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Planted Forests and Biodiversity

By J-M Carnus1, J Parrotta2, EG Brockerhoff3, M Arbez4, H Jactel1, A Kremer1, D Lamb5, K O'Hara6 & B Walters7

For delivery at: UNFF Intersessional Experts Meeting on the Role of Planted Forests in Sustainable Forest Management, 24-30 March 2003, New Zealand

Abstract

Forest ecosystems shelter a major part of terrestrial biological diversity, and over the past decades, conservation of biodiversity has become a key element in national forest policies and planning. Plantation forests are cultivated forest ecosystems established primarily for wood biomass production but also for soil and water conservation or wind protection. During the past decade, the global forest plantation area has increased by an estimated 32 million ha while the area of natural forests has declined by 126 million ha.

Biodiversity is an issue of increasing relevance to the development and management of plantation forests and their long-term sustainability. Plantations can and do play a vital role in forest conservation by providing a substitute for wood from unsustainable harvesting of natural forests. In many parts of the world plantations also play a key role in restoring local ecosystem services and by reducing runoff and erosion on previously degraded sites. Despite these positive attributes, plantation forests are widely viewed in a negative light in relation to biological diversity conservation, especially when intensive monocultures of exotic species are involved.

While a plantation stand will, in general, support fewer native species than a native forest at the same site, plantations are increasingly replacing other human-modified ecosystems (e.g., degraded pasture) and often support a greater diversity of native species, particularly in understorey communities. As such, plantations can play an important role in conserving or even restoring native biodiversity in production landscapes. As well as providing habitat in their own right, plantations play particularly important roles in buffering native forest remnants and in enhancing connectivity between areas of native ecosystems. In doing so, these plantation forests may help foster the overall sustainability of agriculture and other land uses across these landscapes.

However, to sustain health and productivity of planted forests, managers need to preserve genetic diversity through adapted breeding strategies and controlled deployment of improved genetic material, and enhance interspecific diversity using a greater variety of planted species (exotic and native) and alternative forest management regimes and practices, such as the extension of rotation lengths in some stands, and adoption of a variety of harvesting approaches.

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1 INRA Bordeaux Pierroton, 33610 Gazinet Cestas (France); carnus@pierroton.inra.fr
2 USDA Forest Service, Research & Development, 1601 N. Kent Street, Arlington VA 22209 (USA); jparrotta@fs.fed.us
3 Forest Research , PO Box 29237, Christchurch (New Zealand); Eckehard.Brockerhoff@ForestResearch.co.nz
4 IEFC- EFI Regional Project Centre, 69 route d'Arcachon, 33 610 Cestas (France); aquitaine@iefc.net
5 Department of Botany, University of Queensland, Brisbane, Queensland 4072 (Australia): d.lamb@botany.uq.edu.au
6 Division of Forest Science, 145 Mulford Hall #3114, University of California, Berkeley CA 94720-3114 (USA); ohara@nature.berkeley.edu
7 Department of Geography, Mount Allison University, 144 Main Street, Sackville, New Brunswick, E4L 1A7 (Canada); bwalters@mta.ca

1. Introduction

Plantation forests, or planted forests, are cultivated forest ecosystems established by planting or/and seeding in the process of afforestation and reforestation, primarily for wood biomass production but also for soil and water conservation or wind protection. Though the total area of plantation forest (187 million ha) represents only 5% of the global forest cover (FAO, 2001), their importance is rapidly increasing as individual countries move to establish sustainable sources of wood fibre to meet the increasing demand for wood and pulp. This is particularly the case in Asia, where an estimated 62% of the global plantation forest estate is located. Industrial plantations (supplying industrial wood and fibre) account for 48% of the global plantation estate; these typically consist of intensively managed, even-aged and regularly-spaced stands of a single tree species (indigenous or exotic), often genetically improved, and are characterised by relatively short rotations when compared with natural forests. Non-industrial plantations, established for fuelwood, soil and water conservation (e.g., watershed rehabilitation), and wind protection, account for 26% of the world's plantation forests, while an additional 26% of plantation forests are established for other, unspecified, purposes (FAO, 2001).

During the past decade, while natural forest and total forest areas have continued to decline at the global level, forest plantation areas have increased in both tropical (+20 million ha) and non-tropical (+12 million ha) regions. In both tropical and non-tropical regions, the conversion of natural forests and reforestation of non-forest areas have contributed in roughly similar proportions to these increases in forest plantation areas during this period (FAO, 2001). It is worth noting that between 1990-2000, the rate of conversion of natural to plantation forests in tropical regions was about equal to the increase in natural forest resulting from natural reestablishment (i.e., forest succession) of non-forest areas, and only 7% of the area of natural forest converted to non-forest land uses. In non-tropical areas the net increase in natural forest areas was more than three times the rate of conversion of natural to plantation forests.

About 60 % of plantation forests are located in four countries (China, India, Russian Federation and the United States). Species in the genera Pinus and Eucalyptus are the most commonly used in plantations (30%), though the overall diversity of planted tree species is increasing (FAO, 2001). Table 1 provides a summary of plantation forest areas and their geographic distribution.

