The advantages and challenges of integrating tree legumes into pastoral systems 146
in the production of food and forage; (iii) contributing to the sequestration of C in soils; and (iv) providing a viable source of biomass for biofuel (Jensen et al., 2012). Carbon cycles are often coupled with N cycling. Kirkby et al.
(2011) demonstrated similar soil C:N ratios across different soil types. Thus, BNF by tree legumes also increases the potential for C sequestration in the soil (Nair et al., 2010).
In addition to BNF and recycling of N via litter deposition and animal excreta, trees can also recycle nutrients from deeper soil layers to soil surface (Buxbaum et al., 2005). In low-P soils, the establishment of an organic P pool in the soil is critical to assuring long-term productivity. Nitrogen-fixing tree legumes associated with mycorrhizae is one possible strategy to increase organic soil-P and revegetate degraded soil receiving low fertilizer inputs (Franco and Faria, 1997). Hydraulic redistribution involving hydraulic lift by trees is the most commonly observed and takes place when shallow soil layers are drier than deep layers (Prieto et al., 2012). Aboveground benefits of hydraulic lift come from increased soil moisture in dry soil layers, which in turn affect plant physiology and water relations. Increased soil moisture enhances root growth and function as well as rhizosphere processes, with relevant implications in ecosystem nutrient cycling (Sanderson et al., 2004; Prieto et al., 2012). Pang et al. (2013) demonstrated that hydraulic redistribution by deep-rooted legumes improves survival of a drought-stressed, shallow-rooted, legume companion.
Thus, tree legumes might benefit the herbaceous vegetation by transporting water from deeper soil layers. This may not be the case in all associations, with competition for moisture occurring between some trees and herbaceous vegetation (Gea-Izquierdo et al., 2009).
Challenges in the adoption of tree legumes in pastoral
systems is challenging (Dagang and Nair, 2003). Unless trees are already an integral part of local silvopastoral systems, their introduction into purely herba-ceous pastures faces numerous biotic and abiotic hurdles which limit adoption of innovative silvopasture trees in many regions.
Abiotic Challenges
A common abiotic hurdle to introducing silvopastoral innovation is land manager reluctance to implement changes in their pastures. This reluctance arises from conservatism that can only be overcome with heavy investment in research, education, and demonstration prior to and following pasture mana-gement innovation (Pengelly et al. 2003; Peters et al. 2003; Dagang and Nair, 2003). Research that drives change must be convincing and unequivocal as well as practical, a hurdle more easily overcome with on-farm research compa-red with less realistic experiment station efforts that do not include farmers or extension (Borel and Romero, 1991). Innovative silvopastoral research takes years if not decades due to the slow establishment rate of most trees compared with herbaceous components (see below). A slow diffusion process follows in which demonstrations and neighbor emulation are required to convince land managers that the risk is tolerable.
Sustainability and resilience to a silvopasture land manager mean econo-mics first and biology second. This balance requires annual income, often more stable over time when income is diversified (Martins et al., 2011); maintaining an income stream is a challenge, however, when initially introducing or main-taining trees in a system (Kizos et al., 2013). Exterior financial incentives such as ecosystem services payments are rare but, where they are available, help to mitigate silvopasture establishment costs (Garback et al., 2012). Normal short-term market or socio-cultural demands are more difficult adjustments in silvopastoral systems that require decades to establish but days to destroy (García-Tejero et al., 2013). This does not, however, necessarily favor recruit-ment of economically stable land managers to silvopastoral systems. In some cases the poor can better afford to risk investment in tree establishment because they have less to lose (Pagiola et al., 2008).
Secondary benefits from trees within a silvopastoral system can offset forage yield and animal product reduction through income diversification. The-se include food items such as nuts or fruit harvested from the tree component (Martins et al., 2011; Kallenbach et al., 2006). Fuel from tree branches and
The advantages and challenges of integrating tree legumes into pastoral systems 148
trunks can also generate income, especially in peri-urban and urban regions where wood is used for cooking and heating (Muir, 1999). Lumber is another source of income from diversified silvopastoral systems (Bird et al., 2010).
