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A study of wind damage on Pinus patula stands in Southern Tanzania
Forest Ecology and Management, 63 (1994) 13-21
13
Elsevier Science Publishers B.V., Amsterdam
A study of wind damage on Pinus patula stands in Southern Tanzania P.K.T. Munishi*, S.A.O. Chamshama Department of Forest Biology, Faculty of Forestry, Sokoine University of Agriculture, P.O. Box 3010, Chuo Kikuu, Morogoro, Tanzania (Accepted 8 July 1993)
Abstract
Between February and March 1990 a study was carried out at Kiwira Forest plantations in Southern Tanzania to investigate wind damage to Pinus patula stands, and to relate the damage to various factors that influence windthrow in forest plantations. It was found that wind damage at Kiwira occurred mainly during the rainy season. The damage occurred mainly by stem breakage and/or severe bending (19.4-32.6%) and to a lesser extent by uprooting (4.5-9.2%). A positive and significant relationship between the height/diameter ratio and the wind damage by stem breakage plus severe bending and the total damage was found. Overstocking was a major factor in increasing the risk of wind damage, as it contributed to increasing the height/diameter ratio, whereas the influence of soil on wind damage was not evident. Over 30% of the trees in the study area have been wind damaged before the rotation age. Wider initial spacing (3 m × 3 m) followed by early and heavy thinnings will produce wind-firm trees and reduce the risks of wind damage.
Introduction
Damage to forests by wind is a major problem in many countries. However, studies on factors affecting stand stability to windfall have mainly been carried out in temperate countries. These studies include aerodynamic effects, movement and mechanical properties of stems and the force needed to pull trees over by winching (Coutts, 1983; Petty and Swain, 1985; Blackburn et al., 1988; Milne and Blackburn, 1989; Milne, 1991 ), measurements on root systems in relation to stability (Coutts, 1983; Deans, 1983; Deans and Ford, 1983 ), effects of silvicultural treatments such as spacing (Irvine, 1970; Bryndum, 1986; Blackburn and Petty, 1988 ), site fertility and fertilization (Mackenzie, 1974; Persson, 1975), thinning (Persson, 1975; Cremer et al., 1982; Bryndum, 1986), cultivation and drainage (Booth, 1974; Savill, 1976; Deans, 1983; Hendrick, 1986), effects of site type (soil, rainfall, wind, exposure, aspect, slope and topography) (Ruth and Yoder, 1953; Busby, 1965; Wen*Corresponding author.
SSDI0378-1127 (93)03280-V
14
P.K.T. MunishL S.A, O. Chamshama / ForestEcology and Management 63 (1994) 13-21
delken, 1966; Mackenzie, 1974) and species differences in wind firmness (Ruth and Yoder, 1953; Bouchon, 1986; Neckelmann, 1986; Lohmander and Helles, 1987). In Tanzania, establishment of commercial plantations dates back to the 1950s. The present area is about 82 000 ha. Although some of these plantations have experienced wind damage, no studies have been undertaken to determine the causes of the damage. Kiwira Forest Project in Southern Tanzania is among the softwood plantations which have been seriously damaged by wind. A study of wind damage on Pinus patula stands at this project was carried out to relate wind damage to various factors that influence windthrow in plantations. The specific objectives of the study were: ( 1 ) to determine the height/diameter (H/D) ratio and its relationship with wind damage; (2) to determine the extent of previous windfalls (number and age of trees affected) and possible relationship with stocking, previous stand treatments and topography; (3) to determine the depth to an impenetrable soil layer and its relationship with wind damage. Materials and methods
The study site The Kiwira Forest Project (around 9°03'S, 38°42'E) is located in the Southern Highlands of Tanzania, between Rungwe and the Livingstone Mountains. The land is rolling, with gentle to very steep slopes. Most of the area lies between 2225 and 2440 m above sea-level, and the highest point is about 2712 m. The area has dark volcanic soils with mixed silt and organic matter, which freely compacts when wet and is friable when dry. The mean annual rainfall is about 1846 m m and falls mainly from November to May. The mean monthly minimum and maximum temperatures are 16.9°C and 38.6 ° C, respectively. Winds are southerlies during the dry season, north-easterlies during the rainy season, and easterlies during the cold period.
Data collection The study focused on wind-damaged stands. Three compartments of ages 17, 18 and 21 years were selected randomly from the plantation map. Compartment 10 (21 years) had two sub-compartments, Compartment 11 ( 18 years) had four sub-compartments and Compartment 12 (17 years) three sub-compartments. Data were collected in these compartments from 59 circular sample plots each measuring 0.04 ha. Each compartment was allocated plots proportional to its area. The plots were established equidistant along the longest diagonal of each compartment. In each plot, four dominant trees were
P.K.T. Munishi, S.A.O. Chamshama / Forest Ecology and Management 63 (1994) 13-21
15
measured for height using a Suunto hypsometer and diameter at breast height using callipers. All trees were counted to allow calculations of stocking. The number of wind-damaged trees in each plot was assessed using the following categories: ( 1 ) no damage; (2) stem breakage; (3) severe bending (i.e. irreversible bending of the stem ); (4) uprooting. Eleven pits were dug to about 2 m depth in randomly selected plots within the surveyed compartments to determine the depth to an impenetrable layer. Data were also collected on previous windfalls from records of the number of trees damaged, age and stocking.
