Forest plantations have direct ecological effects in addition to the positive impact of reducing pressure on natural forests. Generalizations are difficult, however, in part because plantation management regimes are diverse and the appropriate comparison is not always to unmanaged natural forests. In worst-case scenarios, natural forests or savannas on fragile soils are converted to plantations of exotic species that lower groundwater tables, decrease biodiversity, and develop extreme nutrient deficiencies in successive rotations. While this scenario overstates the impact of plantations, their generally monoculture nature and intensive management raises concerns about the effect of plantations on biodiversity, water, long-term productivity and nutrient cycling, and susceptibility to insects and diseases.
Biodiversity illustrates the complicated ecological impact of forest plantations; although biodiversity encompasses genetic, species, structural, and functional diversity, much of the focus in discussions about diversity has been at the genetic, species, and local ecosystem levels. As has occurred in agriculture, the introduction of genetically improved exotic or native species in forestry increases productivity and carbon-fixation efficiency. In some regions, this introduction has also increased interspecies diversity at landscape and regional scales. In France, compared with 70 natural forest tree species, 30 introduced species that are commonly used in forest plantations have helped increase the interspecies genetic diversity of forests at the local level. In Europe, at least, there is no doubt that the introduction of new tree species has increased the species richness of forests. Nevertheless, exotic species, even those long naturalized species such as Douglas-fir (Pseudotsuga menziesii) are unacceptable in nature conservation schemes.
Exotic species can have negative impacts on native species and communities. For example, fast-growing species can replace native forest species because of their natural invasive potential, as have been observed with Eucalyptus in northwestern Spain and Portugal. As the introduction of exotic species has potential risks, confirmation of long-term adaptation to local environmental conditions and pest resistance is necessarily the first step for the use of exotic species in extensive plantation programs.
Plantations tend to be even-aged and managed on relatively short rotations; thus, simple stand structures are common. When repeated across a landscape, large areas of similar species and low structural complexity result in a loss of habitat for taxa that require the kind of conditions provided by naturally regenerated stands or old forests. It has been reported that the bird fauna of single-species plantation forests is less diverse than that of natural and seminatural forests. In other cases, however, bird species diversity in plantation forests is comparable with that in naturally generated stands. For example, cottonwood (Populus deltoides) plantations in the Mississippi River Valley in the southern United States are intensively managed (rotation lengths of 10-15 years), reaching crown closure in 2 years. In comparison to natural stands, bird species diversity and abundances are similar for all guilds except cavity nesters.
Where avian diversity is decreased in managed forests generally, loss of structure following harvest is usually the cause. In plantations, simplified structure may be exacerbated further by use of exotic species or by monoculture. Because plantations are harvested at or near economic optima, rather than at biological maturity, plantations seldom develop much beyond the stem exclusion stage of stand development and do not re-establish characteristics of old forests or complex stand structures such as snags and coarse woody debris. Strategies to compensate for the simplifying tendencies of plantations and integrate biodiversity considerations include complex plantations composed of multiple species, varying planting spacing, thinning to variable densities, and retaining uncut patches and snags after harvest. Such biological legacies should benefit invertebrates such as saproxylic beetles as well as fungi, small mammals, and birds.
Silvicultural and site management practices of site preparation, competing vegetation control, and fertilization may reduce understory and groundcover vegetation diversity, although the effects of previous land use such as agriculture may play a larger role. For example, in southern United States industrial pine plantations, understory diversity was correlated with previous land use; lower diversity of native forest species occurred in plantations established on former farmland and higher diversity in plantations on cutover forest land.
Some species can benefit from forest plantations. For example, clear-cutting and short rotations favor the occurrence of ruderal plant species over some long-lived climax species. Forest plantations accommodate edge-specialist bird species and generalist forest species such as deer. Some rare and threatened species have been found to occupy forest plantations, especially when they lost most of their habitat to agricultural and urbanized land uses. For example in the UK, the native red squirrel is out-competed in native woodlands by the gray squirrel introduced from North America but the red squirrel thrives in conifer plantations, which are poor habitat for the gray squirrel.
