Convention on biological diversity
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Temperate Forest BiomesOverview of the functioning of the ecosystemOn a large geographic scale, the natural biodiversity of deciduous forests is the result of the distribution of species along different moisture and temperature gradients. At site level, only one or a few species gain dominance (Schulze et al., 1996). These trends are also reflected in relation to elevation. Temperate forests are dominated by deciduous species and to a lesser extent by evergreen broad-leaf and conifer species. More than 1200 tree species are represented in this biome (UNEP, 1995). Soil texture and fertility (determined by the base saturation) is a major factor in determining temperate forest biodiversity at site level. Soil acidity and nitrogen are important in determining the pattern of biodiversity, with more species growing on neutral or alkaline soils than on acid soils (Schulze et al., 1996). Under natural conditions, the highest diversity is reached at high base saturation and high nutrient levels (Schulze et al., 1996). However, decreases in woodland plant species diversity have been observed as a result of anthropogenic nitrogen deposition, for example in parts of Europe. Although high nitrogen deposition could potentially offset harvesting losses, it is also likely to exacerbate the acidification of soils (Schulze, 1989). Effect of species and genetic diversity on forest ecosystem processesEffect on productivity and on nutrient cyclingThere is no evidence for an effect of biodiversity on net primary productivity in semi-natural forest stands. Productivity responds to available light and nutrients regardless of the number of species present in the stand (Schulze et al., 1996). Some authors have shown that productivity is instead related to soil fertility and stand age. The factor most strongly correlated with productivity and nitrogen cycling is soil texture (Aber et al., 1991) and the correlation is stronger in stands that have never been harvested. This suggests that disturbance by human activities can affect biogeochemical cycles by changing soil properties (Schulze et al., 1996). Effect of biodiversity on forest resilienceIn general, greater species diversity increases the resilience of ecosystems against external disturbances (Tilman and Downing, 1994). In a stand containing diverse species, the loss of one species can be compensated for by the remaining ones. For example, the extinction of Castanea dentata in eastern North America (caused by a fungus) left a gap, which was filled by Quercus spp., suggesting redundancy within a diverse system. The same situation applies in the case of Ulmus, which was largely eradicated by Dutch elm disease in Europe, Canada and the USA, and was then replaced by Acer, Fraxinus and other species (Schulze et al., 1996). Forest resilience appears to diminish with growing anthropogenic effects such as air pollution and nitrogen deposition. Conifer stands in some parts of Europe, were seriously affected by anthropogenic pollution caused by sulphur and nitrogen deposition (Schulze, 1989). Abies alba showed signs of decline, but there was no loss of stand productivity where it grew in close association with Fagus sylvatica, which compensated for the loss. Abies alba disappeared over vast areas, apparently without any major effect on ecosystem functioning. In the case of Picea decline, Fagus and Quercus also showed decline symptoms and increased susceptibility to parasite damage (Schulze et al., 1996), which had more obvious effects on forest structure. Impact of human activities and their consequences on the delivering of good and servicesOnly 3% of old growth temperate forest remain, and in many regions temperate forests have a history of management and exploitation which has occurred over a large proportion of their area. Europe has a particularly long history of human use of forests and there are very few, if any, truly natural forests remaining. Many large predators have been lost from large areas for centuries, and domestic and wild grazing animals often have a strong influence on woodland composition and structure (Mayle, 1999). Woodland biodiversity has become adjusted to the traditional management and land-use patterns, with a higher proportion of species associated with open and edge habitats and younger growth stages, leading to more generalist species than in the past. However, historical management systems, notably coppice, wood pasture have declined over recent decades and a high forest structure is becoming more predominant. Ceasing management altogether, especially in fragmented woodland areas, is therefore likely to lead to loss of biodiversity and also other goods and services, such as recreation values, which depend on maintaining a range of age classes and open and wooded areas. Therefore, although there is a need for minimum intervention in some protected forest areas, (and there will be a place for new and restored natural forest in which the aim is to allow development with little intervention) the need in much of the temperate forest in Europe is to adjust forest management to modern conditions, rather than question whether to manage at all. Effect of management on ecosystem functionsDifferent types of management systems can have different types of impacts on the forest ecosystem functioning. For example, patch felling may have a stronger local impact compared to selective logging, especially on soil fertility, as organic matter is more rapidly decomposed. Also, patch felling produces gaps in the forest, creating an edge effect and modifying the micro-climatic conditions: the air and soil without the shade and protection of trees tend to be dryer and warmer [reference needed?]