Developing legume shade trees for Sustainable coffee production in Puerto Rico

Coffee originated from dense forests in the Ethiopian highlands, where it grows under the shade of a thick canopy. The Spaniards introduced the crop to Puerto Rico in the 17th century from Haiti. Of the three major crops at the beginning of the 20th century (coffee, sugarcane, and tobacco), only coffee survives as a major crop today. Traditionally, it was grown at low densities under shade in the Western and Central uplands. With its ideal climate, Puerto Rico is famous for producing some of the highest quality coffee beans in the world.
However, since the 1960’s, public policy (PR Department of Agriculture) has tried to substitute the traditional agricultural practice of producing coffee under the shade of trees with an open (sun) system, having higher plant density and intensive use of fertilizers and agrochemicals. Recently, this practice has been criticized since production not only declines after a few years, but also there are numerous negative environmental problems (soil erosion, water pollution, loss of plant and wildlife diversity). In spite of that, traditional farmers maintain shade-produced coffee, and some new sustainable agriculture-minded farmers are re-introducing shade trees. An ecosystem approach to sustainable management needs to consider ecological processes and minimize environmental impacts. This approach must be developed on a crop-by-crop basis.
There is insufficient scientific research conducted in the Caribbean and Puerto Rico regarding the best species of shrubs and trees to use in a multilayer, protective cover for coffee production. Further studies should be designed to test the effects of substrate quality and macroclimate on long-term decomposition and nutrient release dynamics, such as envisioned by the Long Term Ecological Research (LTER/NSF).
Nitrogen fertilizer is used for coffee growth. However, its use depends on economic considerations and the existence of effective infrastructure for fertilizer production and distribution. Because the maximum-recorded N fertilizer use-efficiency is only 50% and the increasing problem of environmental pollution of water from fertilizers, the use of Biological N Fixation (BNF) is both an economically and environmentally sound practice. Although fertilizers and soil uptake can supply most of the coffee plant requirements, the exploitation of soil N reserves is not sustainable. The use of N-fixing trees for shade results in the elimination or significant reduction of chemical fertilizer application.
Long-term sustainability of agricultural systems must relay, as much as possible, on the use and effective management of farm internal resources, with an emphasis on devising strategies for legume cultivation that optimize N2 fixation, conserve soil N, and augment the pool of soil N for non-leguminous crops such as coffee (Zhang and Smith, 2002). BNF by tropical legume shrubs and trees can be a major source of N for coffee production when optimally integrated in the overstory. This type of agroforestry system also promotes biodiversity.
Tropical soils (like those in the mountainous region of Puerto Rico) present a challenge to maximum BNF because of their acidity, and adequate combinations of legume and N-fixing bacteria are necessary to provide maximum benefits for the sustainable production of coffee under shade. More research is needed to recommend the best combinations and management practices for this particular agroforestry system.
The international price for coffee is at a historic low, and the future perspectives are not favorable, since several countries have overstocks and are selling coffee at extremely low prices. Coffee production in Puerto Rico cannot compete with these international prices and other measures such as improvement in quality and crop diversification need to be considered. Intercropping coffee with other crops is one possible option to increase productivity. Because soil conservation is a requisite, trees must play a significant role in this multiple cropping option. Besides offering shade, several N-fixing trees produce excellent wood, and a few species produce fruit. Although a large proportion of coffee in Puerto Rico is still produced under shade (Monroig-Inglés, 2001), yields are low, and more research is needed to improve sustainable production. Therefore, the introduction of new trees and shrubs for sustainable coffee production in Puerto Rico should be evaluated.
An additional benefit of integrating more trees into the present coffee production agro environment is that they will also act as sinks for carbon sequestration, a much-needed environmental positive measure. Forests represent a large sink for CO2 sequestration and the growth of N-fixing trees is not limited by soil fertility constraints. Significant contributions to soil C pools are mediated by roots, and their roots symbionts (mycorrhizae and rhizobia) respond to elevated atmospheric CO2 (Treseder et al., 2003). Improvement in the soil quality (soil organic matter) under shaded coffee is also expected (Doran et al, 1996).

