Professor Julian Evans
Julian Evans is a Professor
of Tropical Forestry at Imperial
College of London University and holds an honorary chair of
forestry at the University of Wales, Bangor. He graduated
from Bangor with his first degree in forestry in 1968 and since
has received a doctorate (PhD) in 1972 on sustainability of
plantations in Swaziland, Africa, and the senior doctorate
degree (D.Sc.) in 1988 for work on plantation sustainability.
Prof. Evans has worked widely in the tropics as an academic
and consultant, and is author of the standard textbook on
Plantation Forestry in the Tropics. He has written 7 other books
and over 120 scientific and technical papers, mainly on plantation
forestry. Until 1997, he was the British Forestry Commission's
chief research officer. He currently chairs the advisory committee
on forest research funded by the UK's Department for
International Development (DFID). In 1997, he was honoured by the
Queen and appointed OBE for "services to forestry and the third world".
Forest plantations are an increasingly important resource worldwide, a trend that is expected to continue strongly. But is growing trees in plantations a technology that can work in the long term? Is plantation silviculture biologically sound or are there inherent flaws, which will eventually lead to insuperable problems?
The principal conclusions are as follows.
1. Plantations and plantation forestry operations do impact the sites on which they occur. Under certain conditions nutrient export may threaten sustainability, but usually more important for maintaining site quality is care with harvesting operations, conservation of organic matter, and management of the weed environment. Plantation forestry appears entirely sustainable under conditions of good husbandry, but not where wasteful and damaging practices as permitted.
2. Plantations are at risk from damaging pests and diseases. New threats will inevitably arise and some plantations may become more susceptible owing to climate change factors, but the history of plantation forestry suggests that these risks are containable with vigilance and the underpinning of sound biological research.
3. Measurements of yield in successive rotations of trees suggest that, so far, there is no significant or widespread evidence that plantation forestry is unsustainable in the narrow-sense. Where yield decline has been reported poor silvicultural practices and operations appear to be largely responsible. There is some evidences that recent plantations are more productive than older ones owing to climate change and silvicultural impacts.
4. There are several interventions in plantation silviculture, which point to increasing productivity in the future, providing management is holistic and good standards maintained. Genetic improvement offers the prospect of substantial long-term gains over several rotations.
Overall, plantation forestry is likely to be sustainable on most sites provided good standards of silviculture are carried out.
Forest plantations are an increasingly important resource worldwide, a trend that is expected to continue strongly. This study examines the evidence concerning the 'narrow-sense' sustainability of forest plantations. It asks the question: is growing trees in plantations a technology that can work in the long term or are there inherent flaws biologically which will eventually lead to insuperable problems for such silviculture?
The question of sustainability in plantation forestry has two components. There are the general or broad issues of whether using land and devoting resources to tree plantations is a sustainable activity from the economic, the environmental or from the social sense. They can be labelled 'broad sense' sustainability.
The second component, 'narrow-sense' sustainability, is largely a biological and silvicultural issue. The question raised is: can tree plantations be grown indefinitely for rotation after rotation on the same site without serious risk to their well being? More specifically, can their long-term productivity be assured, or will it eventually decline over time? These questions are pertinent owing to the increasing reliance on plantation forestry, but are also scientifically challenging since in previous centuries trees and woodlands were seen as 'soil improvers' and not 'impoverishers'. Are today's silvicultural practices more damaging because of greater intensity and the high timber yields achieved, typically 2-4 times that of natural forest increment? And, of course, are resources such as genetic improvement, targeted fertiliser application, and sophisticated manipulation of stand density, along with rising atmospheric carbon dioxide, likely to lead to crop yield improvement, or could they disguise evidence of genuine site degradation or increasing risk of damaging pests and diseases?
This paper looks at evidence world-wide, but focuses on developing countries, to address four elements of narrow-sense sustainability: a fuller analysis is in Evans (1999b). (a) What changes to a site may plantation forestry induce and hence threaten future rotations? (b) What risks are tree plantations exposed to? (c) What factual evidence is there for and against productivity change over time? (d) What silvicultural interventions can help sustain yields?
Two important questions are (1) do the silvicultural practices commonly applied, such as exotic species, monocultures, clear felling systems etc., cause site change, and (2) are such changes more or less favourable to the next crop? Does growing one crop influence the potential of its successor?