Spatial considerations play a role in maintaining biodiversity at the landscape scale. Landscape diversity can meet the habitat needs of wildlife and be achieved by varying the size and shape of plantations and incorporating adjacency constraints into harvest scheduling models (i.e., a plantation adjacent to a recently harvested or young stand cannot be harvested until the adjacent stand reaches a certain age or crown height). Retaining areas of naturally regenerated forest, riparian buffers, or open habitat creates a landscape mosaic that combined with prescribed burning in fire-affected ecosystems, adds to landscape diversity. Landscape connectivity that provides dispersal corridors for mobile species is fostered by careful placement of forest roads and firebreaks.
Concerns about plantations and water are as varied as the issues surrounding biodiversity but generally relate to water use, water quality, or alteration of natural drainage. Species of Eucalyptus planted outside their native Australia have attracted the most negative attention for their putative excessive water use, especially in Africa and India but Populus species have similarly been accused in China of lowering local water tables and adding to drought. Species such as Eucalyptus camaldulensis, E. tereticornis, and E. robusta (and hybrids of these and other eucalypts) are drought tolerant and able to transpire even under considerable moisture stress. On balance they probably do not use more water than adjacent natural forests but certainly use more of the available water than grasslands or agricultural crops. There is little evidence that they can abstract groundwater; however, there is no recharge below the root zone. In the Wheatbelt of Western Australia, removal of the deep-rooted native vegetation including eucalypts and conversion to cereal crops has caused water tables to rise with subsequent salinization of soils and surface water bodies. Plantations of oil mallee crops (E. polybractea, E. kochii subsp. plenissima, and E. horistes) are planted to restore natural hydrology and counteract salinization.
Negative effects of plantations on water quality and aquatic resources are more due to intensive management than to use of exotic species. Intensive mechanical site preparation, especially on sloping sites, can result in sediment movement into streams. Chemical herbicides are used to control competing vegetation at various stages in the plantation growth cycle, but usually for site preparation in place of mechanical treatments or early in the life of the stand to release crop species from competitors. Less intense site preparation, formulations of herbicides that are not toxic to insects or other aquatic organisms and break down in soil, careful placement of chemicals to avoid direct application to water bodies, and designation of riparian buffers all have contributed to protection of water quality.
Harvesting practices, especially placement and construction of harvest roads and layout of skidding trails, potentially can degrade water quality. In developed nations, forest practices such as site preparation, harvesting, use of herbicides, and even choice of species may be regulated to some extent. In the United States, best management practices (BMPs) to address nonpoint source pollution and protect water quality have been codified by state agencies and landowners follow them voluntarily. Research shows generally high rates of compliance. Certification schemes substitute the coercive power of the marketplace for that of government; the various certification bodies differ in how they regard plantations, especially with regard to the use of herbicides, exotic species, or genetically modified trees.
Use of inorganic fertilizers to overcome fertility deficiencies, promote rapid growth, and sustain biomass accumulation generally has been found to have little impact on aquatic systems unless fertilizers are applied directly to streams, lakes, rivers, or adjacent riparian zones. Greater attention has focused on nutrient removals in harvests and the potential for intensive management to reduce site fertility and cause a fall-off in productivity of subsequent rotations. Claims of later-rotation productivity declines have been hard to substantiate, however, as general improvements in seed and seedling quality, genetic makeup, site preparation and competition control, and more careful harvesting that conserves site fertility have raised, rather than lowered yields. Nevertheless, there exist documented cases of lowered fertility caused by export of nutrients in the harvested wood. These localized cases have been caused by low initial fertility, often of phosphorus, potassium, or micronutrient deficiencies inherent in the soil parent material that are easily overcome by application of inorganic fertilizers.
