importance of establishing crops with high water use efficiency.
There is an immediate land opportunity of almost 600 thousand ha globally for Agave bioenergy crops that would not incur indirect land use changes (FAO, 2010). Assuming maximum yields of 26 Mg ha−1 yr−1 in semiarid conditions and a conversion efficiency of 380 L ethanol Mg−1 of lignocellulosic biomass (Farrell et al., 2006), 6.1 billion L of ethanol could be produced from Agave with minimal increases in environmental impacts. Although this is a small percentage of the total global biofuel production (Table 2), the assumptions used in this calculation are conservative because they only include Agave production on lands that have come out of previous Agave cultivation. In reality, if Agave production on other semiarid lands were economically and environmentally sustainable, Agave could add substantially more value to land-based resources in Africa, Australia, and Mexico. In some places, this new commodity could stimulate local economies, but there are political and legislative obstacles to overcome before Agave production can expand globally. For local economies in some regions, it will also be critical not to displace current beverage and fiber production from Agave. More research is needed to identify Agave species that are optimal for maximizing biomass yields regionally.
We would like to thank Rhea Kressman for collecting literature sources and all of the participants of the Agave and Bioenergy Workshop held on May 12–13, 2010 in Guadalajara, Mexico for sharing their insights about Agave production. This work was supported by the Energy Biosciences Institute.