Performance of Prosopis species for short rotation fuelwood forestry on degraded sites - H.M. Behl and V.L. Goel

Biomass Research Centre, National Botanical Research Institute, Lucknow 226001, India

Introduction

Forests are under an unprecedented pressure for fuel, fodder and timber, resulting in a rapid depletion of forest cover, ecological imbalances and desertification (Anonymous, 1993). There is an urgent need to identify fast growing multi-purpose tree species for degraded sites which can meet this ever-increasing demand. Afforestation with these species will also give an added advantage of soil amelioration.

Prosopis species (Leguminosae, subfam. Mimosoideae) are dominant in several tropical and sub-tropical countries, including India. Use of Prosopis for afforestation of wastelands, erosion control, soil amelioration and fuelwood production is well documented (Palmberg, 1981). There are 44 species within the genus, consisting of trees or shrubs, distributed in the drier, warmer areas of America, Africa and western Asia (Burkart, 1976). Some species are capable of growing rapidly, even on barren and degraded lands while others have slower growth rates.

There have been few studies conducted on the screening of Prosopis species for fuelwood production on degraded soil sites in Asia (Felker et al., 1982). This prompted this investigation of a wider range of Prosopis germplasm suitable for the afforestation of alkaline soils. In the present study, Prosopis germplasm from within the country as well as from other parts of the world were screened to select the best performing genotypes/species on the basis of growth, establishment, productivity, wood energy harvests and nutrient utilisation for use in short rotation fuelwood plantations on degraded sodic sites.

Materials and methods

Germplasm of seven species of Prosopis; P. alba, P. cineraria, P. glandulosa, P. juliflora, P. pubescens, P. tamarugo and P. velutina were procured from various agencies, with the natural occurrence of the species and seed sources given in Table 1. Plants were raised in polythene bags (15 x 25 cm when full), filled with a mixture of sand, soil and farm yard manure in a 1:1:1 ratio. Seedling survival and variation in thorn characters were observed. After 6 months in the nursery, young seedlings were transplanted to a field site, with an alkaline soil (pH 8.6-10.5) of low porosity and with a pan of calcareous depositions at a depth of 50 cm. Plantings was carried out at a spacing of 1.5 x 1.5 m (4,444 plants/ha), arranged into plots of 90-100 trees, and periodic monitoring of survival and growth traits was conducted. Randomly selected trees were sampled for biomass estimations using a stratified sampling procedure. Linear regression equations relating plant height and diameter to biomass yield were developed. Frequency class distribution on the basis of tree diameter was calculated. Basal area, mean annual increment (MAI) and biomass yields were calculated after 3 years. A regression equation of the following form was fitted:

y = d²h * b + a

where;

y = biomass yield (kg)

d = diameter at 50 cm above ground level (dm)

h = height of the plant (dm)

a,b = regression constants

Leaves were harvested during October for studying mineral composition, with samples dried at 70^oC and processed for analysis. Nitrogen was estimated on a Tecator kjeltec auto 1030 N-analyzer after sulphuric acid digestion (Kalra and Maynard, 1991). Phosphorus was determined on a flow injection analyser 5010 using chemifold type-II, while sodium, potassium and calcium were analysed on a flame photometer after di-acid digestion (Jackson, 1967).

Wood samples were collected to study the heat of combustion of different plant components of various species. The calorific value of samples was measured using an oxygen bomb calorimeter (Parr Instruments, USA). To compare the magnitude of wood energy harvested per hectare by different species, the calorific value of wood of different species was multiplied by their respective woody biomass yield.

Results

Nursery performance

Germination and survival percentages are given in Table 1. P. tamarugo had very poor germination, with most seedlings dying within a few days and those that survived were not healthy. Accordingly, this species was not included for further studies. Felker (1988) reported that young seedlings of P. tamarugo died off when they were about 15 cm tall. All species except P. tamarugo showed good seedling survival (82-98%). The highest germination (74-86 %) was recorded in the two accessions of P. alba, with 32-50% recorded for the other species, including the native P. cineraria.

