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Carbon storage potential of short rotation tropical tree plantations
Forest Ecology and Management, 50 ( 1 9 9 2 ) 3 I - 4 1 Elsevier Science Publishers B.V., A m s t e r d a m
31
Carbon storage potential of short rotation tropical tree plantations Paul Schroeder Man Tech Environmental Technology, US EPA Environmental Research Laboratory. 200 S W 35th St., Corvallis, OR 97333, USA (Accepted 21 May 1991 )
ABSTRACT Schroeder, P., 1992. Carbon storage potential o f short rotation tropical tree plantations, For. Ecol. Manage., 50: 31-41. Forests are a major sink for carbon and play an important role in the global carbon cycle. Not only do forests contain huge a m o u n t s o f carbon, they exchange it very actively with the atmosphere. Expanding the world's forests, therefore, may present an opportunity to increase the terrestrial carbon sink, and slow the increase in atmospheric CO2 concentration. The tropical zones o f the world seem particularly attractive for forestation because o f the high rates o f productivity that can potentially be attained there, and because there appear to be large areas o f land that would benefit from tree planting. The analysis described here examines the carbon storage potential o f short rotation tropical tree plantations in particular. Mean long-term carbon storage over multiple rotations was calculated for several c o m m o n l y grown species. Rotation length, and hence the potential to accumulate biomass, is shown to be a key factor in the ability of plantations to remove carbon from the atmosphere over the long-term.
INTRODUCTION
A major focus of research in the environmental sciences for the decade of the nineties will be the likelihood and potential magnitude of global climate change. While the debate continues over the possibilities for future changes in climate, the phenomenon which first raised concerns about climate change, increasing atmospheric carbon dioxide, is now a well-accepted fact. The many possible responses to the perceived threat from increasing atmospheric CO2 fall into two broad classes: those that reduce the emission of CO2 into the atmosphere, and those that remove CO2 from the atmosphere and store it on land or in the oceans. Forests are a major sink for carbon and play an important role in the global carbon cycle. Not only do forests contain huge amounts of carbon, they exC o r r e s p o n d e n c e to: P. Schroeder, M a n T e c h E n v i r o n m e n t a l Technology, U S E P A E n v i r o n m e n tal Research Laboratory, 200 SW 35th St., Corvallis, O R 97333, USA.
© 1992 Elsevier Science Publishers B.V. All fights reserved 0 3 7 8 - 1 1 2 7 / 9 2 / $ 0 5 . 0 0
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change it very actively with the atmosphere. On average the equivalent of the entice CO2 content of the atmosphere passes through the earth's terrestrial vegetation every 7 years, and about 70% of the entire exchange occurs through forest ecosystems (Waring and Schlesinger, 1985). Because this exchange is so active, expanding the world's forests could present an opportunity to increase the terrestrial carbon sink, and slow the increase in atmospheric CO2 concentration. A number of analyses have been published that assess the feasibility and potential of forestation at least to slow the rise of atmospheric CO2 (Dyson, 1977; Marland, 1988, 1989; Sedjo, 1989a, 1989b; Sedjo and Solomon, 1989; Schroeder and Ladd, 1991 ). (Forestation is defined to include both reforestation, planting trees on a site that was recently in forest, and afforestation, planting trees on a site that has not been in forest for 50 years or more, if ever. ) These studies present somewhat different results, but they all arrive at essentially the same conclusion: a very large area of new forest is required to have a significant effect on the concentration of CO2 in the atmosphere. Most of these published reports also acknowledge the temporary nature of the forestation option. When trees and forests are in their active growth phase, they remove carbon from the atmosphere. As they age, however, their growth slows and eventually stops, and they are no longer a net sink of carbon (although they continue to store it). Several studies of forestation potential also suggest that the tropics may offer a good opportunity to fix and store large amounts of carbon, and thereby reduce the area required to store a given amount of carbon (Marland, 1988; Myers, 1989; Sedjo, 1989a, 1989b; Schroeder and Ladd, 1991 ). Because of potentially very high growth rates, Marland (1988) estimated that the area required to capture annual carbon emissions from fossil fuel combustion worldwide could be reduced by 25% if forestation efforts were centered in the tropics. There also appears to be a very large area of land available for forestation in the tropics. The most widely cited study on land availability in the tropics (Grainger, 1988) estimated that the tropics contained over 2 billion ha of depleted or degraded land, and of those, 758 million ha were once forested and could theoretically be planted with trees (Table 1 ). These consist of 137 million ha of logged tropical moist forests, 203 million ha of forest fallow of various kinds in the humid tropics, an estimated 87 million ha of deforested watershed areas, and 331 million ha of rainfed and irrigated croplands in need of rehabilitation in the arid and semi-arid zones. All are areas that are not only suitable for trees, but would be improved by the planting of trees. Indeed, local motives for tree planting will more likely be soil improvement and conservation, rather than carbon storage. However, there is a dilemma here in that many of the well-known and commonly grown tropical plantation species are relatively short-lived, and are grown on rotations of less than 20 years. When a stand is cut, much, or per-
SHORT ROTATIONTROPICALTREE PLANTATIONS
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TABLE I Areas of tropical land potentially available for reforestation (ha 103 ) Region
Logged forests
Forest fallow
Deforested watersheds
Descrtified drylands
All lands
A~ica Asia Latin America Total
38956 53574 43993 136523
59292 58770 84754 202816
3126 56494 27230 86850
740900 748000 162000 1650900
842274 916838 317977 2077089
Source: Grainger, 1988.
