Biodiversity and carbon sequestration

P. juliflora and P. pallida woodlands are important habitats for a variety of plant and animal species (e.g. Mooney et al 1977, Hardy et al 1999), several of which are collected or hunted for human uses, have become endangered and require protection. Many such woodlands have suffered as a result of fuelwood collection or the expansion of agricultural land.

While no plantations have been established specifically for the purpose of wildlife conservation, some native Prosopis stands have been conserved primarily for this reason. Wildlife benefits from ameliorated environmental conditions and the physical protection offered by the trees. In the USA, P.

glandulosa woodland and scrubland is maintained as a refuge for wild game and occasionally stocked with exotic game, specifically for commercial hunting purposes. P. juliflora is

also an important source of food for wild animals where it has been introduced, such as for large wild mammals in Gujarat, India (Chavan 1986, Prajapati and Singh 1994).

Some areas of native Prosopis woodland or dominated ecosystems such as the UNESCO Biosphere site in La Monte, Argentina, have been given special protected status, while several national reserves throughout the native range of P.

juliflora and P. pallida include these species as a major vegetation component. The genetic diversity of the trees themselves is conserved, as are the other ecosystem components. The positive benefits of maintaining or increasing biodiversity on a global level by protecting existing Prosopis woodlands or establishing others is stressed, while also considering the negative aspects of Prosopis as an invasive weed and its negative impacts on biodiversity, e.g.

in Brazil (Pasiecznik and Harris 1998) and India (Jadhav et al 1993).

Prosopis trees and woodlands world-wide may account for a significant amount of sequestered carbon, though tree species in arid and semi-arid zones are not considered when calculating carbon balances at present. Felker et al (1990) estimated that carbon stored as woody biomass was equivalent to 2-20 t C/ha in P. glandulosa stands in the USA, with an additional 1.4-18.4 t C/ha sequestered as reserves of soil carbon, assuming 25% canopy cover (Geesing et al 1999).

While such data is expected to vary greatly between sites and species, they indicate the considerable quantities of carbon stored in woody biomass and soil reserves. Even a single rotation of P. juliflora or P. pallida would lead to significant amounts of total carbon sequestered. Using the figures above, it may be estimated that Prosopis plantations could sequester carbon in excess of 1000 kg/ha/yr, with a yet unseen value in the emerging global market for ‘carbon credits’. Arid zones presently contain the lowest levels of carbon in the world on a per hectare basis, and it is necessary to consider the role of arid zone forests in carbon sequestration on a regional and global level.

4.1 Nursery propagation

4.1.1 Propagation by seed Seed collection and storage

Seed should be collected only from trees that have been identified as having desirable characteristics, such as high yields of sweet pods, lack of thorns, erect form and freedom from pests and diseases. The fruiting period of P. juliflora and P. pallida varies throughout the year and can be seasonal or almost continuous. Pods can be collected from the trees or gathered off the ground. Pods that have been on the ground for more than a few days should not be collected as they are more likely to have seeds that have reduced viability or to have been destroyed by predators. In many regions, insects can destroy a substantial percentage of the seed crop.

Collection of the best seed generally involves collecting pods that can be picked easily off lower branches, or fall when boughs are shaken. A hooked stick or rope thrown over the tree or branch can be used to avoid injury from thorns, and sticks can be used to remove pods that remain on the tree even after shaking.

Only the largest, undamaged pods should be collected, rejecting discoloured pods or those showing signs of herbivory, such as the exit holes of bruchid beetles. This is the first selection procedure. Collection is facilitated by large numbers of long pods and, when picked from the tree, a large number of pods per hand (i.e. the number from a single inflorescence). Access to the trees and tree form are very important. Pods are more easily collected from plants with a tree form, with a broad crown facilitating access to principal branches and access under the canopy. The amount and type of undergrowth also affects accessibility. Pods are collected easily from bare ground and, where weeds are present, cultivation around seed trees with a tractor drawn disc harrow has been recommended in the USA prior to pod maturity. In weedy invasions of dense, multi-stemmed Prosopis, or where there is substantial undergrowth including other woody species, some clearing would be required in order to access the trees and allow for an adequate harvest.

Prosopis pods are gathered into sacks weighing up to 30 kg when full.

Pods are transported to a central processor, storage unit or nursery, where they are often dried before storage or processing. Sun drying is sufficient for some processing methods, and is achieved by laying pods out thinly on the ground during three consecutive sunny days. If left overnight, however, pods are likely to re-absorb some moisture and gathering in of the pods is recommended each evening.

