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Early response of Eucalyptus saligna to site preparation and fertilisation at Sao Hill, Tanzania
Forest Ecology and Management, 62 (1993) 303-311
303
Elsevier Science Publishers B.V., Amsterdam
Early response of Eucalyptus saligna to site preparation and fertilisation at Sao Hill, Tanzania M.L. Mhando *,a, S.M. Maliondo b, A.G. Mugasha b aTanzania Forestry Research Institute, P.O. Box 1854, Morogoro, Tanzania bFaculty of Forestry, Sokoine University of Agriculture, P.O. Box 3010, Morogoro, Tanzania (Accepted 4 May 1993)
Abstract
The effect of site preparation and fertilisation on the survival and growth of Eucalyptus saligna over 4 years and 10 months is discussed. The tree survival percentage was not significantly affected by either treatment, ranging from 88 to 100%. However, addition of NPK or B to tillage treatments improved root-collar diameter, diameter at breast height and height growth compared with unfertilized tillage treatments. In comparison, strip tillage with or without the addition of NPK increased the incidence of tree terminal die-back. However, terminal die-back was not increased by complete tillage with or without addition of B. In general, the results suggest that for improved initial performance of E. saligna at Sao Hill, complete tillage should replace strip tillage, and NPK be applied in addition to B.
Introduction
Tanzania, like many other tropical countries, has embarked on an ambitious plantation forestry programme to meet the increasing d e m a n d for industrial wood and fuelwood, as well as reducing exploitation pressure on indigenous forests. As a result, the area under industrial plantation forestry has doubled from 39 000 ha in 1976 to 80 000 ha in 1986 (Ahlback, 1986). Most of this expansion has, however, occurred in the Sao Hill Forestry Project (21 000 ha in 1976 to 42 000 ha in 1991 ) in southern Tanzania. The main species planted are Pinus patula, Pinus caribaea, Pinus elliottii, Cupressus lusitanica, and several Eucalyptus species. This project mainly caters for a sawmill and a pulp and paper mill which annually use 45 000 m 3 and 300 000 m 3 of wood, respectively. The relatively high productivity of trees under low intensity management during the first rotation at Sao Hill, typifies many plantations in Tanzania, *Corresponding author: Telefax: + 255 056 3725.
© 1993 Elsevier Science Publishers B.V. All rights reserved 0378-1127/93/$06.00
304
M.L, Mhando et al, / Forest Ecology and Management 62 (1993) 303-311
whereby the average annual increment for species such as C. lusitanica, P. patula,y P. caribaea, and P. elliottii is about 25 m 3 h a - ~. This was the result of appropriate species choice and selecting the more favourable sites for planting which were often the fertile agricultural lands or newly cleared natural forests/woodlands. More marginal sites were deliberately avoided. Furthermore, the small annual planting targets ( 10-100 ha) and the low labour costs often ensured that high initial planting failures due to poor establishment techniques and other causes could be redressed immediately by beatingup, or complete replanting. In addition, modest after-planting care often ensured reasonably high survival and growth. Recently there has been an increase in planting targets for most plantation tree species owing to an increase in the demand for wood products. The higher planting targets coupled with the altered government policy since the late 1970s restricting the clearing of natural forest to establish exotic plantation species has meant that expansion planting will mostly occur on more marginal sites, either with sparse natural vegetation or abandoned and exhausted agricultural land, or replanting of clearcut plantation forests. This fact, together with the resulting higher labour costs, requires the adoption of more intensive site preparation and tending techniques for survival, growth and productivity to equal or exceed that of the first rotation. Inadequate site preparation before planting and lack of sufficient boron in the soil have been reported (Procter, 1967; Waring, 1982; Cannon, 1985) as some of the causes of a die-back disorder of pines and eucalypts at Sao Hill. The objective of the research reported here was to study the effect of different intensities of tillage and the addition of N P K and B on the early performance of Eucalyptus saligna at Sao Hill. Materials and methods
The experimental site The field experiment was established in the Sao Hill Forestry Project in the southern highlands of Tanzania (8 ° 20' S, 35 ° 18' E, 1900 m above sea level). The area generally receives a mean annual rainfall of about 1000 mm, most of it falling between November and April. A mean annual temperature of about 16 ° C is experienced with some occasional frosts (Nykvist, 1976a). The experimental site was almost fiat (less than 2% slope), and the soil is generally of sandy clay loams derived from granitic parent material. The clay minerals are mainly kaolinitic with low cation exchange capacity and low base saturation. The original vegetation in the area was characterised by grassland with scattered short shallow-rooted bushes dominated by Brachystegia and Julbernardia species.