Some systems target animal products derived from browsers such as wild ungulates or goats, in which case trees become an essential forage component vis-à-vis grass production for grazers (Addlestone et al., 1999). Ecosystem services such as soil conservation (Alonso et al., 2006; Devkota et al., 2009) or C sequestration (Nair et al., 2010; Garback et al., 2012) can provide addi-tional benefits that offer income over the long run. All of these compensate for the investment costs and pastoral losses resulting from introducing trees into animal production systems.
A third abiotic hurdle is the steep learning curve for researchers, educators, and land managers. If silvopastoral systems are already a tradition in the region, expanding or improving them is relatively easy (Martins et al., 2013). If it is a novel innovation to the region, learning how to manage the comparatively complex mixture of multiple plant and animal species becomes a challenge.
Information sharing, such as extension services or on-farm demonstrations, can help overcome some of this adoption delay (Dagang and Nair, 2003; Garback et al., 2012).
Biotic Challenges
Biotic challenges to establishing silvopastoral systems, especially novel ones, are myriad. The need to understand and manage multiple plant species in the same landscape is primary among these. Single-species pastures are gene-rally easier to establish and manage than multiple-species pastures, especially in tree-herb mixtures, because of the physiological and palatability differences among species (Sanderson et al., 2007). Because resilience of such systems is key, maintaining a balance among the various components while extracting multiple products (plant and animal) requires far greater understanding, skills, and experience for researchers, educators and land managers (Callaway and Lawrence, 1997).
Trees establish and mature at a slower rate than herbaceous plants. Land managers must therefore be willing to defer reaping the benefits from their land by protecting trees from animals until they are fully mature (Sibbald et al., 2001; Lehmkuhler et al., 2003). Planting trees in greater density may shorten time to initial use if the trees are a primary source of browse forage within the
silvopastoral system (Addlestone et al., 1999). In some cases, low tree browse palatability, especially for grazing animals as opposed to browsers, can protect them during establishment in pre-existing herbaceous pastures (Lehmkuhler et al., 2003; Apolinário et al., 2015). Soil microbial population health, especially rhizobia or mycorrhiza (Keyser, 1992; Mauricio Molina et al., 2005), suppres-sion of herbaceous weeds (Campbell et al., 1994; Gakis et al., 2004), or strategic fertilization (Campbell et al., 1994) can facilitate persistence and shorten tree establishment periods, thereby lessening the cost of silvopasture establishment.
Compared with purely herbaceous pastures, silvopastoral systems may produce less forage and, consequently fewer animal products. This is especially apparent at greater tree densities in cooler (Bird et al., 2010) or drier (Yama-moto et al., 2007) climates. An increase in quality (Kallenbach et al., 2006) and seasonal availability (Yamamoto et al., 2007) of the herbaceous forages or the selection of tree species that provide greater nutritive value vis-à-vis other species (Addlestone et al., 1999) or feed during critical seasons (Muir et al., 1995) can sometimes offset sacrificed herbaceous canopy forage. Mana-gement therefore becomes important in mitigating negative effects on animal production, for example the use of tree shade for milk cows during hot warm seasons or times of the day.