haps all, of its carbon returns to the atmosphere within a short time. Some long-lived species, both tropical and temperate, can be grown for a single rotation of 50-100 years. If forestation is considered as a temporary option to store carbon for several decades, the amount of carbon removed from the atmosphere over the decades-long rotation (but prior to harvest) is the same as the standing crop of carbon at maturity, and the average amount of carbon removed annually is equivalent to the mean annual growth increment (MAI). Because of regular and frequent cutting, however, this is not true for short rotation plantations over a similar extended period of time (e.g. 100 years). The MAI for a tropical plantation may be very high, but that carbon is stored for a relatively short time. What is required is an estimate of the amount of carbon that can be stored indefinitely, or at least over many decades and many rotations. Concentrating on growth rate and MAI that may be sustained only for a few years can be misleading (Schroeder and Ladd, 1991 ). This paper will concentrate on short rotation plantations because they are widely used and sometimes suggested as a way to store carbon. It will: ( 1 ) demonstrate a simple, yet valid computational method to estimate long-term carbon storage for short rotation plantations; (2) estimate examples of carbon storage for plantations of selected common tropical plantation species; (3) examine the carbon storage potential of the areas of available land shown in Table 1. MATERIALS AND METHODS
Carbon storage and calculation Even though short rotation plantations may be cut frequently, they still represent an amount of carbon removed from the atmosphere and stored. If they are promptly replanted after harvest, plantations should always be covered with trees. The relevant estimate in terms of carbon cycle implications, therefore, is the average amount of carbon on-site over an indefinite number of rotations. Graham et al. (1990) used similar logic in assessing the potential
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for plantation forestry in Africa to store carbon. If we assume that the system is sustainable a n d there is no yield reduction in later rotations, this is the same as the average a m o u n t o f carbon on-site over the life of the stand for one full rotation. This calculation can be m a d e by s u m m i n g the carbon standing crop for every year in the rotation and dividing by the rotation length, the simple calculation of a mean:
Rotation
)
Z" C standing crop~
i=O ~¢C a r b o n storage =
R o t a t i o n length
(1)
This analysis assumes that at, or shortly after, harvest all stored carbon returns to the atmosphere. T h e implications o f this a s s u m p t i o n are explored in the Discussion section below. C a r b o n standing crop for any particular time can be e s t i m a t e d by starting with stem wood v o l u m e data a n d multiplying by wood density to obtain stem w o o d biomass. D a t a on stem w o o d v o l u m e are generally the most readily available of any p l a n t a t i o n d i m e n s i o n because it is the o u t p u t t h a t is m o s t frequently sought or utilized. A g e - d e p e n d e n t expansion factors have been published by Brown et al. ( 1 9 8 6 ) that account for biomass in other plant c o m p o n e n t s such as branches a n d roots. Total b i o m a s s can be converted to total carbon content by a s s u m i n g that biomass is a p p r o x i m a t e l y 50% carbon (Brown and Lugo, 1982 ). Carbor, ~tanding crop can then be calculated as: C standing crop, = S V D F O.5
(2)
W h e r e S V is the stem w o o d volume; D is the w o o d density; F is the age-dep e n d e n t expansion factor; a n d 0.5 is the constant p r o p o r t i o n of carbon. RESULTS
Examples for plantation species T h e above calculations were c o m p l e t e d for nine c o m m o n l y grown tropical plantation species (Table 2). T h e species were chosen to represent a wide range of e n v i r o n m e n t s and uses, a n d to c o r r e s p o n d generally to the categories o f available land in Table 1. Pinus caribaea, Leucaena spp. a n d Casuarina spp. represent moist conditions of logged forest a n d forest fallow. Pinus patula, Cupressus lusitanica a n d Acacia mearnsii represent m o n t a n e or u p l a n d watershed conditions. Cassia siamea, Acacia nilotica a n d Azadirachta indica represent dryland e n v i r o n m e n t s . This is certainly not an exhaustive list a n d other species could have been included, but these selections are good examples of different varieties of plantation species. They are also species for which data were available.
SHORT ROTATION TROPICAL TREE PL ANTATIONS
35
TABLE 2 Final yield, rotatiCa length, wood density and mean carbon storage potential over a rotation for selected tropical tree plantar,on species Species
Final yield (m 3 h a - i )
Rotation length (years)
Mean annual growth ( m 3 h a - t year -~)
Wood density (gem -3)
Mean carbon storage t C h a -~
Sources
Pinus caribaea
300
15
20
0.46
59
Y~: 3, 5 D2: I
Leucaena spp. Poor site
72
8
9
0.60
21
140
7
20
0.60
42
Y : D: Y : D:
7 6 6 6
Casuarina spp. Moderate site
140
10
14
0.83
55
Degraded site
50
10
5
0.83
21
Pin us patula
400
20
20
0.45
72
Cupressus lusitanica
340
20
17
0.43
57
Acacia mearnsii
250
10
25
0.60
78
Cassia siamea
1O0
l0
l0
0.58
28
Y : D: Y : D: Y : D: Y : D: Y : D: Y : D:
9 2 4 2 3,5 2 5,8 2 5 2 5 l
Acacia nilotica Moderate site
60
10
6
0.60
17
Degraded site
45
15
3
0.60
12
Azadirachta indica
40
8
5
0.52
8
Fuelwood crop
Y : 5 D :10 Y : 5 D :i0 Y: 5 D: 3
ty, source for yield and rotation information. 2D, source for wood density information. Sources: ( ! ) Brown and Lugo, 1984, ( 2 ) Chudnoff, 1979; (3) Evans, 1982, ( 4 ) FAO, 1985; ( 5 ) Pandey, 1983; ( 6 ) P o u n d and Cairo, 1983, (7) Ravilla, 1982; ( 8 ) T s c h i n k e l , 1972; ( 9 ) Vivekanandan, 1981; (10) as for Acacia mearnsii.
Data were obtained on stem wood volume yield, rotation length and wood density from various literature sources as cited in Table 2. Yield data were carefully selected primarily for sites of moderate quality. As indicated in the table, some yield information was also from poor or degraded site conditions, where available. This was an attempt to produce realistic estimates of carbon storage under the kinds of conditions that would most likely be encountered in the field. Since much of the land that is potentially available for reforestation is depleted or degraded to some extent, using data from very highly productive, good quality sites would result in an over-estimate of carbon storage potential.