M.L. ,~lhando et al. / Forest Ecology and Management 62 (1993) 303-311
305
Treatments Four site preparation treatments were studied with and without fertilisation. Mechanical tillage using a light farm tractor was used, and tillage treatments were as follows: ( 1 ) complete tillage to a depth of 20 cm, i.e. tillage, harrowing and disking to a depth of 20 cm; (2) strip disking to a depth of 15 cm; (3) strip disking to a depth of 25 cm; (4) as Treatment 1 + 50 g of NPK per tree; (5) as Treatment 2 + 50 g of NPK per tree; (6) as Treatment 3 + 50 g of NPK per tree; (7) as Treatment 1 + 60 g of borax per tree; (8) as Treatment 4 + 60 g of borax per tree. Long narrow plots of size six by ten rows were planted at a spacing of 2.7 m × 2 . 7 m, on 28 February 1980, with 8-month-old nursery-raised seedlings of Eucalyptus saligna. These seedlings were raised from seeds collected from a local source (land race ). The seedlings were sorted for uniformity in height. Fertilisers were applied on 2 April 1980 (NPK) and 13 April (borax 12). NPK (25:5:5) was mixed with triple superphosphate (46%) on a weight by weight basis and applied at a rate of 50 g per tree (i.e. 12.5 g N, 6.18 g P and 1.25 g K); borax was applied at a rate of 60 g (i.e. 7.2 g B) per tree. Application was done at 45 cm from the tree base on opposite sides in V shaped holes. Each plot was surrounded by one guard row leaving a net assessment plot of 32 trees. The eight treatments were laid out in a randomised block design and replicated three times. During the first year spot weeding was conducted in all plots followed by slashing during the second and third years. Assessment for height, survival, diameter at root collar and die-back incidence was carried out after I year and 9 months and breast height diameter (DBH) at 58 months. Root-collar diameter was measured at 15 cm above the ground, and die-back incidence was assessed as the percentage of trees in a plot exhibiting terminal shoot die-back. Also at 58 months after planting, ten trees per plot covering a wide range of DBH were measured for height in order to develop regression equations to estimate the height (H) of trees only measured for DBH. Based on minimum standard error and maximum R 2, the following equation was selected In H=0.9172+0.6552 In DBH
SE=0.161, R2=0.61
The data were subjected to analysis of variance, and the means were further compared using the least significant difference test at P < 0.025.
306
M.L. Mhando et aL / Forest Ecology and Management 62 (1993) 303-311
Results
Survival rate Overall survival averaged 96% and was not significantly affected by the different treatments, although complete tillage plus NPK, with or without boron, gave 100°/o survival (Table 1).
Root-collar diameter at 21 months and breast height diameter at 58 months Root-collar diameter ranged from 4.0 to 7.2 cm, and was significantly affected by treatments ( P < 0.05 ). Complete tillage alone, and strip tillage to a depth of 15 or 25 cm gave the poorest performance, whereas the application of N P K to all tillage treatments, or borate alone in the case of complete tillage, increased diameter growth substantially (Fig. 1 ). Treatments affected DBH ( P < 0.05 ). Both the shallow and deep strip tillage treatments were inferior to all others (Table 1 ), whereas complete tillage with, or without the application of N P K or B, enhanced DBH growth. The beneficial effect of N P K addition was significant in the deep but not shallow strip tillage.
Height at 21 and 58 months after planting As with root-collar diameter, height at 21 months was affected by treatments ( P < 0.001 ). Again, complete tillage (Treatment 1 ) and deep strip tillage with N P K and borate resulted in greater tree height growth (Fig. 1 ). The mean height of trees at 58 months closely mirrored the trend exhibited by DBH. Table 1 R e s p o n s e of Eucalyptus saligna to tillage a n d fertiliser t r e a t m e n t s at Sao Hill Variable
Survival (%) DBH (cm)
Treatment
1
2
3
4
5
6
7
8
88" (8) 13,4 "be (0.5)
93 (4) 11.1 d (0.8)
97 ~ (1) 12.2 bed (1.1)
100" (0) 13.9 ab (0.6)
97 a (1) 11.7 ~d (0.5)
96 a (0) 13.0 ab (0.5)
97 a (1) 14.1 a (0.5)
100" (0) 13.6 ab (0.3)
Figures in parentheses are s t a n d a r d errors o f the m e a n . M e a n s followed by the s a m e superscript do n o t differ significantly at P < 0.05.
307
M.L. Mhando et al. / Forest Ecology and Management 62 (1993) 303-311
46-
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I I
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3
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I
0
4
5
6
7
8
4
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6
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Fig. 1. Height, root-collar diameter and die-back incidence of at Sao Hill, Tanzania. See text for explanation of treatments.