Table 1: Plantation forests area by region, 2000 (from FAO, 2001)

Region	Total forest area (million ha)	Natural forest area (million ha)	Forest plantation area (million ha)	% of total plantation area
Africa	650	642	8	4
Asia	548	432	116	62
Europe	1039	1007	32	17
North & Central America	549	532	18	9
Oceania	198	194	3	2
South America	886	875	10	6
World total	3869	3682	187	100

What is biodiversity?

Biodiversity is defined as "the variability among living organisms from all sources …[including] diversity within species, between species and of ecosystems" (Convention on Biological Diversity: United Nations 1992). Forest ecosystems shelter a major part of terrestrial biological diversity, including an estimated 80% of all terrestrial species; approximately 12% of the world's forests are presently in protected areas (FAO, 2001). The importance of maintaining biodiversity in forest ecosystems has been emphasised in the past decade at political levels through many international conventions and agreements promoting sustainable forest management (SFM) including the Montreal or Pan European Processes, and at commercial levels as part of forest certification schemes (e.g., FSC, PEFC). Biodiversity is thus an issue of increasing relevance to plantation forests and their long-term sustainability, and as a criterion for SFM, it is becoming clear that maintenance of biological diversity has direct implications for plantation forests and their management.

Biodiversity in a forest ecosystem is determined and influenced by climatic and soil conditions, evolution, changes in species' geographical ranges, population and community processes, and natural or human-related disturbances. Ecological processes and biodiversity change over time as ecosystems recover from natural or human-caused disturbances. Disturbances can either increase or decrease biological diversity depending on the scales and measures of biodiversity being considered; for many measures, the highest levels of biodiversity are found in forests that have been subjected to intermediate frequencies, scales and intensities of disturbance (Kimmins, 2000).

Four components of biological diversity are of particular relevance to discussions on planted forests and their environmental impacts:

• Genetic diversity: the genetic variation within a population or a species.
• Species diversity: the number of species in a particular area or community (species richness) or the relative abundance of the species therein (species evenness).
• Structural diversity: how forest plant communities are structured both horizontally and vertically, which changes continuously as stand development proceeds and is particularly significant in plantation forests. Structural diversity can be as important for animal species diversity as is the diversity of plant species in the forest plant communities.
• Functional diversity: variation in functional characteristics of trees and other plant species, i.e., evergreen vs deciduous, shade tolerant vs light demanding, deep-rooted vs shallow-rooted, etc....

The measures of biological diversity defined above can be applied at various spatial scales (local ecosystem and stand level, landscape level, regional and beyond) and are dynamic, changing over time. This change can be quite rapid as a result of disturbance or slow as a result of climate change or species evolution. Much of the focus in discussions about biodiversity has been at the local ecosystem level; however, biodiversity measures at this level exhibit the greatest temporal variation.

In the following sections we will discuss and attempt to summarize the current state of scientific knowledge regarding the impacts of planted forests and their management on biodiversity. We will consider key issues related to intraspecific diversity, focusing on genetic diversity within tree plantations, as well as the influence of planted forests on interspecific diversity within planted forests and in surrounding landscapes. Further, we will consider the role of biodiversity in planted forests and the strategies for managing planted forests to conserve and/or enhance biological diversity at various spatial scales from the forest stand to the landscape level.

2 Genetic Diversity

Characterisation of genetic diversity in tree plantations

As a fundamental component of global biodiversity, genetic diversity includes the intraspecific variations between individual trees, e.g. genes, within populations and between populations (races, ecotypes and provenances). This genetic diversity largely controls adaptability and resistance to abiotic and biotic disturbances.

In the past decade, the rapid development of tools (e.g. molecular markers) for analysing the genetic variability of forest trees (Petit et al., 1997), has enabled scientists to better characterise and assess pollen fluxes between individuals and populations, spatial distributions of genetic diversity within stands, and to better understand the effects of silvicultural practices on the long-term evolution of genetic diversity of forest trees. Also, the molecular characterisation of the plantation tree populations and improved varieties enable us to better manage and control the movements of forest reproductive materials (FRM). (Ribeiro et al. 2002)

Modification of genetic pools (new species, seed transfer)

Although there is an increasing body of scientific information available to assess the possible impact of plantations on intraspecific genetic diversity of forest trees, there is no single satisfactory answer to this question. This impact is clearly influenced by the type of forest reproductive material (FRM) used in plantations, the quality of available and registered FRM genetic information, and the feasibility of the control applied to it (Ditlevsen, 1993). The impact of plantations on genetic diversity depends on the level of genetic variability of the FRM itself, as well as on the possible gene exchanges between the planted FRM and surrounding forest tree gene pools. The final impact of plantations set up with a controlled FRM, at the regional forest tree diversity level, depends also on the total area afforested with this FRM and duration of its use. There is a real challenge for sustainable forest management to take into consideration what would happen if highly selected FRM (especially hybrid, clonal or GM varieties), initially planned to be clearfelled and re-planted, were to regenerate naturally and spread outside plantation areas by lack of control over time.

As has occurred earlier in agriculture, the introduction of genetically improved exotic species in forestry increases productivity and carbon-fixation efficiency but also interspecific diversity at landscape and regional scales (see chapter 3). In France for example, compared to 70 natural forest tree species, 30 introduced species are commonly used in plantation forestry and often help to increase the interspecific genetic diversity of forests at the local level (Le Tacon et al., 2000; 2001). More generally in Europe, the forest flora was very diverse at the end of the tertiary era, and numerous species disappeared during the successive glacial periods. There is no doubt that the introduction of new species has partly restored this species richness.

