Woodland Ecosystems As A Global Resource

Woodland ecosystems are a major planetary resource, involving about a third of the land surface. Forests and woodlands provide a habitat for biological diversity. They contribute substantially to the global carbon cycle and are the natural venue for many derived pharmaceutical, practical, and edible products. Woodlands are also an environment for indigenous peoples, and provide much aesthetic and amenity value. Alarmingly, our forests are being lost at a rate of around 14.6 X106 ha/yr due to pollution, disease, and excessive exploitation (Anon 2001).

Wood accounts for around 80% of the global organic carbon reservoir. About 70% of woodland net primary production is deposited annually as litter, whilst the remaining 30% accumulates at least transiently as woody biomass (Boddy 1991; Rayner and Boddy 1988). Below ground, persistent lignified roots may contribute between 18 and 22% to the total subterranean biomass (Boddy and Watkinson 1995). Thus, approximately 2 ton of plant biomass may be deposited annually per hectare of forest in temperate regions due to the accumulation of leaf litter, fallen branches and periodically, whole trees. Together with carbon reserves, deadwood litter fall is estimated to retain between 1.1-3.7 and 0.1-0.8kg/(hayr) of nitrogen and phosphorus, respectively, which serves to immobilize essential nutrients until decay is achieved (Boddy and Watkinson 1995; Dighton and Boddy 1989). Without decay, nutrient reserves would accumulate and eventually halt ecosystem productivity.

Saprotrophic wood-decay fungi play a pivotal role in the ecology of forests as they are the principal agents of wood and chitin decomposition and hence nutrient and energy fluxes (Dighton 1997). Unsurprisingly, fungi therefore represent the dominant microbial biomass of the forest floor and soils of many forest ecosystems, of which 60% may be due to decomposer basidiomycetes. Energy flow through woodland ecosystems is ultimately dependent on a limited mineral nutrient availability. Consequently, the balanced cycling of mineral nutrients within woodlands is central to ecosystem functioning, and fungal participation in mineral cycling is extensive (Boddy 1991; Dighton and Boddy 1989; Wainwright 1992). Fungi have been shown to be capable of all major nitrogen transformations except nitrogen fixation, although certain fungi may sequester nitrogen from nitrogen fixing bacteria within wood and fungal sporocarps (Barron 1992). Fungi can utilize organic (during decay) and inorganic (solubilized by organic acids and siderophores) phosphorus sources and many fungi produce siderophores to sequester limiting supplies of bioavailable iron (Cromack and Caldwell 1992). Mineral resources are transformed into new fungal biomass, organic acids, and humic substances. Indeed significant quantities of nutrients may be immobilized as fungal biomass, and this immobilization has resulted in the adoption of management practices such as burning to eliminate woody debris from forestry sites prior to re-plantation, thereby reducing potential competition for mineral nutrients between fungal biomass and tree transplants (Dighton 1995). Nutrients sequestered as fungal biomass may be released subsequently as a result of processes such as interactions with other fungal individuals or bacteria, by physical damage or invertebrate grazing and withstanding these, by senescence and death of the mycelium (Boddy and Watkinson 1995).

The biological diversity contained within woodland ecosystems may be exploited for practical and aesthetic gain. Traditional methods of exploitation have involved collection or cultivation of fungi for food, e.g., truffle fungi, Lentinula edodes (Shii-take), and Pleurotus ostreatus (oyster mushroom) as well as many other edible woodland fungi. Wood colonized by certain fungal species may be employed to generate valuable timber products. For example, "Brown oak" veneer timber (pourriture rouge dur) is produced by Fistulina hepatica colonizing heartwood, Chlorosplenium aeruginascens is used in the commercial production of Tunbridge ware, and wood containing interaction zone lines is turned to produce decorative artifacts. Novel methods of exploitation may involve the application of fungal decay systems to convert a range of renewable lignocellulosics into protein, fermentable sugars, and other products, or to bioremediate certain recalcitrant pollutants. Appreciation and understanding of fungal activities, relationships, and distributions in natural ecological contexts may provide insight into methods of searching and screening for useful products and conversion systems, and support the management and conservation of our valuable global resource. As Hawksworth and Colwell (1992) succinctly expressed as part of the Microbial Diversity 21 statement, saprophytic fungi, in common with other microbes, "are vital to the function of the Earth's ecosystems and biosphere. As major contributors in biogeochemical cycles, they perform unique and indispensable activities in the circulation of matter in the world, on which all larger organisms, including humans, depend. They constitute a genetic resource of great potential for contributing to the sustainable development of the planet as well as human, animal, and plant health. Urgent attention is required to redress our ignorance on many key aspects of their scientific understanding, their distribution and functions."

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