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Nutrient cycling in tropical rainforests: Implications for management and sustained yield
Forest Ecology and Management, 22 (1987) 297-300 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
297
Short Communication
Nutrient Cycling in Tropical Rainforests: Implications for Management and Sustained Yield GUY HILTON Department of Forestry, University of Technology, Lae (Papua New Guinea)
(Accepted 14 April 1987)
ABSTRACT Hilton, G., 1987. Nutrient cycling in tropical rainforest:implications for management and sustained yield.For. Ecol. Manage., 22: 297-300.
It is now understood that much of the world's tropical rainforest exists on very poor soils, only able to do so by retaining a high proportion of the available nutrients within the biomass. The proportion which is lost is replaced through nutrients imported to the site largely in rainfall. The disturbance of the rainforests by logging can disrupt this mechanism and cause further impoverishment of the site.As the better-qualityforestis consumed and poorer areas are exploited,techniques have to be developed to reduce disturbance and conserve the nutrient regimes within the forest.
The species diversity, structural complexity and apparent luxuriance characteristic of tropical rainforests give the impression that they occupy goodquality fertile sites. It is now recognised, however, that rainforest soils tend to be of less-than-average fertility, which can be attributed to the very high rainfalls and temperatures experienced in those areas (Fournier, 1978). These factors encourage the rapid breakdown and incorporation of organic matter derived from the forest itself, mostly in the form of leaf- and litter-fall. The high rainfall means that many nutrients are leached out of the soil very quickly ( Stark, 1971a). In response to this, the distribution of nutrients within a tropical rainforest is markedly different to that of other forest types. In most forests the majority of the nutrients is accumulated in the soil (Westman and Rogers, 1977; Miller, 1979). In a tropical rainforest a high proportion of the nutrients are found in the biomass, mostly in bark and foliage (Stark, 1971a; Ruddle and Manshard, 1981 ), with an equal proportion occurring in the litter. This can be thought of as part of the biomass, since it may never be incorpo-
0378-1127/87/$03.50
© 1987Elsevier Science Publishers B.V.
298 rated into the soil but acted on directly by mycorrhizal hyphae and nutrients transferred directly back into the vegetation ( Stark, 1971b). This mechanism enables rainforest to survive on very poor and even almost sterile soils (Biswas, 1984). However, no system is ever entirely closed and nutrients will be continually escaping from it. The tea-coloured water of many rainforest rivers is due to the organic materials leaking from the forests, through their free-draining and highly leached soils (Klinge and Ohle, 1962). This suggests that many of the usual soil processes of mineral replacement are unable to function normally and that there are external nutrient sources for the forest. There are two possible mechanisms for this: the deposition of airborne dust or in solution in rainwater. The dust-free air and high rainfalls and humidities characteristic of tropical rainforests indicates that rainwater is the more important. There have been a number of studies into the nutrient content of rainwater (Miller, 1979; Jordan et al., 1980; Hingston et al., 1981 ) including my own preliminary work in Papua New Guinea ( Hilton, 1985). All show that rainfall can make a considerable contribution to forest nutrition and that nutrients removed from the forest by felling can be replaced. However, replacement depends entirely on their being a residual forest to capture the nutrients. Neither food crops nor grass seem to be as efficient at nutrient capture as trees, probably because they simply lack the physical size. Unable to make use of rain as a source of nutrients or to retain those in the soil, an area of cleared forest rapidly loses its fertility. There is so little hard information available that no comparison can be made between past and present yields in those areas of rainforest which are being logged for the second time. Although the growing stock may have been accurately assessed before the first harvest the rate of growth remains unknown. Thus it is impossible to see whether this nutrient-capture process is working or how productivity has been affected, if it has, by logging. However, a much more fundamental change which will affect future yields from the forests is the large area of lowland rainforest being cleared for other uses, often agriculture. These areas contain some of the most fertile and productive sites and, as in Malaysia (Lee, 1982), forestry has been reduced to the less-fertile sites and slopes. On such poorer sites replacement of nutrients removed through logging becomes more critical and reliance on external sources more important. This has serious implications for the way in which these less-robust forests are managed, particularly in view of the trend to increase productivity by shortening rotations. There are several aspects of forest operations which have to be carefully considered. There is a trend to take a wider range of species and to take smallerdiameter trees, both of which result in the removal of more trees per hectare. These improve operational efficiency and forest utilization and favour the lesstolerant and faster-growing species, thus allowing shorter rotations. Such species require less of a residual stand above them for successful regeneration and