Eucalyptus saligna at 21
months
308
M.L. Mhando et al. / Forest Ecology and Management 62 (1993) 303-311
Die-back incidence Treatment effects on the die-back incidence were significant ( P < 0 . 0 5 ) , ranging from 4.4 to 48.1% (Fig. 1 ). In the two strip tillage treatments, application of N P K resulted in the highest die-back incidence, whereas the lowest die-back incidence was recorded in the complete tillage treatment with or without B. However, the application of B alone to complete tillage did not lower the die-back incidence significantly. Discussion
Effect of tillage methods Survival was consistently high (more than 80%) irrespective of treatment. This exceeded the minimum acceptable survival rate (80%) for plantations in moist areas of Tanzania (Anonymous, 1982). All treatments, therefore, seem effective in ensuring high seedling survival of Eucalyptus saligna at Sao Hill. This result contrasts sharply with the strong influence of tillage treatments on the survival of P. patula at Sao Hill (Kalaghe and Manssy, 1989). They observed that complete tillage resulted in higher survival. Chamshama and Hall ( 1987 ) reported an increase in the survival of Eucalyptus tereticornis due to intensive site preparation. It is not clear whether the differences between the results of these studies are due to species differences, or other causes. For example, McKimm and Flinn (1979) showed that of the four eucalypt species tested, Eucalyptus nitens exhibited the best survival and early growth response to intensive tillage and fertilisation. Eucalyptus regnans and Eucalyptus globulus ssp. bicostata showed poor survival. In contrast to survival, height and diameter (root collar and DBH ) growth was strongly influenced by the various treatments adopted. The results obtained in this experiment clearly demonstrate the ineffectiveness of shallow strip tillage. In particular, the standard method of strip tillage to a depth of 15 cm proved inferior to the other treatments, with only a very slight tree growth improvement when strip tillage to a depth of 25 cm was adopted. However, complete tillage treatments proved more effective and facilitated higher diameter and height growth. The strong positive effect on tree growth owing to complete tillage has previously been documented for P. patula (Schonau et al., 1981; Boden, 1984; Kalaghe and Manssy, 1988 ) and for Eucalyptus grandis (Boden, 1984 ). In the case of tree die-back, there seemed to be an interaction between tillage method and fertilisation, although it was not possible to test its significance in this study. For example, the highest incidence of die-back when no fertiliser was added occurred in the strip tillage method (approximately 16%), whereas complete tillage to a depth of 20 cm dramatically lowered the die-
M.L. Mhando et al. / Forest Ecology and Management 62 (1993) 303-311
309
back incidence to 4.7%. These results can be interpreted by postulating that complete tillage, unlike strip tillage, improved access to, or the availability of, some limiting factor in the soil, most likely as a result of one or more of the following: reduced or delayed competition to trees by weeds; reduced soil moisture and nutrient stress arising from increased nutrient availability and water holding capacity; root proliferation due to loosened soil. Thus, complete tillage, by ensuring increased soil rooting volume, might have permitted early site capture by seedlings with a well developed and vigorous root system well adapted to compete effectively for soil resources. In contrast, in shallow strip tillage, the shallow rooting soil volume and early onset of weed competition might explain the poor seedling growth recorded, as it has been shown in many previous studies that the seedlings of most eucalypt species are very sensitive to early weed competition, resulting in early mortality (Goor and Barney, 1976), or as in the current study, reduced growth rates.
The effect of fertiliser addition As in the case of tillage methods, fertiliser addition did not affect survival, contrary to the findings of Chamshama and Hall ( 1987 ) who found that fertilisation increased the survival of E. tereticornis in Morogoro, Tanzania. In most cases, the addition of fertiliser improved seedling growth over the effect of the tillage methods. The effect of fertilisers was, however, less obvious with strip tillage than with complete tillage. For example, while diameter and height growth were only slightly improved by NPK, the highest growth resulted from N P K addition with complete tillage (Fig. 1 ). In contrast, Schonau et al. ( 1981 ) reported greater fertiliser response by E. grandis when site preparation was poor. The results of our study suggest that complete tillage mostly improved soil moisture conditions, while the addition of N P K increased nutrient availability, both ensuring improved nutrient uptake and tree growth. These results are consistent with Nykvist's (1976a,b) hypothesis that both rainfall and soil nutrients are likely to limit growth in the more marginal extension areas of Sao Hill Forest Project area. The strong positive influence of NP fertilizer on Eucalyptus has been demonstrated by many workers (Knudson et al., 1970; Kadeba, 1978; Boden, 1984; Schonau and Herbert, 1989), especially when combined with other intensive cultural operations (Birk and Turner, 1992). The high growth response to N P K and B under complete tillage is similar to the results obtained for E. saligna in Brazilian savanna (Knudson et al., 1970). In contrast to the strong positive effect of N P K addition and tillage method on tree growth, the addition of N P K fertiliser significantly increased the incidence of tree die-back, with the highest figures being recorded from the two strip tillage methods (48% and 45% for Treatments 5 and 6, respectively).