This is a much-researched topic and only the main themes are summarised. Two recent books have presented the science: Dyck et al. (1994) 'Impacts of forest harvesting on long-term site productivity', and Nambiar and Brown (1997) 'Management of soil, nutrients and water in tropical plantation forests'. More dated but still relevant is Chijioke's (1980) review of the impacts of fast-growing species on tropical soils. However, it is important to be cautious: tree rotations are long, even in the tropics, compared with most research projects!
Demonstrating that soil changes may be caused by forestry practices, is usually difficult to establish conclusively both in fact and in scale. An absence of sound baseline data is common and is the reported change actually induced by plantation silviculture?
The second question is whether the observed changes represent degradation or improvement. There are remarkably few examples of changes supposedly induced by growing trees that lead to less favourable conditions for that species. Equally, the irreversibility of changes has rarely been demonstrated, apart from obvious physical losses such as erosion of topsoil. A gradual trend, perhaps observed over several decades, can be quickly reversed as stand conditions change. As Nambiar (1996) points out "the most striking impacts on soils and hence productivity of successive crops occur in response to harvesting operations, site preparation, and early silviculture from planting to canopy closure."
Most reports of site change in plantation forestry derive from matched plots. Increasingly today long term observational experiments are being specifically designed to investigate change, e.g. CIFOR's tropics-wide study (Tiarks et al., 1998), the network in USA (Powers et al. 1994), and those monitoring gross environmental change such as the Europe-wide extensive and intensive forest monitoring plots (level 1 and level 11). Modelling is widely used but suffers in precision at site level because of assumptions made.
The observational approach suffers bias in that investigation is often carried out specifically because there is a problem, which has already revealed itself in poor tree growth or health. It also suffers from soils being notoriously variable, a difficulty exacerbated on many sites by the kind of ground often used for plantations. A second, little known, source of variability is that measured values of many soil parameters can change radically during one year.
The above points underline the danger of drawing conclusions from limited investigations covering only a few years of a rotation. Short-term studies can be grossly misleading especially when extrapolating over whole rotations and successive rotations.
Plantations may have three impacts: nutrient removal from soil as trees grow and then are harvested; changes in the chemistry of the soil surface as the litter layer and organic matter are dominated by one species and hence uniform composition and decay characteristics; and site preparation practices such as ploughing, drainage and fertilising which directly affect soil physical parameters and in turn nutrient and moisture availability.
Soil as a mineral store
Soils vary enormously in their role as a nutrient reservoir. Thinking has been conditioned by arable farming that treats soils as a medium in which to grow crops where nutrient supply is largely maintained by annual fertiliser inputs; and by the fact that in most temperate soils the store of plant nutrients far exceeds that in the above-ground biomass. In forestry, where fertiliser inputs are limited and trees perennial and generally deep rooting, the focus is less exclusively on soil reserves and more on where the dynamics of nutrient supply is mediated - i.e. largely at the soil surface. Indeed, forests are highly efficient re-cyclers of nutrients and in the tropics, where recycling can be at its most efficient, nutrients in mineral soil often no longer represent the dominant proportion of the ecosystem. The soil often plays only a small part in the nutrient exchange and it is the surface organic, root-bearing zone, especially the annual turnover of fine roots, which is important in concentrating energy flow from decomposing organic matter back into living organic matter. The integrity of this layer and how it is handled in plantation silviculture is critical to sustainability.
Nutrient removal in plantation forestry occurs when any product is gathered or harvested. Many studies have been made; Goncalves et al. (1997) alone list 12 tropical examples. Critical to plantation sustainability is what proportion the nutrients lost represent of the whole store. This ratio of nutrient export : nutrient store is advocated as key measure of long-term ecosystem stability, (though it rather begs what is the store and how it can be measured?). For example Lundgren (1978) found that Pinus patula plantations in Tanzania led to annual removals of 40, 4, 23, 25 and 6 kg ha-1 of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) respectively. These rates of removal are about one-third of those of maize (Sanchez, 1976) and in the Tanzania study represented less than 10 per cent of soil store i.e. a stability ratio of <0.1. In contrast, Folster and Khanna (1997) report data for Eucalyptus urophylla x grandis hybrid stands with three very different site histories at Jari in NE Amazonia suggesting imminent impoverishment: "most of the previously grown Gmelina, Pinus or Eucalyptus had already extracted their share of base cations from the soil and left it greatly impoverished." with an unsustainable stability ratio of >1. However, caution is needed. Others (e.g. Rennie, 1955; Binns, 1962; Johnson and Todd, 1990) have predicted from comparison of removals in harvested biomass with available quantities in soil that calcium nutrition will be a problem; yet trees continue to grow on soil where conventional soil analysis suggests there is virtually no calcium.