While popular in the past, introduction of exotic species has been limited more recently because there were and are still risks associated with these introductions. Long-term confirmation of adaptation to local soil and climate conditions is necessary for the use of exotic species in extensive plantation programmes, to avoid severe damage - summer drought and winter frost resistance, tolerance to hydromorphic soil condition, resistance to insects and diseases all require verification. Also, exotic fast-growing species can replace native forest tree species because of their natural invasive potential, as observed for example with eucalypts in northwestern Spain and Portugal.

Impact of using genetically improved FRM

FRM collected from registered seed stands results in plantation forests with a level of genetic diversity most often similar to the wild population from which it originates. The main genetic impacts depend on the level of adaptation of the introduced population to its new environment and the possible gene transfers from it to the surrounding native population; in this respect, the possible undesirable impacts of long-distance seed transfer requires special consideration. With the development of selection programmes for plantation tree species, the level of genetic diversity of the planted material has been progressively restricted, as with single or controlled mixtures of full-sib families, clonal varieties, or genetically modified (GM) trees that may be used in the future. Consequently such FRM could be expected to have a lower adaptability and an increased ecological risk over the same rotation time (Gadgil & Bain, 1999; Evans, 1999; Wingfield, 1999). On the other hand, the genetic information is much greater, allowing the forest-owner to better balance the expected economic gains and the ecological risks, and there are some relevant and known breeding strategies and gene conservation procedures able to maintain the genetic variability of the plantation species over several generations.

Clonal varieties

A major concern arising from the use of clonal plantation forestry is the maintenance of stand adaptability, i.e. the ability to face an unexpected catastrophic perturbation due to biotic or abiotic causes. Does the increased use of clonal planting stock contribute to a decrease in stand viability? These questions have been investigated theoretically by considering simplified situations in which susceptibility to the unknown hazard (Bishir & Roberds, 1999) is controlled by one single diallelic locus. The results varied according to the frequency of susceptible genotypes and the level of acceptable stand mortality. If the former is higher than the latter, then increasing the number of clones will increase the susceptibility of the multiclonal variety. If the former is low, then increasing the number of clones increases the probability of success, but the increase of probability of success occurs mainly up to 10 genotypes. To cover most of the situations, Bishir & Roberds (1999) recommend using clonal mixtures including between 30 to 40 genotypes.

Genetically modified (GM) trees in commercial varieties

Gene transfer is currently being tested in most forest species undergoing intensive breeding activities (Radiata pine, Scots pine, Maritime pine, Sitka spruce, Norway Spruce, Eucalyptus, poplars…). In conjunction with other biotechniques such as somatic embryogenesis, rapid and important genetic gains can potentially be transferred to forestry. Transgenesis has been considered as an attractive tool for genetically improving trees for pest and insect resistance, wood properties and lignin content (Jouanin, 2000). Benefits expected from transgenesis are increased ecological and economic efficiency of wood production by improving and homogenising target traits, increased adaptability and resistance to biotic and abiotic stresses, and reductions in the use of undesirable insecticides and pesticides. For example, poplar, European larch, and white spruce have been engineered for a gene encoding an insecticide toxin from the soil bacterium Bacillus thuringiensis (Bt). To date there are a total of 117 experimental plantations with GM trees belonging to 24 trees species around the world, but no commercial plantation has been reported. The main risks for biodiversity (Kremer, 2002) are related to the dissemination of GM material which might result in introgression with related tree species (Matthews and Campbell, 2000) and in the spread, through natural regeneration, of GM trees that are potentially better adapted to site conditions (Hayes, 2001). As for annual crops, the potential use of transgenic trees in forestry has raised concerns in the public and among foresters and scientists, and has motivated vandalism and other criminal acts. These unfortunate events illustrate the sharp controversy existing between the public and the scientific community, and also within the scientific community. There is an urgent need for an in-depth debate on benefits and risks associated to transgenic technology in forestry, considering scientific, economic, social and ethical aspects. This debate is a motivation for this contribution.

3 Interspecific Diversity

Species diversity in plantation forests versus natural forests and other habitats

It is widely thought that plantation forests are, on average, less favourable as habitat for a wide range of taxa, particularly in the case of even-aged, single-species stands involving exotic species (Hunter, 1990; Hartley, 2002). In support of this notion, the bird fauna of single-species plantation forests has been reported to be less diverse than that of natural or semi-natural forests (Helle and Mönkkönen, 1990; Baguette et al., 1994; Gjerde and Sætersdal, 1997; Fischer & Goldney, 1998; Twedt et al., 1999). Carabid beetles were found to be more abundant and diverse in natural or semi-natural forest than in spruce plantations in Ireland (Fahy & Gormally, 1998) and Hungary (Magura et al., 2000). Similar results were obtained in studies of beetles in South Africa (Samways et al., 1996), dung beetles in Borneo (Davis et al. 2000), and arthropods in general in Brazil (Chey et al., 1998) and New Zealand (Anderson and Death, 2000). The vegetation in conifer plantations was found to be less diverse than that in semi-natural woodlands in Ireland (Fahy & Gormally, 1998) and in Great Britain (Humphrey et al., 2002).