Data analysis Means for the measured variables were computed for each plot and averaged per sub-compartment. The H/D ratio was calculated from the mean dominant height and mean diameter at breast height. A simple linear regression analysis was used to determine the correlation between the H/D ratio and number of damaged stems per hectare for each of the damage categories. Damage by stem breakage and severe bending were combined in the analysis. The significance of each regression equation developed was determined using t-tests. Results and discussion
Background to windthrow problems at Kiwira Windthrow incidence at Kiwira started in 1973. At first, only a few plantations including some research blocks greater than 12 years old were slightly affected. Since then, damage has occurred every year and with greater intensity. Wind damage has been observed only during the rainy season (Mganga, 1985 ). There are no records of wind speed at Kiwira, but the speed has been observed to be higher during the rainy season than during the dry season. Windthrow occurred randomly in patches ranging from 0.04 to 0.07 ha. It has been estimated that up to 1981 about 50% of all trees had been wind damaged, especially in stands over 13 years old, and that up to 1985, approximately 1349 ha out of the total project area of 2637 ha had been seriously wind damaged (Y.K. Mganga, unpublished work, 1985 ). It has also been observed (Forestry and Beekeeping Division (Tanzania), unpublished work, 1979 ) that wind damage intensifies beyond the age of 12 years and that the intensity of damage tends to increase with increase in age (Table 1 ). The greater resistance of young or low stands to windthrow is well documented (Busby, 1965; Cremer et al., 1982 ). Three factors are suggested to explain the increasing incidence of wind damage with increasing age (or tree height): increase in turning moment at the base of the tree with increasing height; increase in wind speed with height, so that taller trees are subjected
16
P.K.T. Munishi, S.A.O. Chamshama / ForestEcology and Management 63 (1994) 13-21
Table 1 Results of a 1979 survey of wind damage at Kiwira Forest Project, Southern Tanzania Age of stand (years)
Stocking (stems ha - ~)
Damage (%)
13 14 15 16 17 18 19
1100 1100 1100 740 740 740 740
20.6 25.9 31.0 14.0 15.6 62.9 72.4
to more wind; the increase in diameter in fully stocked stands being disproportionately small relative to height increase, resulting in slender stems which are more vulnerable to both stem and root failure (Cremer et al., 1982 ).
Stocking and thinning Table 2 shows the stocking in the sampled compartments and the number of damaged trees. It can bee seen that stocking is far higher than if the thinning schedules were followed, and that overall wind damage is high (25.740.4%). Plantation records show that most stands had received one to two thinnings only. The superiority in resistance to both stem and root failure by trees grown at low stocking and the reduction in the H/D ratio with the resultant increase in stability of trees have been widely reported (Cremer et al., 1982; Petty and Swain, 1985; Blackburn and Petty, 1988; Blackburn et al., 1988). However, the quantitative evidence to support this was reported only recently (Blackburn and Petty, 1988 ). Where low stocking was associated with higher incidence of stem and root failures, the affected stands had been thinned within the previous 5 years or so (Busby, 1965; Booth, 1974; Persson, 1975; Cremer et al., 1982). The situation in the forest plantations in Tanzania (Kiwira inclusive) is that thinning operations do not follow the schedules, as a result of lack of funds, lack of markets for small logs and lack of processing plants (Chamshama and Philip, 1980). As a result, stands are overstocked and/or thinning is delayed unduly. Plantation records at Kiwira show that none of the sampled compartments had received thinnings in the previous 5 years or so. Overstocking therefore seems to explain the windthrow cases at Kiwira. As pointed out by Wendelken (1966), Irvine (1970), Booth (1974), Persson ( 1975 ), Bryndum ( 1986 ) and Neckelmann ( 1986 ), wind-firmness of stands and resistance to breakage can to a considerable extent be promoted by wide initial spacing and early and heavy thinnings.
P.K.T. Munishi, S.A.O. Chamshama /Forest Ecology and Management 63 (1994) 13-21
17
0
0
m
m
,a n9 e-r /-al
.=. ~_~
~
O0
O0
O0
O0
r~
~.
I-~
ccJ ~0 e9 o
~9
o
?