310
M.L. Mhando et al, /Forest Ecology and Management 62 (1993) 303-311
Addition of NPK to the complete tillage treatment quadrupled the incidence of die-back from 4.7 to 17%, but this was lowered to 4.0% when boron was added (Treatment 8 ). However, while these changes are biologically important, they were statistically non-significant. These findings seem to suggest that complete tillage alone can be effective against tree die-back (Waring, 1982), but the addition of N P K seems to induce boron deficiency through dilution of the latter as a result of increased growth. This dilution of boron aggravates the incidence of die-back as the addition of borate to the latter treatment reduced the incidence of die-back to 11%. In a previous study, Procter ( 1967 ) showed that boron deficiency was responsible for the die-back ofP. patula at Sao Hill. In another study, Savory (1962) attributed Eucalyptus die-back in Zambia to boron deficiency. Kadeba ( 1978 ) showed that boron deficiency was responsible for the incidence of die-back of several eucalypt species grown in the savanna zone of Nigeria, and the die-back was corrected by applying 40 g of borate per tree. However, the results obtained in this study indicate that unless N P K is added, complete tillage without boron addition is effective in counteracting die-back of E. saligna at Sao Hill. The difference between our results and those of Procter (1967) may be a consequence of the minimal site preparation adopted by Procter, i.e. hoed spots (holes) for planting in uncultivated grassland. This treatment was later found to be inferior to, and was replaced by, strip tillage. Conclusion
Although this study had some limitations on statistical grounds, thus complicating the interpretation of the results, and no foliar nutrient analysis was made, it has revealed some important findings relevant to plantation establishment, mainly the need for intensive site preparation. The important findings can be summarized as follows. ( 1 ) Complete tillage with N P K or B ensured the best tree growth, while strip tillage was always inferior. (2) Application of N P K to the strip tillage treatments strongly increased the incidence of die-back, while complete tillage treatments with or without B, resulted in the lowest die-back incidence. (3) Application of N P K to the complete tillage treatment increased the dieback incidence, which was reduced by B addition. (4) It is therefore recommended that in establishing E. saligna stands at Sao Hill, complete tillage replace strip tillage, and NPK be applied with B. References
Ahlback, A.J., 1986. Industrial plantations forestry in Tanzania. Facts, problems and challenges. Ministry of Natural Resources and Tourism, Planning Division, Dar-es-Salaam. (Unpublished.)
M.L. Mhando et aL / Forest Ecology and Management 62 (1993) 303-311
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Anonymous, 1982. Management of conifer plantations in Tanzania: Notes on forest operations. Tanzania Ministry of Natural Resources and Tourism, Forest Division, 68 pp. Birk, E.M. and Turner, J., 1992. Responses of flooded gum (Eucalyptus grandis) to intensive cultural treatments: biomass and nutrient content of eucalypt plantation and native forests. For. Ecol. Manage., 47: 1-28. Boden, D.I., 1984. Early responses to different methods of site preparation for three commercial tree species. In: IUFRO Symposium on Site Productivity of Fast Growing Plantations, Pretoria and Pietermarizburg, South Africa, 30 April-11 May 1984, South Africa Forestry Research Institute, Pretoria, pp. 565- 578. Cannon, P., 1985. Studies in fire protection, stumpage and dieback. Technical Cooperation Programme--United Republic of Tanzania. Doc. No. 2, FAO, Rome. Chamshama, S.A.O. and Hall, J.B., 1987. Effects of site preparation and fertilizer application at planting of Eucalyptus tereticornis at