Variability in early growth and biomass accumulation in a progeny trial of Prosopis alba - L.N. Harsh, J.C. Tewari, N.K. Sharma and M.D. Bohra

Central Arid Zone Research Institute, Jodhpur 342003, India

Introduction

The vegetation of the hot desert region of India, classified as dry tropical thorn forests (Champion and Seth, 1968), is experiencing a high biotic pressure due to ever increasing human and livestock population (densities of 64 and 113 individuals/km² respectively). As a consequence of this, the forest cover has been reduced to only 1% of the total land surface area of the region, with the remainder highly degraded forms (Tewari et al., 1993). Furthermore, hostile environmental conditions do not support the regeneration of natural vegetation. The desert landscape of western Rajasthan, known as the Thar desert, accounts for more than 61% of the total area of Indian hot arid regions. According to a recent estimate, the fuelwood requirement at present is more than 520,000 t and the projected requirement for 2001 is around 680,000 t, whereas availability of fuelwood at present is only 70,000 t (Anonymous, 1987). To bridge this gap between demand and supply of woody biomass, it is essential to raise large scale plantations of fast growing multi-purpose tree species (MPTS) in this desert tract (Mann and Muthana, 1984). Various species of the genus Prosopis are found to be very adaptable to the environmental conditions encountered in this hot arid region of India (Mann and Saxena, 1982). The present investigation was carried out on a progeny trial of 30 accessions of P. alba, to test their acclimatisation, adaptability and genetic variability; and also to assess the possibility of selection of plus trees for future plantation programmes in the arid environmental conditions of India.

Materials and methods

The experiment was conducted at the silvaetum of the Central Arid Zone Research Institute, Jodhpur (26^o18’N, 73^o08’E). The climate is typically arid, characterised by exceptionally hot dry summers, sub-humid monsoons and cold dry winters, with climatic details recorded at the experimental site during the period of study presented in Table 1. The soil is a sandy loam (Camborthid) with a pH of 8.1 and has low nutrient levels, with 0.23% organic carbon, 0.03% nitrogen and 0.02% phosphorus (Dhir, 1984). Thirty accessions of P. alba, obtained from Texas A&M University, Kingsville, U.S.A. were used in the this investigation. All were of Argentinean origin, from Cordoba, Salta and

Table 1. Mean monthly rainfall and temperatures recorded at the experimental site during the entire period of experimentation.

Catamarca, from areas receiving 250-700 mm rainfall per year. The term accession is used here to describe seeds from single tree representative of one location. Seeds were scarified with concentrated sulphuric acid for 15 minutes and sown into polythene bags (25 x 10 cm) containing a 1:2:1 ratio of manure, sand and clay in February 1991. Five month old seedlings were then out-planted in July, at a spacing of 2.5 x 4.0 m in a randomised complete block design with four replications, each having five plants. Plants were watered fortnightly in the summer and once a month during the winter (15 l/plant), and manual weeding was carried out twice, in the first year after planting.

Data for survival, plant height and collar diameter of all accessions were recorded at 6, 18 and 30 months, at the end of each growing season. At 30 months, the above ground biomass was also estimated using diameter at 15 cm above ground level on single stemmed and multi-stemmed trees. With multi-stemmed trees, the diameters of different stems (up to a maximum of 5 stems/plant) were measured, and the biomass of each stem was estimated before being pooled to obtain the total above ground biomass of the plant. The following empirical equation developed by Felker et al., (1989) for P. alba was used for the estimation of biomass in both single stemmed and multi-stemmed trees:

log₁₀ dry weight (kg) =

2.1905 {log₁₀ stem diameter (cm)} - 0.9811.

Replication means of registered data were used for statistical analysis using the computer software “SPAR” (Doshi and Gupta, 1991).

Results and discussion

Treatment means were found to be significant for plant height and estimated biomass but not for collar diameter (Table 2). Of the 30 accessions of P. alba, only a few consistently showed improved performance. Acc. no. 65 was found to be consistently better performing in terms of plant height, collar diameter and biomass, height being 60 cm, 102 cm and 225 cm, while the collar diameter was 1.06 cm, 1.76 cm and 4.19 cm, over the 3 growing periods. The estimated biomass was 7.05 kg/tree after 30 months. This accession was never in first place in the rank order for any character but performed better than average for all three characters and maintained good growth throughout the trial.