0
b.,
Z
18
P.K. 72 MunishL S.A.O. Chamshama/Forest Ecology and Management 63 (1994) 13-21
Relationship between H / D ratio and wind damage
Both the damage (stems ha- 1) by stem breakage plus severe bending and the total damage showed a positive and significant linear relationship with H~ D ratio in Compartments 10 and 11 (Table 3 ). The positive and significant relationship between wind damage and H/D has been reported elsewhere (Cremer et al., 1982; Petty and Swain, 1985 ). Compartment 12 did not show a significant relationship in either damage category, although the damage increased with increase in the H/D ratio (Table 1). In all compartments, there were no statistically significant relationships between H/D and uprooting. Lack of significant correlation between H/D ratio and wind damage is an indication that the risk of wind damage depends not only on the sturdiness of the tree population but also on the aerodynamic properties of the stand, as well as on wind and soil conditions (Cremer et al., 1982 ). The H/D ratios at Kiwira ranged from 93.3 to 97.7 in Compartment 10, from 89.0 to 101.4 in Compartment 11 and from 95.9 to 101.1 in Compartment 12 (Table 2). These high H/D ratios indicate fast height growth and suppressed diameter growth, with high risk of wind damage. Studies by Cremer et al. ( 1982 ) have shown that H/D ratios below 74 indicate no damage, whereas ratios above 90 indicate complete damage (dominant height averaging 25 m). The low incidence of uprooting, and the non-significant relationship between H/D and uprooting, may be because the extreme H/D ratios make the trees susceptible to stem breakage and/or severe bending. The total damage Table 3 H / D ratios in relation to number o f damaged stems per hectare o f P i n u s patula stands at Kiwira Forest Project, Southern Tanzania Compartment no.
Age (years)
Damage category
l0
21
( 1 ) Stem breakage plus severe bending (2) Uprooting (3) Total damage ( 1 ) Stem breakage plus severe bending (2) Uprooting (3) Total damage ( 1 ) Stem breakage plus severe bending (2) Uprooting (3) Total damage
11
12
18
17
Regression equation
R
R2
Y= - 327.2 + 5.5X Y= 2 1 4 . 1 - 1.6X Y= - 113.1 + 3.9X
0.68 -0.31 0.55
0.46** 0.10NS 0.30*
Y= - 218.3 + 4.0X Y= 47.5-0.1X Y= - 199.9+4.2X
0.70 -0.05 0.74
0.49"** 0.0025NS 0.55***
Y= - 371.5 + 5.8X Y= 5 3 8 . 5 - 4 . 9 X Y= - 166.9+0.8X
0.40 -0.39 0.07
0.16NS 0.15NS 0.0049NS
Y is the number o f stems per hectare damaged by ( 1 ) stem breakage plus severe bending, (2) uprooting or (3) total damage. X is the H / D ratio. NS, not significant. *Significant at P < 0.05. "Significant at P < 0.01. ***Significant at P < 0.001.
P.K.T. MunishL S.A.O. Chamshama / ForestEcology and Management 63 (1994) 13-21
19
ranged from 25.7% to 40.4%. This damage is alarmingly high, as the plantations have not reached the rotation age of 25 years, and may necessitate clearfelling of these plantations before rotation age. According to Miller (1985), when wind damage within a stand reaches 40%, clearfelling is normally desirable to allow recovery of the full productive capacity of the site.
Effect of soil on wind damage Examination of soil pits indicated that the soils permit deep rooting and generally lack compact layers within the upper 2 m. Compact layers were observed only in two of the 11 pits examined. In all cases, roots were observed below 2 m, indicating firm anchorage. This may, to some extent, explain the low incidence of wind damage by uprooting (Table 2) even though most of the damage occurred during the rainy season when soils were wet. Windthrow by uprooting is more likely on shallow soils or soils with impeded drainage (Day, 1949; Busby, 1965; Wendelken, 1966; Mackenzie, 1974; Cremer et al., 1982). Such soils are not common at Kiwira. The volcanic soils at Kiwira are free draining, and this is enhanced by the sloping ground, which provides topographical drainage.
Topography and elevation in relation to wind damage Topography plays a role in determining the extent of windthrow. The land at Kiwira is rolling, with gentle to very steep slopes and valleys. The unevenness of the configuration of the land would be expected to reduce the seriousness of wind damage (Busby, 1965 ). However, despite the unevenness of land, wind damage at the study site is high. This is possibly due to the concentration and local acceleration of wind along valleys as a result of the wind being forced to change direction within the plantations and the resulting damage occurring randomly along the zone of the new wind direction. Indeed, most damage was observed along or near the sides of valleys. This is in agreement with the literature on the influence of topography on wind damage (Ruth and Yoder, 1953; Busby, 1965; Mackenzie, 1974).