LITERATURE REVIEW
Coffee can be cultivated as a monocrop under sunny conditions or mixed with other species, as one of the classical agroforestry systems (shade). Since new planted coffee will not start to produce until its third year, farmers usually plant a cash crop (such as beans or plantains) intercropped, until coffee achieves its full size. The system is, therefore, very complex, and research usually centers on a specific topic or objective. Unfortunately, there is a cheap coffee glut in the international market, and farmers are starting to increase quality and enhance value (organic coffee, gourmet, etc.) to generate more income.
Most of the research conducted on coffee under shade has been done in Mexico and Costa Rica. In Puerto Rico, since the 60’s and following published results (Abruña et al., 1965 and 1966, and Vicente Chandler et al., 1984) farmers began to grow coffee without a shade canopy.
In 1978, the government of Puerto Rico started to implement a plan to increase coffee production, by a so-called “modernization” of the industry. The plan was based on sun-grown coffee, heavy fertilization and liming, high inputs of herbicides and chemical control, and gathering grain in plastic nets (Vicente-Chandler et al., 1968, Vicente-Chandler et al., 1969, and Vicente-Chandler et al., 1969). The use of plastic nets to harvest was never widely adopted, but the area sown with unshaded coffee expanded, because traditional shaded coffee had low yields. Excess shade from not pruned overstory was one of the causes.
In recent years, worldwide increases in coffee production caused a drastic reduction in the price of coffee paid to the farmer. A large proportion of this coffee is of very low quality, although special and organic coffees still maintain reasonable prices. Local production is unaffected by the international price (the government imports around 100,000 quintals at low price and pays the farmer a higher price), better quality coffee could maintain the industry and even permit exportation of a small amount). Still, many old coffee farms have been abandoned in the Cordillera Central of Puerto Rico. Although secondary forests develop on abandoned farms, particularly in shaded plantations, only a few native species become common (Marcano-Vega et al., 2002).
Many factors indicate that good quality coffee can yield economic benefits and contribute to a sustainable production. In the first place, an agroforestry system in which coffee is produced under the shade of trees, can maintain biodiversity of the ecosystem. For example, fruit set depends on cross-pollinization by bees, and the distance to forest, the variety of trees and number of flowering herbs increases the number of bees’ species that facilitate pollination (Klein et al., 2003). The importance of biodiversity has also been shown by research conducted in Costa Rica in capital-intensive monoculture plantations where all shade trees were eliminated. Perfecto et al. (1997) showed that arthropod biodiversity was lost when samples were collected in coffee monoculture compared to traditional plantations with many species of shade trees. In another study conducted in Southern Mexico (Chiapas), Perfecto et al (2003) compared species richness of birds, butterflies, and ants along a coffee intensification gradient represented by a reduction in the number of species of shade trees and percentage of shade cover in coffee plantations. Although the three taxa responded differently, there was a reduction in species richness along the gradient. Bird species (more mobile) where correlated with distance to forest fragments and not the habitat type. Their results imply that levels of shade and landscape structure are very important for biodiversity conservation. Another study also from Southern Mexico (Peeters et al, 2003) showed that replacement of a diverse shade vegetation of native tree species by the monoculture of Inga significantly reduced firewood products and secondary production (e.g., timber, woods, medicines,) and that coffee production was similar in both plantation types. Neotropical army ants were also studied in Panama in intact moist forest, shade coffee plantations and sun coffee plantations by Roberts et al (2000). Shade coffee plantations provided additional habitat for the ants, which act as critical links between birds and arthropods, providing an easily exploited food resource that would otherwise be unavailable for many birds. Continued conversion of shaded coffee plantations to sun coffee plantations could have negative effects on army ants and associated biodiversity. Ant biodiversity was also reduced in a shaded (Inga) coffee monoculture as compared to traditional polyculture plantation when the distance to the forest increased (Ambrecht and Perfecto, 2003).
In Costa Rica, an agroforestry system that includes very fast-growing trees such as Eucalyptus deglupta is recommended to increase productivity of coffee plantations. Since the benefits could be outweighed by competition for light and soil nutrients, Schaller et al., (2003) studied the factors that regulate the association. They found no evidence of negative effect of the trees on coffee growth, yields, and mineral nutrition. It seems that trees and coffee differ in their roots depth and distribution, resulting in complementary use of soil resources.