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P. SCHROEDER
Estimates of final yield are more readily available for more species than functions of yield over the entire length of the rotation. However, calculating standing crop on an annual or periodic basis from estimates of final yield at rotation assumes a pattern of equal linear growth each year. In actuality, growth changes from year to year, being rapid in the earlier years and then slowing. This changing growth rate is responsible for the curvilinear relationship between yield and time that is familiar to foresters (Smith, 1962). Assuming that this relationship is linear probably results in a slight under-estimate of standing crop for any particular year. The assumption was made in order to increase the number of species included in the analysis. Table 2 illustrates the problem that can arise by trying to use mean annual growth rate to characterize carbon removals from the atmosphere over an extended period of time. Three entries in the table show the same annual growth rate. Pinus caribaea, Leucaena as a fuelwood crop, and Pinus patula are all capable of a mean annual growth rate of about 20 m a h a - ~ year- ~. At the same time, the ability of these three species to store carbon on a long-term basis is vastly different. Pinus patula can store over 70% more carbon than Leucaena, 72 tonnes of carbon ha-1 (tC ha -1 ) vs. 42 tC ha-~ (without even considering differences in wood density which would make this difference even greater). The reason for the differences in mean carbon storage between these three species is the length of time that a growth rate of 20 m 3 h a - ~yearcan be sustained, the rotation length. Pinus patula can sustain this rate over a period of 20 years, whereas Pinus caribaea sustains it for 15 years, and Leucaena for only about 7 years. The longer the rotation length, the higher the accumulation of biomass over time, and therefore the greater the mean carbon standing crop. This is the key concept to be kept in mind when considering plantation carbon dynamics. Rotation length and growth rate interact to determine storage. Growth rate alone cannot adequately characterize carbon storage potential.
Potential carbon storage in the tropics A first approximation of the amount of carbon that could on average be stored indefinitely in tropical plantations can be made by combining Tables I and 2. Each of the four general land types in Table 1 was multiplied by both the highest and lowest carbon storage values of the appropriate species in Table 2. For drylands, Grainger's (1988) figure of 331 million ha total area available for reforestation was used, although it does not appear directly in Table 1. The results are shown in Table 3. All ofthe forest fallow land in Table 1 could not be reforested without greatly disrupting shifting cultivation. The full area was used in the calculations for Table 3 to provide an upper boundary. These results indicate that somewhere between about 15 and 36 billion tons of carbon could be stored in tropical plantations (Table 3). If anthropogenic
SHORT ROTATIONTROPICALTREE PLANTATIONS
37
TABLE 3 Potential carbon storage in the tropics Land type
Logged forests Forest fallow Deforested watersheds Drylands Total
Area (ha 103 )
High' estimate carbon storage ( t C h a -~ )
Total carbon storage (tC 109)
Low 2 estimate carbon storage ( t C h a -= )
Total carbon storage (tC 109)
135523
59
8.06
21
2.87
202816
59
11.97
21
4.26
86850 331000
78 28
6.77 9.27 36.07
57 8
4.95 2.65 14.73
'Values from Table 2 for Pinus caribaea, Acacia mearnsii and Cassia siamea. 2Values from Table 2 for Leucaena spp. (poor site), Cupressus lusitanica and Azadirachta indica.
carbon emissions from burning fossil fuels are estimated to be in the range of 5-6 billion tons annually, this means that tropical plantations could store the equivalent of 2.5-7 years worth of such emissions. But, as stated earlier, we would not be looking to forestation to capture the entire fossil fuel emission. It would only be part of a more comprehensive response strategy. In that context, forestation in the tropics could store the equivalent of 10% of 25-70 years of anthropogenic carbon emissions at current emission rates. Another way to put carbon storage by short rotation plantations in perspective is to make a comparison with the carbon stored in natural forests. A typical closed tropical rainforest contains approximately 176 tC ha -~ (Brown and Lugo, 1984). This means that it would take approximately 3.0 ha of short rotation Pinus caribaea, with mean carbon storage of 59 tC h a - 1, to recapture and store an equivalent amount of carbon. Even if each hectare of tropical rainforest that is cut down could be replanted as a plantation, each hectare of tropical forest conversion would still represent a net release of carbon to the atmosphere of about 1 !7 tC ha -~ or more. In the mid-!980's, the rate of deforestation was greater than reforestation by a ratio of 10:1 (FAO, 1982). DISCUSSION
The results presented here should be considered in the light of a number of sources of uncertainty. The largest of these is, perhaps, the land areas available for reforestation presented in Table I. Although these estimates are among the best currently available, they were described by the author as "'tentative'" because the data on which they were based were not fully reliable (Grainger, 1988 ). The rapid pace of deforestation in the tropics further complicates the
38
P. SCHROEDER
task of quantifying land in need of reforestation. Better data are needed before more accurate estimates can be attempted. Another source of uncertainty is the plantation growth and yield data presented in Table 2. These are mean values that can be expected to vary with site conditions and genotype. Since Table 3 was derived from both Tables 1 and 2, those uncertainties persist in the estimates of total potential carbon storage by short rotation tropical plantations. Nonetheless, the values presented in Table 3 are useful because they give an indication of the potential scale of' carbon storage that has not been estimated before. The analysis described in this paper also depends on a number of key as= sumptions. One is that once a plantation is cut, all of its carbon returns to the atmosphere within a relatively short pedod~ of time. This does not account for any carbon that may go into durable products and thus be removed from the atmosphere for a longer time. A study of old-growth temperate forests (Harmon et al., 1990) showed that on average only about 42% of stem wood carbon was converted to products like lumber and plywood that have a life-span of more than 5 years. If stem wood represents only about 60% of total carbon (Brown et al., 1986), that means that only about 25% of total carbon goes into durable products. By their very nature, short rotation tropical plantation tree crops are less suited for lumber and plywood products than very large old-growth temperate trees. The primary uses for tropical plantation wood are pulp and paper products, from which most carbon is returned to the atmosphere via incineration. One way to increase the net carbon storage resulting from tropical plantations would be to increase the amount of wood going into some kind of durable product that would store carbon for many decades or more. For example, if 10% of the carbon in the final harvest of a Pinus caribaea plantation is converted to durable products, the mean carbon storage indicated in Table 3 would be doubled after six rotations, or about 90 years. If25% of carbon from the final harvest could be stored ove:- a long period of time, a doubling would take place after about two rotations, or 30 years. To take another example, carbon storage by Pinus patula would be doubled after about five rotations, or 100 years, if 10% of final harvest carbon goes into durable products. For 25% conversion to durable products, carbon storage doubling would occur after about two rotations, or 40 years. Achieving a 10-25% conversion of total carbon to durable product, however, could be very challenging. One useful product that is often suggested in this context is wood as a fuel to replace fossil fuel. The carbon benefit would be due to not burning fossil fuel and therefore not causing emissions. In tropical countries of the world, however, wood is already the predominant fuei source. It is the developed countries of the temperate zones that have the highest output of CO2 from fossil fuels. Because of the distances involved and the low-energy density of wood, it seems unlikely that substitute wood fuels will be shipped from the