Management recommendations Light thinning or absence of thinning is regarded as having contributed greatly to wind damage, as this has led to overstocking and increases in H/D ratios. Adopting a wider initial spacing ( 3 m × 3 m) would reduce the cost of thinning, as there would be fewer thinnings than the present four (Malimbwi et al., 1992). Further, early and heavy thinnings would increase wind-firm-
20
e.g.T. Munishi, S.A.O. Chamshama / Forest Ecology and Management 63 (1994) 13-21
ness. With Pinus patula at a spacing of 3 m × 3 m it is even possible to practise a no-thinning regime while maintaining the production of large sawlogs at a rotation age of 25 years (Malimbwi, 1987 ). Acknowledgements
We thank Sokoine University of Agriculture for funding this study, the management of Kiwira Forest Project for their assistance during data collection and Dr. J. Deans for sending us some of the literature and for comments on the manuscript. This paper is based on a special project report submitted by the senior author in partial fulfilment of the requirements for the B.Sc. (For.) degree. References Blackburn, P.S. and Petty, J.A., 1988. Theoretical calculations of the influence of spacing on stand stability. Forestry, 61: 235-244. Blackburn, P., Petty, J.A. and Miller, K.F., 1988. An assessment of the static and dynamic factors involved in windthrow. Forestry, 61:29-43. Booth, T.C., 1974. Silvicultural management of high risk forests in Great Britain. Irish For., 31: 145-153. Bouchon, J., 1986. Susceptibility of different conifer species to blow down. Proceedings of the Workshop on Minimizing Wind Damage to Coniferous Stands. Commission of the European Communities/Danish Forest Experiment Station, l-~venholm Castle, Denmark, pp. 33-34. Bryndum, H., 1986. Influence of silvicultural treatment of crops on the risk of windblow. Proceedings of the Workshop on Minimizing Wind Damage to Coniferous Stands. Commission of the European Communities/Danish Forest Experiment Station, l_4venholm Castle, Denmark, pp. 35. Busby, J.A., 1965. Studies on the stability of conifer stands. Scott. For., 19: 87-102. Chamshama, S.A.O. and Philip, M.S., 1980. Thinning Pinus patula plantation at Sao Hill, Southern Tanzania. Record 13. Division of Forestry, University of Dar-es-Salaam, Morogoro, Tanzania, 16 pp. Coutts, M.P., 1983. Root architecture and tree stability. Plant Soil, 71: 171-188. Cremer, K.W., Borough, C.J., McKinnell, F.H. and Carter, P.R., 1982. Effects of stocking and thinning on wind damage in plantations. NZ J. For. 12: 244-268. Day, W.R., 1949. The soil conditions which determine wind throw in forests. Forestry, 23: 9095. Deans, J.D., 1983. Distribution of thick roots in Sitka spruce plantation 16 years after planting. Scott. For. 37: 17-31. Deans, J.D. and Ford, E.D., 1983. Modelling root structure and stability. Plant Soil, 71:189195. Hendrick, E., 1986. Appropriate cultivation and drainage techniques for sites liable to windthrow. Proceedings of the Workshop on Minimizing Wind Damage to Coniferous Stands. Commission of the European Communities/Danish Forest Experiment Station, L~venholm Castle, Denmark, pp. 30-32. Irvine, R.E., 1970. The significance of windthrow for Pinus radiata management in the Nelson District, New Zealand. NZ J. For. Res., 15: 57-68.
P.K.T. Munishi, S.A.O. Chamshama / Forest Ecology and Management 63 (I 994) 13-21
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Lohmander, P. and Helles, F., 1987. Windthrow probability as a function of stand characteristics and shelter. Scand. J. For. Res., 2: 227-238. Mackenzie, R.F., 1974. Some factors influencing the stability of Sitka in Northern Ireland. Ir. For. 31: 110-129. Miller, K.F., 1985. Windthrow hazard classification. Forestry Commission Leaflet 85, 14 pp. Milne, R., 1991. Dynamics of swaying ofPicea sitchensis. Tree Physiol., 9: 383-399. Milne, R. and Blackburn, P., 1989. The elasticity and vertical distribution of trees within stems ofPicea sitchensis. Tree Physiol., 5:195-205. Neckelmann, J., 1986. Choice in species, topping and high pruning as means to prevent windthrow in permanent or recent stand edges. Proceedings of the Workshop on Minimizing Wind Damage to Coniferous Stands. Commission of the European Communities/Danish Forest Experiment Station, l_~venholm Castle, Denmark, pp. 42-45. Persson, P., 1975. Windthrow in forests. Royal Research Note 36, Department of Forest Yield Research, Royal College of Forestry, Stockholm, pp. 234-248. Petty, J.A. and Swain, C., 1985. Factors influencing stem breakage of conifers in high winds. Forestry, 58: 75-84. Ruth, R.H. and Yoder, R.A., 1953. Reducing wind damage in the forests of the Oregon Coast range. US For. Serv. Pac. Northwest For. Range Exp. Stn. Res. Pap. 7, 30 pp. Savill, P.S., 1976. The effects of drainage and ploughing of surface water glays on rooting and windthrow of Sitka spruce in Northern Ireland. Forestry, 49:133-14. Wendelken, W.J., 1966. Eyrewell forest: Search for stable management. NZ J. For., 11: 43-65.