Understanding these dynamics helps identify at what points on the continuum of plantation growth throughout the world of sites, species and productivities the ratio becomes critical for long-term stability. There appears to be few examples of reaching such limits. It is worth remembering that nutrient removals by forest crops are typically only one-fifth to one-tenth that of arable farming, see Miller (1995).
Litter and Residues
The influence of litter on soil chemical status may be important since leaves of different species decay at different rates. For example, in southern Africa substantial accumulations may develop under P. patula on certain sites (see Morris, 1993b) while this is unusual beneath the more lightly canopied P. elliottii. In broad-leaved stands, accumulation of litter is uncommon though not unknown. Even under teak and Gmelina, which usually suppress all other vegetation, the large leaves readily decay. Similarly under the light crowns of eucalyptus and the nitrogen rich foliage of leguminous trees such as Acacia, Leucaena and Prosopis sp. and non-legume N-fixers such as casuarinas litter build up is rare owing to rapid decay of the rich organic matter.
Measured changes in soil chemistry
The above processes indicate that plantation forestry practice could influence soil chemical status, but what has been observed? Most studies have either compared conditions in plantation sites with those before establishment or examined trends as a plantation develops. Few have examined changes over successive rotations. Few consistent trends emerge.
In the many tropical studies both increases and decreases in pH, carbon, nitrogen and macro-nutrients under plantations compared with natural forest or pre-existing conditions have been reported - see references in Evans (1999b). Recent investigations have concerned acid rain impacts, though distinguishing these from direct tree effects on soil acidity is difficult. On the whole, tree impacts are relatively small compared with the soil nutrient store.
Plantation forestry may impact soil physical conditions, and hence sustainability through (1) site preparation and establishment operations, (2) the effects of tree growth itself for example on water uptake, and (3) harvesting practices. They are discussed in Evans (1999a, 2000) and comment is only made here on vegetation suppression.
Indirect impact of vegetation suppression
Plantations of teak and Gmelina and also many conifers in both tropical and temperate regions may suppress all ground vegetation. Where this exposes soil, perhaps because litter is burnt or gathered, erosion rates increase. Under teak, Bell (1973) found soil erosion 2½ to 9 times higher than under natural forest. The protective function of tree cover derives more from the layer of organic matter that accumulates on the soil surface than from interception by the canopy. In India, raindrop erosion was 9 times higher under Shorea robusta plantations where litter had been lost through burning (Ghosh, 1978). Soil erosion beneath Paraserianthes falcataria stands was recorded as 0.8 t ha-1y-1 where litter and undergrowth were kept intact but an astonishing 79.8 t ha-1y-1 where it had been removed (Ambar 1986). Wiersum (1983) found virtually no soil erosion under Acacia auriculiformis plantations with litter and undergrowth intact, but serious where local people gathered the litter.
What happens to the litter and organic matter layer at the soil surface is critical to the question of sustainability for three reasons:
1. The surface litter layer helps prevent soil
2. Litter and organic matter represent a significant nutrient store, albeit a dynamic one;
3. The litter:organic matter:mineral soil interface is the seat of nutrient cycling and microbial activity.
Any activity that disturbs these roles in the ecosystem can have large effects of which perhaps most serious of all, and still practised in some countries, is regular and frequent litter raking. In commercial plantation forestry, the cost of managing debris and site preparation when restocking plantations is expensive and a high proportion of the establishment costs, but as Nambiar (1996) points out 'one shoddy operation can leave behind lasting problems'.
Establishment of plantations greatly affects ground vegetation with many operations designed directly or indirectly to reduce weed competition to ensure that the planted tree has sufficient access to site resources. A neglected but critical phase is managing the weed problem through crop harvesting and restocking. In subsequent rotations, the weed spectrum often changes. Owing to past weed suppression, exposure of mineral soil in harvesting, and the accumulation of organic matter, conditions for weed species change. Birds and animals may introduce new weed species, grass seed may be blown into plantations and accumulate over several years, and roads and rides in plantations can become sources of weed seeds. Weed management must be a holistic operation. As with a failure to handle organic matter carefully, where yield declines have been reported, often the significance of weeds has been insufficiently recognised on restocked sites in second or third rotations.