In the most intensive management of pine plantations for pulpwood in the southern United States, some companies routinely apply complete nutrient mixes containing all macro- and micronutrients as a precaution, despite lack of demonstrated deficiency of most nutrients except phosphorus and a responsiveness to added nitrogen. A stand may be fertilized with nitrogen up to five times in a 25-year rotation, sometimes in combination with phosphorus. These stands occur mostly on relatively infertile Ultisols and Spodosols developed on old marine sediments. On better soils (Alfisols, Entisols, and Vertisols), cottonwood plantations managed on 10-year rotations receive only an initial application of nitrogen at planting to promote rapid height growth to better compete with herbaceous competitors. Management of site nutrients in intensive plantations is critical to high yields as well as to protect long-term productivity and may require attention to retaining soil organic matter, especially on sandy soils. Factors to consider include inherent soil fertility (nutrient stocks as well as transformations and fluxes), plant demand and utilization efficiency, and nutrients export in products removed as well as leakages.
It is common wisdom that monoculture plantations are more susceptible than natural forests to insect and disease attacks, yet there is little evidence this is generally true. On the one hand, single-species stands occur naturally and some of these natural vegetation types are the product of periodic, catastrophic disturbances such as pine bark beetles or spruce budworm. On the other hand, one explanation for the often greater productivity of exotic tree species than attained in their native habitat is the lack of yield-reducing insects and diseases. But diversity in the abstract is not a guarantor of lessened risk; diverse, multiple-species stands themselves are not immune to devastating attack by introduced pests, a situation likely to increase in frequency as a result of globalization of trade in timber products.
Often the practices associated with intensive management are the causes of insect and disease problems. For example, the desire to maximize wood production may set the level of tolerable damage from native pests lower than the stable equilibrium levels for the pest; attempts to control the pest at lower levels may cause unstable population growth cycles. The potential risks of plantations stem from their uniformity: the same or a few species, planted closely together, on the same site, over large areas. Pests and pathogens adapted to the dominant species may build up quickly due to food supply and abundant sites for breeding or infection. Proximity of the branches and stems in closely spaced stands may favor buildup of species with low dispersal rates or small effective spread distances. Conversely, the same uniformity of plantations that contributed to the risks of insects and diseases also confers some advantages. Species can be chosen that have resistance to diseases, for example, the greater resistance of loblolly pine (Pinus taeda) compared to slash pine (P. elliottii) to Cronartium rust was one reason loblolly was favored by forest industry in the US South. The shorter rotation length of plantations relative to naturally regenerated stands means trees are fallen before they become overmature and become infected. The compact shape and uniform conditions in plantations facilitate detection and treatment of economically important pests and pathogens.
Plantations may negatively impact adjacent communities - because of invasive natural regeneration of planted trees in adjacent habitat or alteration of local and regional hydrologic cycles and poor management practices may damage aquatic systems. Plantations are certainly simpler and more uniform than naturally regenerated stands or native grasslands, and may support a less diverse flora and fauna. Nevertheless, plantations can contribute to biodiversity conservation at the landscape level by adding structural complexity to otherwise simple grasslands or agricultural landscapes and by fostering the dispersal of forest-dwelling species across these areas.
Further, comparisons of plantations to unmanaged native forests or even naturally regenerated secondary forests are not necessarily the most appropriate comparisons to make. Although the conversion of old-growth forests, native grasslands, or some other natural ecosystem to forest plantations rarely will be desirable from a biodiversity point of view, in that forest plantations often replace other land uses including degraded lands and abandoned agricultural areas. Objective assessments of the potential or actual impacts of forest plantations on biological diversity at different temporal and spatial scales require appropriate reference points. Forest plantations can have either positive or negative impacts on biodiversity at the tree, stand, or landscape level depending on the ecological context in which they found. Impacts on water quantity and quality can be minimized if sustainable practices are followed; similarly with soil resources and long-term site productivity. Both complex plantations for wood production and environmental plantations can beneficially impact local and regional environments.
Lastly, managing forest plantations to produce goods such as timber while at the same time enhancing ecological services such as biodiversity involves tradeoffs; this can be made only with a clear understanding of the ecological context of plantations in the broader landscape. Tradeoffs also require agreement among stakeholders on the desired balance of goods and ecological services from plantations. Thus, there is no single or simple answer to the question of whether forest plantations are 'good' or 'bad' for the environment.
See also: Boreal Forest; Temperate Forest; Tropical Rainforest.
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