SHORT ROTATION TROPICAL TREE PLANTATIONS
39
tropics to the temperate zones. However, fuels produced from wood, like methanol for instance, might find a place in the world market. Another key assumption is that all of the area under discussion will in fact be managed on short rotations. The intention ofthis analysis was to examine specifically the potential of short rotation plantations. In reality, some areas, for example watersheds in need of reforestation for conservation purposes, may be left uncut indefinitely. The carbon stored in these areas would remain out of the atmosphere for an extended period. Another alternative method for increasing carbon storage is to intentionally leave some of the area uncut, or to manage it on long rotations rather than short. For example, teak plantations ( Tectona grandis) are grown on rotations of 50 years or more. There is no a priori reason to choose short rotations. It may also be preferable to manage forests to produce multiple resources for use by local inhabitants (Peters et al., 1989). Thirdly, the mean carbon storage values in Table 2 represent potential net carbon storage only if we assume that plantations are installed on unvegetated bare ground. In reality, net carbon storage will be the difference between current on-site carbon and additional or replacement carbon resulting from growth of the plantation. That difference will be less than calculated carbon storage values presented here. Plantation trees would replace the vegetation on abandoned pastures and Imperata grasslands, but would possibly supplement woody vegetation on some forest fallows. The best choice in some cases, however, could be to allow natural regrowth of fallows without direct intervention. The land areas in Table 1 may be technically suitable for forestation, but they may not actually be available (another assumption), because they are in use as agricultural lands. They cannot be devoted to plantation forestry because they are needed to support local populations. In these cases, agroforestry practices, growing trees in conjunction with agricultural crops, may be a more appropriate land use. The carbon storage potential of agroforestry systems is presently unknown and represents a research need. Better estimates of land area and land use patterns are also needed. Problems related to land tenure and ownership can be reasons why technically suitable land is not reforested or used for agroforestry. People are understandably reluctant to invest scarce resources in land they do not own for a benefit that may not accrue for several years. Reforestation projects on common land or government land may fail because of local pressures from grazing and firewood removal. The concept of forestation as a method of storing atmospheric CO2 has social and economic facets that are at least as complex as the biological ones. The results presented here further emphasize the conclusion reached by several authors that the forestry option is not the sole solution to the CO2
40
P. SCHROEDER
problem (Marland, 1988; Sedjo, 1989b; Myers, 1989; Schroeder and Ladd, 1991 ). Even if the estimates of carbon storage potential in Table 3 were doubled or tripled, it would still only be possible to capture and store the equivalent of a relatively few years of emissions from fossil fuel burning. A comprehensive approach should include forestation because it can make a significant contribution, and because of the important non-carbon benefits that can be derived. An overall strategy, however, will also need to encompass a reduction of CO2 inputs into the atmosphere from both fossil fuels and deforestation. ACKNOWLEDGEMENTS
This research has been funded by the US Environmental Protection Agency. This document has been prepared at the EPA Environmental Research Laboratory in Corvallis, OR, through contract number 68-C8-0006 to ManTech Environmental Technology Inc. It has been subjected to the Agency's peer and administrative review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
REFERENCES Brown, S. and Lugo, A.E., 1982. The storage and production of organic matter in tropical forests and their role in the global carbon cycle. Biotropica, 14:161-187. Brown, S. and Lugo, A.E., 1984. Biomass of tropical forests: a new estimate based on forest volumes. Science, 223:1290-1293. Brown, S., Lugo, A.E. and Chapman, J., 1986. Biomass of tropical tree plantations and its implications for the global carbon budget. Can. J. For. Res., 16: 390-394. Chudnoff, M., 1979. Tropical Timbers of the World. USDA Forest Service, Forest Products Laboratory, Madison, WI, 750 pp. Dyson, F.J., 1977. Can we control the carbon dioxide in the atmosphere? Energy, 2:287-291. Evans, J., 1982. Plantation Forestry in the Tropics. Clarendon Press, Oxford, 472 pp. FAO, 1982. Tropical forest resources. FAO Forestry Paper No. 30, Food and Agriculture Organization of the United Nations, Rome, 106 pp. FAO, 1985. Intensive multiple use forest management in the tropics. FAO Forestry Paper No. 55, Food and Agriculture Organization of the United Nations, Rome, 180 pp. Graham, R.L., Perlack, R.D., Prasad, A.M.G., Ranney, J.W. and Waddle, D.B., 1990. Greenhouse gas emissions in sub-saharan Africa. Oak Ridge National Laboratory Report ORNL6640, 135 pp. Grainger, A, 1988. Estimating areas of degraded tropical lands requiring replenishment of forest cover. Int. Tree Crops J., 5: 31-61. Harmon, M.E., Ferrell, W.K. and Franklin, J.K., 1990. Effects on carbon storage of conversion of old-growth forests to young forests. Science, 247: 699-702. Marland, G., 1988. The prospect for solving the CO= problem through reforestation. US Department of Energy, Office of Energy Resources Report DOE/NBB-0082, 66 pp. Marland, G., 1989. The role of forests in addressing the CO2 greenhouse. In: J.C. White (Edi-
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for), Global climate change linkages: acid rain, air quality and stratospheric ozone. Elsevier Publishing, New York, 262 pp. Myers, N., 1989. The greenhouse effect: a tropical forestry response. Biomass, 18: 73-78. Pandey, D., 1983. Growth and yield of plantation species in the tropics. Forest Resources Division, Food and Agriculture Division of the United Nations, Rome, 115 pp. Peters, C.M., Gentry, A.H. and Mendelsohn, R.O., 1989. Valuation of an Amazonian rainforest. Nature, 339: 655-656. Pound, B. and Cairo, M.L., 1983. Leucaena: Its cultivation and uses. Overseas Development Administration, London, 287 pp. Ravilla, A.V., 1982. Wood-yield prediction models for Leucaena plantations in the PhiUipines. In: Proceedings of a Workshop held in Singapore, 23-26 November 1982. Organized by the Nitrogen Fixing Tree Association and the International Development Research Centre, Ottawa, 192 pp. Schroeder, P.E. and Ladd, L., 1991. Slowing the increase of atmospheric carbon dioxide: a biological approach. Climatic Change, 19: 283-290. Sedjo, R., 1989a. Forests to offset the greenhouse effect. J. For., 87:12-15. Sedjo, R., 1989b. Forests: a tool to moderate global warming? Environment 31: 14--20. Sedjo, R. and Solomon, A., 1989. Climate and forests. In: N.J. Rosenberg, W.E. Easterling, P.R. Crosson and J. Darmstadter (Editors), Greenhouse Warming: Abatement and Adaptation. Proceedings of a Workshop held in Washington, DC, June 14-15, 1988. Resources for the Future, Washington, DC, 185 pp. Smith, D.M., 1962. The Practice of SilvicuRure. John Wiley, New York, 578 pp. Tschinkel, H.M., 1972. Growth, Site Factors and Nutritional Status of Cupressus lusitanica Plantations in the Highlands of Colombia. Ph.D. dissertation, University of Hamburg, Hamburg, Germany. Vivekanandan, K., 1981. The status of Casuarina in Sri Lanka. In: S.J. Midgely, J.W. Turnbull and R.D. Johnson (Editors), Casuarina ecology, management, and utilization. In: Proceedings of an International Workshop, Canberra, Australia, 17-21 August 1981, CSIRO, Melbourne, 286 pp. Waring, R.H. and Schlesinger, W.H., 1985. Forest Ecosystems: Concepts and Management. Academic Press, New York, 340 pp.