Management practices such as deforestation, shifting cultivation, use of very steep lands for cropping, reduced fallow periods and cutting woody perennials for fuel and wood causes erosion problems in tropical soils. Erosion severity depends on soil properties, topography, climate, and use of the land. In Venezuela the government has also promoted changes to the traditional way of cultivating coffee under shade. Ataroff and Monasterios (1997) conducted a study in the State of Merida to analyze the possible consequences of this change on the soil erosion in coffee plantations located on steep slopes. They showed a significant loss of the soil mineral fraction during the first year and that human activities within the plantations increased erosion. Trees provide a solution for wind problems, and although this aspect is not often considered in the Americas, it has been suggested as the agroforestry solution for several countries in Africa such as Tanzania (Stigter et al., 2002).
Soil erosion affects not only land productivity, but also sediments from rivers are transported to the oceans and affect corals. Since reefs are essential for aquatic life, marine productivity is also affected in this chain of events. Appropriate soil erosion control measures, such as the use of multiple-layer plant canopies should be taken in steep slopes such as the mountainous region of Puerto Rico. Data from the U. S. Geological Survey show that lakes capacity has been reduced by sedimentation from 12 to 81 percent of the reservoirs’ original storage capacities (Soler-López, L. R., 2002).
Coffee diseases can reduce quality and yield of the crop. Shade affects each disease differently. For example, Fusarium oxysporum causes severe economic losses in coffee, and the use of resistant varieties is the only practical measure to control the disease (Pietro et al., 2003). More research on Integrated Pest Management (IPM) needs to be conducted in shaded coffee agrosystems. Vandermeer et al (2002) studied the role of Azteca ants as potential biological control agents (BCA) in an organic farm in Chiapas, Mexico. Their results indicate that ants have potential as pests through their positive effect on scale insects, but also potential as BCA through their negative effect on potential herbivores. The arrival of coffee seed borer (CSB) in 2007 in Puerto Rico seems to have reduced significantly the 2008 coffee harvest and many farmers are considering abandoning coffee production.
To obtain good prices for coffee, farmers need to improve quality. A study conducted in Costa Rica found that fruit weight and bean size increased significantly when shade intensity was increased from 0% to more than 80% under unprunned Erythrina poeppigiana. The author hypothesizes that shade promotes slower and more balanced filling and uniform ripening of berries, thus yielding a better-quality product than unshaded coffee plants (Muschler, 2001).
To achieve agricultural sustainability, the use of organic manure and biological nitrogen fixation (BNF) is an efficient biological way of managing soil fertility. Such practices could be the main source of N inflow to agroecosystems and thus a substitute to expensive chemical fertilizer use, especially under low-input agriculture. Besides supplying N for coffee, organic manure imparts beneficial effects on physical soil properties (Kadiata et al., 2002).
Significant higher yields of coffee are obtained with adequate NPK fertilization. In coffee plantations with abundance of N fixing trees, organic matter mineralization supplies, enough N (nitrate and ammonium) for plant nutrition, and N fertilization would be both an economic waste and a source of contamination for water. Besides, owing to its sensitivity to available soil mineral N, any N2-fixing legume may be affected in its nodulation and N2 fixation. This effect can be either stimulation or inhibition, depending on the rate as well as the nature of the fertilizer.
Further studies are needed since legume residues (mostly leaf fall), even if they have high N concentration, may still cause a net immobilization of N, since they might decompose slowly during early stages. Materials with high C/N ratio cause strong N immobilization.
Agroforestry systems such as coffee with shade trees, life fences and tree enrichment of fallows are technically, socially, and economically viable and have a C-sequestration potential varying from 17.6 to 176.3 Mg C ha-1. A project to evaluate and monitor the carbon mitigation effect was established in Mexico (De Jong et al., 1997).