Thorn character

A wide variation was detected among seedlings with respect to the presence or absence of thorns on the stem. Maximum number of thornless plants (55%) was found in P. pubescens (Table 1). P. alba I had 30% thornless plants while other species had low numbers (2.6-8.8%) of plants without thorns. No thornless plants were detected in P. cineraria and P. tamarugo. Thorn character can be used for taxonomic characterisation as well as a criterion for tree selection. Thornlessness is a very favourable trait when wood is used directly for fuelwood.

Field evaluation

Field performance was evaluated on the basis of plant establishment (survival %), growth (height and basal area) and productivity (Table 2). P. cineraria had the highest survival (92%), although its growth was relatively slow in the initial years. All other species had survivals ranging from 51% to 74%. P. alba I out-performed the other species in terms of mean height and diameter, with a basal area of 7.5 m²/ha, which was followed by P. juliflora with a basal area of 3.3 m²/ha. All other species had basal areas of below 2.4 m²/ha. Correlation coefficients ® were calculated for height and diameter, with all species exhibiting a strong positive correlation (Table 2). P. alba II was distinct in having poor growth and the lowest r value (0.63). The frequency class distribution identified differences within as well as between species (Table 3). P. alba I followed by P. juliflora had the highest numbers of individuals over 5 cm in basal diameter. P. velutina and P. cineraria were intermediate, while P. pubescens and P. glandulosa had the lowest numbers of plants above 5 cm basal diameter. P. alba II had no plants at all above 5 cm (data not presented). Only P. alba I had trees above 11 cm diameter, with 5 % in this group. Basal area and frequency class distributions suggest that P. alba I followed by P. juliflora were the best performers in terms of growth, with P. velutina and P. cineraria also showing some promise.

Table 1. Natural occurrence, seed source and nursery performance of Prosopis germplasm

NARI = Nimbkar Agriculture Research Institute, Phaltan, India

CSSRI = Central Soil Salinity Research Institute, Karnal, India

UCR = University of California, Riverside, U.S.A.

Table 2. Establishment and growth of Prosopis species.

Biomass estimations

Biomass production and its allocation to stem, branch and leaf is shown in Table 4. The values of the regression constants (a and b) and r² values are given in Table 5. Among the six species of Prosopis, P. alba I gave the maximum dry biomass of 5.0 t/ha/yr, with approximately 90% of the biomass allocated to stem and branch wood, with the largest individual of this species having a biomass of 133 kg. P. juliflora and P. velutina had biomass yields of 3.5 and 3.1 t/ha/yr respectively, while biomass yields of the other species were 1.1-1.5 t/ha/yr. The largest P. juliflora tree had a biomass of 43.5 kg. P. alba I also showed the highest dry stem wood biomass (2.6 t/ha/yr), while P. pubescens and P. glandulosa had relatively high proportions of leafy biomass.

Table 3: Frequency distribution of height in six Prosopis species.

Table 4. Dry biomass production and allocation into stem, leaf and branch components in six species of Prosopis.

Table 5. Regression constants and r² values for biomass production and partitioning in six species of Prosopis.

Nutrient analysis of leaves

Data on the mineral composition of leaves on dry weight basis is shown in Table 6. Mean nitrogen content varied from 3.3% in P. pubescens to 4.7% in P. velutina, and potassium levels ranged from 1.9% in P. cineraria to 3.75% in P. velutina. Calcium levels also varied from 0.4% and 0.5% in P. alba I and P. alba II respectively, to 1.75% in P. velutina. The variation in phosphorus and sodium concentrations between species were negligible. P. velutina outperformed other species with respect to its nutrient status, with no other species having a higher concentration of any of the nutrients tested. P. juliflora also had high nutrient levels. It was noted that the leaf nutrients of P. alba II were very similar to those of P. alba I, although the two accessions performed differently in terms of growth. A direct correlation between nutrient content and productivity was not established.