Acc. no. 67 performed relatively poorly in the first two growing periods but by 30 months it had attained a height of 259 cm and was placed first in the overall ranking, with increments in collar diameter also following a similar trend. The low order of estimated biomass (2.39 kg/tree) for this accession may perhaps be due to a lower number of stems per plant. Acc. nos. 28, 65, 73, 78, 120, 147 and 151 performed poorly for plant height after 6 months but later grew considerably. Acc. nos. 28, 78, 120, 147 and 151 showed relatively better growth in terms of collar diameter. Acc. no. 78 had a collar diameter of only 0.97 cm after 6 months, but performed very well later and registered maximum collar diameter (4.36 cm) thus acquiring first place in the ranking after 30 months. Acc. nos. 67 and 73 exhibited poor performance in the first two growing periods but after 30 months they had achieved 2nd and 3rd ranks respectively. Maximum estimated biomass (7.84 kg/tree) was recorded for acc. no. 146, followed by acc. no. 65 (7.05 kg/tree). However, the performance of acc. no. 146 was relatively poor in terms of plant height and collar diameter, the high biomass production in this accession perhaps due to its vigorous multi-stemmed form.

Concerning genetic parameters, the magnitude of PCV was more than GCV in all the three growing periods for both plant height and collar diameter, and also for biomass after 30 months, indicating interactions of some environmental factors (Table 3). In general, PCV showed an increasing trend with age for both plant height and collar diameter. For plant height, the magnitude of GCV was similar throughout the trial, but for collar diameter it was at a maximum (10.79%) after 18 months and minimum (5.68%) after 6 months. Heritability estimates in general showed a decreasing trend with age for both the characters.

For plant height, results after 6 months appeared most suitable for effective selection as maximum heritability as well as genetic advance as a percentage of the mean were recorded at this stage (Table 3). In the case of collar diameter, results after 18 months appeared to be more suitable, also true if simultaneous selection is practised, for both plant height and collar diameter. At this stage, GCV and heritability were relatively higher for both traits, which is further substantiated by the value of genetic advance as a percentage of the mean. The biomass GCV, heritability and genetic advance as a percentage of the mean were 26.07%, 20.10% and 23.96% respectively. Genetic parameters computed in the present investigation indicated that through simple selection, more gain is possible in biomass production than for plant height and collar diameter. Such changes in genetic parameters with age (from juvenile to mature stages) have also been reported with P. pallida (Sharma et al., 1993), Liquidambar styraciflua and Platanus occidentalis (Schultz, 1983) and Tecomella undulata (Jindal et al., 1992).

Table 2. Plant height, collar diameter, crown diameter and biomass of 30 accessions of P.alba over 3 years.

Table 3. Analysis of variance and genetic parameters, calculated as mean squares, for plant height (Ht.), collar diameter (SBD), crown diameter (Crown) and biomass of P. alba over 3 years (* p<0.05; ** p<0.01).

Significant positive correlation coefficients between plant height and collar diameter across all growing periods indicated that selection practised for one trait will be automatically meaningful for a second trait and vice versa (Table 4). The associations were more pronounced with increasing age at all the three levels (genotypic, phenotypic and environmental). The magnitude of correlations at all three levels were similar after 30 months. Environmental correlations include the effects of soil heterogeneity and cultural irregularities, such factors causing harmonic changes in tree behaviour and may be easily interpreted in terms of physiological adjustments (Tewari et al., 1994).

The application of non-hierarchical Euclidean cluster analysis using plant height and collar diameter resulted in the formation of four clusters across all growing periods. The clustering pattern of different accessions changed with time (Table 5). Acc. no. 126 performed the best and remained in cluster IV throughout. The performance of acc. nos. 28 and 147 were relatively poorer after 6 months and so were positioned in cluster III, but they attained good growth in subsequent years and moved to cluster IV. Acc. nos. 65, 67, 73, 78 and 120 did not perform well in the first two growing periods, but by 30 months the had grown appreciably and as such they also moved to cluster IV. The degree of distance was found to be maximum between clusters I and IV across all the three growing periods (Table 6). However, the peak (3.48) observed after 30 months may perhaps be due to high variable values. For intra-cluster distances no particular trend was evident.

Table 4. Correlation coefficients between plant height and collar diameter of P. alba in 3 different years (*p<0.05; **p<0.01).

Table 5. Clustering pattern of 30 accessions of P. alba over 3 growing periods using non-hierarchical Euclidean cluster analysis.

Conclusions

The material used in the present investigation showed a wide range of variability and differential growth patterns, therefore early performance may not always mean similar performance levels in subsequent years and vice versa. The selection of accessions showing relatively better and consistent performance throughout the trial (e.g. acc. nos. 28, 65, 78, 120, 147 and 151) may be beneficial in terms of productivity in future plantations programmes. Similar findings have also been reported with P. cineraria (Kackar, 1988) and Tecomella undulata (Jindal et al., 1992), the two most prominent native species of the Indian arid tract.

Table 6. Average distance of cluster members from cluster centroids (diagonal) and distances between cluster centroids of four clusters having 30 accessions of P. alba in 4 different years.

Acknowledgements

Funding support from the Indo-US, PL-480 project, grant no. FG-IN-749 is gratefully acknowledged. The authors also wish to thank Dr. A.S. Faroda, director of CAZRI, for providing all the necessary facilities as and when required.

References

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