However, such findings cannot be generalised because in some cases the wildlife or other biota in plantation forests may be as diverse as in natural forests. For example, species richness of indigenous birds in New Zealand was only slightly lower in pine plantation forests (Clout and Gaze, 1984), and in some cases bird counts in these plantations exceed those of most natural forests (Brockie, 1992). Bird species richness in a Lophostemon plantation was similar to that in secondary forest (Kwok and Corlett, 2000). In Great Britain, the fungal and invertebrate communities in conifer plantations have been found to be similar to those in natural woodlands (Humphrey et al., 1999; 2000; 2002).

Furthermore, an analysis of the impact of plantation forestry on biodiversity based simply on comparisons with natural forests in the same area is not always appropriate. While the conversion of old-growth forest, native grassland or some other natural ecosystem to plantation forests will rarely be desirable from a biodiversity point of view, planted forests in fact often replace other land uses. Where they are established on abandoned pastures or degraded land, plantation forests are usually more beneficial to biodiversity than such modified agricultural areas. For example, in New Zealand pasture is known to be dominated by exotic species and to be a particularly poor habitat for indigenous species whereas the understorey of pine plantations usually includes many indigenous plant species (Brockerhoff et al., 2001).

Numerous studies carried out during the past 15 years have demonstrated that planted forests can accelerate natural forest regeneration on degraded sites where persistent ecological barriers to succession would otherwise preclude recolonization by native forest species. This facilitative role of planted forests is due to their influence on understorey microclimatic conditions, vegetation structural complexity, and development of litter and humus layers during the early years of plantation growth. Examples of the "catalytic effect" of forest plantings on degraded landscapes (summarized in Parrotta & Turnbull, 1997; Parrotta, 2002) can be found in many tropical and subtropical countries, including India, China, Indonesia, Australia, Uganda, Malawi, Congo, South Africa, Puerto Rico, Costa Rica, and Brazil. In the Mediterranean region, artificial forests created at the end of the 19th century to rehabilitate overgrazed grasslands and for watershed protection, and subsequently thinned and/or harvested, revert naturally to mixed conifer-broadleaf forests similar in structure and species composition to those that existed prior to their degradation caused by overgrazing, overharvesting, and fire. These examples highlight the need for consideration of the land use history when evaluating species richness in plantation forests.

The differences in species composition and diversity between plantations and natural forests can be attributed to a number of factors. The use of exotic tree species in plantations has implications for indigenous forest species (Kholi 1998), which may have certain requirements that are not met by the exotic tree species or the habitat they create. For example, exotic tree species in Britain are inhabited by far fewer herbivorous insects than are found in indigenous forests (Kennedy and Southwood, 1984). By contrast, vascular plant species are generally not as discriminative and can colonise plantation forests regardless of the identity of the canopy species, provided the physical characteristics of the habitat are appropriate. Some plantations can have a surprisingly diverse understorey of indigenous species (Allen et al. 1995, Keenan et al., 1997, Oberhauser 1997, Viisteensaari et al., 2000, Yirdaw, 2001). However, there is considerable variation in the richness and abundance of understorey plants among planted forest stands. Some of this variation can be attributed to the amount of light available to understorey plants (Cannell 1999). Particularly dense stands of spruce and Douglas fir can cast so much shade that they appear to literally shade out the understorey vegetation (Humphrey et al. 2002). Likewise, single-species plantations of Rhizophora may prevent site colonization of other, non-planted mangrove species (Walters, 2000). The harvesting method of clearfelling places a strong constraint on species inhabiting plantations. For example, clearfelling dramatically changes the species composition of understorey plants (Allen et al. 1995), although the subsequent succession often restores the pre-clearfell understorey vegetation (Brockerhoff et al. 2001). Generally, site management practices in planted forests have direct impacts on biodiversity. Fertiliser use can lead to reductions in the populations of some native plant species but increases in the populations of others, especially if the site was degraded prior to reforestation. Fertilisation may also induce an increase in microbial diversity by accelerating turnover of organic matter (Nys, 1999). There is limited knowledge of effects of planted forests on the diversity of soil biota compared to other land_uses; it has been shown that longer rotations foster soil biodiversity for loblolly pine plantations in the southeastern U.S. (Johnston et.al. 2002) and also that short-rotation plantations have positive effects on biological soil fertility in the Congolese savanna environment (Bernhard-Reversat, 2001). Herbicide or insecticide application which are often associated with intensive management of plantation forests can also result in a temporary decrease in plant, fungi and insect biodiversity (Dreyfus, 1984). Short rotation management can also reduce the quantity of dead wood that are beneficial to saproxylic insect species (Jukes et al. 2002) or bryophyte species (Ferris et al., 2000) and may decrease the opportunities for colonisation by poorly dispersed, late-successional native plant species (Keenan et al., 1997). Short rotations will also limit the extent to which structurally complex understorey development will occur which can limit the suitability of plantation for some wildlife species.