A serious threat to plantations can arise from a massive build-up of a pest or disease. It has been much disputed whether monoculture itself is more susceptible to devastation from these causes. The broadly accepted ecological principle of stability dates back to the 1950s and is that the stability of a community and its constituent species is positively related to its diversity. Following this reasoning foresters have stressed that substitution of natural forest by even-aged monoculture plantations may remove many of the natural constraints on local tree pest and pathogens and thus increase risk of attack. Some evidence supports this, see Gibson and Jones (1977), though these authors point out that increased susceptibility mostly arises from conditions in plantations rather than because only one tree species is present.
The relative susceptibility of monocultures to organic damage is complex ecologically. The influence of diversity on stability of (say) insect populations depends on what population level is deemed acceptable. Often stable, equilibrium levels are too damaging and so artificially low populations sought through control. Speight and Wainhouse (1989) stress that artificially created diversity, i.e. mixed crops, does not necessarily improve ecological stability and is certainly inferior to naturally occurring diversity, complexity of organisation and structure is as important (Bruenig 1986).
It is prudent, nevertheless, to spell out why plantations are perceived to be in danger.
1. Plantations of one or two species offer an enormous food source and ideal habitat to any pest and pathogen species adapted to them.
2. Uniformity of species and closeness of trees including branch contact above ground and root lesions in the soil, allow rapid colonization and spread of infection.
3. Narrow genetic base in plantations e.g. one provenance or no genetic variation (e.g. clones) reduces the inherent variability in resistance to attack.
4. Trees grow on a site for many years and permit pest or disease to build up over time.
5. Many plantations are of introduced species and while without the insect pests and pathogens that occur in their native habitat also missing are the many natural agencies controlling pests and diseases. Thus, many argue that exotic plantations experience a period of relative freedom from organic damage, perhaps for the first one or two rotations. Zobel's et al (1987) analysis of the threat to exotics concluded that evidence does not confirm that stands are more at risk, other than clonal plantations, and that problems arise mainly when species are ill suited to a site.
Examples of devastating outbreaks of fungal disease and insect pests are listed in many publications, see for example Ciesla and Donnaubauer (1994) and Evans (1999a), and they illustrate the scale and potential threat pest and diseases represent. They have prevented the planting of some species, impaired the productivity of others, but overall have not caused such widespread damage as to seriously question plantation silviculture as a practice.
There remain two serious concerns. 1. Environmental change - changing climate, increasing atmospheric pollutants of CO2 and nitrogen compounds, will add stress to established plantations while higher nitrogen inputs may increase insect pest risk and diseases problems (Lonsdale and Gibbs, 1996). 2. New pests and diseases will emerge: a) from new hybrids or mutations; b) from new introductions arising from increasing global trade e.g. Cryphonectria canker in eucalyptus in S. Africa and new phytophthoras in Britain; and c) from native pests adapting to introduced trees.
Many pest and disease problems in plantations arise from the nature of forest operations, and not directly from growing one tree species in a uniform way (monoculture). They are only briefly touched on here.
Harvesting and other residues
Large amounts of wood residue from felling debris and the presence of stumps are favourable for colonization by insect pests and as sources of infection. Usually modification of silviculture or application of specific protection measures can contain such problems.
Site and species selection
Extensive planting of one species, whether indigenous or exotic, inevitably results in some areas where trees are ill suited to the site and suffer stress. This may occur where large mono-specific blocks are planted or where exotics are used extensively before sufficient experience has been gained over a whole rotation e.g. Acacia mangium in Malaysia and Indonesia and the discovery of widespread heart rot.
Thinning and pruning damage
Thinning and pruning can damage trees and provide infection courts for disease. Neither practice seriously threatens plantation sustainability.
Problems with data
For forest stands (crops) hard evidence of productivity change over successive rotations is meagre with few reliable data. The long cycles in forestry make data collection difficult. Records are rarely maintained from one rotation to the next, funding for long term monitoring is often a low priority, detection of small changes is difficult, and often the exact location of sample plots is poorly recorded (Evans, 1984). Also, few plantations are second rotation, and even fewer third or later rotation, thus the opportunity to collect data is limited.