Morogoro, Tanzania. For. Ecol. Manage., 18:103112. Goor, A.Y. and Barney, C.W., 1976. Tree planting in arid zones. Ronald Press, New York, 409 PP. Kadeba, O., 1978. Nutritional aspects of afforestation with exotic tree species in the savanna region of Nigeria. Commonw. For. Rev., 57: 191-199. Kalaghe, A.G. and Manssy, W., 1989. Effect of different site-preparation intensities on the growth of Pinuspatula at Sao Hill, Tanzania. For. Ecol. Manage., 29: 29-38. Knudson, D., Yahner, J.E. and Correa, H., 1970. Fertilizing eucalypts in Brazilian savanna soils. Commonw. For. Rev., 49: 30-40. McKimm, R.J. and Flinn, D.W., 1979. Eucalypt species, site preparation and fertilizer requirements for reforestation of Toorongo plateau in Central Victoria. Aust. For., 42:117-124. Nykvist, N., 1976a. Reconnaissance soil survey at Sao Hill, Mufindi area. Silv. Tech. Note (New Ser. ) No. 28, Mimes, Lushoto, Tanzania. Nykvist, N., 1976b. Meteorological data for Sao Hill, Mufindi area. Silv. Tech. Note (New Ser. ) No. 26, Mimes, Lushoto, Tanzania. Procter, J.E.A., 1967. A nutritional disorder of pine. Commonw. For. Rev., 46(2): 145-154. Savory, B.M., 1962. Boron deficiency in Eucalyptus in Northern Rhodesia. Emp. For. Rev., 41: 118-126. Schonau, A.P.G., Verloren van Thermaat, R. and Boden, P.I., 1981. The importance of complete site preparation and fertilizing in the establishment of Eucalyptus grandis. S. Afr. For. J., 6: 1-10. Schonau A.P.G. and Herbert, M.A., 1989. Fertilizing eucalypts at plantation establishment. For. Ecol. Manage., 29: 221-244. Waring, H.D., 1982. Dieback of pines and Eucalyptus. Sao Hill Forest Protection. Technical Cooperation Programme--United Republic of Tanzania. Doc. No. 2, FAO, Rome.
303
Elsevier Science Publishers B.V., Amsterdam
Early response of Eucalyptus saligna to site preparation and fertilisation at Sao Hill, Tanzania M.L. Mhando *,a, S.M. Maliondo b, A.G. Mugasha b aTanzania Forestry Research Institute, P.O. Box 1854, Morogoro, Tanzania bFaculty of Forestry, Sokoine University of Agriculture, P.O. Box 3010, Morogoro, Tanzania (Accepted 4 May 1993)
Abstract
The effect of site preparation and fertilisation on the survival and growth of Eucalyptus saligna over 4 years and 10 months is discussed. The tree survival percentage was not significantly affected by either treatment, ranging from 88 to 100%. However, addition of NPK or B to tillage treatments improved root-collar diameter, diameter at breast height and height growth compared with unfertilized tillage treatments. In comparison, strip tillage with or without the addition of NPK increased the incidence of tree terminal die-back. However, terminal die-back was not increased by complete tillage with or without addition of B. In general, the results suggest that for improved initial performance of E. saligna at Sao Hill, complete tillage should replace strip tillage, and NPK be applied in addition to B.
Introduction
Tanzania, like many other tropical countries, has embarked on an ambitious plantation forestry programme to meet the increasing d e m a n d for industrial wood and fuelwood, as well as reducing exploitation pressure on indigenous forests. As a result, the area under industrial plantation forestry has doubled from 39 000 ha in 1976 to 80 000 ha in 1986 (Ahlback, 1986). Most of this expansion has, however, occurred in the Sao Hill Forestry Project (21 000 ha in 1976 to 42 000 ha in 1991 ) in southern Tanzania. The main species planted are Pinus patula, Pinus caribaea, Pinus elliottii, Cupressus lusitanica, and several Eucalyptus species. This project mainly caters for a sawmill and a pulp and paper mill which annually use 45 000 m 3 and 300 000 m 3 of wood, respectively. The relatively high productivity of trees under low intensity management during the first rotation at Sao Hill, typifies many plantations in Tanzania, *Corresponding author: Telefax: + 255 056 3725.