Characteristics of species that can benefit from planted forests

As a habitat for other species, plantation forests are characterised by some constraints resulting from their more or less intensive management (see above). Clearfelling and comparatively short rotations favour the occurrence of ruderal plant species whereas some long-lived climax species may not be present, and harvesting disturbance may enable invasive exotic plants to invade plantation forests (Allen et al. 1995). However, older stands can provide habitat for indigenous shade-tolerant species that are typical of natural forest understories (c.f., Allen et al., 1995; Brockerhoff et al., 2001). Similar patterns have been observed for birds (Clout and Gaze, 1984), typically for relatively common species. All such species benefit from the additional habitat provided by plantation forests if they have replaced less suitable habitat. Plantation forests can also accommodate edge-specialist species (Davis et al. 2000) and generalist forest species that would benefit from any forest type (Christian et al. 1998, Ratsirarson et al. 2002). "Planting tree monocultures" has even been suggested as a method to restore forest vegetation on degraded land, by providing a sheltered forest environment that allows colonisation of forest tree species (Lugo, 1997).

Rare or threatened species are not often reported from plantation forests, but this is perhaps due to a lack of scientific study. Some notable cases of occurrence of such species exist, and these are often significant findings both as conservation issues and because they can have implications for the management of affected plantations. For example, large populations of threatened kiwi inhabit some pine plantations in New Zealand (Kleinpaste, 1990). The occurrence of these flightless endemic birds and other threatened species challenges plantation forest managers (Brockerhoff et al., 2001).

Spatial considerations

The role of plantation forests in benefiting biodiversity at a regional level depends very much on the location of the plantation within the landscape. In some circumstances, plantation forests can potentially have negative effects on adjacent communities because of invasive natural regeneration of planted trees in adjacent habitats (Engelmark, 2001) or alteration of hydrological properties. On the other hand, they can also make an important contribution to biodiversity conservation at the landscape level by adding structural complexity to otherwise simple grasslands or agricultural landscapes, and fostering the dispersal of species across these areas (Parrotta et al., 1997; Hunter, 1990; Norton, 1998). Even plantation forests that are less diverse than natural forests can increase bird diversity at landscape and regional scales, when they have habitat characteristics that are favoured by some species (Gjerde and Sætersdal 1997). In most tropical regions, wildlife species (especially bats and birds) are of fundamental importance as dispersers of seeds and soil microorganisms. Their effectiveness in facilitating plantation-catalyzed biodiversity development on deforested, degraded sites depends on the distances they must travel between seed sources (remnant forests) and plantations, the attractiveness of the plantations to wildlife (ability of plantations to provide habitat and food), and the condition of the forests from which they are transporting seeds (c.f. Wunderle, 1997). Plantation forests adjacent to exposed remnants of indigenous forest can therefore be beneficial because they provide shelter, reduce edge effects, and enlarge the habitat for some species, and they can also serve to increase connectivity among forest fragments (Norton, 1998). Such effects are most important in regions with sparse indigenous forest vegetation.

Of course not all plantations generate benefits such as these, and there is still much uncertainty about just how these outcomes might be achieved. Little is known, for example, of just how much of a deforested landscape must be reforested to allow biodiversity and self-sustaining forest ecosystems to be re-established. Likewise, little is known of where trees might be re-planted in a fragmented landscape to achieve an optimal biodiversity outcome.

4 Role of Biodiversity in Planted Forests

Biodiversity and ecosystem functioning

It is well known that living organisms, through their metabolism and growth, drive energy and matter flows that contribute to the structuring and functioning of ecosystems. It is more difficult to understand how the diversity of these organisms, i.e., biodiversity, affects these ecosystem processes. This question is a key issue in modern ecology but has also practical implications for agriculture and forest management. It is indeed of great interest to understand how changes in biodiversity can affect ecosystems functions (e.g. primary productivity, soil fertility or trophic interactions) that in turn can affect crop yields.

Three main hypotheses have been proposed to link biodiversity to ecosystem functioning (Naeem et al., 2002). The first is the 'species redundancy' hypothesis, which postulates that species lost from a system can be substituted by others and compensate for the functional role of the lost species without affecting the functioning of the ecosystem. The 'species singularity' hypothesis states that each species performs a unique contribution to ecosystem processes and that the loss of any species will cause a potentially significant change in these processes. The 'context-dependent role of species' hypothesis accounts for the unpredictability of the effect of the loss (or the addition) of species on ecosystem functioning, which can be either beneficial or detrimental, depending on the local and temporal context.

Functional role of biodiversity in planted forests

Most of the experimental studies that demonstrated increasing biomass production with richer species diversity involved grassland, wetland or microbial species (Naeem et al., 1994; Yachi and Loreau (1999); Tilman et al., 2002). Due to obvious technical difficulties in manipulating long-lived species like trees, relatively few manipulative experiments have so far addressed this issue in forests, but similar mechanisms are likely to apply in forest ecosystems. One area where manipulative experiments involving forests have been conducted is in the use of nitrogen-fixing trees to overcome nitrogen deficiencies. These studies, mostly involving two-species mixtures, have shown mixed-species stands improve plantation productivity (Binkley et al., 1992; Khanna, 1997, DeBell et al., 1997; Parrotta, 1999). In addition, several observational studies indicate higher growth performances in mixed than in pure stands of oaks (Bartsch et al.,1996) and spruce (Wang et al., 1995).