. However, selective logging and patch felling at scales and intensities suited to the regeneration ecology of the main tree species are generally not disruptive to the ecosystem and should not significantly affect the future potential to supply most goods and services. Larger scale clear-felling, for example over areas of several hectares, causes substantial fluctuations in microclimate and soil and species composition. Logging roads can have a strong impact on the overall forest ecosystem (an impact which can be even greater than the tree harvesting itself), especially when layout and use do not take into account the restrictions of topography, soils conditions and hydrology. The conversion of broad-leaf forest into coniferous plantations to increase timber production rates was prominent during the twentieth century in deciduous forests in some temperate countries. In these countries, large scale conversion began more than 100 years ago but its intensity is now reduced with new approaches advocating greater use of broad-leaf and native species. For example, Fagus sylvatica forest was massively converted to Picea abies plantations in Germany and deciduous forests in Japan are still being converted into Cryptomeria japonica in the south and Abies and Picea in the north (Schulze et al., 1996). Nearly 40% of the ancient semi-natural broad-leaf woodland in the UK was converted to conifer plantation between around 1930 and 1985; after 1985 policy shifted towards conserving and restoring native woodlands (Kirby et al., 1989). The impacts of such changes on ecosystem functioning are important. The change from deciduous forest to conifer species often results in a decrease in the cycling of nitrogen between plant canopies and the forest soil, which can be 75% or more (Schulze et al., 1996). The immediate consequence is increased loss of nitrate to ground water, which can affect water quality, notably where management involves large scale clear felling. There may be a long-term risk of soil acidification (Tsutsumi, 1977). Restoration of acidified soils often implies liming, which can have further impact on ground water quality by increasing nitrogen leaching. Other impacts of coniferous conversion on diversity and related goods and services include reduced light penetration associated with evergreen foliage, which maintains drier soil conditions and this, together with the reduced litter decomposition rates caused by the chemistry of the needles, acts to inhibit the herb layer (Schulze and Gerstberger, 1993) at least during the younger stages of the forest rotation. FragmentationForest fragmentation can lead to an increase in species-richness through edge effects, but often this is achieved by adding generalist woodland/agricultural edge species at the expense of the loss of forest interior specialists (Andren, 1994). Agriculture, urbanization and road building have substantially altered temperate forest integrity and ecosystem functioning. Fragmentation increases ecosystem vulnerability to other impacts such as invasive species, drift of fertilisers and herbicides from agricultural land, and climate change, which may cause localised extinctions of vulnerable species in future. Roads, including logging roads, introduce corridors, which promote the introduction of invasive plants, insects and pathogens (Schulze et al., 1996), in addition to fragmenting the natural habitat for wildlife and modifying the drainage pattern. Studies have shown a correlation between the density of forest roads and the decrease of some animal populations (Brocke et al., 1989). Furthermore, poorly designed or operated logging roads can have a strong negative impact on the quality of the water as a result of erosion (Aber et al., 2000). Effect on carbon sink capacityTemperate forests, along with the forests of the boreal biome, represent an important terrestrial net carbon sink (net uptake of carbon) as forest regrowth absorbs carbon dioxide from the atmosphere, where absorption rates exceed respiration rates. This compensates, to some extent, for the net emissions resulting from land use changes in the tropics (UNEP, 2000). This situation is explained by land use practices, natural regrowth, the indirect effects of human activities (e.g., atmospheric CO2, fertilisation and nutrient deposition) and changing climate (UNEP, 2000). Expected growth in plantation areas will absorb more carbon, but the likely continued rate of deforestation will mean the world’s forests remain a net source of CO2 emissions and a contributor to global climate change (WRI et al., 2000). A plantation forest, managed for maximum volume yield, will normally contain substantially less carbon than the same area of unmanaged primary forest (Cannell, 1999). Multi-purpose planted forests, where some areas are allowed to mature and remain unharvested for biodiversity and other benefits should increase carbon sequestration compared to single purpose timber plantations and provide the best overall mix of goods and services. Water distribution and qualityWatershed deforestation and subsequent inappropriate agriculture practices and soil erosion have had important effects on water quality and water quantity of streams and rivers. In some mountainous regions watershed protection is regarded as the main function of forests, for example in parts of Austria, Switzerland and Japan. |