299
so offer much less protection to the forest floor. Increased exposure will change the nature of litter decomposition and the reduced residual stand will mean fewer roots to tap that nutrient store. There will be a greatly increased leakage of nutrients from the litter to the soil and out of the forest altogether. To overcome these problems it may be necessary to modify present silvicultural systems or even devise new ones. Systems like clearfelling and the more-open forms of shelterwood which greatly increase exposure will have to be carefully monitored for their impact on site fertility. This could also encourage the use of the selection system and the more-closed forms of shelterwood which simulate natural forest gap dynamics. Such techniques as would help to conserve the nutrient store within the litter while debarking the logs in the forest would retain another valuable nutrient reserve on site. However, as well as conserving nutrients within the forest, methods would have to be devised to protect the roots of the residual stand which tap them. Loggers will have to be more closely controlled and be more highly skilled so as to minimize the disturbance they cause. Serious damage to the remaining standing trees is often caused when skidding logs from stump to roadside. Much damage can be done to the soil itself by skidding and by the movement of heavy equipment over the site. To reduce this, changes will have to be made to many logging practices and particularly to extraction methods. More use will have to be made of winches, to limit the movement of tractors on the site. Some form of skyline-winch system, with the logs lifted right off the ground, would be ideal but these tend to be too expensive and too inflexible at present. Any solution would, however, be expensive and require far more of a commitment to the site than is usual at the moment. The present systems of relatively short-term concessions and leases mean that loggers seldom feel much responsibility for the forest as they will not benefit from any investment they might make. What is needed is a way to make responsible forest management and harvesting profitable and worthwhile. In order to maintain the productivity of much of what is presently rainforest it is essential that it remains forest. With a better understanding of the way nutrients are scavenged, stored and cycled within a rainforest it should be possible to manage those forests more efficiently. Increased management efficiency should increase both the productivity and the value of the forest, making it less likely that they would be cleared for some other use. REFERENCES Biswas, A.K. (Editor), 1984. Climate and Development. Tycooly/UNEP, Dublin, 13 pp. Fournier, F., 1978. Water balance and soils. In: Tropical Forest Ecosystems. UNESCO, Paris, pp. 256-269. Hilton, R.G.B., 1985. Nutrients in rainwater: Do they make a significant contribution? Klinkii, 1: 106-109.
300 Hingston, F.J., Dimmock, G.M. and Turton, A.G., 1981. Nutrient distribution in a jarrah (Eucalypus marginata Donn ex Sin.) ecosystem in south-west Western Australia. For. Ecol. Manage., 3: 183-207. Jordan, C., Golley, F., Hall, J. and Hall, J., 1980. Nutrient scavenging of rainfall by the canopy of an Amazonian rain forest. Biotropica, 12: 61-66. Klinge, H. and Ohle, W., 1964. Chemical properties of rivers in the Amazonian area in relation to soil conditions. Verh. Int. Vet. Limnol., 15: 1067-1076. Lee, H.S., 1982. The development of silvicultural systems in the hill forests of Malaysia. Malay. For., 1: 1-9. Miller, H.G., 1979. The nutrient budget of even-aged forests. In: E.D. Ford, D.C. Malcolm and J. Atterson (Editors), The Ecology of Even-Aged Forest Plantations. Proc. IUFRO Conference, 1978, Cambridge. Institute of Terrestial Ecology, pp. 221-245. Ruddle, K. and Manshard, W., 1981. Renewable Natural Resources and the Environment. United Nations University/Tycooly, Dublin, pp. 23-26. Stark, N., 1971a. Nutrient cycling. I: nutrient distribution in some Amazonia soils. Int. J. Trop. Ecol., 12: 24-50. Stark, N., 1971b. Nutrient cycling. II: nutrient distribution in Amazonian vegetation. Int. J. Trop. Ecol., 12: 177-201. Westman, W.E. and Rogers, R.W., 1977. Nutrient stocks in a subtropical eucalypt forest, North Stradbroke Island. Aust. J. Ecol., 2: 447-460.