Diverse forests can be healthier than monocultures, and thus the trophic dimension of the biodiversity-ecosystem functioning relationship needs to be considered. Several reviews indicate that forest monocultures in all climatic regions may experience insect outbreaks that cause considerable damage (Gibson & Jones, 1977; Barthod, 1994). Until recently, the evidence in support of the view that insect pest outbreaks occur more frequently in plantation forests as a result of their poor tree diversity was controversial (Gadgil & Bain, 1999) because, in plantation forestry, confounding factors may occur such as even-age structure (Géri, 1980; Schwerdfeger, 1981), use of exotic species (Watt & Leather, 1988; Speight & Wainhouse, 1989) and intensive silviculture (Gibson & Jones, 1977; Ross & Berisford, 1990; Jactel et al., 1997). However, a recent review, based on a meta-analysis of more than fifty field experiments which compared pure stand vs. mixed stand of the same tree species, demonstrated a significant increase in insect pest damage in single-tree species forests (Jactel et al., 2002). Three main factors related to single-species forestry can predispose forest plantations to insect attack (Jactel et al., 2002). Firstly, the lack of physical or chemical barriers provided by other associated plant species that could reduce access of herbivores to the large concentration of food resources, i.e., the high density of host trees in the forest monoculture. Secondly, the low abundance or diversity of natural enemies often observed in forest plantations can result in limited biological control of pest insects. The third explanation is the potential absence of a diversion process, i.e., the disruption effect on pest insects resulting from the presence in the same stand of another more palatable host tree species.

Because of technical and economic constraints, it is unlikely that plantation managers will convert single-species stands into mixed-species stands simply in order to reduce pest damage that was normally only of minor significance. On the other hand they might if the commercially attractive tree species was especially valuable and the insect damage was significant. Thus Keenan et al. (1995) described the advantages and required trade-offs involved in using a temporary tree cover crop to minimise tip borer attack in red cedar (a member of the Meliaceae) in north Queensland. In this case the multi-species plantation reduced insect damage on the target species to an extent sufficient to make the plantation viable. On the other hand, the overhead cover also reduced the growth rate so that care had to be taken to balance survival against growth increment.

Alternative ways of achieving the functional benefits of diversity might be to increase plant diversity in the plantation understorey, but proper field experiments are needed to test whether this would be effective. A second option might be to consider increasing tree diversity at the landscape level. Growing evidence suggests that enhancing habitat diversity in plantation forest landscapes may prevent the development of pest insect outbreaks. For example, a study on spruce budworm, Choristoneura fumiferana, reported lower balsam fir mortality in stands surrounded by non-host deciduous forest than in stands within large conifer-dominated forest (Cappucino et al. 1998). Similarly, Jactel et al. (2002) demonstrated that pure stands of maritime pines bordered by a mixed woodland of broad-leaved species were less attacked by the stem borer Dioryctria sylvestrella than pure stands among a monoculture of pine trees. These findings indicate that the preservation or restoration of mixed-species woodlands, for example in gaps where site conditions or stand accessibility make timber production less profitable, could provide the basis for a more sustainable management of plantations forests.

5 Managing Planted Forests to Enhance Biodiversity: Suggestions for the Future

Genetic resources

By combining current scientific knowledge in the area of forest and tree genetics with common-sense forest management, general suggestions for preserving and enhancing genetic diversity in plantation forestry can be elaborated (Arbez, 2000):

• Monitoring and improving genetic diversity in breeding populations. The main concerns associated with the use of improved FRM are whether genetic gain and diversity can be simultaneously maintained at reasonable levels over successive generations during the whole selection programme. As many operational tree breeding programs conducted on fast growing species are entering their third or even more advanced generations, these questions have raised theoretical and experimental approaches which provide guidelines to geneticists for maintaining genetic diversity (Namkoong, 1988; Eriksson, 1993; White et al., 1993). Furthermore, conservation strategies can enrich the genetic base at any moment and must be used as a necessary complement of the breeding process.
• Controling quality of Forest Reproductive Material (FRM). Quality of a given FRM is directly related to the quality of the genetic information available, allowing its final user to optimally balance expected gains and possible risks. It means precise and reliable information on: (i) geographic origin of the parent gene pool (natural population or selected genotypes), (ii) identities, number, genetic characteristics of the parents and crossing scheme used to obtain the commercial variety, and (iii) selection procedures (description of the mono- or multi-site experimental design, selected traits and levels of genetic superiority assessed by comparison with well known reproducible standards). This information can be used to control quality of FRM and to favour FRM resulting from long term breeding scheme combining recurrent selection and gene resource conservation.
• Diversifying genetic resources at stand or landscape levels through the parallel development of available genetically improved varieties and limited utilisation of a given variety in space and time in order to prevent genetic uniformisation. The risks associated with improved FRM and decreased genetic diversity can be minimised by (a) using multi-clonal mosaic schemes, where genetic diversity within a stand at a given time is replaced by genetic diversity between stands at the landscape level; (b) limiting the monoclonal plantation area at the regional scale as well as the time during which a given clonal variety is permitted to be used.
• Evaluating genetic risks, in particular developing risk simulation methods and secured long-term trials to monitor impacts of introduction of GM trees in forest plantations prior to any commercial deployment and use (Kremer, 2002). Economic and biological constraints limit the number of GM trees created and impose to deploy them through clonal varieties; main recommendations for their use include: (a) male sterility, preventing pollen contamination of the surrounding forest related tree species; (b) testing not only in classical clonal tests (comparing one clone with a limited number of standard other clones, in well controlled conditions of experimental plantation) but also in long-term experimental field trials to evaluate environmental risks .