13
Elsevier Science Publishers B.V., Amsterdam
A study of wind damage on Pinus patula stands in Southern Tanzania P.K.T. Munishi*, S.A.O. Chamshama Department of Forest Biology, Faculty of Forestry, Sokoine University of Agriculture, P.O. Box 3010, Chuo Kikuu, Morogoro, Tanzania (Accepted 8 July 1993)
Abstract
Between February and March 1990 a study was carried out at Kiwira Forest plantations in Southern Tanzania to investigate wind damage to Pinus patula stands, and to relate the damage to various factors that influence windthrow in forest plantations. It was found that wind damage at Kiwira occurred mainly during the rainy season. The damage occurred mainly by stem breakage and/or severe bending (19.4-32.6%) and to a lesser extent by uprooting (4.5-9.2%). A positive and significant relationship between the height/diameter ratio and the wind damage by stem breakage plus severe bending and the total damage was found. Overstocking was a major factor in increasing the risk of wind damage, as it contributed to increasing the height/diameter ratio, whereas the influence of soil on wind damage was not evident. Over 30% of the trees in the study area have been wind damaged before the rotation age. Wider initial spacing (3 m × 3 m) followed by early and heavy thinnings will produce wind-firm trees and reduce the risks of wind damage.
Introduction
Damage to forests by wind is a major problem in many countries. However, studies on factors affecting stand stability to windfall have mainly been carried out in temperate countries. These studies include aerodynamic effects, movement and mechanical properties of stems and the force needed to pull trees over by winching (Coutts, 1983; Petty and Swain, 1985; Blackburn et al., 1988; Milne and Blackburn, 1989; Milne, 1991 ), measurements on root systems in relation to stability (Coutts, 1983; Deans, 1983; Deans and Ford, 1983 ), effects of silvicultural treatments such as spacing (Irvine, 1970; Bryndum, 1986; Blackburn and Petty, 1988 ), site fertility and fertilization (Mackenzie, 1974; Persson, 1975), thinning (Persson, 1975; Cremer et al., 1982; Bryndum, 1986), cultivation and drainage (Booth, 1974; Savill, 1976; Deans, 1983; Hendrick, 1986), effects of site type (soil, rainfall, wind, exposure, aspect, slope and topography) (Ruth and Yoder, 1953; Busby, 1965; Wen*Corresponding author.
SSDI0378-1127 (93)03280-V
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P.K.T. MunishL S.A, O. Chamshama / ForestEcology and Management 63 (1994) 13-21
delken, 1966; Mackenzie, 1974) and species differences in wind firmness (Ruth and Yoder, 1953; Bouchon, 1986; Neckelmann, 1986; Lohmander and Helles, 1987). In Tanzania, establishment of commercial plantations dates back to the 1950s. The present area is about 82 000 ha. Although some of these plantations have experienced wind damage, no studies have been undertaken to determine the causes of the damage. Kiwira Forest Project in Southern Tanzania is among the softwood plantations which have been seriously damaged by wind. A study of wind damage on Pinus patula stands at this project was carried out to relate wind damage to various factors that influence windthrow in plantations. The specific objectives of the study were: ( 1 ) to determine the height/diameter (H/D) ratio and its relationship with wind damage; (2) to determine the extent of previous windfalls (number and age of trees affected) and possible relationship with stocking, previous stand treatments and topography; (3) to determine the depth to an impenetrable soil layer and its relationship with wind damage. Materials and methods
The study site The Kiwira Forest Project (around 9°03'S, 38°42'E) is located in the Southern Highlands of Tanzania, between Rungwe and the Livingstone Mountains. The land is rolling, with gentle to very steep slopes. Most of the area lies between 2225 and 2440 m above sea-level, and the highest point is about 2712 m. The area has dark volcanic soils with mixed silt and organic matter, which freely compacts when wet and is friable when dry. The mean annual rainfall is about 1846 m m and falls mainly from November to May. The mean monthly minimum and maximum temperatures are 16.9°C and 38.6 ° C, respectively. Winds are southerlies during the dry season, north-easterlies during the rainy season, and easterlies during the cold period.
Data collection The study focused on wind-damaged stands. Three compartments of ages 17, 18 and 21 years were selected randomly from the plantation map. Compartment 10 (21 years) had two sub-compartments, Compartment 11 ( 18 years) had four sub-compartments and Compartment 12 (17 years) three sub-compartments. Data were collected in these compartments from 59 circular sample plots each measuring 0.04 ha. Each compartment was allocated plots proportional to its area. The plots were established equidistant along the longest diagonal of each compartment. In each plot, four dominant trees were
P.K.T. Munishi, S.A.O. Chamshama / Forest Ecology and Management 63 (1994) 13-21
15
measured for height using a Suunto hypsometer and diameter at breast height using callipers. All trees were counted to allow calculations of stocking. The number of wind-damaged trees in each plot was assessed using the following categories: ( 1 ) no damage; (2) stem breakage; (3) severe bending (i.e. irreversible bending of the stem ); (4) uprooting. Eleven pits were dug to about 2 m depth in randomly selected plots within the surveyed compartments to determine the depth to an impenetrable layer. Data were also collected on previous windfalls from records of the number of trees damaged, age and stocking.
