WFL Publisher Science and Technology Meri-Rastilantie 3 B, FI-00980 Helsinki, Finland e-mail:
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Journal of Food, Agriculture & Environment Vol.11 (1): 1147-1151. 2013
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Effect of thinning on the aboveground biomass accumulation and branch characteristics in Acacia salicina trees Said S. Hegazy 1, Pervaiz R. Khan 1, Ibrahim M. Aref 1 and Muhammad Iqbal 2* Department of Plant Production, College of Food and Agricultural Sciences, King Saud University, P. O. Box 2460, Riyadh, 11451, Saudi Arabia. 2 Department of Botany, Faculty of Science, Hamdard University, Tughlaqabad, New Delhi 110062, India. *e-mail:
[email protected] 1
Received 8 September 2012, accepted 28 January 2013.
Abstract This study, undertaken in the Riyadh region of Saudi Arabia, was focused on high initial density plantation and the subsequent thinning of Acacia salicina trees, to ensure high individual tree growth suitable for saw timber production. It investigated the effect of high stand density on restricting early branch development. Trees were planted in 1998, at a density of 6400 trees ha-1 (with spacing of 1.25 m x 1.25 m). After two years, half of the experiment was subjected to annual mechanical thinning up to 2003. From 2004 to 2010, the densities of 400 and 3200 trees ha-1 were maintained for the thinned and unthinned stands; respectively. In 2010, five trees from each of the two stands were selected randomly, felled, and processed for estimating the biomass of stem, branches, and foliage. Tree height, tree diameter, log diameter along the stem, number of branches, branch diameter and distance of each branch from tree base were measured. Thinning significantly increased the diameter at breast height (dbh) and the total tree height at the age of 12 years, the mean values being 25.2 cm, and 16.9 m for diameter and height, respectively, in thinned population (400 trees ha-1), while these were 12.2 cm and 10.7 m, respectively, in unthinned population (3200 stem trees ha-1). Though the individual stem volume increased significantly in thinned population, the total stem stand volume per hectare was far greater in the unthinned population. The thinned trees had a lower form factor (or stronger taper) than the unthinned trees. Thinning significantly increased the dry biomass of different plant parts and had a significant effect on the diameter of the thickest branch. Key words: Stand biomass, thinning regimes, tree biomass, tree density.
Introduction Saudi Arabia has only few indigenous tree species, including the acacias, which are extremely slow growing. Plantation of fastgrowing species and use of improved recycling techniques are, therefore, a need of the hour in the kingdom. Forestry Unit at the King Saud University has focused on importing high-yielding species and defining their growth and biomass characteristics through standardization of initial density, thinning and the final state of plantation. Estimating tree biomass is an important way of measuring the energy potential of forests 14, and biomass accumulation is enhanced through short-rotation forestry. Many aspects of stand dynamics are kept in view while developing a regime for growing a short-rotation tree crop. It is necessary to determine which spacing promotes early canopy closure so as to control weed growth, reduce coarse branching and optimize wood production on individual tree as well as stand basis 26. Planting at higher initial stockings and delay in thinning until the live crown base rises to a commercial log length restrict early branch development 20, 34. A compromise has to be sought between stand volume and tree quality, and branch development pattern has to be understood for developing suitable thinning regimes. A study was undertaken to establish guidelines for high initial density plantation of Acacia salicina trees, by investigating the use of subsequent thinning to ensure high individual tree growth suitable for saw timber production and to understand the effectiveness of stand density in restricting early branch Journal of Food, Agriculture & Environment, Vol.11 (1), January 2013
development. The research was based on the hypothesis that an early and high-intensity thinning rapidly and markedly increases the size of the remaining trees. Acacia salicina Lindley, a vigorous and fast-growing thornless tree, is native to Australia. Normally 5-12 m tall, it occasionally grows up to 18 m 19. Its major branches are acutely upright, with pendulous ends. It is a drought- and salt-tolerant species capable to grow on a wide range of soils 18. Its wood is very hard and forms a high-value furniture timber because of its dark reddish brown heartwood with attractive markings 5. Materials and Methods Field operations: Six-month-old seedlings of Acacia salicina Lindley were planted in the field in October 1998, accommodating 330 seedlings in 15 rows, with 22 plants in each row. The space between and within the rows was 1.25 m x 1.25 m, with a starting density of 6400 trees ha-1. The plantation was irrigated once a week in winter and twice a week in summer. In February 2000, the planted area was divided into three blocks, each comprising of two plots, using a randomized complete block design. Three plots were then chosen randomly and subjected to mechanical thinning, in which each second tree was removed from the rows to reduce the plot density to 3200 trees ha-1. These plots were thinned twice again in the next two years to reduce tree density to 800 trees ha-1. El-Juhany 9 studied the effect of early thinning on tree growth and biomass production in this plantation, 1147
when it was 4.5-year-old. In 2004, in order to evaluate thinned plots for producing highquality logs suitable for solid wood, these plots were re-thinned to a final density of 400 trees ha-1, while unthinned plots had a density of 3200 trees ha-1 with 2.5 m x 1.25 m spacing. Both these stand densities were maintained till the end of experiment. In 2010, five trees from each tree-density stand were selected randomly and felled (at 10 cm above ground level) to determine the total tree height, bole diameter at breast height (1.4 m), number of branches above 1.4 m height, diameter of each branch at its base (above the base swell), and distance of each branch from tree base. Stems were cut into logs, first at breast height and then successively at each one-meter interval. Stem logs were weighed separately and diameter up to bark surface was recorded for each log. Fresh weight was determined in the field with a spring balance to the nearest 0.2 kg. Wood disk samples as well as the branch and foliage samples were taken to the laboratory for dry matter estimation and biomass related calculations for each component. Laboratory operations: All the disks and other tree samples were weighed fresh to the nearest 0.1g, and left to air dry for few days. Samples of stem and branches were then oven-dried at 102- ± 2°C, and the foliage at 70°C, until constant weights. Dry weight to fresh weight ratio of each component was obtained and multiplied by the total fresh mass in order to get the total dry biomass of the tree. The dry weights of different plant parts, and of the total aboveground biomass, were determined. The volume of each log was calculated using the Smalian formula 3, as given below: V=
A1 + A2 xL 2
(1)
where V is the log volume, A1 is the upper cross section area (πr2) of the log, A2 is the lower cross section area of the log, and L is the length of the log. The total volume of the stem was then calculated by summing up all the log volumes.
Statistical analysis: Analysis of variance test was applied to the data obtained, using the SAS computer software 28. Means were compared by the L.S.D. test (P