Stand management

Enhancing biodiversity in plantations can generally be achieved by increasing variability when plantations are established or tended (Hartley, 2002). The emphasis in the past has been on reducing variability to improve predictive capabilities and efficiency of establishment, tending, and harvesting operations. As a result, there is little experience with enhancing variability in plantation management settings. It seems likely however that many future plantation owners, especially those operating on a small scale, will be seeking more than just timber production from their plantations and might be willing to trade off efficiency and predictability for the sake of ecological services such as enhanced biodiversity.

This increased variability can be achieved in several ways. Perhaps the most obvious is to use multi-species plantations rather than monocultures. Random species assemblages are unlikely to be successful and care is needed to design mixtures that are stable as well as productive (FAO, 1992; Montagnini et al., 1995; Lamb, 1998). The choice of species and the number to use will also be affected by economic considerations. One of the potential advantages of diversity is that it provides insurance against future changes in market values but all potential species must have broadly similar values; if not, the opportunity cost of reducing the stocking of high value species to use lower value species may be too high. Various planting arrangements have been tested but alternate row plantings appear the most common. Plantations with more than one species planted in alternate rows may increase yields and facilitate removal of the slower growing species in an intermediate thinning. These mixed species plantation systems may also provide higher wood quality through mutual shading of lower limbs (Oliver and Larson, 1996).

Another way of achieving enhanced variability and diversity is by taking advantage of the "catalytic effect" referred to earlier. In many areas, single-species stands may be the intention, but natural regeneration of other species is inevitable and adds to diversity (Parrotta & Turnbull, 1997). In these situations, such as in the Douglas-fir region of North America, this natural regeneration could be encouraged during the vegetation control process. Similar biodiversity enhancement could be also achieved favouring a diverse plant understorey (Chey et al., 1998; Lamb, 1998). Given sufficient time this understorey community may grow up and join the canopy layer. This means it could compete with the original plantation trees and reduce their productivity. Some of the management options are reviewed in Keenan et al. (1997).

Even in plantation monocultures there is considerable scope for enhanced variability. Less uniform site preparation treatments, variations in tree spacing and thinning treatments can also enhance stand structure variability. Structural complexity of the planted forest is an important determinant of subsequent biodiversity enrichment due to the importance of habitat heterogeneity for attracting seed-dispersing wildlife and microclimatic heterogeneity required for seed germination for a variety of species (Parrotta et al., 1997). This suggests that broadleaf species yield generally better results than conifers, and that mixed-species plantings are preferable to monocultures, due in part to their increased structural complexity. Two-aged stands may also be a viable alternative in situations where clearcutting is aesthetically unpopular. Extending rotation length could also benefit biodiversity, particularly favouring diversity of soil biota and species associated with dead wood or leaf litter (Ferris et al., 2000; Magura et al., 2000). Maintaining snags, logs and other woody debris on site can also enhance habitat values for a range of species, from fungi to cavity-nesting birds. Management practices that increase soil organic matter content (such as spot cultivation, use of amendments, retention of harvest residues) and decrease soil disturbance during site preparation and harvest are desirable for maintaining the inherent biological capacity of soils and diversity of soil living organisms which are essential for nutrient conservation and cycling (Johnston et.al., 2002). Although management efficiency may be reduced, these more complex stand structures may be as productive, if not moreso, than comparable even-aged plantations (O'Hara, 1996). Whereas the productivity and actual effects on biodiversity of these structures are not well understood, there is additional uncertainty with regard to current tree breeding and the appropriateness of these trees in complex forest structures.

Landscape level

Forest management needs to consider plantations from a landscape perspective in that they comprise a spatial array of different elements that can be arranged in different ways depending on management goals. The key elements within a plantation forest are individual stands or compartments of different age and species composition, remnants of native ecosystems, including riparian strips, and amenity plantings. Observations suggest that managing plantation densities and creating irregularities within the spatial structures, favouring the proportion of borders and clearings, and preserving natural plant communities along rivers and in swampy areas would logically increase the level of associated plant and animal biodiversity. Retention of broad-leaved species among coniferous plantations (Ferris et al., 2000), or preservation of native remnants, have been proposed as a management tool to enhance biodiversity at the landscape level (Norton, 1998; Fisher et al., 1998).

Some of these elements are fixed in the landscape (e.g., native remnants and riparian strips) but others can be arranged in different ways. Humphrey et al. (2000) suggested locating plantations near existing semi-natural woodland fragments. In North America, spatial modelling tools have been used to optimise timber harvesting in native forests to meet biodiversity conservation goals (Bettinger et al., 1997). Similar modelling could be used to optimise the arrangement of different-aged plantation forest compartments, and different plantation species, to maximise timber production, biodiversity conservation and ecosystem stability. The key feature of this approach is that it considers biodiversity conservation at the landscape scale rather than at the stand scale and thus removes the direct conflict between biodiversity conservation and timber production at any individual site. The major potential difficulty, of course, is that land ownership patterns and consequently management decisions are often made at the local rather than landscape scale. Ways must therefore be found to ensure social outcomes as well as ecological outcomes at the landscape level.

