The role of tree uprooting in soil formation: A critical literature review

The role of tree uprooting in soil formation was evaluated using an analysis of scientific articles published from 1940 to 2009. The potential for generalizing these published results across a range of regions, forest and soil types was assessed. We focused on the following topics: ecological conditions within pit–mound microsites; the area of pit–mounds in different landscapes; the age of pit–mounds; rotation period; and the effect of tree uprooting on soil properties, including the absence of pit–mound dynamics in forests that have been managed long-term. These topics were analysed on the spatial scales of the pit–mound, forest stand, and landscape.The effect of tree uprooting on soil formation has been particularly studied in northern hardwood forests on Podzols in the region of the Great Lakes (USA, Canada). Fewer studies have been done in Europe, Asia or Australia, and we are not aware of any studies from tropical rain forests and temperate forests in South America or Africa. The ecological characteristics of pit–mound microsites with moister, colder and more stable pit conditions are well known and can be generalized. On the other hand, the proportional area of pit–mounds is highly variable, ranging from 0 to 90% depending on forest history and natural conditions, and therefore has only local validity. The maximum age of pit–mounds is most often 200–500 years, though sometimes pit–mounds can be older than 2000 years. The duration of pit–mounds depends on geologic, geomorphologic and climatic conditions, and thus has regional validity. The rotation period (how often an area equivalent to the entire study area is disturbed) is a synthetic characteristic which is largely similar within specific biomes, and usually is as long as 103 years. However, there is still an open question concerning the heterogeneity of the rotation period on fine spatial scales, especially considering the assumed higher susceptibility of some microsites to disturbances.On fine spatial scales, the effect of tree uprooting on soil formation is relatively well understood. However, the traditional method of approximating the development of pit–mound soil properties by using a linear function is a gross simplification. Faster leaching from pits compared with mounds and currently undisturbed soils is a common phenomenon, and is valid across forest and site types. Under specific site conditions and under specific disturbance regimes, however, this phenomenon may not necessarily be applicable.On the scale of the forest stand, the opinion predominates that pit–mound dynamics inhibit the development of soils. This theory is hardly ever supported with quality data, however, and results from some important studies do not fully reflect this. In managed forests, where there is a long-term absence of pit–mound dynamics, studies have indicated that the stage of soil formation is more advanced compared with naturally disturbed forests. The impact of tree uprooting on the development of soil spatial variability is thus still insufficiently explained.At the landscape scale, there are only few relevant studies dealing with the importance of tree uprooting where authors have demonstrated the significant effect of pit–mounds on erosion–sedimentation processes.