Energy content

The heat of combustion as indicated by the calorific value showed little variation between stem, branch, bark and leaves between the six Prosopis species (Table 7). Leaves had slightly higher heats of combustion (23.64-26.02 kJ/g) whereas it was lowest in bark (19.10-23.19 kJ/g). The calorific value of the stem and branch wood exhibited slightly less variation, with P. pubescens having the highest heat of combustion in both (24.16 and 24.47 kJ/g respectively). Differences in wood energy values, biomass production and its allocation to different parts of the tree, led to variations between species in the total energy content harvested per hectare. P. alba I gave the maximum energy harvest of 103.32 GJ/ha, followed by P. juliflora with 69.91 GJ/ha and P. velutina with 62.21 GJ/ha, thus exhibiting the promise of these species for short rotation fuelwood plantations.

Table 6. Nutrient composition of leaves of six Prosopis species.

Table 7. Energy values of different plant components in six Prosopis species

Discussion

Tree growth is influenced by a number of factors including genotype, agro-climatic conditions and cultural practices. Adequate supply of nutrients is of utmost importance and depends on soil conditions. Most nutrients are more available in low pH rather than high pH soils (Brady, 1990), and nutrient availability is therefore a limiting factor in alkaline soil sites. Several attempts have been made to select alkalophilic species and to assess their soil amelioration potential (Yadav, 1980; Chaturvedi, 1987). Species raised on highly alkaline soils generally suffer high mortality (Singh, 1989), although this was not the case for these species of Prosopis. The exotic species showed 27-49% mortality whereas the indigenous P. cineraria had only 8% mortality, indicating the ability of P. cineraria to establish itself by adjusting its micro-environment under stressed conditions. Patel (1986) however, reported that P. juliflora had a wide range of adaptability and could be planted without any soil amendments.

The presence of a high frequency of thornless plants of P. pubescens and P. alba affords the possibility of ease of handling the wood for fuel purposes. Prosopis species have been reported to be self incompatible, and cross pollination results in a great deal of variation among genotypes with respect to their growth, thorniness and other morphological traits (Sandys-Winsch et al., 1991). A high frequency of thornless plants was detected in P. alba by Felker et al. (1983) who proceeded to clone this species. Shroff et al. (1986) reported that such thornless individuals can be used for fodder purposes in sites where a deficit exists.

Felker et al. (1983) predicted the biomass production of several leguminous species including Prosopis spp. under greenhouse conditions after 2 years growth, and found that P. alba (acc. no. 0136) produced the highest biomass (14.5 t/ha) compared with other species such as P. glandulosa (7.1 t/ha). In another study Felker et al. (1989) reported a standing biomass production of 13.1 t/ha/yr with a thornless clone of P. alba. Singh (1989) estimated the above ground biomass of P. juliflora to be 1.06 t/ha after 3.5 years when planted on alkaline soils without any soil treatment, with large improvements in yield with gypsum and pyrite amendments incorporated at planting. In our trials, P. juliflora produced 3.53 t/ha after three years without gypsum treatment.

A comparison of the nursery and field performance of these six species of Prosopis have revealed that P. alba I is a promising species for short rotation fuelwood plantations on alkaline soil sites, followed by P. juliflora and P. velutina. However, the other species (with the exception of P. tamarugo) all exhibited potential either in producing higher numbers of thornless plants or a better survival and establishment, P. glandulosa var. torreyana however, exhibited poor growth.

Species selection can be made depending on the desired end-products required. However, tree improvement studies with these species have not yet been undertaken in India. Provenance trials are essential to identify promising seed sources to finally recommended a particular germplasm for a specific site condition. Thornless material is a good resource for multiplication and future trials.

Acknowledgements

The authors are grateful to Dr. P.V. Sane, director of NBRI for providing guidance, and to the Nimbkar Agricultural Research Institute, Phaltan, and the Central Soil Salinity Research Institute, Karnal for providing seed. The financial support of the Ministry of Non Conventional Energy Sources for Biomass Research Centre is gratefully acknowledged.

References

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