Traditional drying methods used in the Americas included covering sun dried pods with hot ashes from a fire, sometimes with periodic turning (Felger 1977). This would also kill the seed feeding insects which could otherwise destroy a store of pods. Insects can also be killed by freezing, roasting or toasting, or immersion of the pods in cold water (Díaz Celis 1995).

Heat-treatment to reduce the moisture content to approximately 8% also prevents the high sugar content pods

‘sticking’ during mechanical processing (Cruz 1999). Grain dryers blowing air through stacked pods have been used successfully for drying Prosopis pods to 10-12% moisture content in 2 days. Several wood-fired ovens and driers have been designed for drying P. juliflora and P. pallida pods, generally for human or animal feed (e.g. IPA 1988, Cruz 1999). Some large processors can dry batches of several tonnes of Prosopis pods to acceptable moisture levels in a single day. A second selection procedure may minimise secondary infection by removing all pods with visible bruchid exit holes.

Optimal conditions for storage of Prosopis seed, along with many other tropical legume seed, is refrigeration at 0°C (Díaz Celis 1995) or 4°C (Ffolliot and Thames 1983). Dusting of the pods with fungicides and, particularly, insecticides is generally recommended. Naphthalene and paradichlorbenzene were suggested for use in Prosopis seed stores in India by Griffith (1945), who found that these chemicals were effective and had no detrimental effect on seed germination even at high concentrations. Muthana and Arora (1983) recommended fumigation and Luna (1996) suggested carbon disulphide, hydrocyanic acid and naphthalene. The incorporation of neem (Azadirachta indica) leaves in seed and pods stores at rates of 10-20 g neem leaf powder or 2-3 ml neem oil/kg of seed has been suggested (Tewari et al 2000). A large number of traditional, bio-control methods for bio-controlling insect pests in seed stores is given by Johnson (1983).

Pods have been stored in rooms and barns, either ventilated to keep pods dry and free from fungal attack, or sealed to prevent insect infestation (see 3.2.2). Pods are often placed in large sacks to facilitate transport. However, it is generally recommended that for nursery use, seeds are extracted before storage. These will have a much smaller volume, and should be stored in sealed metal tins or plastic bags. Dry pods can be stored for years if kept dry and free from insects and animals. Whole Prosopis pods have been stored for over 15 years in sub-optimal conditions, with some insect damage and variable temperatures and humidities, and the extracted seed still had viability of 40-80% (Pasiecznik and Felker

Chapter 4

Husbandry and management

1992). However, Killian (1990) noted reduced viability of seed stored in pods rather than as cleaned seed.

Seed extraction

Whole pods, pods broken into segments or seed in their endocarps can be used to propagate Prosopis, but resulting seed germination and emergence is generally poor. Removal of the mesocarp and exocarp can be most simply achieved by soaking pods in water for 1-14 days (Saxena and Khan 1974) or leaving freshly collected pods in sealed bags and allowing the mesocarp to be broken down by fungal attack (Díaz Celis 1995). Controlled fungal action, although effective in seed extraction, was damaging to the seed (Kandaswamy and Venkata-Rao 1956). Pods can also be mixed with cow dung for 14 days before removal (Luna 1996). Following any of these treatments, the mesocarp pulp can be easily washed off, leaving seed still enclosed in their endocarp shells. The mesocarp may also be removed by lightly pounding the pods, or rubbing between coarse material such as sand or coarse cloth. The endocarps are rigid and fibrous and require opening to obtain clean seed.

Manual extraction of seed is a very time-consuming operation with a knife or other cutting implement, and is greatly facilitated using batch methods.

Endocarps can more easily be separated after soaking in acid or alkaline solutions. Soaking is much more effective if a small amount of sodium hydroxide (NaOH) is added to the water, followed by rubbing between two folds of a coarse cloth to loosen the endocarps (Saxena and Khan 1974).

Pimentel (1982) suggested soaking in a 4% solution of NaOH. Endocarp removal was also facilitated by soaking pod parts in a mild solution of hydrochloric acid (HCl) for 24 hours and washing, although all seed were killed with a 24 hour soak in concentrated HCl (Vasavada and Lakhani 1973). Valdivia (1972) found soaking with 5% HCl and rubbing with sand effective. Muthana and Arora (1983) recommended shaking in concentrated H₂SO₄ for 5 hours to completely destroy the endocarps, while a 30 minute soak was also effective (Saxena and Khan 1974, Souza et al 1983).

However, owing to the corrosive nature of concentrated acids, use of mild alkaline solutions is recommended. Seed extracted using either acid or alkaline chemical methods may show improved germination (Saxena and Khan 1974).