© 1993 Elsevier Science Publishers B.V. All rights reserved 0378-1127/93/$06.00
304
M.L, Mhando et al, / Forest Ecology and Management 62 (1993) 303-311
whereby the average annual increment for species such as C. lusitanica, P. patula,y P. caribaea, and P. elliottii is about 25 m 3 h a - ~. This was the result of appropriate species choice and selecting the more favourable sites for planting which were often the fertile agricultural lands or newly cleared natural forests/woodlands. More marginal sites were deliberately avoided. Furthermore, the small annual planting targets ( 10-100 ha) and the low labour costs often ensured that high initial planting failures due to poor establishment techniques and other causes could be redressed immediately by beatingup, or complete replanting. In addition, modest after-planting care often ensured reasonably high survival and growth. Recently there has been an increase in planting targets for most plantation tree species owing to an increase in the demand for wood products. The higher planting targets coupled with the altered government policy since the late 1970s restricting the clearing of natural forest to establish exotic plantation species has meant that expansion planting will mostly occur on more marginal sites, either with sparse natural vegetation or abandoned and exhausted agricultural land, or replanting of clearcut plantation forests. This fact, together with the resulting higher labour costs, requires the adoption of more intensive site preparation and tending techniques for survival, growth and productivity to equal or exceed that of the first rotation. Inadequate site preparation before planting and lack of sufficient boron in the soil have been reported (Procter, 1967; Waring, 1982; Cannon, 1985) as some of the causes of a die-back disorder of pines and eucalypts at Sao Hill. The objective of the research reported here was to study the effect of different intensities of tillage and the addition of N P K and B on the early performance of Eucalyptus saligna at Sao Hill. Materials and methods
The experimental site The field experiment was established in the Sao Hill Forestry Project in the southern highlands of Tanzania (8 ° 20' S, 35 ° 18' E, 1900 m above sea level). The area generally receives a mean annual rainfall of about 1000 mm, most of it falling between November and April. A mean annual temperature of about 16 ° C is experienced with some occasional frosts (Nykvist, 1976a). The experimental site was almost fiat (less than 2% slope), and the soil is generally of sandy clay loams derived from granitic parent material. The clay minerals are mainly kaolinitic with low cation exchange capacity and low base saturation. The original vegetation in the area was characterised by grassland with scattered short shallow-rooted bushes dominated by Brachystegia and Julbernardia species.
M.L. ,~lhando et al. / Forest Ecology and Management 62 (1993) 303-311
305
Treatments Four site preparation treatments were studied with and without fertilisation. Mechanical tillage using a light farm tractor was used, and tillage treatments were as follows: ( 1 ) complete tillage to a depth of 20 cm, i.e. tillage, harrowing and disking to a depth of 20 cm; (2) strip disking to a depth of 15 cm; (3) strip disking to a depth of 25 cm; (4) as Treatment 1 + 50 g of NPK per tree; (5) as Treatment 2 + 50 g of NPK per tree; (6) as Treatment 3 + 50 g of NPK per tree; (7) as Treatment 1 + 60 g of borax per tree; (8) as Treatment 4 + 60 g of borax per tree. Long narrow plots of size six by ten rows were planted at a spacing of 2.7 m × 2 . 7 m, on 28 February 1980, with 8-month-old nursery-raised seedlings of Eucalyptus saligna. These seedlings were raised from seeds collected from a local source (land race ). The seedlings were sorted for uniformity in height. Fertilisers were applied on 2 April 1980 (NPK) and 13 April (borax 12). NPK (25:5:5) was mixed with triple superphosphate (46%) on a weight by weight basis and applied at a rate of 50 g per tree (i.e. 12.5 g N, 6.18 g P and 1.25 g K); borax was applied at a rate of 60 g (i.e. 7.2 g B) per tree. Application was done at 45 cm from the tree base on opposite sides in V shaped holes. Each plot was surrounded by one guard row leaving a net assessment plot of 32 trees. The eight treatments were laid out in a randomised block design and replicated three times. During the first year spot weeding was conducted in all plots followed by slashing during the second and third years. Assessment for height, survival, diameter at root collar and die-back incidence was carried out after I year and 9 months and breast height diameter (DBH) at 58 months. Root-collar diameter was measured at 15 cm above the ground, and die-back incidence was assessed as the percentage of trees in a plot exhibiting terminal shoot die-back. Also at 58 months after planting, ten trees per plot covering a wide range of DBH were measured for height in order to develop regression equations to estimate the height (H) of trees only measured for DBH. Based on minimum standard error and maximum R 2, the following equation was selected In H=0.9172+0.6552 In DBH
SE=0.161, R2=0.61
The data were subjected to analysis of variance, and the means were further compared using the least significant difference test at P < 0.025.
306
M.L. Mhando et aL / Forest Ecology and Management 62 (1993) 303-311
Results
Survival rate Overall survival averaged 96% and was not significantly affected by the different treatments, although complete tillage plus NPK, with or without boron, gave 100°/o survival (Table 1).
Root-collar diameter at 21 months and breast height diameter at 58 months Root-collar diameter ranged from 4.0 to 7.2 cm, and was significantly affected by treatments ( P < 0.05 ). Complete tillage alone, and strip tillage to a depth of 15 or 25 cm gave the poorest performance, whereas the application of N P K to all tillage treatments, or borate alone in the case of complete tillage, increased diameter growth substantially (Fig. 1 ). Treatments affected DBH ( P < 0.05 ). Both the shallow and deep strip tillage treatments were inferior to all others (Table 1 ), whereas complete tillage with, or without the application of N P K or B, enhanced DBH growth. The beneficial effect of N P K addition was significant in the deep but not shallow strip tillage.