Tree shade is a factor in most silvopasture, intercepting from 25 to 88% of the sunlight (Devkota et al., 2009; Veras et al., 2010) and reducing herbaceous canopy yields up to 40% compared to treeless pastures. Pruning trees can mitigate this loss to the herbaceous species (Devkota et al., 2009) as well as increase tree forage yield and productivity (Muir, 1999). Selecting herbaceous species that tolerate shade (Hagedorn and Pearson, 1984; Muir and Alage, 2001) or manipulating grazing height of this herbaceous component (Veras et al., 2010) can actually turn shade into an advantage. For example, Cenchrus ciliaris produces greater biomass under full sunlight, but it can compensate for lower leaf area index and etiolation under shading by building denser leaf photosynthetic capacity (Mishra et al., 2010). Some grass species such as Pa-nicum maximum (Muir and Alage, 2001) and Andropogon gayanus (Veras et al., 2010) as well some (Muir and Pitman, 1989) but not all (Interrante et al., 2004) herbaceous legumes respond positively to moderate shade with improved nutritive value. A two-year field study showed that leaf and stem digestibility and crude protein were lower and fiber component and lignin concentrations greater for shaded rhizoma peanut (Arachis glabrata Benth.) than when grown in full sun, but they were not so low as to limit use of rhizoma peanut as an
The advantages and challenges of integrating tree legumes into pastoral systems 150
understory forage crop (Johnson et al., 2002). Among legumes, several studies have pointed to the existence of both shade-tolerant and shade-intolerant types (Wong et al., 1985), with capacity to nodulate and fix N at high rates in low-light environments being keys to persistence and N concentration of shade-tolerant legumes (Izaguirre-Mayoral et al., 1995). One of the challenges of interpreting the literature on shading is the preponderance of short-term studies conducted with plants growing in pots in the greenhouse under artificial shade. The results from these studies may or may not reflect longer term understory responses to shade from a tree canopy under field conditions.
Tree shade may result in greater soil moisture (Interrante et al., 2004), greater relative humidity (Mishra et al., 2010), and less herbaceous evapo-transpiration due to canopy cooling and lower soil temperatures (Baliscei et al., 2012). For example, during a period of drought stress in spring, rhizoma peanut herbage accumulation was less when growing at 100% than 78% percent incident photosynthetic photon flux density (Johnson et al., 1994). Measures of leaf water and turgor potentials confirmed that plant water deficits occurred in the 100% treatment. In the absence of drought stress in subsequent growth intervals, herbage accumulation was greatest for the 100% treatment. In a ma-ture silvopasma-ture, however, trees may extract greater soil moisma-ture deeper in the profile compared to open grassland, offsetting the positive effects of shade on soil moisture (Lei et al., 2011). Likewise, the presence of herbaceous species can also take surface soil moisture away from deeper profiles (Chang et al., 2002) where tree roots extract most of their moisture. Greater rainfall could mitigate the effects of trees in silvopastoral systems but few studies compare the same species in similar edapho-climatic conditions. Results from seasonal studies indicate that negative effects of tree canopy on herbaceous production can increase during higher rainfall periods (Rusch et al., 2014), indicating that a denser tree canopy may play a greater role than soil moisture in limiting herbaceous forage production growing under trees.
Dynamics of soil nutrients between arboreal and herbaceous silvopastoral components can be complex. In some cases, the presence of herbaceous forages increases soil C and N compared with tree groves with bare ground (Chang et al., 2002). Likewise, deep-soil nutrient extraction and leaf litter will also increase surface soil organic components under tree canopies (Chang et al., 2002). This can favor herbaceous layer productivity, especially in deciduous savannahs (Muir and Alage, 2001).
Tree species selection and spacing can be an important determinant of her-baceous canopy production (Roa t al., 1998). Some trees have greater negative effects on herbaceous species yield, nutritive value and diversity than others (Buergler et al., 2005). Canopy architecture may be a differentiating factor (Rusch et al., 2014), so directed pruning may mitigate this influence. Selecting tree species such as Faidherbia albida (Delile) A. Chev. (syn. Acacia albida Delile), which shed their leaves at critical times to allow greater understory light penetration when the herbaceous canopy most needs it (Vanderbeldt and Williams, 1992) can also minimize competition. Management options such as wider tree spacing (Buergler et al., 2005; Bird et al., 2010) and more severe tree pruning (Devkota et al., 2009) usually result in diminished herbaceous species’
yield reduction. There may be a tradeoff, however, as secondary production from the tree component will suffer (see below).