Spacing effects on relative growth rates of height , diameter and biomass production of Leucaena leucocephala ( Lam . ) de Wit . before and after pruning

Seedlings of Leucaena leucocephala (Lam.) de Wit. were planted in the field at The Agricultural Experiments Station of the college of Agriculture, King Saud University, Riyadh, Saudi Arabia in October 1995. Spacing between rows or trees within row was either 0.70 or 1.40 or 2.10 meter. The trees were pruned after two years of planting. Two trees from each plot were felled at time of pruning and divided into stem, branches and leaves to determine dry weights of above-ground part of tree. The relative growth rates of both diameter and height increased with increasing the distance between trees but decreased with age. The effects of both diameter and height growth on total above-ground biomass before pruning were similar for all spacing regimes. After pruning the height had the most effect on above-ground biomass of the trees in particular those in both the 1.4 and 2.1m spacing. Key wards: Leucaena leucocephala, relative growth rate, spacing, pruning, diameter and height. Introduction Introducing fast-growing tree species has been practiced in arid and semi-arid regions as well as modified planting techniques for decades. However, some fast-growing tree species are water demanding so that they can not be recommended in such conditions. Multipurpose tree species with reasonable water requirements and heat-resistant ability may be the most suitable for arid environments. Many species, of course, are sharing these characteristics. The final decision for adopting a species, therefore, has to be taken depending, in addition to the aforementioned, on providing maximum yield in the concerned site. Stem growth is a function of both height and diameter growth. The growth status of the seedlings, therefore, may be expressed in terms of total height and current height growth increment (Jobindon, 1994). Tree volume also is a function of the square of diameter (Van Den Beldt, 1982). Thus, exploring the variability in early diameter and height growth rates is important when interpreting development of the stands. Trees of the same age grow in height at roughly the same rate, provided site conditions are uniform. The diameter growth of trees is much more variable than that of height, therefore the trees in an even-aged stand are not as uniform in diameter as they are in height. Pruning is used to produce knot-free wood and also to lengthen the branch-free bole of trees. It is possible to plant new stands at a very wide spacing with a small number of trees and employ drastic pruning not only to get rid of branches but also to prevent boles from becoming too tapering. A fast RGR can be achieved through an efficient uptake and/or use of resources such as water, nutrients and light. Higher stem-wood production can be achieved through a faster rate of total biomass production and/or by allocating a larger proportion of the biomass produced to stem growth (Olov Norgren (1996). The present study sought to investigate the effects of spacing on stem height and diameter of leucaena trees before and after pruning. Materials and Methods Plant material Seedlings of Leucaena leucocephala (Lam.) de Wit. were planted in the field at King Saud University Experiments Station near Riyadh, Saudi Arabia in October 1995. Site characteristics The location where leucaena seedlings were planted is 24? 6 ` N, altitude; 46? 5` E, attitude; 650 m above sea level with average temperatures ranging between 0 ?C in Winter and 37 ?C in Summer and 50 mm annual rainfall. The soil is sandy loam with average 61, 23 and 15 % of sand, silt and clay, respectively (Aref, 1987). Experimental design and treatments The seedlings were planted in the field using a randomized complete block design with three blocks. Each block had 108 seedlings distributed into three experimental plots, where seedlings were planted in 6 rows with spacing either 0.70 or 1.40 or 2.10 meter (between rows and trees within row). The seedlings were irrigated once a week. Stem diameter at 30 cm above soil surface and total tree height were measured every two months for each tree. After one year of the planting time the trees were subjected to low pruning and two trees from each plot were felled. The aboveground parts of the felled trees were divided into leaves and stems. These were dried at 70 ?C for 48 h and their dry weights were determined. In order to have more details about the growth of the trees, the relative growth rates (RGRs) of both diameter and height were quantified every six months through four sequential intervals. This quantification was done twice before pruning and twice after pruning. Statistical analysis The obtained data were statistically analyzed through analysis of variance for a randomized complete block design using SAS computer programme. Results The statistical analysis of the obtained data revealed that spacing had significant effects upon the growth of Leucaena leucocephala trees. The relative growth rates (RGRs) of both diameter and height increased significantly with increasing the distance between trees (P<0.0001) and (P<0.001), respectively (Figure 1 and 2). On the other hand, the RGRs of both diameter and height decreased steadily with time (P<0.0001) and (P<0.0001), respectively, but the reductions were not the same for the different spacing regimes. For instance, by the second interval, Just before pruning, there were marked reductions in the RGR of diameter accounted for by 61, 60 and 64% in the close, medium and wide spacing, respectively (P<0.0001) (Figure 1 and 2). However, the magnitude