Feeding pods to stalled livestock may allow clean seeds to be collected from faeces (e.g. Gomes 1961, Muthana and Arora 1983). This system is recommended for obtaining large numbers of clean seed where machinery and labour are limiting. Clean seeds are, however, not always used in large nurseries in Africa and Asia, and processing involves only removing the mesocarp and then sowing seeds in their endocarp shells. In these cases, pod soaking or feeding methods that remove the pulp and improve germination are used. Feeding or milling may be preferred as the pods then have a feed value rather than the mesocarp being wasted.

Some form of mechanical extraction, generally a modified form of existing machinery used to thresh or grind cereals, is often preferred over manual or chemical methods (e.g.

Souza et al 1983). Equipment used includes some form of rotating wheel or drum combined with mesh screens and/or air blowers to separate pod parts and produce clean seeds.

Flynt and Morton (1969) described an early modified grain thresher used to separate seeds of North American Prosopis pods. Pasiecznik and Felker (1992) were able to process 2 kg of pods/h, yielding 0.25 kg of clean seeds from a number of Prosopis species with a modified meat grinder and seed separators. Smaller quantities can be cut by hand. The seed yield by weight from dry Prosopis pods is approximately 10-15% by hand or small machines (Pasiecznik and Felker 1992).

Large scale threshers are also used to separate seeds in India (Kanzaria and Varshney 1998), Peru (Cruz 1999) and Argentina (Galera et al 1990). Some of these have been developed for milling pods into flour for animal feed but can also be used to separate clean seed (e.g. Grados and Cruz 1996). These larger machines can process over 50 kg of pods/

h, more than enough to supply the seed for a large afforestation campaign. Smaller machines, such as those tested by Flynt and Morton (1969) and Pasiecznik and Felker (1992) are also suitable for large nurseries. Although a small amount of seed is destroyed during mechanical processing, coincidental scarification during the process leads to improved germination of seed.

Germination pretreatment

It is preferable if more than 90% of seed sown in the nursery germinates in a synchronised manner within seven days.

The hard seed coat of P. juliflora and P. pallida prevents water uptake and subsequent germination, even when seed has been removed from the pod and endocarp (e.g. Killian 1990, Diagne 1996) (see 2.3.3). Scarification of the seed coat to aid water permeability and encourage germination, known as pretreatment, is usually required. However, in some circumstances, pretreatment is not required. Fresh seed have been seen to exhibit high germination rates without treatment, probably as a result of the seed coat not yet hardening (e.g. Ffolliot and Thames 1983, Luna 1996).

Also, mechanically processed seed and seed collected from animal faeces may germinate without further treatment. Old seeds often germinate without pretreatment, probably because the seed coat has weakened over time. Seed in the middle of the pods have a higher viability than seeds in the distal or proximal ends (Killian 1990).

Pretreatment can be classified into heat, chemical and mechanical treatments. Seed from all species of section Algarobia tested are similar in their size, morphology and responses to pretreatment. Standard techniques have been developed using accepted seed testing methodologies (e.g.

Catalán and Macchiavelli 1991). Most tests involve the germination of seeds on filter paper, although there is a noted

variation in germination of P. juliflora seed on a sand medium (e.g. Torres et al 1994) or in soil (e.g. Harris et al 1996a).

Sowing seeds into nursery bags introduces other factors such as sowing depth which affects germination and seed emergence. However, germination tests indicate approximately 10% germination of untreated seed, increasing to 50% with old or damaged seed and to over 90% with fresh or pretreated seeds.

Heat treatment of seed generally involves immersion of seeds in hot water and is often recommended as a simple and effective pretreatment. Pouring boiling water onto seeds and leaving to cool for 24 hours has been recommended for P.

juliflora and P. pallida and used successfully with other Prosopis species (Catalán and Macchiavelli 1991). However, Pasiecznik et al (1998) found this method gave germination of only 52% with P. juliflora. Leaving the seed to soak for up to 5 days in the same water did not further increase final germination significantly (Pasiecznik et al 1998), but improved synchronisation, with a high proportion of the total germination occurring within 24 hours of removal from the water (Harris et al 1996a).

Scalding of seeds by leaving them immersed in actively boiling water for 5-10 seconds was found to increase germination, but longer periods of scalding led to seed mortality (Pasiecznik et al 1998). Incubating seed at unnaturally high temperatures can break dormancy, with P.

juliflora seeds incubated at temperatures up to 60°C for 12-24 hours exhibiting improved germination (Tewari et al 1998). P. juliflora seed heated to 90°C for 7 days had a germination of 48% while 95°C was lethal (Sacheti 1996) and seed of a number of Prosopis species showed decreased germination following a dry heat of 60°C for 24 hours (Lópes and Avilés 1990).