In his analysis of the role of industrial plantations in large-scale restoration of degraded tropical forest lands, Lamb (1998) suggests a number of management approaches by which forest productivity (and profitability) and biodiversity objectives may be harmonized at the landscape level. These include: increased use of native rather than exotic species, creation of species mosaics by matching species to particular sites, embedding plantation monocultures in a matrix of intact or restored vegetation, using species mixtures rather than monocultures, or modifying silvicultural management practices to encourage development of diverse understories beneath plantation canopies.

Conclusions

There is no single, or simple, answer to the question of whether planted forests are good or bad for biodiversity. Plantations can have either positive or negative impacts on biodiversity at the stand or landscape level depending on the ecological context in which they are found. Objective assessments of the potential or actual impacts of planted forests on interspecific biological diversity at different spatial scales require appropriate reference points. In this regard, it is important to consider in particular the (biodiversity) status of the site (and surrounding landscape) prior to establishment of planted forests, and the likely alternative, land-use options for the site (i.e., would or could a site be managed for biodiversity conservation and other environmental services or be converted to agriculture or other non-forest uses?). For example, the establishment of an industrial pine plantation on a particular site will clearly have a more negative impact on stand-level biodiversity if it replaces a healthy, diverse, old-growth native forest ecosystem than if it replaces a degraded abandoned pasture system that was the result of earlier forest conversion. Thus, the ecological context of planted forest development, as well as the social and economic context shaping land-use change, must be considered in the evaluation of biodiversity impacts (Romm, 1989; Walters, 1997; Rudel, 1998; Clapp, 2001; Rudel et al., 2002).

The need to pay more attention to biodiversity issues in plantation design and management is supported by observational, experimental and theoretical studies that indicate that biodiversity can improve ecosystem functioning. While plantation monocultures have economic advantages, the need to ensure their long-term sustainability argues for greater research effort to develop design and management strategies that enhance plantation understory and soil biodiversity as well as their functional benefits. Many plantations are being established for the contribution they can make to overcome ecological degradation (e.g. soil salinity, erosion) and improve the long-term sustainability of land uses such as agriculture. Faced with the unpredictable, enhancing species diversity may improve adaptability of all managed forest ecosystems to changing environmental conditions (Hooper et al., 2002).

The primary management objective of most plantation forests has traditionally been to optimise timber production. This will continue to be the primary objective in most (though perhaps not all) industrial plantation programs but it will not necessarily be the case in many smaller scale plantations owned by farmers and other non-industrial groups. In these circumstances the management objectives may place greater weight on the provision of non-timber products and ecological services such as biodiversity. This will require the development of a new range of silvicultural tools to establish and manage these plantations.

Where managers are seeking to produce goods as well as ecological services, there are invariably difficulties in making the necessary trade-offs. These trade-offs operate at all levels of biological diversity. In the case of genetic diversity, for example, a balance must be struck between the need to identify the most productive forest reproductive material to plant at a particular site and the desire to re-establish the biodiversity represented in the original genotypes. Should a manager use highly productive planting material with a narrow genetic base that has been developed from an intensive selection program, clonal material or even genetically modified varieties? Or, should one rely instead on natural seed sources with a wider genetic diversity because these will confer greater resilience to the plantation enabling it to cope better with future environmental changes such as insect attacks or climatic events? Judicious use of relevant, well-known tree breeding strategies and gene conservation strategies can greatly facilitate efforts by managers to maintain genetic variability of plantation species over several generations and thus achieve better balance between economic and environmental benefits and risks.

Likewise, at the species level, should managers establish plantation monocultures or should they give greater emphasis to multi-species plantations? There are, of course, no simple answers to questions such as these because much depends on the fertility of the soils being planted (are they still able to support the original native species and the soil biota required for maintaining soil fertility and nutrient cycling processes?) and on the present objectives of the landowner. Usually some compromise between the two extremes is chosen.

A critical issue for the future of plantation forests is how to combine biodiversity maintenance and wood production at various spatial scales (i.e., stand, forest, landscape). One way to achieve a balance between biodiversity and productivity/profitability is through improved practices at the stand level or alternative silvicultural regimes (species mixture at different scales from individual trees to compartments of different sizes, age and clone mosaic) combined with biodiversity management at landscape level. This would include, for example, modification of extensive clear-felling practices to reduce coup sizes (i.e., plan for smaller compartments of same-aged stands that are dispersed within the plantation landscape) to achieve a better balance between economic and environmental objectives. Thus, it may be possible to achieve a degree of biodiversity at the landscape scale through diversification of plantation landscapes to create mosaics of different planted forest and natural vegetation habitats, even if each of the individual plantation stands within that landscape are established as simple monocultures. In many parts of the world, this will require a reorientation of current practices and, in particular, a shift from a stand-level to a forest- or landscape-level approach to the planning of all aspects of plantation management.

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