The few comparisons of productivity between rotations have mostly arisen because of concern over yields, namely 'second rotation decline', or stand health. Thus, the focus has been on problems: the vast extent of plantations where no records are available suggest no great concern and no obvious decline problems. Thus data in the older literature may be biased to problem areas while more recent studies may be less so, such as the European Forestry Institute survey (Spiecker et al. 1996) and CIFOR's 'Site management and productivity in tropical forest plantations', that incorporates systematic establishment of sample plots.
Four major studies have reported productivity in successive rotations along with some anecdotal evidence and occasional one-off investigations. These are grouped by region with emphasis here on developing country experience
Spruce in Saxony and Other European Evidence
In the 1920s Wiedemann (1923) reported that significant areas of second and third rotation spruce (Picea abies) in lower Saxony (Germany) were growing poorly and showed symptoms of ill health. In 8 per cent of plantations, there was a fall of two quality classes in second and third rotation stands. It is now clear that this mainly arose from planting spruce on sites to which it was ill suited. Today, young spruce stands in Saxony and Thuringia are growing more vigorously than equivalent stands 50 or 100 years ago (Wenk and Vogel, 1996).
Elsewhere in Europe, comparisons between first and second rotations are limited. In Denmark Holmsgaard et al. (1961) indicated no great change for either Norway Spruce or beech, though today second rotation beech is growing significantly better (Skovsgaards and Henriksen, 1996). In the Netherlands second rotation forest generally grows 30 per cent faster than the first (van Goor, 1985) and in Sweden second rotation Norway Spruce shows superior growth (Eriksson and Johanssen, 1993; Elfing and Nystrom, 1996). In France decline was reported in successive rotations on Pinus pinaster in the Landes (Bonneau, et al. 1968). In Britain most second rotation crops are equal to or better than their predecessor and no decrease in growth is expected (Dutch, pers. comm.) and recent evidence points to UK conifer forests growing faster than they used to (Cannell, et. al. 1998).
Pinus radiata in Australia and New Zealand
Significant yield decline in second rotation Pinus radiata appeared in South Australia in the early 1960s (Keeves, 1966) with an average 30 per cent drop in most forests in the state. In the Nelson area in New Zealand, on a few impoverished ridge sites there was transitory second rotation yield decline (Whyte, 1973). These reports, particularly from South Australia, were alarming and generated much research. By 1990 it was clear for South Australia that harvesting and site preparation practices which failed to conserve organic matter and an influx of weeds, especially grasses, in the second rotation were the main culprits. By rectifying these problems and using genetically superior stock second and third rotation pine now grow substantially better than the first crop (Boardman, 1988; Nambiar, 1996; Woods, 1990). Elsewhere in Australia, second rotation crops are mostly equal or superior to first rotation - see summary in Evans (1999b)
In New Zealand, the limited occurrence of yield decline was mostly overcome by cultivation and use of planted stock rather than natural regeneration (Whyte pers comm). On most sites successive rotations gain in productivity. However, Dyck and Skinner (1988) do conclude that inherently low quality sites, if managed intensively, will be susceptible to productivity decline.
Pines in Swaziland
Long-term productivity research by the writer in the Usutu forest, Swaziland began in 1968 as a direct consequence of second rotation decline reports from South Australia. For 32 years, measurements have been made over three successive rotations of Pinus patula plantations, grown for pulpwood, from a forest-wide network of long-term productivity plots. Plots have not received favoured treatment, but simply record tree growth during each rotation resulting from normal forest operations by SAPPI Usutu .
The most recent analysis appear in Evans (1996, 1999a) and in Evans and Boswell (1998). Tables 1 and 2 (simplified and updated from Evans, 1999a) show second and third rotation growth data obtained from plots on exactly the same sites. [First rotation data were derived from stem analysis and paired plots and are less accurate: some are reported in Evans (1996).]
Table 1 Comparison of second and third rotation Pinus patula on granite and gneiss derived soils at 13/14 years of age (means of 38 plots).
Mean ht. (m)
Mean tree vol. (m3)
Table 2 Comparison of second and third rotation Pinus patula on gabbro dominated soils at 13/14 years of age (means of 11 plots)
Mean ht. (m)
Mean tree vol. (m3)
Source: modified from Evans (1999a)
Tables 1 & 2 summarise results from arguably the most accurate datasets available on narrow-sense sustainability. Over most of the forest on granite derived soils (Table 1) third rotation height growth is significantly superior to second and volume per hectare almost so. There had been little difference between first and second rotation (Evans, 1978). In a small part of the forest (about 13% of area), on phosphate-poor soils derived from slow-weathering gabbro, decline occurred between first and second rotation, but this has not continued into the third rotation where there is no significant difference between rotations (Table 2).