31
Carbon storage potential of short rotation tropical tree plantations Paul Schroeder Man Tech Environmental Technology, US EPA Environmental Research Laboratory. 200 S W 35th St., Corvallis, OR 97333, USA (Accepted 21 May 1991 )
ABSTRACT Schroeder, P., 1992. Carbon storage potential o f short rotation tropical tree plantations, For. Ecol. Manage., 50: 31-41. Forests are a major sink for carbon and play an important role in the global carbon cycle. Not only do forests contain huge a m o u n t s o f carbon, they exchange it very actively with the atmosphere. Expanding the world's forests, therefore, may present an opportunity to increase the terrestrial carbon sink, and slow the increase in atmospheric CO2 concentration. The tropical zones o f the world seem particularly attractive for forestation because o f the high rates o f productivity that can potentially be attained there, and because there appear to be large areas o f land that would benefit from tree planting. The analysis described here examines the carbon storage potential o f short rotation tropical tree plantations in particular. Mean long-term carbon storage over multiple rotations was calculated for several c o m m o n l y grown species. Rotation length, and hence the potential to accumulate biomass, is shown to be a key factor in the ability of plantations to remove carbon from the atmosphere over the long-term.
INTRODUCTION
A major focus of research in the environmental sciences for the decade of the nineties will be the likelihood and potential magnitude of global climate change. While the debate continues over the possibilities for future changes in climate, the phenomenon which first raised concerns about climate change, increasing atmospheric carbon dioxide, is now a well-accepted fact. The many possible responses to the perceived threat from increasing atmospheric CO2 fall into two broad classes: those that reduce the emission of CO2 into the atmosphere, and those that remove CO2 from the atmosphere and store it on land or in the oceans. Forests are a major sink for carbon and play an important role in the global carbon cycle. Not only do forests contain huge amounts of carbon, they exC o r r e s p o n d e n c e to: P. Schroeder, M a n T e c h E n v i r o n m e n t a l Technology, U S E P A E n v i r o n m e n tal Research Laboratory, 200 SW 35th St., Corvallis, O R 97333, USA.
© 1992 Elsevier Science Publishers B.V. All fights reserved 0 3 7 8 - 1 1 2 7 / 9 2 / $ 0 5 . 0 0
32
P. SCHROEDER
change it very actively with the atmosphere. On average the equivalent of the entice CO2 content of the atmosphere passes through the earth's terrestrial vegetation every 7 years, and about 70% of the entire exchange occurs through forest ecosystems (Waring and Schlesinger, 1985). Because this exchange is so active, expanding the world's forests could present an opportunity to increase the terrestrial carbon sink, and slow the increase in atmospheric CO2 concentration. A number of analyses have been published that assess the feasibility and potential of forestation at least to slow the rise of atmospheric CO2 (Dyson, 1977; Marland, 1988, 1989; Sedjo, 1989a, 1989b; Sedjo and Solomon, 1989; Schroeder and Ladd, 1991 ). (Forestation is defined to include both reforestation, planting trees on a site that was recently in forest, and afforestation, planting trees on a site that has not been in forest for 50 years or more, if ever. ) These studies present somewhat different results, but they all arrive at essentially the same conclusion: a very large area of new forest is required to have a significant effect on the concentration of CO2 in the atmosphere. Most of these published reports also acknowledge the temporary nature of the forestation option. When trees and forests are in their active growth phase, they remove carbon from the atmosphere. As they age, however, their growth slows and eventually stops, and they are no longer a net sink of carbon (although they continue to store it). Several studies of forestation potential also suggest that the tropics may offer a good opportunity to fix and store large amounts of carbon, and thereby reduce the area required to store a given amount of carbon (Marland, 1988; Myers, 1989; Sedjo, 1989a, 1989b; Schroeder and Ladd, 1991 ). Because of potentially very high growth rates, Marland (1988) estimated that the area required to capture annual carbon emissions from fossil fuel combustion worldwide could be reduced by 25% if forestation efforts were centered in the tropics. There also appears to be a very large area of land available for forestation in the tropics. The most widely cited study on land availability in the tropics (Grainger, 1988) estimated that the tropics contained over 2 billion ha of depleted or degraded land, and of those, 758 million ha were once forested and could theoretically be planted with trees (Table 1 ). These consist of 137 million ha of logged tropical moist forests, 203 million ha of forest fallow of various kinds in the humid tropics, an estimated 87 million ha of deforested watershed areas, and 331 million ha of rainfed and irrigated croplands in need of rehabilitation in the arid and semi-arid zones. All are areas that are not only suitable for trees, but would be improved by the planting of trees. Indeed, local motives for tree planting will more likely be soil improvement and conservation, rather than carbon storage. However, there is a dilemma here in that many of the well-known and commonly grown tropical plantation species are relatively short-lived, and are grown on rotations of less than 20 years. When a stand is cut, much, or per-
SHORT ROTATIONTROPICALTREE PLANTATIONS
33
TABLE I Areas of tropical land potentially available for reforestation (ha 103 ) Region
Logged forests
Forest fallow
Deforested watersheds
Descrtified drylands
All lands
A~ica Asia Latin America Total
38956 53574 43993 136523
59292 58770 84754 202816
3126 56494 27230 86850
740900 748000 162000 1650900
842274 916838 317977 2077089
Source: Grainger, 1988.