Chemical treatments that improve germination mostly involve acids that weaken the seed coat. Sulphuric acid (H₂SO₄) is effective with P. juliflora and P. pallida (e.g.

Muthana 1988, Silva 1990b, Tewari and Harsh 1998).

Maximum germination rates of over 95% were achieved with P. juliflora seed by a 15 or 30 minute soak in 97%

(concentrated) sulphuric acid, or a 30 minute soak in a 60%

solution (Pasiecznik et al 1998). Large heats of reaction are noted following washing of acid treated seed in cold water which may also improve germination. Hydrochloric acid (HCl) has also been used but with less success.

Passage through the digestive tract is known to assist in the breaking of seed dormancy. The high rate of P. juliflora and P. pallida seedling emergence from animal faeces shows that not only are seeds cleaned, but that some weakening of the seed coat has occurred. Pasiecznik et al (1998) found a slight increase in the germination of clean P. juliflora seed following passage through a cow, while Danthu et al (1996) found germination rates much higher, as has been seen with other Prosopis species (e.g. Peinetti et al 1993). Different animals are known to have different effects on germination, with cattle having the most beneficial effects, followed by goats,

while ingestion by sheep, camels and pigs leads to reductions in germination (Harding 1991, Danthu et al 1996) (see also 2.3.4). While these effects are often assumed to be due to the action of stomach acids, other chemicals in the gut may also have weakened the seed coat.

Mechanical scarification techniques have been assessed for their effectiveness in breaking P. juliflora and P. pallida seed dormancy. All methods aim to puncture or weaken the hard seed coat allowing water to enter. Seeds can be cut individually with a blade, scissors or nail clippers, to remove a portion of the seed coat. Abrasion by manually drawing the seed across a sheet of sandpaper is also very effective.

Burning with a hot iron is the standard method of pre-treating small batches of seed at DANIDA (Stubsgaard 1990). Such techniques are highly effective, equal to the best chemical treatments, with over 95% germination found in most studies. However, treating seeds individually is time consuming. Pasiecznik et al (1998) found that a maximum of 1200 seeds/h could be individually scarified. Also, as the seeds are small, they are difficult to handle individually, and minor damage to fingers can occur.

Batch scarification involving mechanical means would be preferred for pretreating seeds, but little has been done to develop and promote the use of appropriate equipment.

Machines for milling pods tend to increase the germination of seed, presumably by mechanical scarification during processing. Rotating drums with P. juliflora seed alone, or with sand or small stones, have been suggested for batch pretreatment. Simply pounding seeds with sand was also effective but less easy than using a drum or container for the seeds (Nambiar 1946). Firing seeds at a hard surface also improved germination markedly (Stubsgaard 1990). Shaking P. juliflora seed in a square tin for 10-15 minutes increased germination to over 95% (Nambiar 1946). Bandyopadhyay et al (1990) found that gamma radiation decreased the germination of P. juliflora as radiation dose increased from 1 kR to 16 kR. Other unsuccessful pretreatments have included soaking in cold water and stratification at low temperatures.

While heat treatments are not as effective as chemical or mechanical scarification, the use of boiling water is a safe, simple, cheap and straightforward technique which leads to acceptable levels of germination (over 50%). Acid scarification is very effective and simple but also more expensive and somewhat dangerous. Mechanical scarification is equally effective but can often be time consuming and thus more expensive when undertaken manually.

Mechanical scarification techniques that can be applied to large samples of seed are preferable, such as those using a rotating or shaking drum or container. The choice of pretreatment will depend on availability of labour, chemicals or equipment, and on the quantity and value of the seed to be used. For small seed samples, manual extraction and mechanical scarification is adequate, while for larger amounts of seed, either mechanical scarification by batch treatment,

or boiling water treatments are recommended. Soaking of seed produced by any of these methods for at least 24 hours will improve the rate and sychronisation of germination but not final germination.

4.1.2 Vegetative propagation Rooted cuttings

While concentrating on P. juliflora and P. pallida, references will be made to literature available on other Prosopis from section Algarobia sharing similar characteristics, although some differences in the rooting response of different species has been observed. Vegetative propagation of P. juliflora by

While concentrating on P. juliflora and P. pallida, references will be made to literature available on other Prosopis from section Algarobia sharing similar characteristics, although some differences in the rooting response of different species has been observed. Vegetative propagation of P. juliflora by