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The sustainability of wood production in plantation forestry

J. Evans

Accurate evidence about the sustainability of plantation forestry is needed, since future supplies of forest products will increasingly come from intensively managed forests and plantations. This article reviews available evidence and reports recent findings from a comparison of three rotations of pine in the Usutu Forest, Swaziland.

Professor Julian Evans is a consultant with the United Kingdom Overseas Development Administration.
Note: This article is an adaptation of a voluntary paper submitted to the Eleventh World Forestry Congress, 1322 October 1997, Antalya, Turkey.

The concept of sustainability is central to sound forest management and the subject of much current debate. In the case of forest plantations established with the specific purpose of producing wood, whether for industrial purposes or domestic use, the question must be asked, can supplies be maintained in perpetuity? This biological or "narrow sense" sustainability must first be satisfactorily addressed, then the technology of plantation silviculture can fulfil the worldwide potential expected of it with some assurance.

The importance of plantation forestry and sustainability

Although estimates vary, the total area of forest plantations in the world amounts to between 120 million and 140 million ha. What is more certain is that this area is increasing in both temperate and tropical countries. In the tropics especially, the present rate of plantation establishment (2 million to 3 million ha per year) is double that recorded in the 1960s and 1970s (FAO, 1992; Evans, 1992). The main purpose of such plantations is either for industrial production or domestic use as building poles, fuelwood and fodder.

Third rotation Chinese fir (Cunninghamia lanceolata) growing among dense grass and bamboo in Fujian Province, China. The third rotation is often much inferior to the second

A great majority of forest plantations are of uniform age and composition (monoculture) and most are managed to optimize the yield of wood from a site. Clear-felling and replanting is the most common silvicultural system although, where appropriate, coppicing is used as a means of restocking. These features of plantation silviculture have raised concern that many of the sites on which trees are planted may be incapable of sustaining their productivity. Models of nutrient export, examination of physical damage of soil structure and claims of greater risk from pests and diseases have all been advanced as hypotheses for the inherent unsustainability of intensive plantation forestry.

The question of sustainability, at least in the narrow biological sense, has long been a concern in agriculture, particularly with arable cropping. Several long-term experiments are being carried out in different countries, of which the oldest and most famous is Broadbalk Field at Rothamsted Experimental Station, Harpenden, United Kingdom. Since 1843, successive crops of wheat have continuously been grown and assessed. Over a long period, yields from the control treatment, which has received no fertilizer and only minimal cultural treatment to control weeds, have remained low but stable (Johnston, 1994). This work has shown that, even after 150 years, the land itself has not become "wheat sick" and that low yields arise from low external inputs (although these are rising, notably the anthropogenically derived nitrogen mainly found in rainfall which has now reached as much as 30 kg/ha-1/year-1).

Factual evidence concerning long-term productivity of forest plantations remains meagre by comparison but, without it, foresters cannot properly demonstrate how robust their silviculture is and cannot refute suggestions that successive rotations of fast-growing trees inevitably lead to soil deterioration. This article examines the evidence of yield decline and reports in detail on the best data sets in the world which describe the performance of three successive rotations on the same site (Evans, 1996). The subject was reviewed by the author at the Eighth World Forestry Congress held in Jakarta in 1978 (Evans, 1978). Since then significant new information has arisen which is germane to the question of sustainability.

Mensurational evidence of the productivity of successive tree crops

A full review of this subject in Evans (1990) revealed how few examples exist of demonstrable and widespread yield declines, excluding those attributed to pollution and pathogen-related dieback (Ciesla and Donaubauer in FAO, 1994; Freer-Smith, in press). Three main examples have been reported in the forestry literature.

Spruce in Saxony and other European evidence

Reports by Weidemann (1923) suggested that in the 1920s significant areas of second and third rotation spruce (Picea abies) in lower Saxony, Germany, were growing poorly and showed symptoms of ill-health. This much-researched decline was attributed to insect defoliation, air pollution, the effects of monoculture and, simply, the intensive form of forestry practiced. It is now clear that much of the problem arose from planting spruce on sites to which it was ill-suited, as also happens with silver fir (Abies alba).

Litter raking under Pinus caribaea in Leishou, southern China. The loss of organic matter and the consequent break in nutrient recycling are likely to lead to yield decline in the future

Twelve-year -old second rotation Pinus patula in the Usutu Forest, Swaziland - the following crop grew even better

Clear-felling of Pinus patula on steep hills in Swaziland. Mules are used to minimize extraction damage to the soil

Elsewhere in Europe reports of localized yield decline in Denmark, the Netherlands and the Landes area of France have appeared but neither the extent nor magnitude of yield change were cause for alarm. In Great Britain most second rotation crops are equal to or better than those of the first rotation and, in the case of restocking with Sitka spruce (Picea sitchensis), by far the most important species in the uplands, there has been no need to reapply phosphate fertilizer even though it was essential for establishing the first rotation (Taylor, 1990).