Data analysis Means for the measured variables were computed for each plot and averaged per sub-compartment. The H/D ratio was calculated from the mean dominant height and mean diameter at breast height. A simple linear regression analysis was used to determine the correlation between the H/D ratio and number of damaged stems per hectare for each of the damage categories. Damage by stem breakage and severe bending were combined in the analysis. The significance of each regression equation developed was determined using t-tests. Results and discussion
Background to windthrow problems at Kiwira Windthrow incidence at Kiwira started in 1973. At first, only a few plantations including some research blocks greater than 12 years old were slightly affected. Since then, damage has occurred every year and with greater intensity. Wind damage has been observed only during the rainy season (Mganga, 1985 ). There are no records of wind speed at Kiwira, but the speed has been observed to be higher during the rainy season than during the dry season. Windthrow occurred randomly in patches ranging from 0.04 to 0.07 ha. It has been estimated that up to 1981 about 50% of all trees had been wind damaged, especially in stands over 13 years old, and that up to 1985, approximately 1349 ha out of the total project area of 2637 ha had been seriously wind damaged (Y.K. Mganga, unpublished work, 1985 ). It has also been observed (Forestry and Beekeeping Division (Tanzania), unpublished work, 1979 ) that wind damage intensifies beyond the age of 12 years and that the intensity of damage tends to increase with increase in age (Table 1 ). The greater resistance of young or low stands to windthrow is well documented (Busby, 1965; Cremer et al., 1982 ). Three factors are suggested to explain the increasing incidence of wind damage with increasing age (or tree height): increase in turning moment at the base of the tree with increasing height; increase in wind speed with height, so that taller trees are subjected
16
P.K.T. Munishi, S.A.O. Chamshama / ForestEcology and Management 63 (1994) 13-21
Table 1 Results of a 1979 survey of wind damage at Kiwira Forest Project, Southern Tanzania Age of stand (years)
Stocking (stems ha - ~)
Damage (%)
13 14 15 16 17 18 19
1100 1100 1100 740 740 740 740
20.6 25.9 31.0 14.0 15.6 62.9 72.4
to more wind; the increase in diameter in fully stocked stands being disproportionately small relative to height increase, resulting in slender stems which are more vulnerable to both stem and root failure (Cremer et al., 1982 ).
Stocking and thinning Table 2 shows the stocking in the sampled compartments and the number of damaged trees. It can bee seen that stocking is far higher than if the thinning schedules were followed, and that overall wind damage is high (25.740.4%). Plantation records show that most stands had received one to two thinnings only. The superiority in resistance to both stem and root failure by trees grown at low stocking and the reduction in the H/D ratio with the resultant increase in stability of trees have been widely reported (Cremer et al., 1982; Petty and Swain, 1985; Blackburn and Petty, 1988; Blackburn et al., 1988). However, the quantitative evidence to support this was reported only recently (Blackburn and Petty, 1988 ). Where low stocking was associated with higher incidence of stem and root failures, the affected stands had been thinned within the previous 5 years or so (Busby, 1965; Booth, 1974; Persson, 1975; Cremer et al., 1982). The situation in the forest plantations in Tanzania (Kiwira inclusive) is that thinning operations do not follow the schedules, as a result of lack of funds, lack of markets for small logs and lack of processing plants (Chamshama and Philip, 1980). As a result, stands are overstocked and/or thinning is delayed unduly. Plantation records at Kiwira show that none of the sampled compartments had received thinnings in the previous 5 years or so. Overstocking therefore seems to explain the windthrow cases at Kiwira. As pointed out by Wendelken (1966), Irvine (1970), Booth (1974), Persson ( 1975 ), Bryndum ( 1986 ) and Neckelmann ( 1986 ), wind-firmness of stands and resistance to breakage can to a considerable extent be promoted by wide initial spacing and early and heavy thinnings.
P.K.T. Munishi, S.A.O. Chamshama /Forest Ecology and Management 63 (1994) 13-21
17
0
0
m
m
,a n9 e-r /-al
.=. ~_~
~
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O0
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~.