haps all, of its carbon returns to the atmosphere within a short time. Some long-lived species, both tropical and temperate, can be grown for a single rotation of 50-100 years. If forestation is considered as a temporary option to store carbon for several decades, the amount of carbon removed from the atmosphere over the decades-long rotation (but prior to harvest) is the same as the standing crop of carbon at maturity, and the average amount of carbon removed annually is equivalent to the mean annual growth increment (MAI). Because of regular and frequent cutting, however, this is not true for short rotation plantations over a similar extended period of time (e.g. 100 years). The MAI for a tropical plantation may be very high, but that carbon is stored for a relatively short time. What is required is an estimate of the amount of carbon that can be stored indefinitely, or at least over many decades and many rotations. Concentrating on growth rate and MAI that may be sustained only for a few years can be misleading (Schroeder and Ladd, 1991 ). This paper will concentrate on short rotation plantations because they are widely used and sometimes suggested as a way to store carbon. It will: ( 1 ) demonstrate a simple, yet valid computational method to estimate long-term carbon storage for short rotation plantations; (2) estimate examples of carbon storage for plantations of selected common tropical plantation species; (3) examine the carbon storage potential of the areas of available land shown in Table 1. MATERIALS AND METHODS
Carbon storage and calculation Even though short rotation plantations may be cut frequently, they still represent an amount of carbon removed from the atmosphere and stored. If they are promptly replanted after harvest, plantations should always be covered with trees. The relevant estimate in terms of carbon cycle implications, therefore, is the average amount of carbon on-site over an indefinite number of rotations. Graham et al. (1990) used similar logic in assessing the potential
34
P. SCHROEDER
for plantation forestry in Africa to store carbon. If we assume that the system is sustainable a n d there is no yield reduction in later rotations, this is the same as the average a m o u n t o f carbon on-site over the life of the stand for one full rotation. This calculation can be m a d e by s u m m i n g the carbon standing crop for every year in the rotation and dividing by the rotation length, the simple calculation of a mean:
Rotation
)
Z" C standing crop~
i=O ~¢C a r b o n storage =
R o t a t i o n length
(1)
This analysis assumes that at, or shortly after, harvest all stored carbon returns to the atmosphere. T h e implications o f this a s s u m p t i o n are explored in the Discussion section below. C a r b o n standing crop for any particular time can be e s t i m a t e d by starting with stem wood v o l u m e data a n d multiplying by wood density to obtain stem w o o d biomass. D a t a on stem w o o d v o l u m e are generally the most readily available of any p l a n t a t i o n d i m e n s i o n because it is the o u t p u t t h a t is m o s t frequently sought or utilized. A g e - d e p e n d e n t expansion factors have been published by Brown et al. ( 1 9 8 6 ) that account for biomass in other plant c o m p o n e n t s such as branches a n d roots. Total b i o m a s s can be converted to total carbon content by a s s u m i n g that biomass is a p p r o x i m a t e l y 50% carbon (Brown and Lugo, 1982 ). Carbor, ~tanding crop can then be calculated as: C standing crop, = S V D F O.5
(2)
W h e r e S V is the stem w o o d volume; D is the w o o d density; F is the age-dep e n d e n t expansion factor; a n d 0.5 is the constant p r o p o r t i o n of carbon. RESULTS
Examples for plantation species T h e above calculations were c o m p l e t e d for nine c o m m o n l y grown tropical plantation species (Table 2). T h e species were chosen to represent a wide range of e n v i r o n m e n t s and uses, a n d to c o r r e s p o n d generally to the categories o f available land in Table 1. Pinus caribaea, Leucaena spp. a n d Casuarina spp. represent moist conditions of logged forest a n d forest fallow. Pinus patula, Cupressus lusitanica a n d Acacia mearnsii represent m o n t a n e or u p l a n d watershed conditions. Cassia siamea, Acacia nilotica a n d Azadirachta indica represent dryland e n v i r o n m e n t s . This is certainly not an exhaustive list a n d other species could have been included, but these selections are good examples of different varieties of plantation species. They are also species for which data were available.
SHORT ROTATION TROPICAL TREE PL ANTATIONS
35
TABLE 2 Final yield, rotatiCa length, wood density and mean carbon storage potential over a rotation for selected tropical tree plantar,on species Species
Final yield (m 3 h a - i )
Rotation length (years)
Mean annual growth ( m 3 h a - t year -~)
Wood density (gem -3)
Mean carbon storage t C h a -~
Sources
Pinus caribaea
300
15
20
0.46
59
Y~: 3, 5 D2: I
Leucaena spp. Poor site
72
8
9
0.60
21
140
7
20
0.60
42
Y : D: Y : D:
7 6 6 6
Casuarina spp. Moderate site
140
10
14
0.83
55
Degraded site
50
10
5
0.83
21
Pin us patula
400
20
20
0.45
72
Cupressus lusitanica
340
20
17
0.43
57
Acacia mearnsii
250
10
25
0.60
78
Cassia siamea
1O0
l0
l0
0.58
28
Y : D: Y : D: Y : D: Y : D: Y : D: Y : D:
9 2 4 2 3,5 2 5,8 2 5 2 5 l
Acacia nilotica Moderate site
60
10
6
0.60
17
Degraded site
45
15
3
0.60
12
Azadirachta indica
40
8
5
0.52
8
Fuelwood crop
Y : 5 D :10 Y : 5 D :i0 Y: 5 D: 3
ty, source for yield and rotation information. 2D, source for wood density information. Sources: ( ! ) Brown and Lugo, 1984, ( 2 ) Chudnoff, 1979; (3) Evans, 1982, ( 4 ) FAO, 1985; ( 5 ) Pandey, 1983; ( 6 ) P o u n d and Cairo, 1983, (7) Ravilla, 1982; ( 8 ) T s c h i n k e l , 1972; ( 9 ) Vivekanandan, 1981; (10) as for Acacia mearnsii.
Data were obtained on stem wood volume yield, rotation length and wood density from various literature sources as cited in Table 2. Yield data were carefully selected primarily for sites of moderate quality. As indicated in the table, some yield information was also from poor or degraded site conditions, where available. This was an attempt to produce realistic estimates of carbon storage under the kinds of conditions that would most likely be encountered in the field. Since much of the land that is potentially available for reforestation is depleted or degraded to some extent, using data from very highly productive, good quality sites would result in an over-estimate of carbon storage potential.
