Forest Cover
Native plantation for biodiversity restoration
New plantations should utilize indigenous species to accelerate biodiversity.
Tropical forests are considered the most endangered due to deforestation, degradation, fragmentation, shifting cultivation, clear cut, illegal logging and other types of anthropogenic disturbances. Now these forests consist of only 10% of their former areas and 12% of their former primary vegetations. About 1.8% (in Amazonian forests, probably 2.6%) of the tropical forests is disappearing annually. These rates indicate that one Florida per year is being destroyed; one football field is logged per second. Tropical forests are also subject to destructive natural forces -- cyclones, landslides, floods, mud flows, volcanic eruptions, fire, drought, and climate change.
On the other hand, in some areas the forest and tree coverage have been increased through afforestation and reforestation. According to FAO (2006) timber plantations are being expanded at a rate of 2.6 million hectares per year. The researchers are finding low levels of biodiversity in plantation forests compared to protected natural forests. Some plantation species provide critical habitat for endangered species. It is true that they provide wildlife corridors and play an important role in sustainable development. Plantations may be a “lesser-evil” alternative to agriculture or urban development in terms of conserving species diversity.
Plantations can do little to conserve biodiversity, but they act as a carbon sink and conserve soil. In fact, environmental outcomes of plantation forests, including effects on soil carbon and biodiversity depend upon the characteristics of both the plantations and the previous land uses. Amphibians and reptiles can be easily colonized in the plantation forests. Plantation trees can work as the basis of food chain and contribute to important ecosystem services including climate regulation, water purification, and pollination
A number of factors such as land use, plantation species, plantation age and establishment influence biodiversity. Afforestation of natural ecosystems usually alters habitat substantially for native flora and fauna. The loss of plant diversity and richness with afforestation of natural and semi-natural grasslands and shrublands is attributed to a number of factors including site preparation, exclusion of shade intolerant native species by plantation canopy cover and allelopathy. It can affect plant diversity, community structure and native species richness of grasslands. Changes in community structure are also reflected in changing proportion of exotic and native species.
Primary forests usually support higher levels of native species richness and abundance than plantation forests. Poor species richness in plantations compared to primary forests is likely due, in part, to the high level of structural complexity in natural forests that is required for seed germination in some plant species, particularly late seral and animal dispersed species, and for the paucity of seed sources.
Generally plantation forests contain a subset of primary forest species with lower levels of diversity and richness. On the other hand, plantations (particularly young plantations) also tend to favour establishment of ruderal or exotic species over large, gravity dispersed or late seral species. It is clear from the biodiversity perspective that primary forests (and other non-forested natural lands) should not be converted to plantations.
Rapid growth plantation may be beneficial for some wildlife species. Generally plantation forests have less developed understories due to tillage and intercultural operations. Very old plantations can play an important role in biodiversity conservation. Plantation forests can perform like natural forests if they are composed of locally occurring native tree species, and in some cases it may be difficult to distinguish older stands from natural forest. The species used in plantation can play a particularly important role in secondary forest to plantation conversions. Exotic species plantations support lower levels of plant diversity while native plantations support more diversity. This is important to facilitate on natural regeneration and plantation establishment; in tropical regions, the area of natural forest is converted to plantations.
Plantation forests often support intermediate levels of biodiversity, which are lower than the natural ecosystems but higher than other human-modified landscapes. Considering the economic point of view, plantations aid restoration in degraded areas where native regeneration may otherwise be inhibited, by improving soil conditions through increased organic matter and litter production, by shading out competitive grasses and other light-demanding species, and by creating a microclimate more favourable for seed dispersal and colonisation, particularly for animal-dispersed species.
Effective restoration of biodiversity depends on past land use pattern, distance to native seed sources, persistence of root stocks and seed bank, and presence of seed dispersing wildlife, as well as plantation species, age, and management. Native plantations may be a better choice for the restoration of degraded or exotic grasslands as well.
In many cases plantation is the only economic means to overcome large scale degradation. In this condition the issue is not whether to establish plantations but, rather, what kind of plantation to establish. Native species plantations may create better canopy cover and soil chemistry conditions that favours native over exotic species to be colonized.
Exotic plantations are less species rich than natural and semi-natural ecosystems, and support a less diverse flora and fauna. Native plantations if increase floral diversity or not, but have extra value for faunal diversity due to mating cycles and fruit and nectar quality. Plantations with native species are important for the endangered faunal species providing an important restoration tool that balances environmental and economic goals. A vast number of invertebrates and microorganisms only survive in native plant species.
Native plantations are also important from landscape perspectives as they preclude the risk of exotic trees associated with exotic plantations. Nevertheless, native species are increasingly recognized as valuable timber species in many countries like China and USA.
Older plantations established on previously forested lands support the higher levels of diversity and developed structural complexity. It creates congenial microclimates and litter and humus layers that are favourable for native plant colonization. Contrarily, plantations established on natural or semi-natural shrublands and grasslands have a negative effect on native species with age, increasing canopy cover, and with multiple rotations.
Biodiversity in the plantation forests can be enhanced through (1) conserving remnant native trees, snags, and cavity trees during harvest, (2) planting long rotational tree species, (3) utilizing native species over exotics and polycultures over monocultures, (4) avoiding intensive site preparation, and (5) thinning some plantations heavily and others not to maintain a mosaic of open to non-open areas to encourage native species colonization.
In many cases regeneration of native species in plantations may depend on colonization from adjacent or nearby native ecosystems. Canopy closure is also an important factor for influencing understory richness. But thinning may facilitate establishment of shrubs and herbaceous flora. It can also favour primarily generalist and exotic species which thrive with increased light and space. Medium level of human disturbances can be somehow beneficial for biodiversity, but severe disturbance creates conditions few plants can tolerate. The short rotational plantations discriminate against old forest succession species, decreasing the value of plantations as compared to natural forests.
The plantations are more likely to positively benefit biodiversity (particularly in terms of favouring native over exotic species) on degraded or exotic land covers rather than as a replacement of natural ecosystems, whether those systems were originally forested or not. New plantations should utilize indigenous tree species to accelerate biodiversity within plantations. Plantations can play an important role in biodiversity conservation and recuperation, particularly at the landscape level.