I-~
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P.K. 72 MunishL S.A.O. Chamshama/Forest Ecology and Management 63 (1994) 13-21
Relationship between H / D ratio and wind damage
Both the damage (stems ha- 1) by stem breakage plus severe bending and the total damage showed a positive and significant linear relationship with H~ D ratio in Compartments 10 and 11 (Table 3 ). The positive and significant relationship between wind damage and H/D has been reported elsewhere (Cremer et al., 1982; Petty and Swain, 1985 ). Compartment 12 did not show a significant relationship in either damage category, although the damage increased with increase in the H/D ratio (Table 1). In all compartments, there were no statistically significant relationships between H/D and uprooting. Lack of significant correlation between H/D ratio and wind damage is an indication that the risk of wind damage depends not only on the sturdiness of the tree population but also on the aerodynamic properties of the stand, as well as on wind and soil conditions (Cremer et al., 1982 ). The H/D ratios at Kiwira ranged from 93.3 to 97.7 in Compartment 10, from 89.0 to 101.4 in Compartment 11 and from 95.9 to 101.1 in Compartment 12 (Table 2). These high H/D ratios indicate fast height growth and suppressed diameter growth, with high risk of wind damage. Studies by Cremer et al. ( 1982 ) have shown that H/D ratios below 74 indicate no damage, whereas ratios above 90 indicate complete damage (dominant height averaging 25 m). The low incidence of uprooting, and the non-significant relationship between H/D and uprooting, may be because the extreme H/D ratios make the trees susceptible to stem breakage and/or severe bending. The total damage Table 3 H / D ratios in relation to number o f damaged stems per hectare o f P i n u s patula stands at Kiwira Forest Project, Southern Tanzania Compartment no.
Age (years)
Damage category
l0
21
( 1 ) Stem breakage plus severe bending (2) Uprooting (3) Total damage ( 1 ) Stem breakage plus severe bending (2) Uprooting (3) Total damage ( 1 ) Stem breakage plus severe bending (2) Uprooting (3) Total damage
11
12
18
17
Regression equation
R
R2
Y= - 327.2 + 5.5X Y= 2 1 4 . 1 - 1.6X Y= - 113.1 + 3.9X
0.68 -0.31 0.55
0.46** 0.10NS 0.30*
Y= - 218.3 + 4.0X Y= 47.5-0.1X Y= - 199.9+4.2X
0.70 -0.05 0.74
0.49"** 0.0025NS 0.55***
Y= - 371.5 + 5.8X Y= 5 3 8 . 5 - 4 . 9 X Y= - 166.9+0.8X
0.40 -0.39 0.07
0.16NS 0.15NS 0.0049NS
Y is the number o f stems per hectare damaged by ( 1 ) stem breakage plus severe bending, (2) uprooting or (3) total damage. X is the H / D ratio. NS, not significant. *Significant at P < 0.05. "Significant at P < 0.01. ***Significant at P < 0.001.
P.K.T. MunishL S.A.O. Chamshama / ForestEcology and Management 63 (1994) 13-21
19
ranged from 25.7% to 40.4%. This damage is alarmingly high, as the plantations have not reached the rotation age of 25 years, and may necessitate clearfelling of these plantations before rotation age. According to Miller (1985), when wind damage within a stand reaches 40%, clearfelling is normally desirable to allow recovery of the full productive capacity of the site.
Effect of soil on wind damage Examination of soil pits indicated that the soils permit deep rooting and generally lack compact layers within the upper 2 m. Compact layers were observed only in two of the 11 pits examined. In all cases, roots were observed below 2 m, indicating firm anchorage. This may, to some extent, explain the low incidence of wind damage by uprooting (Table 2) even though most of the damage occurred during the rainy season when soils were wet. Windthrow by uprooting is more likely on shallow soils or soils with impeded drainage (Day, 1949; Busby, 1965; Wendelken, 1966; Mackenzie, 1974; Cremer et al., 1982). Such soils are not common at Kiwira. The volcanic soils at Kiwira are free draining, and this is enhanced by the sloping ground, which provides topographical drainage.
Topography and elevation in relation to wind damage Topography plays a role in determining the extent of windthrow. The land at Kiwira is rolling, with gentle to very steep slopes and valleys. The unevenness of the configuration of the land would be expected to reduce the seriousness of wind damage (Busby, 1965 ). However, despite the unevenness of land, wind damage at the study site is high. This is possibly due to the concentration and local acceleration of wind along valleys as a result of the wind being forced to change direction within the plantations and the resulting damage occurring randomly along the zone of the new wind direction. Indeed, most damage was observed along or near the sides of valleys. This is in agreement with the literature on the influence of topography on wind damage (Ruth and Yoder, 1953; Busby, 1965; Mackenzie, 1974).