36
P. SCHROEDER
Estimates of final yield are more readily available for more species than functions of yield over the entire length of the rotation. However, calculating standing crop on an annual or periodic basis from estimates of final yield at rotation assumes a pattern of equal linear growth each year. In actuality, growth changes from year to year, being rapid in the earlier years and then slowing. This changing growth rate is responsible for the curvilinear relationship between yield and time that is familiar to foresters (Smith, 1962). Assuming that this relationship is linear probably results in a slight under-estimate of standing crop for any particular year. The assumption was made in order to increase the number of species included in the analysis. Table 2 illustrates the problem that can arise by trying to use mean annual growth rate to characterize carbon removals from the atmosphere over an extended period of time. Three entries in the table show the same annual growth rate. Pinus caribaea, Leucaena as a fuelwood crop, and Pinus patula are all capable of a mean annual growth rate of about 20 m a h a - ~ year- ~. At the same time, the ability of these three species to store carbon on a long-term basis is vastly different. Pinus patula can store over 70% more carbon than Leucaena, 72 tonnes of carbon ha-1 (tC ha -1 ) vs. 42 tC ha-~ (without even considering differences in wood density which would make this difference even greater). The reason for the differences in mean carbon storage between these three species is the length of time that a growth rate of 20 m 3 h a - ~yearcan be sustained, the rotation length. Pinus patula can sustain this rate over a period of 20 years, whereas Pinus caribaea sustains it for 15 years, and Leucaena for only about 7 years. The longer the rotation length, the higher the accumulation of biomass over time, and therefore the greater the mean carbon standing crop. This is the key concept to be kept in mind when considering plantation carbon dynamics. Rotation length and growth rate interact to determine storage. Growth rate alone cannot adequately characterize carbon storage potential.
Potential carbon storage in the tropics A first approximation of the amount of carbon that could on average be stored indefinitely in tropical plantations can be made by combining Tables I and 2. Each of the four general land types in Table 1 was multiplied by both the highest and lowest carbon storage values of the appropriate species in Table 2. For drylands, Grainger's (1988) figure of 331 million ha total area available for reforestation was used, although it does not appear directly in Table 1. The results are shown in Table 3. All ofthe forest fallow land in Table 1 could not be reforested without greatly disrupting shifting cultivation. The full area was used in the calculations for Table 3 to provide an upper boundary. These results indicate that somewhere between about 15 and 36 billion tons of carbon could be stored in tropical plantations (Table 3). If anthropogenic
SHORT ROTATIONTROPICALTREE PLANTATIONS
37
TABLE 3 Potential carbon storage in the tropics Land type
Logged forests Forest fallow Deforested watersheds Drylands Total
Area (ha 103 )
High' estimate carbon storage ( t C h a -~ )
Total carbon storage (tC 109)
Low 2 estimate carbon storage ( t C h a -= )
Total carbon storage (tC 109)
135523
59
8.06
21
2.87
202816
59
11.97
21
4.26
86850 331000
78 28
6.77 9.27 36.07
57 8
4.95 2.65 14.73
'Values from Table 2 for Pinus caribaea, Acacia mearnsii and Cassia siamea. 2Values from Table 2 for Leucaena spp. (poor site), Cupressus lusitanica and Azadirachta indica.
carbon emissions from burning fossil fuels are estimated to be in the range of 5-6 billion tons annually, this means that tropical plantations could store the equivalent of 2.5-7 years worth of such emissions. But, as stated earlier, we would not be looking to forestation to capture the entire fossil fuel emission. It would only be part of a more comprehensive response strategy. In that context, forestation in the tropics could store the equivalent of 10% of 25-70 years of anthropogenic carbon emissions at current emission rates. Another way to put carbon storage by short rotation plantations in perspective is to make a comparison with the carbon stored in natural forests. A typical closed tropical rainforest contains approximately 176 tC ha -~ (Brown and Lugo, 1984). This means that it would take approximately 3.0 ha of short rotation Pinus caribaea, with mean carbon storage of 59 tC h a - 1, to recapture and store an equivalent amount of carbon. Even if each hectare of tropical rainforest that is cut down could be replanted as a plantation, each hectare of tropical forest conversion would still represent a net release of carbon to the atmosphere of about 1 !7 tC ha -~ or more. In the mid-!980's, the rate of deforestation was greater than reforestation by a ratio of 10:1 (FAO, 1982). DISCUSSION
The results presented here should be considered in the light of a number of sources of uncertainty. The largest of these is, perhaps, the land areas available for reforestation presented in Table I. Although these estimates are among the best currently available, they were described by the author as "'tentative'" because the data on which they were based were not fully reliable (Grainger, 1988 ). The rapid pace of deforestation in the tropics further complicates the
38
P. SCHROEDER
task of quantifying land in need of reforestation. Better data are needed before more accurate estimates can be attempted. Another source of uncertainty is the plantation growth and yield data presented in Table 2. These are mean values that can be expected to vary with site conditions and genotype. Since Table 3 was derived from both Tables 1 and 2, those uncertainties persist in the estimates of total potential carbon storage by short rotation tropical plantations. Nonetheless, the values presented in Table 3 are useful because they give an indication of the potential scale of' carbon storage that has not been estimated before. The analysis described in this paper also depends on a number of key as= sumptions. One is that once a plantation is cut, all of its carbon returns to the atmosphere within a relatively short pedod~ of time. This does not account for any carbon that may go into durable products and thus be removed from the atmosphere for a longer time. A study of old-growth temperate forests (Harmon et al., 1990) showed that on average only about 42% of stem wood carbon was converted to products like lumber and plywood that have a life-span of more than 5 years. If stem wood represents only about 60% of total carbon (Brown et al., 1986), that means that only about 25% of total carbon goes into durable products. By their very nature, short rotation tropical plantation tree crops are less suited for lumber and plywood products than very large old-growth temperate trees. The primary uses for tropical plantation wood are pulp and paper products, from which most carbon is returned to the atmosphere via incineration. One way to increase the net carbon storage resulting from tropical plantations would be to increase the amount of wood going into some kind of durable product that would store carbon for many decades or more. For example, if 10% of the carbon in the final harvest of a Pinus caribaea plantation is converted to durable products, the