Today, many forest stands in Europe are showing growth increases compared with the past for reasons that are not entirely clear but may reflect better silviculture and rising CO2 and NOX concentrations in the atmosphere (Spiecker et al., 1996)

Pinus radiata in South Australia and New Zealand

The first reports of significant productivity decline emerged in the early 1960s (Keeves, 1966) and by the end of the decade it was clear that, throughout the state, a productivity falloff of about 30 percent was occurring in the second crop. Not surprisingly, the problem generated a great deal of research and it gradually became clear that a combination of factors was causing the poorer growth of the replanted second rotation. Harvesting and site preparation practices, e.g. windrowing, were causing great losses of organic matter from sites and high weed-seed loads combined with inadequate weed control were leading to extensive grass invasion. Experiments showed that the conservation of organic matter and more gentle handling of a site, along with adequate weed control, greatly improved performance of the second rotation and largely overcame the decline problem. Today, these and other changes in silviculture have eliminated decline altogether (Woods, 1990).

TABLE 1. Yield of second and third rotation Pinus patula at age 14 years, from 24 plots on Lochiel Hood granite or ancient gneiss complex soils (86 percent of the forest)

Rotation

Stems per hectare

Mean height
(m)

Basal area
(m2/ha-1)

Volume
(m3/ha-1)

Mean annual increment
(m3/ha-1/year-1)

First: 1R

[1266]1

17.52

-

[297.4]1

[21.2]1

Second: 2R

1298

17.28

42.63

284.16

20.30

Third: 3R

1273

18.01

43.58

302.36

21.60

3R:2R (%)

-1.9

+4.2

+2.2

+6.4

+6.4

't' statistic

-

2.94

1.28

1.89

-

Significance

-

P <0.01

n.s.

P <0.1

-

1 Unreliable figures thought to be slight overestimates.
Source: Evans (1996).

TABLE 2. Yield of second and third rotation Pinus patula at age 14 years, from ten plots in Forest Block A on Usushwana complex soils (13 percent Or the forest)

Rotation

Stems per hectare

Mean height
(m)

Basal area
(m2/ha-1)

Volume
(m3/ha-1)

Mean annual increment
(m3/ha-1/year-1)

First: 1R

[1239]1

18.18

-

[301.3]1

1215]1

Second: 2R

1170

17.03

37.57

248.22

17.73

Third: 3R

1025

16.87

38.93

255.12

18.22

3R:2R (%)

8.8

-0.9

+3.6

+2.8

+2.8

't' statistic

-

-0.26

0.55

0.18

-

Significance

-

n.s

n.s.

n.s.

-

1 Unreliable figures though to be slight overestimates.
Source: Evans (1996).

In restricted areas of New Zealand, second rotation decline in P radiata was also reported (Whyte, 1973) but was confined to impoverished sites in the Nelson area of the South Island. Elsewhere, a carefully investigated study of second rotation P. elliottii in Queensland, Australia, showed no evidence of yield decline. Nor has any such decline been reported among subtropical pines in southern Africa, apart from a highly localized and site-related occurrence in Swaziland (Evans, 1996).

Cunninghamia lanceolata in China

About 6 million ha of plantations of Chinese fir (C. lanceolata) have been established in subtropical China. Indeed it is the most widely planted species. Most plantations are monoculture and are worked on short rotations to produce small poles, although the tree itself, its foliage, bark and sometimes even its roots, are all utilized in some way. Reports of significant yield decline began to appear some years ago: accounts by Li and Chen (1992) and Ding and Chen (1995) suggest a drop in productivity between first and second rotation of around 10 percent and up to a further 40 percent between the second and third rotations. Data have been difficult to obtain to indicate how widespread this kind of decline is, but the importance attached to it by Chinese foresters is evidenced by a large amount of research into questions of monoculture, allelopathy, soil changes, etc. It appears that the practice of whole-tree harvesting, the almost total removal of organic matter from a site after harvesting and conditions that favour extensive grass and bamboo invasion all contribute substantially to the problem. The effect of allelopathy and recruitment of coppice shoots for restocking on productivity remains unresolved. Yield decline in Chinese fir has been the subject of a cooperative research investigation by the United Kingdom Overseas Development Administration and the Chinese Academy of Forestry.

Coppice

Plantations of some species, e.g. eucalypts, are often managed by coppicing for the second, third and sometimes fourth rotation. Much evidence indicates that, typically, the first coppice crop is the most productive, followed by poorer yields in each subsequent crop until replanting. Kaumi's 1983 report from Kenya and that of Jacobs from India in 1981 are typical (Kaumi and Jacobs, cited in Evans, 1992) although in the charcoal coppice plantations of Brazil the first coppice crop is not always the best. This diminution in productivity mostly arises from stump death and poorer stocking per hectare as well as from the physiological feature of coppice shoots exhibiting "mature" characteristics at an earlier stage when grown on an increasingly old root system. There has been little evidence to suggest that the practice of coppicing itself depresses site productivity.

