Forest plantations are in most situations artificial ecosystems, designed to be simple so that they can be easily and efficiently managed for the purpose of wood production. However, artificial ecosystems must not be simple or have a narrow focus. Recently, designer ecosystems have been proposed to create well-functioning communities of organisms that optimize the ecological services available from coupled natural-human ecosystems (Palmer et al, 2004). Here, the design may not just be concerned with the choice of tree species and the arrangement of trees, but with the creation of entire ecological communities and functioning landscapes to meet specific services, and the use of complex biotic interactions becomes the key technology (Shiyomi and Koizumi, 2001; Kirschenmann, 2007). Such designed plantation ecosystems are not modelled after historical references of ecosystem structure and function for a given location, as is typically the case in ecosystem restoration efforts. Instead, such systems may be designed to mitigate unfavourable conditions by means of novel mixtures of native and non-native species with particular traits that favour specific ecosystem functions (Palmer et al, 2004). They are thus not a substitute for natural systems, but in our highly modified world, they can take over functions of natural systems or ease the pressure on natural systems. In addition, these designed systems can take account of changing environmental conditions in the future, if the knowledge exists, how well the different species and communities will cope with or perform under the new conditions that may be brought about by climate change, species invasions or other forces shaping ecosystems (van der Meer at al, 2002; Seastedt et al, 2008). In more complex settings, plantations may take over the functions of completely different ecosystems such as wetlands or of technical solutions such as sewage treatment plants. For example, in an increasingly urbanized world, safe and environmentally sound ways to treat and dispose of effluent or biosolids produced by industries and urban centres may include the use of irrigated tree plantations (e.g. Hopmans et al, 1990; Myers et al, 1999; Borjesson and Berndes, 2006). In addition to being a safe and low-cost solution to the cleaning of wastewater and recycling of nutrients, these plantations can provide energy, timber and even amenity values. The combination of these functions is likely to be particularly promising in peri-urban areas of arid and semi-arid regions, where a large proportion of available water is appropriated by humans and where the increasing demand for firewood has led to the degradation and depletion of natural forest and woodland systems.
Plantations composed of salt-tolerant species and placed in specific locations in the landscape can assume important ecosystem functions, such as the maintenance of hydrological balance, to uphold the viability of agricultural landscapes in many dry regions of the world. Here, elevated saline groundwater tables, which may be the consequence of irrigation or of clearing deep-rooted perennial vegetation for pastures and cropping or of irrigation, threaten the continuation of agricultural land use (Johnson et al, 2009). Planting of trees to reduce the recharge of groundwater and to increase the discharge from groundwater can help to lower the water table or stop it from rising further to the surface (Chapter 4 in this book; Nambiar and Ferguson, 2005). Depending on the salt concentration of the groundwater and the height of the water table, different types of trees (or shrubs) may be most suitable for this purpose, in many cases non-native species (e.g. Mahmood et al, 2001).
These are just a few examples, where the main purpose of plantations may be on specific ecosystem services, and wood production assumes a secondary role. The conditions under which such purposefully designed plantations are established may be adverse or sub-optimal for tree growing, so that the costs for establishment and maintenance may not be recouped through the production of timber. Where the plantation owner is not also the direct beneficiary of the ecosystem service(s) provided, mechanisms such as payment for ecosystem services (PES) must be developed to reward plantation owners for providing these services. PES is often defined as voluntary, conditional transactions between the buyer and the seller for well-defined environmental services or corresponding land-use proxies (Wunder, 2005). A comparative study carried out by Wunder et al (2008) analysed 14 PES or PES-like programmes in developing and developed countries. The vast majority of the programmes studied were related to management or protection of watersheds through natural forests. Only three of the cases studied included forest and tree plantations (afforestation, reforestation, agroforestry) as eligible activity of the programme, indicating that so far plantations have not been used much within such schemes.
Payments for ecosystem services may be in the form of carbon credits and other credits for specific ecosystem services, e.g. salinity credits. However, where plantations on marginal land for tree growing are required over large areas to restore landscape or watershed functioning, the amount of financial assistance required for tree growers to break even can quickly reach very large sums. For example, Ferguson (2005) calculated for the Murray-Darling Basin in Australia that for eucalypt or pine plantations on land of lower productivity (mean annual increment of 15m/ha/yr) up to AU$2000-3000/ha respectively may be required in the form of salinity credits to achieve a net present value of zero, after accounting for revenues from timber and carbon credits.
One of the important challenges to facilitate schemes such as PES or other reward systems and to facilitate sound landscape planning is to develop ways to measure the ecological functions of plantations (Hartley, 2002; Dudley, 2005). If in future, plantation design and management should consider more fully the whole range of ecosystem goods and services, both agreed systems for valuing goods and services (see Chapter 2) and a good understanding about the compatibility, and possible trade-offs and synergies among ecosystem goods and services are required.
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