Height at 21 and 58 months after planting As with root-collar diameter, height at 21 months was affected by treatments ( P < 0.001 ). Again, complete tillage (Treatment 1 ) and deep strip tillage with N P K and borate resulted in greater tree height growth (Fig. 1 ). The mean height of trees at 58 months closely mirrored the trend exhibited by DBH. Table 1 R e s p o n s e of Eucalyptus saligna to tillage a n d fertiliser t r e a t m e n t s at Sao Hill Variable
Survival (%) DBH (cm)
Treatment
1
2
3
4
5
6
7
8
88" (8) 13,4 "be (0.5)
93 (4) 11.1 d (0.8)
97 ~ (1) 12.2 bed (1.1)
100" (0) 13.9 ab (0.6)
97 a (1) 11.7 ~d (0.5)
96 a (0) 13.0 ab (0.5)
97 a (1) 14.1 a (0.5)
100" (0) 13.6 ab (0.3)
Figures in parentheses are s t a n d a r d errors o f the m e a n . M e a n s followed by the s a m e superscript do n o t differ significantly at P < 0.05.
307
M.L. Mhando et al. / Forest Ecology and Management 62 (1993) 303-311
46-
q4'
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i
4.
~6_ 34 -r
I
!
42 ¸
II
I
i
I i
i
i, I
t
32 31
I
I
I
28' 2.6 24
Ii
II
I I
2
3
II
22 2
I
0
4
5
6
7
8
4
5
6
7
8
6.5 6 5.5 5 4.5 4
I, ii
3,5 0
1
2
50-
3
I
45'
I
40
i
3~
,
i
3)
q
25----
i c~
i~,
O'
I
--
|
III 2
3
4 5 Treatment
~
i 7
Fig. 1. Height, root-collar diameter and die-back incidence of at Sao Hill, Tanzania. See text for explanation of treatments.
Eucalyptus saligna at 21
months
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Die-back incidence Treatment effects on the die-back incidence were significant ( P < 0 . 0 5 ) , ranging from 4.4 to 48.1% (Fig. 1 ). In the two strip tillage treatments, application of N P K resulted in the highest die-back incidence, whereas the lowest die-back incidence was recorded in the complete tillage treatment with or without B. However, the application of B alone to complete tillage did not lower the die-back incidence significantly. Discussion
Effect of tillage methods Survival was consistently high (more than 80%) irrespective of treatment. This exceeded the minimum acceptable survival rate (80%) for plantations in moist areas of Tanzania (Anonymous, 1982). All treatments, therefore, seem effective in ensuring high seedling survival of Eucalyptus saligna at Sao Hill. This result contrasts sharply with the strong influence of tillage treatments on the survival of P. patula at Sao Hill (Kalaghe and Manssy, 1989). They observed that complete tillage resulted in higher survival. Chamshama and Hall ( 1987 ) reported an increase in the survival of Eucalyptus tereticornis due to intensive site preparation. It is not clear whether the differences between the results of these studies are due to species differences, or other causes. For example, McKimm and Flinn (1979) showed that of the four eucalypt species tested, Eucalyptus nitens exhibited the best survival and early growth response to intensive tillage and fertilisation. Eucalyptus regnans and Eucalyptus globulus ssp. bicostata showed poor survival. In contrast to survival, height and diameter (root collar and DBH ) growth was strongly influenced by the various treatments adopted. The results obtained in this experiment clearly demonstrate the ineffectiveness of shallow strip tillage. In particular, the standard method of strip tillage to a depth of 15 cm proved inferior to the other treatments, with only a very slight tree growth improvement when strip tillage to a depth of 25 cm was adopted. However, complete tillage treatments proved more effective and facilitated higher diameter and height growth. The strong positive effect on tree growth owing to complete tillage has previously been documented for P. patula (Schonau et al., 1981; Boden, 1984; Kalaghe and Manssy, 1988 ) and for Eucalyptus grandis (Boden, 1984 ). In the case of tree die-back, there seemed to be an interaction between tillage method and fertilisation, although it was not possible to test its significance in this study. For example, the highest incidence of die-back when no fertiliser was added occurred in the strip tillage method (approximately 16%), whereas complete tillage to a depth of 20 cm dramatically lowered the die-
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back incidence to 4.7%. These results can be interpreted by postulating that complete tillage, unlike strip tillage, improved access to, or the availability of, some limiting factor in the soil, most likely as a result of one or more of the following: reduced or delayed competition to trees by weeds; reduced soil moisture and nutrient stress arising from increased nutrient availability and water holding capacity; root proliferation due to loosened soil. Thus, complete tillage, by ensuring increased soil rooting volume, might have permitted early site capture by seedlings with a well developed and vigorous root system well adapted to compete effectively for soil resources. In contrast, in shallow strip tillage, the shallow rooting soil volume and early onset of weed competition might explain the poor seedling growth recorded, as it has been shown in many previous studies that the seedlings of most eucalypt species are very sensitive to early weed competition, resulting in early mortality (Goor and Barney, 1976), or as in the current study, reduced growth rates.