To evaluate the effect of shade on yield of coffee, it is necessary to investigate the shade structure. Results from on-farm research conducted in Chiapas, Mexico, showed that shade cover percentage and coffee shade density had significant effects on yields Soto-Pinto et al., 2000). Shade tree cover had a positive effect between 23 and 38% shade cover and yield was the maintained up to 48%. Production may decrease under shade cover >50%. They found a total of 61 shade species with an average density of 260 trees per hectare, the majority of them being indigenous species, used as a source of food, construction material and as firewood.
The shaded coffee agroforestry systems are more efficient in maintaining soil fertility than monocropping systems. Certain shaded coffee plantations have remained productive for many decades but more research on the causal factors is needed. Roskoski (1981) studied the nodulation and N2-fixation of Inga jinicuil in Veracuz, México. She estimated annual nitrogen fixation as approximately 35 kg/ha, since NPK fertilization was used. However, coffee production was 37% higher with I. jinicuil than with I. vera that lacked nodulated roots. Van Kessel and Roskoski (1981 and 1983) estimated that high levels of phosphorus produced maximum nodule biomass but neither magnesium nor molybdenum affected nodulation or acetylene reduction of I. jinicuil seedlings. Favorable effects on soil fertility and crop nutrition have been observed in associations including N2 fixing legume trees (Schroth, G et al., 2001). Somarriba et al (2001) have proposed research trusts on the responses of coffee to shade and competition and socioeconomic analysis of both traditional and new shade-coffee combinations vs. monocultures. Budowski and Russo (1997) discussed the role of N-fixing trees is sustainable agriculture, particularly with examples from Mexico and Costa Rica. Research has involved Leucaena, Alnus acuminata and Erythrina spp. Results with E. poeppingiana indicate considerable benefits to the soil associated crops, and research projects in this area are promising.
In Africa (Burundi), Snoeck (1996) investigated the optimization of BNF for coffee by legumes. From his results, he recommended Leucaena diversifolia for sites over 1500 m and Leucaena leucocephala under 1500 m. More recently, Snoeck et al., (2000) used isotopic techniques to demonstrate nitrogen transfer from legumes to coffee. They showed that N availability for coffee depends on the conditions. When the legume crop can fix more than 20-25% of its needs from BNF, the N transfer is effective from the first year. The importance of BNF has probably being underestimated. An innovative labelling technique to measure 15N recovery (Meyer et al, 2003) suggests that significant rhizodeposition (RD) not identified with visible roots or root fragments occur. The RD can increase the soil nitrogen pools by 40-68 kg ha-1 and represents a significant addition to the soil. Although nitrogen fixation occurs in the leaves of coffee by epiphytic lichens, the amounts provided for coffee growth is very small (Roskoski, 1980). However, the phyllospheric N fixation occurring in shade trees in coffee ecosystems has not been measured.
All factors previously discussed interact in specific sites, therefore the relative importance depends upon site conditions, component selection, below-ground and above-ground characteristics of the tree and coffee variety, and management practices. They have been reviewed by Beer et al (1997), who indicate that economic evaluations are needed to judge the best system. Therefore, we will not be able to recommend a single “Magic Tree”; it is necessary to use a mix of species. We need to have a list of several species, adapted to different soil and environmental conditions and management practices. This tree biodiversity is also recommended for wildlife conservation. Besides providing shade for coffee, some trees have other uses such as providing fruits, soil conservation; can be used as windbreak and for alley cropping.
In Puerto Rico, public interest in supporting sustainable agriculture is high or increasing.
However, information on NFT is very scarce (mostly botanical aspects) of shade trees. Much more research is needed. A study conducted in Chiapas, Mexico, indicated that farmer knowledge of legumes and the process of BNF were limited, although they had seen root nodules (Grossman, 2003). The situation is possible similar in Puerto Rico, since soil microorganisms, unlike earthworms, cannot be seen directly. Grossman et al, (2005) characterized the diversity of strains isolated from Inga oerstediana, Inga pavonina and an unknown species of Inga and showed they belong to Bradyrhizobium. However, growth nodulation and N fixation seem low during initial growth of Inga oerstediana (Grossman et al., 2006). Since more than 300 species of Inga are known, comparative studies should be performed to select more effective species.