The importance of the Swaziland data, apart from the long run of measurements, is that no ameliorative treatment has ever been applied to any long-term productivity plot. According to Morris (1987), some third rotation P. patula is probably genetically superior to the second rotation. However, the 1980s and especially the period 1989-92 have been particularly dry, Swaziland suffering a severe drought along with the rest of southern Africa (Hulme 1996, Morris, 1993a). This will have adversely impacted third rotation growth. These data are also of interest because plantation silviculture practised in the Usutu forest over some 62,000 ha is intensive with pine grown in monoculture, no thinning or fertilising, and on a rotation of 15-17 years which is close to the age of maximum mean annual increment. Large coupes are clearfelled and all timber suitable for pulpwood extracted. Slash is left scattered (i.e. organic matter conserved) and replanting done through it at the start of the next wet season. These plantations are managed as intensively as anywhere and, so far, there is no evidence to point to declining yield. The limited genetic improvement of some of the third rotation could have disguised a small decline, but evidence is weak. In addition, it can be strongly argued that without the severe and abnormal drought growth would have been even better than it is. Overall, the evidence suggests no serious threat to narrow-sense sustainability.
Chinese fir (Cunninghamia lanceolata) in sub-tropical China
There are about 6 million ha of Chinese fir plantations in subtropical China. Most are monocultures and are worked on short rotations to produce small poles, though foliage, bark and sometimes roots are harvested for local use. Reports of significant yield decline have a long history. Accounts by Li and Chen (1992) and Ding and Chen (1995) report a drop in productivity between first and second rotation of about 10 per cent and between second and third rotation up to a further 40 per cent. Ying and Ying (1997) quote higher figures for yield decline. Chinese forest scientists attach much importance to the problem and pursue research into monoculture, allelopathy, and detailed study of soil changes etc. Personal observation suggests that the widespread practices of whole tree harvesting, total removal of all organic matter from a site, and intensive soil cultivation that favours bamboo and grass invasion all contribute substantially to the problem. Ding and Chen (op. cit.) conclude that the problem is "not Chinese fir itself, but nutrient losses and soil erosion after burning (of felling debris and slash) were primary factors responsible for the soil deterioration and yield decline . . . application of P fertilizer should be important for maintaining soil fertility, and the most important thing was to avoid slash burning . . . These (practices) . . . would even raise forest productivity of Chinese fir." (words in parentheses added by writer).
Teak in India and Java
In the 1930s, evidence emerged that replanted teak (Tectona grandis) crops (second rotation) were not growing well in India and Java (Griffith and Gupta 1948). Although soil erosion is widespread under teak and loss of organic matter through burning leaves is commonplace the research into the 'pure teak problem', as it was called in India, did not generally confirm a second rotation problem. However, Chacko (1995) describes site deterioration under teak as still occurring with yields from plantations below expectation and a decline of site quality with age. Four causes are adduced: poor supervision of establishment; over-intensive taungya (intercropping) cultivation; delayed planting; and poor after-care. Chundamannii (1998) similarly reports decline in site quality over time and blames poor management.
In Java, Indonesia, where there are about 600,000 ha of teak, site deterioration is a problem and "is caused by repeated planting of teak on the same sites" (Perum Perhutani, 1992)
Southern pines in the United States
Plantations of slash (P. elliottii) and loblolly (P. taeda) pines are extensive in the southern states. Significant plantings began in mid 1930s as natural stands were logged out (Schultz, 1997) and with rotations usually 30 years or more, some restocking (second rotation) commenced in the 1970s. In general, growth of the second crop is variable - see examples in Evans (1999b). A coordinated series of experiments in USA is assessing long-term impacts of management practices on site productivity (Powers et al. 1994).
Other evidence is limited or confounded. For example, Aracruz Florestal in Brazil has a long history of continually improving productivity of eucalyptus owing to an imaginative and dedicated tree breeding programme so that regularly new clones are introduced and less productive ones discontinued (Campinhos and Ikemori, 1988). The same is true of the eucalypti plantations at Pointe Noire, Congo (P. Vigneron pers comm.). Thus, recorded yields may reflect genetic improvement and disguise any site degrade.
