Impacts of forest harvesting on biological processes in northern forest soils

Soil is the habitat of plant roots and the home of numerous microflora, including viruses, bacteria, fungi and blue-green algae, and a host of animals from unicellular protozoans to small vertebrates. Simple communities of soil organisms, present from the earliest stages of forest soil genesis, become more complex and grow to astronomical numbers in mature forest soils. Here, they are essential for the maintenance and productivity of these soils, which unlike agricultural systems, generally receive less mechanical treatments and chemical inputs. The soil microflora and fauna complement each other in the comminution of litter, mineralization of essential plant nutrients, and conservation of these nutrients within the soil system. Harvesting directly affects these processes through the reduction and redistribution of organic matter, compaction, changes in plant cover, and modification of microclimate, all of which affect the distribution, composition and activity of the soil biological communities. Although most information is available for clearcut forests, the limited data from other systems indicate that severity of impacts of forest harvesting practices generally follow the sequence: clearcuts > shelterwood systems > extended rotation systems. However, all practices generally affect soil organisms over the short term. Changes over the longer-term are less obvious because of gradual recovery of most biological components with canopy closure. Long-term impacts are more difficult to interpret because these are often influenced by natural changes in population dynamics of soil organisms with forest succession, drought, and the invasion of exotic species. Responses are complex, with various groups being affected differently. Even within a taxon, individual species may behave differently. Although the relationships among floral composition, faunal diversity and sustained soil fertility are not always clear, there are indications that a simplified soil biological system will adversely affect nutrient cycling, tree growth, and forest health. Decomposition rate is generally positively correlated with faunal biomass, and a diverse fauna is required for interacting with microorganisms to enhance microbial diversity. Also, feeding on microflora by the fauna concurrently controls certain plant pathogens. Destruction of mycorrhizae, essential for the establishment of coniferous seedlings, can lead to serious reforestation problems. It is therefore prudent to discourage any qualitative or quantitative changes in the soil biota. Concepts in 'new forestry' attempt to mitigate against adverse affects of conventional forest harvesting practices on soil organisms by maintaining plant biodiversity, minimizing soil compaction, avoiding extreme microclimatic conditions, providing refugia for re-colonization by the biota, and directly introducing desirable microflora and fauna. (C) 2000 Elsevier Science B.V. All rights reserved.