Management recommendations Light thinning or absence of thinning is regarded as having contributed greatly to wind damage, as this has led to overstocking and increases in H/D ratios. Adopting a wider initial spacing ( 3 m × 3 m) would reduce the cost of thinning, as there would be fewer thinnings than the present four (Malimbwi et al., 1992). Further, early and heavy thinnings would increase wind-firm-
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e.g.T. Munishi, S.A.O. Chamshama / Forest Ecology and Management 63 (1994) 13-21
ness. With Pinus patula at a spacing of 3 m × 3 m it is even possible to practise a no-thinning regime while maintaining the production of large sawlogs at a rotation age of 25 years (Malimbwi, 1987 ). Acknowledgements
We thank Sokoine University of Agriculture for funding this study, the management of Kiwira Forest Project for their assistance during data collection and Dr. J. Deans for sending us some of the literature and for comments on the manuscript. This paper is based on a special project report submitted by the senior author in partial fulfilment of the requirements for the B.Sc. (For.) degree. References Blackburn, P.S. and Petty, J.A., 1988. Theoretical calculations of the influence of spacing on stand stability. Forestry, 61: 235-244. Blackburn, P., Petty, J.A. and Miller, K.F., 1988. An assessment of the static and dynamic factors involved in windthrow. Forestry, 61:29-43. Booth, T.C., 1974. Silvicultural management of high risk forests in Great Britain. Irish For., 31: 145-153. Bouchon, J., 1986. Susceptibility of different conifer species to blow down. Proceedings of the Workshop on Minimizing Wind Damage to Coniferous Stands. Commission of the European Communities/Danish Forest Experiment Station, l-~venholm Castle, Denmark, pp. 33-34. Bryndum, H., 1986. Influence of silvicultural treatment of crops on the risk of windblow. Proceedings of the Workshop on Minimizing Wind Damage to Coniferous Stands. Commission of the European Communities/Danish Forest Experiment Station, l_4venholm Castle, Denmark, pp. 35. Busby, J.A., 1965. Studies on the stability of conifer stands. Scott. For., 19: 87-102. Chamshama, S.A.O. and Philip, M.S., 1980. Thinning Pinus patula plantation at Sao Hill, Southern Tanzania. Record 13. Division of Forestry, University of Dar-es-Salaam, Morogoro, Tanzania, 16 pp. Coutts, M.P., 1983. Root architecture and tree stability. Plant Soil, 71: 171-188. Cremer, K.W., Borough, C.J., McKinnell, F.H. and Carter, P.R., 1982. Effects of stocking and thinning on wind damage in plantations. NZ J. For. 12: 244-268. Day, W.R., 1949. The soil conditions which determine wind throw in forests. Forestry, 23: 9095. Deans, J.D., 1983. Distribution of thick roots in Sitka spruce plantation 16 years after planting. Scott. For. 37: 17-31. Deans, J.D. and Ford, E.D., 1983. Modelling root structure and stability. Plant Soil, 71:189195. Hendrick, E., 1986. Appropriate cultivation and drainage techniques for sites liable to windthrow. Proceedings of the Workshop on Minimizing Wind Damage to Coniferous Stands. Commission of the European Communities/Danish Forest Experiment Station, L~venholm Castle, Denmark, pp. 30-32. Irvine, R.E., 1970. The significance of windthrow for Pinus radiata management in the Nelson District, New Zealand. NZ J. For. Res., 15: 57-68.
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Lohmander, P. and Helles, F., 1987. Windthrow probability as a function of stand characteristics and shelter. Scand. J. For. Res., 2: 227-238. Mackenzie, R.F., 1974. Some factors influencing the stability of Sitka in Northern Ireland. Ir. For. 31: 110-129. Miller, K.F., 1985. Windthrow hazard classification. Forestry Commission Leaflet 85, 14 pp. Milne, R., 1991. Dynamics of swaying ofPicea sitchensis. Tree Physiol., 9: 383-399. Milne, R. and Blackburn, P., 1989. The elasticity and vertical distribution of trees within stems ofPicea sitchensis. Tree Physiol., 5:195-205. Neckelmann, J., 1986. Choice in species, topping and high pruning as means to prevent windthrow in permanent or recent stand edges. Proceedings of the Workshop on Minimizing Wind Damage to Coniferous Stands. Commission of the European Communities/Danish Forest Experiment Station, l_~venholm Castle, Denmark, pp. 42-45. Persson, P., 1975. Windthrow in forests. Royal Research Note 36, Department of Forest Yield Research, Royal College of Forestry, Stockholm, pp. 234-248. Petty, J.A. and Swain, C., 1985. Factors influencing stem breakage of conifers in high winds. Forestry, 58: 75-84. Ruth, R.H. and Yoder, R.A., 1953. Reducing wind damage in the forests of the Oregon Coast range. US For. Serv. Pac. Northwest For. Range Exp. Stn. Res. Pap. 7, 30 pp. Savill, P.S., 1976. The effects of drainage and ploughing of surface water glays on rooting and windthrow of Sitka spruce in Northern Ireland. Forestry, 49:133-14. Wendelken, W.J., 1966. Eyrewell forest: Search for stable management. NZ J. For., 11: 43-65.