of reductions was less pronounced at the third interval (44, 38 and 21%). Thereafter, there were 36% and 42% reduction in RGR of diameter in the close and wide spacing, respectively whereas no change was found in that in the medium spacing. The RGR of stem height decreased significantly from interval to the next. After pruning, at the third interval, the reductions were 57, 67 and 76% in the close, medium and wide spacing, respectively. However, the RGRs of stem height did not differ between the spacing regimes from the second interval and onward. By the last interval there was a slight increase for the RGR of stem height in the wide spacing. Discussion Increasing stem diameter and height of leucaena trees with increasing the distance between trees is simply a result of exploiting same available below-ground resources (water and nutrient) by less number of trees (Aref et al., 1999). Increasing spacing would also increase biomass of branches, leaves and main roots of trees (Bundit-Hongtong, 1990). As stem growth is a function of both height and diameter growth thus, exploring the variability in early diameter and height growth rates is important when interpreting development of the stand. In the present study relative growth rates of both stem diameter and height of leucaena increased in the close spacing. This result concurs with that of Nilsson, 1994. Increasing diameter growth rate in with increasing the distance between trees was reported by ???????????????????????????????????????????; ???????????; ??????????????????????????? and Effendi and Bachtiar, 1994 while increasing height growth rate of trees in wide spacing was reported by Vicharn-Pruaksakorn, 1993; Jaeghagen, 1997. On the other hand, the RGRs of both diameter and height decreased steadily with time (P<0.0001) and (P<0.0001), respectively, but the reductions were not the same for the different spacing regimes. 1 (2) (PINE) Diameter increment was measured five years after pruning. The growth reduction was statistically significant when 40% or more of the living crown was removed by pruning. Green pruning decreased diameter growth by 0 to 30% at defoliation levels below 40. This loss in diameter growth is compensated by the knotfree timber resulting from pruning 2-3 years earlier (Uotila and Mustonen, 1994). 2 (20) (PE AR) Vegetative growth increment was larger with more sever pruning and the reverse (Trajanovski et al., 1989). 2 (23) (EUCALYPTUS) The study sought to investigate the post-thinning situation in 4-year-old and 7-year-old Eucalyptus camaldulensis plantations. Tree stands with initial spacing of 2 x 8 m were thinned to three spacing levels: 4 x 4 m, 2 x 16 m and 8 x 8 m. Thinned stands showed better growth in both the 4-year-old and the 7-year-old plantation. The study recommended that thinning should not be applied in 4-year-old plantation if the purpose is just to enhance growth of tree stands, but it was shown to be essential in 7year-old plantation (Moton-Jamroenprucksa, 1989). 4 (15) (POPLAR) Exploring the variability in early height growth rates is important when interpreting development of the stands. Sprout-origin stems of Populus grandintata Mich. Grew faster and had much less variable growth rates than did seed-origin stands (Palik and Pregitzer, 1995). 4 (17). predicting losses in tree growth as measured by the relative growth rate (RGR). The severity of the competing vegetation (expressed in terms of density and height). ........ the growth status of the seedlings (expressed in terms of total height and current height growth increment). Measuring the intensity of light reaching the upper one half of the spruce seedlings significantly explains spruce relative growth rates, expressed in terms of height growth increment and basal stem diameter growth increment over the following two growing seasons, Jobindon, 1994). 4 (32) (RED CEDAR) while the crown of the trees were in the Grow-tubes, the rate of height growth increased but their basal diameter growth remained very low, however, when the crown emerged from the Grow-tubes, the rate of height growth decreased while the basal growth rate increased markedly. Grow-tube is a significant tool for the rapid establishment of red cedar in modified environments (Appligate and Bragg, 1989). 4 (43) (EUCALYPTUS) Relative height and diameter growth of irrigated Eucalyptus globulus was greater than that of other treatments once irrigation began. Between January and February 1992 that rate of increase in diameter was up to 5 times greater in the irrigated control compared to water stressed trees. Differences in height growth during this period were not as well defined (White et al. 1994). 5 (1) (BIRCH) The diameter growth of the dominant trees was reduced when the stem number exceeded 1000 trees ha-1. At the lowest density of 400 trees ha-1, birch height increment was reduced. Increasing the row-to-row distance did not influence the height growth of silver birch (Niemistoe, 1995). 5 (2) (PINE) Height increment of planted Pinus silvestris seedlings was reduced when over-story density was high (Jaeghagen, 1997). 5 (4) (CEIBA) Cotton enhanced the tree height growth rate of Ceiba pentandra (Murniati Mindawati, 1990). 5 (6) (RHIZOPHORA) There is a highly significant difference between plant spacing and both diameter and height growth. The largest diameter was for the trees in the 2 m x 2 m while the lowest was for those in the 2m x 1m. On the contrary, the height had the reverse trend (Effendi and Bachtiar, 1993). 8 (4) (NEEM) Diameter at ground level, diameter at breast height, total height and survival rate tended to increase when tree density decreased. Relative growth rate in terms of biomass and stem volume increased against the density. Tree's yield and biomass in unit area tended to decrease when tree density increased (Vicharn-Pruaksakorn, 1993). 