February 2004
Soil Disturbance via Tree Mortality by Root Throw
Waite Osterkamp, United States Geological Survey
Terry Toy, University of Denver
Melanie Lenart, University of Arizona, ISPE
A basic objective of research conducted at the Desert Laboratory is to
identify relations among plants and the hydrologic and geomorphic
conditions that interact with and control the occurrences of those
plants. Recent studies have focused on how tree mortality by root
throw disturbs soil, causes rock veneering at the surface, and adds
to the sediment load of streams draining areas affected by this
sort of bioturbation.
Rock veneers stabilize hillslope surfaces, occur especially in areas of immature soil, and form through a variety of process sets that includes root throw. Near Westcliffe, Colorado, USA, data were collected from a 20- x 500-m transect on the east slope of the Sangre de Cristo Mountains. Ages of pit/mound complexes with exposed rock fragments brought to the surface by root throw ranged from recent (freshly toppled tree) to unknown (complete tree decay). Calculations based on dimensions of the pit/mound complexes, estimated time of tree toppling, sizes of exposed rock fragments, and percent rock covers at pit/mound complexes and within the transect area indicate that recent rates of tree toppling have resulted in only partial rock veneering since late-Pleistocene de-glaciation. Weathering of rock fragments prevents development of an extensive rock veneer and causes a balance, achieved within an estimated 700 years, between the rates of rock-fragment exposure by root throw and clast disintegration by biochemical reduction. The estimated rate of rock-fragment reduction accounts for part of the fluvial-sediment yields observed for forested alpine areas of western North America.
For more information contact Waite Osterkamp at wroster@usgs.gov.