In India, one recent report (Das and Rao, 1999) claims massive yield decline in second rotation clonal eucalypti plantations which the authors attribute to very poor silviculture.
At Jari in the Amazon basin of Brazil, silvicultural practices have evolved with successive rotations since the first plantings between 1968 and 1982. A review of growth data from the early 1970s to present day suggest that productivity is increasing over successive rotations due to silvicultural inputs and genetic improvement (McNabb and Wadouski, in press).
In Venezuela, despite severe and damaging forest clearance practices, second rotation Pinus caribaea shows much better early growth than the first rotation (Longart and Gonzalez, 1993).
Inaccuracy in predicted yield
For long rotation (>20 years) crops it is usual to estimate yield potential from an early assessment of growth rate to identify the site quality or yield class. A change from predicted to final yield can readily occur where a crop has suffered check in the establishment phase or fertiliser application corrects a specific deficiency. However, there is some evidence for very long rotation (>40 years) crops in temperate countries that initial prediction of yield or quality class underestimates final outturn, i.e. crops grow better in later life than expected. Either the yield models used are now inappropriate or growing conditions are 'improving'. Across Europe the latter appears to be the case (Spiecker et al., 1996; Cannell et al. 1998) and is attributed to rises in atmospheric CO2 and nitrogen input in rainfall, better planting stock, and cessation of harmful practices such as litter raking.
However, as noted, the opposite is occurring with teak. High initial site quality estimates do not yield the expected outturn and figures are revised downward as the crops get older.
Relation of quality (yield) class with time of planting
Closely related to the above is the observation that date of planting is often positively related to productivity i.e. more recent crops are more productive than older ones regardless of inherent site fertility. This shift is measurable and can be dramatic, see example from Australia in Nambiar (1998). Attempts to model productivity in Britain based on site factors have often been forced to include planting date as a variable. Maximum mean annual increment of Sitka spruce increased with planting date in successive decades by 1 m3ha-1y-1 (Worrell and Malcolm, 1990) and for Douglas fir, Japanese larch (Larix kaempferi) and Scots pine (Pinus sylvestris) by 1.3, 1.6 and 0.5 m3ha-1y-1 respectively in each succeeding decade (Tyler et al., 1996). This suggests that some process is favouring present growing conditions over those in the past, such as the impact of genetic and silvicultural improvements (and again cessation of harmful ones) and possibly, the 'signature' of atmospheric changes mentioned above. Broadmeadow (2000) confidently predicts an increase in productivity for forests in United Kingdom owing to climate change.
The impact of these two related observations is that present forecasts of plantation yields are likely to be underestimates; yields generally appear to be increasing.
The steady transition from exploitation and management of natural forest to increasing dependence on plantation forestry is following the path of agriculture. Many of the same biological means to enhance yield are available. They are outlined here only briefly.
The forester only has one opportunity per rotation to change his chosen crop. Change in species, seed origin, use of new clones, use of genetically improved seed and, in the future, genetically modified trees all offer the prospect of better yields in later rotations.
There are surprisingly few examples of wholesale species change from one rotation to the next, which suggests that in most cases foresters have been good silviculturists. Examples of changes are cited in Evans (1999a, b, 2000).
Better seed origins, provenances, and land races
The impact of all these genetic improvements will affect yield and outturn directly and indirectly through better survival and greater suitability to the site which may lead to increased vigour and perhaps greater pest and disease resistance. Countless studies affirm the benefits of careful investment in this phase of tree improvement.
Some of the world's most productive tree plantations use clonal material, including both eucalyptus and poplars. It is clear that both the potential productivity and the uniformity of product make this silviculture attractive. Although clonal forestry has a narrow genetic base, careful management of clone numbers and the way they are interplanted can minimise pest and disease problems. Roberds and Bishir (1997) suggest that use of 30-40 unrelated clones will generally provide security against catastrophic failure.