9 (15) (POPLAR cv. I-214) Intermediate spacing of 4 x 4m and 5 x 5m had a positive effect mainly in the diameter growth of poplar trees while dense spacing of 3 x 2m and 4 x 2m resulted in higher volume production (Kohan, 1997). 9 (22) (Scotch pine) Trees in dense stands have slowed growth than those in more open stands because of competition; stand density has a greater impact on diameter growth than on the height of trees (Taurins, 1997). 9 (26) (PINE) Height growth response was delayed, especially among saplings. Height increment of planted seedlings was reduced when overstory density was high. It is concluded that competition has a great impact on seedling increment and on potential timber quality (Jaeghagen, 1997). 13 (4) (PINE) Increasing planting density slowed diameter and stem growth of individuals, and simulated height growth. Height growth responded to planting density earlier than diameter growth, but diameter growth become more affected as tree grew (Xie, 1995). 13 (6) (PICEA) Both height and diameter growth was significantly lower in the densest spacings compared to the most widely spaced stands. In the dense spacings the relative growth rates of both small and large trees were reduced (Nilsson, 1994). 13 (14) (EUCALYPTUS) Stem diameter and crow stem growth. A fast RGR can be achieved through an efficient uptake and/or use of resources such as water, nutrients and light. Biomass allocation, morphology, chemical composition and rate of assimilation represent physiological functions of seedlings that affect uptake and use of resources. The use of RGR in growth comparisons can compensate for differences in size, but does not function well unless growth is exponential (South, 1990). Seedlings seldom grow it an exponential rate over a period of several years. RGR consequently decreases with time (Van den Driessche and Waring, 1992, Brand et al. 1987, Britt et al. 1991); and smaller plants had higher RGR than larger ones (Britt et al. 1991, Van den Driessche and Waring, 1992). In the growth analysis, comparison of the components of RGR should therefore be conducted with seedlings of similar size. Britt J. R.; Mitchell R. J.; Zutter B. R.; South D. B.; Gjerstad D. H. and Dickson J. F. (1991). The influences of herbaceous weed control and seedling diameter on six years of loblolly pine growtha classical growth analysis approach. For. Sci. 37, p. 655-668. ? Olov Norgren (1996). Growth analysis of Scots pine and lodge pine seedlings. Forest Ecology and Management 86, p. 15-26. Leucaena Research, 1983 Stand volume growth in high-population treatments is probably a reflection of the stems that grow large at the expense of suppressed individuals (Van den Belt, 1982). (p105) As could be expected, the trees at wider spacing had greater diameters than those of the same age at close spacing. As populations decreased from 10,000/ha, the average diameters increased as much as a centimeter or more between populations that differed by only a few thousands stems/ha. This is significant, as tree volume is a function of the square of diameter. Thus, the yearly per-tree increment of widely spaced stems is many times that of close spacing. Height growth is much less affected by spacing than is diameter development. Even at spacing of 60,000 and 80,000/ha, height growth is did not stop until the second year. Overall, height increments get smaller at all spacing as age increases and probably taper off in wider spacing as trees reach a maximum 13-15 m. Height growth in these treatments will not affected stand-volume growth as much as annual increments. (p106) Because of nutrient-poor soils and weed competition, growth of the trees in wide spacings has been slow, and maximum yield has been obtained from the close spacings. (p108) Van Den Beldt R. J. (1982). Effect of spacing on growth of Leucaena. In: Leucaena Research in the Asian-Pacific Region, proceedings of a workshop held in Singapore, 23-26 November 1982, p. 103-108. International Development Research Centre (IDRC), Ottawa, Canada. Norani & Ng 1981 c.f. Ng et al. 1983 Leucaena is very variable in height, depending on site conditions. Norani A. and Ng F. S. P. (1981). Growth of Leucaena leucocephala in relation to soil pH, nutrient levels and Rhizobium concentration. Leucaena Research Reports No. 2, p. 5-10. Wei & Kiang 1983 At age 2-4 years, the trees planted at high densities did not have any advantage, in volume growth, over trees at densities of 5000 and 10 000 trees/ha. Moreover, because the percentage of trees with diameters smaller than 3cm was much higher in densely trees plots than is low-density plots, the economic value of the wood produced in the former was much lower. (p129) Kovitvadhi & Yantasth 1983 Wood weight and volume were negatively correlated with spacing at 1.5 years, but at 3 years, 50cm spacing between hills gave the maximum yields. (p135) Kovitvadhi K. and Yantasth K (1982). Research on Leucaena and other nitrogen-fixing trees at the Thailand Institute of Scientific and Technological Research (TISTR). In: Leucaena Research in the Asian-Pacific Region, proceedings of a workshop held in Singapore, 23-26 November 1982, p. 133-136. International Development Research Centre (IDRC), Ottawa, Canada. Denton 1983Through age 27 months, the giant leucaena shows a remarkably uniform rate of growth in heightand diameter. Monthly growth rates in height and diameter show almost no sign of slowing at leastthrough 24 months. 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