Productivity research over three successive rotations in Swaziland

Research in the Usutu Forest, Swaziland, began in 1968 as a direct consequence of reports emanating from Australia about productivity decline in second rotation Pinus radiata. Since 1968 the productivity of each successive rotation of P. patula has been recorded using a network of sample plots throughout the Usutu Forest. Data from three complete rotations now exist and were reported recently by Evans (1996). The Swaziland yield records are arguably the best data set in the world for comparing three successive rotations of tree plantations on the same site.

The painstaking and carefully recorded measurements show that, for a large part of the forest (86 percent of the total area) where soils derive from a granite and gneiss complex lithology, there has been no decline in yield. Indeed, on these intermediate to slow weathering mineralogies (typically feldspars, biotite and muscovite) there is strong evidence that the third rotation is significantly superior to the second (Table 1). By contrast, in a small part of the forest (13 percent) dominated by gabbro-derived soils from the Uusushwana complex of slow to very slow weathering mineralogy (plagioclase, quartz and hornblende), a significant yield decline occurred between the first and second rotations but not between second and third rotations (Table 2).

Apart from the long-term nature of the research that has been maintained, an important feature of these data from Swaziland is that there has been no genetic improvement nor fertilizer addition from one rotation to the next. Moreover, the 1980s especially the late 1980s - and the early 1990s have been particularly dry in this region, along with the rest of southern Africa (Hulme, 1996), but this has not affected the yield as might have been expected. The data are also important because the plantation silviculture carried out in the Usutu Forest over about 62000 ha is intensive, with P. patula grown in monoculture without thinning and on a rotation of 15 to 17 years, which is close to the age of maximum mean annual increment. Large coupes are clear-felled and all timber suitable for pulpwood is extracted.

These plantations are managed as intensively as any example that can be found and, over three rotations, so far there is no evidence that the practices themselves are leading to yield decline as measured by crop productivity.

The future

The need for research

Although the overview of the Swazi situation presents an encouraging picture, the serious lack of data recording yields in successive rotations is a major problem. It is not a new problem (Evans, 1984) but, at a time when all research budgets are being severely restricted, the maintenance of long-term records that are essential for answering the kind of questions discussed in this article will be increasingly difficult. This is all the more so in forestry research where rotations last from many years to many decades (Evans, 1994). Managers responsible for permanent sample plots must ensure that reestablishment is kept up in successive rotations and that data are recorded and stored for posterity.

The processes that impinge on site productivity are generally more widely researched. For example, the USDA Forest Service (Powers, 1991), the EU Level 11 Network under Europe's Air Pollution regulations and the Centre for International Forestry Research (CIFOR) have established programmes aimed at creating networks of sites to gather data that record the impacts of plantation forestry practices (nutrient supply budgets, physical characteristics of soils, etc.). These networks will be an essential resource for scientific investigation in the future.

Prognosis

Genetic improvement of tree plantations remains largely in its infancy, with a few notable exceptions such as eucalypts growing at Aracruz, poplars and tropical and subtropical pines. It is clear that substantial yield improvement as well as other features such as disease resistance and better stem quality flow from tree-breeding programmes. With plantation forestry practice essentially appearing to be neutral regarding its impact on site productivity - the degree of soil improvement gained from trees is matched by the intermittent export of nutrients away from a site - genetic improvement of the crop should lead to some increase in yields in the future.

A less strong case, but still one leading to growth enhancement, is judicious fertilizer application. Certainly in Swaziland the limited area of forest where yield decline did occur between first and second rotations is being corrected by the application of phosphate on the essentially phosphate-poor soils. Such targeting of inputs to site needs will play a part in the maintenance of productivity, as in the case of magnesium (dolomitic limestone) in Germany. Allied to this amelioration of soil nutrition is the increasing realization that harvesting practices should minimize physical damage to a site and seek to conserve organic matter from one rotation to the next. Attention to weed control as part of good management must also continue.

Conclusions

Overall, it is reasonable to conclude that the outlook is positive and that, as a technology for producing timber efficiently, plantation forestry ought to be sustainable. Evidence from across the world suggests that plantation forestry is likely to be sustainable in terms of wood yield in most situations provided good practice is maintained. Improvements in silviculture and tree genetics may enhance crop productivity. This suggests that, in its simplest form, plantation forestry is a very useful technology, but tints conclusion can be misleading since it is not always an appropriate technology where tree planting is needed. The concept of complex plantation forestry (i.e. multiple species for multiple uses), which is able to deliver a variety of goods, services arid values, will frequently be more appropriate (Kanowski, 1995). By embedding plantation forestry in the broader social arid economic context, its sustainability in the "broad senses is enhanced. This article shows that the "narrow sense" of sustainability should not threaten the achievement of these wider aims.

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