The effect of fertiliser addition As in the case of tillage methods, fertiliser addition did not affect survival, contrary to the findings of Chamshama and Hall ( 1987 ) who found that fertilisation increased the survival of E. tereticornis in Morogoro, Tanzania. In most cases, the addition of fertiliser improved seedling growth over the effect of the tillage methods. The effect of fertilisers was, however, less obvious with strip tillage than with complete tillage. For example, while diameter and height growth were only slightly improved by NPK, the highest growth resulted from N P K addition with complete tillage (Fig. 1 ). In contrast, Schonau et al. ( 1981 ) reported greater fertiliser response by E. grandis when site preparation was poor. The results of our study suggest that complete tillage mostly improved soil moisture conditions, while the addition of N P K increased nutrient availability, both ensuring improved nutrient uptake and tree growth. These results are consistent with Nykvist's (1976a,b) hypothesis that both rainfall and soil nutrients are likely to limit growth in the more marginal extension areas of Sao Hill Forest Project area. The strong positive influence of NP fertilizer on Eucalyptus has been demonstrated by many workers (Knudson et al., 1970; Kadeba, 1978; Boden, 1984; Schonau and Herbert, 1989), especially when combined with other intensive cultural operations (Birk and Turner, 1992). The high growth response to N P K and B under complete tillage is similar to the results obtained for E. saligna in Brazilian savanna (Knudson et al., 1970). In contrast to the strong positive effect of N P K addition and tillage method on tree growth, the addition of N P K fertiliser significantly increased the incidence of tree die-back, with the highest figures being recorded from the two strip tillage methods (48% and 45% for Treatments 5 and 6, respectively).
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Addition of NPK to the complete tillage treatment quadrupled the incidence of die-back from 4.7 to 17%, but this was lowered to 4.0% when boron was added (Treatment 8 ). However, while these changes are biologically important, they were statistically non-significant. These findings seem to suggest that complete tillage alone can be effective against tree die-back (Waring, 1982), but the addition of N P K seems to induce boron deficiency through dilution of the latter as a result of increased growth. This dilution of boron aggravates the incidence of die-back as the addition of borate to the latter treatment reduced the incidence of die-back to 11%. In a previous study, Procter ( 1967 ) showed that boron deficiency was responsible for the die-back ofP. patula at Sao Hill. In another study, Savory (1962) attributed Eucalyptus die-back in Zambia to boron deficiency. Kadeba ( 1978 ) showed that boron deficiency was responsible for the incidence of die-back of several eucalypt species grown in the savanna zone of Nigeria, and the die-back was corrected by applying 40 g of borate per tree. However, the results obtained in this study indicate that unless N P K is added, complete tillage without boron addition is effective in counteracting die-back of E. saligna at Sao Hill. The difference between our results and those of Procter (1967) may be a consequence of the minimal site preparation adopted by Procter, i.e. hoed spots (holes) for planting in uncultivated grassland. This treatment was later found to be inferior to, and was replaced by, strip tillage. Conclusion
Although this study had some limitations on statistical grounds, thus complicating the interpretation of the results, and no foliar nutrient analysis was made, it has revealed some important findings relevant to plantation establishment, mainly the need for intensive site preparation. The important findings can be summarized as follows. ( 1 ) Complete tillage with N P K or B ensured the best tree growth, while strip tillage was always inferior. (2) Application of N P K to the strip tillage treatments strongly increased the incidence of die-back, while complete tillage treatments with or without B, resulted in the lowest die-back incidence. (3) Application of N P K to the complete tillage treatment increased the dieback incidence, which was reduced by B addition. (4) It is therefore recommended that in establishing E. saligna stands at Sao Hill, complete tillage replace strip tillage, and NPK be applied with B. References
Ahlback, A.J., 1986. Industrial plantations forestry in Tanzania. Facts, problems and challenges. Ministry of Natural Resources and Tourism, Planning Division, Dar-es-Salaam. (Unpublished.)
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