Biomass production and partitioning of Prosopis juliflora on a degraded ravinous site - R. Sood and S. Bhatia

Tata Energy Research Institute, New Delhi, India

Introduction

Prosopis juliflora commonly known as mesquite (U.S.A.) or vern or vilayati babul (India), is one of the most important species for the afforestation of arid and semi-arid regions, due to its adaptation to different soils and terrains. It is a fast growing, hardy, drought resistant but frost tender species, with good coppicing ability (NAS, 1980), and has an extensive lateral root system with a stout tap root which can penetrate as deep as 20 m.

Often in dry areas and denuded sites, P. juliflora is multi-stemmed either from ground level or very near to the ground. While the reasons attributed to this characteristic, apart from genetic, are either; repeated cutting of the species for fuelwood, resulting in the production of coppice shoots; or a hard non-porous site often with an underlying calcareous pan, that does not allow the penetration of roots, with the subsequent multiple branching of the main stem. With P. juliflora being a major source of firewood in many parts of the country, it becomes essential to develop a methodology for quick and accurate estimation of the standing biomass. Chaturvedi et al. (1991) working with Acacia farnesiana, a multi-stemmed arid zone shrub, found that measuring the growth parameters of the most prominent stem and counting the number of stems gives a statistically accurate estimate of the biomass of individual plants. The same methodology was used to develop the biometric equations for the assessment of wood biomass of multi-stemmed trees of P. juliflora.

Materials and methods

The investigation was carried out at Gwal Pahari field station, Haryana (23^o35’ N, 77^o12’ E; 255 m altitude). The maximum summer temperature at the site is 47⁰C, and minimum winter temperature 4⁰C. The mean annual rainfall is 450 mm and occasional frosts are experienced in the winter months. The area is cut by ravines with badly eroded slopes. The soil at the site is alkaline, with a sandy loam texture.

In 1988, 116 P. juliflora seedlings were out-planted at 3 x 3 m spacing and raised under rainfed conditions. In 1993 after 5 years, 16 trees were selected representing all the girth classes of the population distribution. Height and girth at 50 cm above ground level were measured for each sample tree, as Chaturvedi (1984) had shown that a strong correlation exists between girth at breast height and girth measured at 50 cm above ground level. Most of the trees had a single stem at this point, but where the stems of a tree had bifurcated before this point, each were considered as individual trees and treated accordingly for biomass estimation.

The selected trees were harvested, and both green and oven dry weights of the samples were recorded. The weight equations were then developed. In addition to the multi-stemmed trees, biometric equations were independently developed for the single stemmed trees also (Chaturvedi, 1985). Of these 16 trees, 4 were selected at random for a root study, whereby the entire root system was dug out and observations made on the length and thickness of roots, spread of root system, and fresh and dry weights. The ratios between weights and lengths of above ground and below ground portions of the trees were calculated.

Results and discussion

The biomass analysis of above ground portions of P. juliflora showed that a strong correlation existed between the above ground biomass and the growth parameters, diameter, height and number of stems. The results are presented in Table 1. On the basis of these observations equations were derived for multi-stemmed trees, and earlier biomass equations developed for single stemmed trees growing on similar saline-alkaline sites given by Chaturvedi (1985), were comparable. From these, the following equations were derived and used:

W = a+ nbD²H

where;

W = weight in kg (green or oven dry)

a = regression constant

b = regression coefficient

n = number of stems

D = diameter at 50 cm decimetres

H = height of the tallest stem in decimetres

Table 1. Tree dimensions, and actual green and oven dry weights (stem only and total: stem + branches + leaves) of sample P. juliflora trees.

Biomass equations for multi-stemmed trees are:

Green weight equations:

W₁ = -0.1685 + 0.3061 n D²H (r² = 0.98)

W₂ = 0.0224 + 0.6882 n D²H (r² = 0.98)

Oven dry weight equations:

W₁ = -0.2627 + 0.1740 n D²H (r² = 0.99)

W₂ = 0.1806 + 0.3383 n D²H (r² = 0.99)

Biomass equations for single stemmed trees are:

Green weight equations:

W₁ = -0.0397 + 0.3786 n D²H (r² = 0.94)

W₂ = 0.1111 + 0.7913 n D²H (r² = 0.92)

Oven dry weight equations:

W₁ = -0.0287 + 0.2284 n D²H (r² = 0.97)

W₂ = 0.0851 + 0.4705 n D²H (r² = 0.96)

where;

W₁ = stem weight in kg

W₂ = total weight (stem+branches+leaves) in kg

Root lengths and thicknesses, and spread of root system are presented in Table 2. The tap root development was pronounced in these trees, with the maximum length of the tap root being 275 cm in tree no. 15 (Table 2). In general, branching of the tap root was observed to be at 50-70 cm below ground level in all trees. In tree nos. 12 and 15, two roots were found to be running parallel to each other from the base of stem. In these trees, the root with the greatest diameter was considered as the main root with the other taken as a branch of the main root. Main branches of the roots of different trees were observed to be further bifurcating into sub-branches with varying diameters and lengths. The lateral root system was found to be well developed in all the trees, the maximum main root lengths varied from 112 cm to 165 cm and diameters from 1.64 cm to 4.12 cm (Table 2). The total volume of the soil entrapped by the roots is given in Table 3. It was observed that the tree which produced the longest tap roots had the lowest spread of lateral roots. Tree no. 15 had the longest tap root with only a 1.60 m spread in lateral roots, whereas tree no. 4 which had the shortest tap root length showed the largest spread of lateral roots of 2.10 m (Table 3). This may be due to the fact that whenever tap root development is restricted for any reason, development of pronounced lateral roots occurs. The volume of soil entrapped by the root system varied from 4.70 m³ to 7.50 m³. The ratios between weights and lengths of above and below ground portions of the trees were found to be fairly consistent (Table 3). The fresh weight of the main stem is almost 3 times the weight of the root, whereas if the weight of leaves and branches is also included then the ratio increases to approximately 6:1. On dry weight basis, the main stem weight is about 2.5 times the weight of the root and the whole tree weight is approximately 4.5 times the weight of the roots. So it appears that there exists a definite correlation between the above and below ground portions of P. juliflora, but more intensive investigations are required to reach definite conclusions. The ratio between tree height and tap root length was also found to be fairly

Table 2. Root development in P. juliflora trees (all measurements in cm).

(A) Vertical root system

(B) Lateral root system

Table 3. Total root weights and dimension, and root/shoot ratios of four P. juliflora trees.

(a) {3.14 x (Column 5)²/4} x Column 4

(b) Column 7 of Table 1/Column 2 of Table 3

(c) Column 8 of Table 1/Column 2 of Table 3

(d) Column 9 of Table 1/Column 3 of Table 2

(e) Column 10 of Table 1/Column 3 of Table 2

(f) Column 11 of Table 1/Column 4 of Table 2 consistent, on average tree height being 2.5 times the length of the tap root. However, this depends on a number of factors like soil type, porosity, moisture status etc., and thus such ratios may hold true only at similar sites.

References

NAS (National Academy of Sciences), 1980. Firewood Crops: Shrubs and Tree Species for Energy Production. National Academy Press, Washington D.C.

Chaturvedi, A.N., 1984. Assessment of biomass production. Indian Forester 110:

Chaturvedi, A.N., 1985. Biomass production on saline soils: a case study. Proceedings of the Bio-Energy Society. First Conventional Symposium. New Delhi.

Chaturvedi, A.N., Shurbhra Bhatia and H.M. Behl, 1991. Biomass assessment for shrubs. Indian Forester 117: 1032-1035.