Linkages Between Tree Diversity, Soil Fauna and Ecosystem Processes

Abstract

The decomposer food web essentially relies on resources built up by primary producers above the ground. While the diversity of the decomposer organisms is not intimately linked to plant diversity, plant traits such as nitrogen and polyphenol concentration are key drivers for belowground community structure. The recycling of plant residues is driven by soil organisms; the above- and belowground food webs therefore complement each other and the interrelationship may be viewed as mutualistic partnership. Traits of plant litter may have evolved to support or suppress certain decomposer organisms and their associated functions. Also, plants may be able to direct resources to foster certain mutualists in the rhizosphere, such as mycorrhiza or bacteria and their associated grazers. Activity and structure of the decomposer food web feed back to plant growth, vegetation development, and the aboveground food web in a variety of ways. Two feedback scenarios have been distinguished: (1) the detritivore-plant-herbivore pathway, i.e., soil organism-mediated changes in plant growth viewed as bottom-up control of the above-ground food web; (2) the detritivore-generalist predator-herbivore pathway, i.e., the fostering of top-down effects exerted on plant herbivores by generalist predators,which also live on prey from the decomposer community. These complex feedback mechanisms suggest that changes in the decomposer community caused by tree diversity and species mixtures feed back to trees themselves and the whole aboveground food web. The decomposer food web therefore not only contributes to ecosystem properties associated with below-ground matter transfer, such as nutrient losses and humus formation, but also to aboveground processes such as the control of pest species. Management practices intending to conserve the biodiversity of forests and the functioning of forest ecosystems therefore need to more closely consider the structure and driving forces of the decomposer food web.