297
Short Communication
Nutrient Cycling in Tropical Rainforests: Implications for Management and Sustained Yield GUY HILTON Department of Forestry, University of Technology, Lae (Papua New Guinea)
(Accepted 14 April 1987)
ABSTRACT Hilton, G., 1987. Nutrient cycling in tropical rainforest:implications for management and sustained yield.For. Ecol. Manage., 22: 297-300.
It is now understood that much of the world's tropical rainforest exists on very poor soils, only able to do so by retaining a high proportion of the available nutrients within the biomass. The proportion which is lost is replaced through nutrients imported to the site largely in rainfall. The disturbance of the rainforests by logging can disrupt this mechanism and cause further impoverishment of the site.As the better-qualityforestis consumed and poorer areas are exploited,techniques have to be developed to reduce disturbance and conserve the nutrient regimes within the forest.
The species diversity, structural complexity and apparent luxuriance characteristic of tropical rainforests give the impression that they occupy goodquality fertile sites. It is now recognised, however, that rainforest soils tend to be of less-than-average fertility, which can be attributed to the very high rainfalls and temperatures experienced in those areas (Fournier, 1978). These factors encourage the rapid breakdown and incorporation of organic matter derived from the forest itself, mostly in the form of leaf- and litter-fall. The high rainfall means that many nutrients are leached out of the soil very quickly ( Stark, 1971a). In response to this, the distribution of nutrients within a tropical rainforest is markedly different to that of other forest types. In most forests the majority of the nutrients is accumulated in the soil (Westman and Rogers, 1977; Miller, 1979). In a tropical rainforest a high proportion of the nutrients are found in the biomass, mostly in bark and foliage (Stark, 1971a; Ruddle and Manshard, 1981 ), with an equal proportion occurring in the litter. This can be thought of as part of the biomass, since it may never be incorpo-
0378-1127/87/$03.50
© 1987Elsevier Science Publishers B.V.
298 rated into the soil but acted on directly by mycorrhizal hyphae and nutrients transferred directly back into the vegetation ( Stark, 1971b). This mechanism enables rainforest to survive on very poor and even almost sterile soils (Biswas, 1984). However, no system is ever entirely closed and nutrients will be continually escaping from it. The tea-coloured water of many rainforest rivers is due to the organic materials leaking from the forests, through their free-draining and highly leached soils (Klinge and Ohle, 1962). This suggests that many of the usual soil processes of mineral replacement are unable to function normally and that there are external nutrient sources for the forest. There are two possible mechanisms for this: the deposition of airborne dust or in solution in rainwater. The dust-free air and high rainfalls and humidities characteristic of tropical rainforests indicates that rainwater is the more important. There have been a number of studies into the nutrient content of rainwater (Miller, 1979; Jordan et al., 1980; Hingston et al., 1981 ) including my own preliminary work in Papua New Guinea ( Hilton, 1985). All show that rainfall can make a considerable contribution to forest nutrition and that nutrients removed from the forest by felling can be replaced. However, replacement depends entirely on their being a residual forest to capture the nutrients. Neither food crops nor grass seem to be as efficient at nutrient capture as trees, probably because they simply lack the physical size. Unable to make use of rain as a source of nutrients or to retain those in the soil, an area of cleared forest rapidly loses its fertility. There is so little hard information available that no comparison can be made between past and present yields in those areas of rainforest which are being logged for the second time. Although the growing stock may have been accurately assessed before the first harvest the rate of growth remains unknown. Thus it is impossible to see whether this nutrient-capture process is working or how productivity has been affected, if it has, by logging. However, a much more fundamental change which will affect future yields from the forests is the large area of lowland rainforest being cleared for other uses, often agriculture. These areas contain some of the most fertile and productive sites and, as in Malaysia (Lee, 1982), forestry has been reduced to the less-fertile sites and slopes. On such poorer sites replacement of nutrients removed through logging becomes more critical and reliance on external sources more important. This has serious implications for the way in which these less-robust forests are managed, particularly in view of the trend to increase productivity by shortening rotations. There are several aspects of forest operations which have to be carefully considered. There is a trend to take a wider range of species and to take smallerdiameter trees, both of which result in the removal of more trees per hectare. These improve operational efficiency and forest utilization and favour the lesstolerant and faster-growing species, thus allowing shorter rotations. Such species require less of a residual stand above them for successful regeneration and