mean carbon storage indicated in Table 3 would be doubled after six rotations, or about 90 years. If25% of carbon from the final harvest could be stored ove:- a long period of time, a doubling would take place after about two rotations, or 30 years. To take another example, carbon storage by Pinus patula would be doubled after about five rotations, or 100 years, if 10% of final harvest carbon goes into durable products. For 25% conversion to durable products, carbon storage doubling would occur after about two rotations, or 40 years. Achieving a 10-25% conversion of total carbon to durable product, however, could be very challenging. One useful product that is often suggested in this context is wood as a fuel to replace fossil fuel. The carbon benefit would be due to not burning fossil fuel and therefore not causing emissions. In tropical countries of the world, however, wood is already the predominant fuei source. It is the developed countries of the temperate zones that have the highest output of CO2 from fossil fuels. Because of the distances involved and the low-energy density of wood, it seems unlikely that substitute wood fuels will be shipped from the
SHORT ROTATION TROPICAL TREE PLANTATIONS
39
tropics to the temperate zones. However, fuels produced from wood, like methanol for instance, might find a place in the world market. Another key assumption is that all of the area under discussion will in fact be managed on short rotations. The intention ofthis analysis was to examine specifically the potential of short rotation plantations. In reality, some areas, for example watersheds in need of reforestation for conservation purposes, may be left uncut indefinitely. The carbon stored in these areas would remain out of the atmosphere for an extended period. Another alternative method for increasing carbon storage is to intentionally leave some of the area uncut, or to manage it on long rotations rather than short. For example, teak plantations ( Tectona grandis) are grown on rotations of 50 years or more. There is no a priori reason to choose short rotations. It may also be preferable to manage forests to produce multiple resources for use by local inhabitants (Peters et al., 1989). Thirdly, the mean carbon storage values in Table 2 represent potential net carbon storage only if we assume that plantations are installed on unvegetated bare ground. In reality, net carbon storage will be the difference between current on-site carbon and additional or replacement carbon resulting from growth of the plantation. That difference will be less than calculated carbon storage values presented here. Plantation trees would replace the vegetation on abandoned pastures and Imperata grasslands, but would possibly supplement woody vegetation on some forest fallows. The best choice in some cases, however, could be to allow natural regrowth of fallows without direct intervention. The land areas in Table 1 may be technically suitable for forestation, but they may not actually be available (another assumption), because they are in use as agricultural lands. They cannot be devoted to plantation forestry because they are needed to support local populations. In these cases, agroforestry practices, growing trees in conjunction with agricultural crops, may be a more appropriate land use. The carbon storage potential of agroforestry systems is presently unknown and represents a research need. Better estimates of land area and land use patterns are also needed. Problems related to land tenure and ownership can be reasons why technically suitable land is not reforested or used for agroforestry. People are understandably reluctant to invest scarce resources in land they do not own for a benefit that may not accrue for several years. Reforestation projects on common land or government land may fail because of local pressures from grazing and firewood removal. The concept of forestation as a method of storing atmospheric CO2 has social and economic facets that are at least as complex as the biological ones. The results presented here further emphasize the conclusion reached by several authors that the forestry option is not the sole solution to the CO2
40
P. SCHROEDER
problem (Marland, 1988; Sedjo, 1989b; Myers, 1989; Schroeder and Ladd, 1991 ). Even if the estimates of carbon storage potential in Table 3 were doubled or tripled, it would still only be possible to capture and store the equivalent of a relatively few years of emissions from fossil fuel burning. A comprehensive approach should include forestation because it can make a significant contribution, and because of the important non-carbon benefits that can be derived. An overall strategy, however, will also need to encompass a reduction of CO2 inputs into the atmosphere from both fossil fuels and deforestation. ACKNOWLEDGEMENTS
This research has been funded by the US Environmental Protection Agency. This document has been prepared at the EPA Environmental Research Laboratory in Corvallis, OR, through contract number 68-C8-0006 to ManTech Environmental Technology Inc. It has been subjected to the Agency's peer and administrative review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
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for), Global climate change linkages: acid rain, air quality and stratospheric ozone. Elsevier Publishing, New York, 262 pp. Myers, N., 1989. The greenhouse effect: a tropical forestry response. Biomass, 18: 73-78. Pandey, D., 1983. Growth and yield of plantation species in the tropics. Forest Resources Division, Food and Agriculture Division of the United Nations, Rome, 115 pp. Peters, C.M., Gentry, A.H. and Mendelsohn, R.O., 1989. Valuation of an Amazonian rainforest. Nature, 339: 655-656. Pound, B. and Cairo, M.L., 1983. Leucaena: Its cultivation and uses. Overseas Development Administration, London, 287 pp. Ravilla, A.V., 1982. Wood-yield prediction models for Leucaena plantations in the PhiUipines. In: Proceedings of a Workshop held in Singapore, 23-26 November 1982. Organized by the Nitrogen Fixing Tree Association and the International Development Research Centre, Ottawa, 192 pp. Schroeder, P.E. and Ladd, L., 1991. Slowing the increase of atmospheric carbon dioxide: a biological approach. Climatic Change, 19: 283-290. Sedjo, R., 1989a. Forests to offset the greenhouse effect. J. For., 87:12-15. Sedjo, R., 1989b. Forests: a tool to moderate global warming? Environment 31: 14--20. Sedjo, R. and Solomon, A., 1989. Climate and forests. In: N.J. Rosenberg, W.E. Easterling, P.R. Crosson and J. Darmstadter (Editors), Greenhouse Warming: Abatement and Adaptation. Proceedings of a Workshop held in Washington, DC, June 14-15, 1988. Resources for the Future, Washington, DC, 185 pp. Smith, D.M., 1962. The Practice of SilvicuRure. John Wiley, New York, 578 pp. Tschinkel, H.M., 1972. Growth, Site Factors and Nutritional Status of Cupressus lusitanica Plantations in the Highlands of Colombia. Ph.D. dissertation, University of Hamburg, Hamburg, Germany. Vivekanandan, K., 1981. The status of Casuarina in Sri Lanka. In: S.J. Midgely, J.W. Turnbull and R.D. Johnson (Editors), Casuarina ecology, management, and utilization. In: Proceedings of an International Workshop, Canberra, Australia, 17-21 August 1981, CSIRO, Melbourne, 286 pp. Waring, R.H. and Schlesinger, W.H., 1985. Forest Ecosystems: Concepts and Management. Academic Press, New York, 340 pp.