Through an array of selection, crossing, and propagating techniques traits can be favoured that may improve vigour, stem and wood quality, pest and disease resistance and other parameters such as frost tolerance. There are many examples of successful tree improvement strategies most of which are only beginning to bear fruit owing to long tree rotations and the slow process of tree breeding, particularly in orchard establishment and promotion of flowering, and in field testing of selections and progenies. Improvements in the order of 20 to 50 per cent are considered relatively easy to achieve (Franklin, 1989). From plus-tree selection alone, based on 24 published reports, Cornelius (1994) reported genetic gain values of 15% in height and 35 % in volume. Genetic tree improvement offers by far the greatest assurance of sustained and improved yields from successive rotations in the medium and long-term
Genetically modified trees
There are no widely planted examples at present where genetic engineering has modified trees. The expectation is that these techniques will be used to develop disease resistance, modified wood properties, cold or drought tolerance rather than increase in vigour.
Silvicultural knowledge continues to increase through research and field trials and greater understanding of tree and stand physiology. While large yield improvements appear unlikely, incremental gains can be expected. Important examples include the following:
1. Manipulation of stocking levels to achieve greater output of fibre or a particular product, by fuller site occupancy, less mortality, and greater control of individual tree growth.
2. Matching rotation length to optimise yield - the rotation of maximum mean annual increment - offers worthwhile yield gain in many cases.
3. In some localities prolonging the life of stands subject to windthrow by silvicultural means will increase yield over time.
4. Use of mixed crops on a site may aid tree stability, may lower pest and disease threats, but is unlikely to raise productivity over growing the best suited species (FAO, 1992).
5. Silvicultural systems that maintain forest cover at all times - continuous cover forestry - such as shelterwood and selection systems are likely to be neutral to slightly negative in production terms while benefiting tree quality, aesthetics, and probably biodiversity value.
6. Crop rotation, as practised in farming, appears unlikely. There are examples of forest plantations benefiting from a previous crop of nitrogen fixing species e.g. Acacia mearnsii but industry is likely to require a similar not a widely differing species when replanting.
Most forest use of fertiliser is to correct known deficiencies e.g. micronutrients such as boron in much of tropics, and macronutrients such as phosphorus on impoverished sites in many parts of both the tropical and the temperate world. In most instances fertiliser is only required once in a rotation. Fertiliser application is likely to be the principal means of compensating for nutrient losses on those sites where plantation forestry practice does cause net nutrient export to detriment of plant growth.
Ground preparation to establish the first plantation crop will normally introduce sufficient site modification for good tree growth in the long term. Substantial site manipulation is unlikely for second and subsequent rotations, unless there was failure first time around, except to alleviate soil compaction after harvesting or measures to reduce infections and pest problems.
Weed control strategies may change from one rotation to the next owing to differing weed spectrum and whether weeds are more or less competitive. The issue is crucial to sustainability since all the main examples of yield decline reflect worsening weed environments, especially competition from grasses and bamboos.
It is clear from many investigations that treatment of organic matter both over the rotation and during felling and replanting is as critical to sustainability as coping with the weed environment. While avoidance of whole tree harvesting is probably desirable on nutrition grounds, it is now evident that both prevention of systematic litter raking or gathering during the rotation and conserving organic matter at harvesting are essential.
If all the above silvicultural features are brought together a rising trend in productivity can be expected. But if any one is neglected it is likely that the whole will suffer disproportionately. For example, operations should not exclusively minimise harvesting costs, but examine collectively harvesting, re-establishment and initial weeding i.e. as an holistic activity, so that future yield is not sacrificed for short term savings. Evidence of a rising trend reflecting the interplay of these gains is reported in Nambiar (1996) for Australia and reproduced in Evans (1999b) along with an example from Swaziland.
Holistic management also embraces active monitoring of pest and disease levels, and researching pest and disease biology and impacts will aid appropriate responses such as altering practices e.g. delayed replanting to allow weevil numbers to fall, and careful re-use of extraction routes to minimise soil compaction and erosion.
Four main conclusions can be drawn from this review.
1. Plantations and plantation forestry practices do affect sites and under certain conditions may cause deterioration, but are not inherently unsustainable. Care with harvesting, conservation of organic matter and management of the weed environment are critical features to minimise nutrient loss and damage to the soil conditions.
2. Plantations are at risk from pests and diseases. History of plantation forestry suggests that most risks are containable with vigilance and underpinning of sound biological research.
3. Measurements of yield in successive rotations of trees suggest that, so far, there is no widespread evidence that plantation forestry is unsustainable in the narrow-sense. Where yield decline has been reported poor silvicultural practices appear largely responsible.
4. Several interventions in plantation silviculture point to increasing productivity in the future, providing management is holistic and good standards maintained. Genetic improvement especially offers the prospect of substantial gains over several rotations.
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