299
so offer much less protection to the forest floor. Increased exposure will change the nature of litter decomposition and the reduced residual stand will mean fewer roots to tap that nutrient store. There will be a greatly increased leakage of nutrients from the litter to the soil and out of the forest altogether. To overcome these problems it may be necessary to modify present silvicultural systems or even devise new ones. Systems like clearfelling and the more-open forms of shelterwood which greatly increase exposure will have to be carefully monitored for their impact on site fertility. This could also encourage the use of the selection system and the more-closed forms of shelterwood which simulate natural forest gap dynamics. Such techniques as would help to conserve the nutrient store within the litter while debarking the logs in the forest would retain another valuable nutrient reserve on site. However, as well as conserving nutrients within the forest, methods would have to be devised to protect the roots of the residual stand which tap them. Loggers will have to be more closely controlled and be more highly skilled so as to minimize the disturbance they cause. Serious damage to the remaining standing trees is often caused when skidding logs from stump to roadside. Much damage can be done to the soil itself by skidding and by the movement of heavy equipment over the site. To reduce this, changes will have to be made to many logging practices and particularly to extraction methods. More use will have to be made of winches, to limit the movement of tractors on the site. Some form of skyline-winch system, with the logs lifted right off the ground, would be ideal but these tend to be too expensive and too inflexible at present. Any solution would, however, be expensive and require far more of a commitment to the site than is usual at the moment. The present systems of relatively short-term concessions and leases mean that loggers seldom feel much responsibility for the forest as they will not benefit from any investment they might make. What is needed is a way to make responsible forest management and harvesting profitable and worthwhile. In order to maintain the productivity of much of what is presently rainforest it is essential that it remains forest. With a better understanding of the way nutrients are scavenged, stored and cycled within a rainforest it should be possible to manage those forests more efficiently. Increased management efficiency should increase both the productivity and the value of the forest, making it less likely that they would be cleared for some other use. REFERENCES Biswas, A.K. (Editor), 1984. Climate and Development. Tycooly/UNEP, Dublin, 13 pp. Fournier, F., 1978. Water balance and soils. In: Tropical Forest Ecosystems. UNESCO, Paris, pp. 256-269. Hilton, R.G.B., 1985. Nutrients in rainwater: Do they make a significant contribution? Klinkii, 1: 106-109.
300 Hingston, F.J., Dimmock, G.M. and Turton, A.G., 1981. Nutrient distribution in a jarrah (Eucalypus marginata Donn ex Sin.) ecosystem in south-west Western Australia. For. Ecol. Manage., 3: 183-207. Jordan, C., Golley, F., Hall, J. and Hall, J., 1980. Nutrient scavenging of rainfall by the canopy of an Amazonian rain forest. Biotropica, 12: 61-66. Klinge, H. and Ohle, W., 1964. Chemical properties of rivers in the Amazonian area in relation to soil conditions. Verh. Int. Vet. Limnol., 15: 1067-1076. Lee, H.S., 1982. The development of silvicultural systems in the hill forests of Malaysia. Malay. For., 1: 1-9. Miller, H.G., 1979. The nutrient budget of even-aged forests. In: E.D. Ford, D.C. Malcolm and J. Atterson (Editors), The Ecology of Even-Aged Forest Plantations. Proc. IUFRO Conference, 1978, Cambridge. Institute of Terrestial Ecology, pp. 221-245. Ruddle, K. and Manshard, W., 1981. Renewable Natural Resources and the Environment. United Nations University/Tycooly, Dublin, pp. 23-26. Stark, N., 1971a. Nutrient cycling. I: nutrient distribution in some Amazonia soils. Int. J. Trop. Ecol., 12: 24-50. Stark, N., 1971b. Nutrient cycling. II: nutrient distribution in Amazonian vegetation. Int. J. Trop. Ecol., 12: 177-201. Westman, W.E. and Rogers, R.W., 1977. Nutrient stocks in a subtropical eucalypt forest, North Stradbroke Island. Aust. J. Ecol., 2: 447-460.