Soil production in forested landscapes (Invited)
Abstract
One of the most fundamental characteristics that defines landscapes is the presence or absence of a soil mantle. In actively eroding terrain, soil (and other natural resources that depend on it) persists only when the rate of soil production is not eclipsed by denudation. Despite successful efforts to empirically estimate long-term rates of soil production, little predictive capability exists as soil formation results from a complex interplay of biological, physical, and chemical processes. Here, we synthesize a suite of observations from the steep, forested Oregon Coast Range (OCR) and anlayze the role of trees in the conversion of bedrock to soil. Pit/mound topography on forest floors attests to the persistent, wholesale overturning of soil by tree root activity. Using airborne LiDAR data for our study site in the western Oregon Coast Range, we calculated how terrain roughness varies with spatial scale. At scales greater than 10m, the well-established ridge/valley structure of the landscape defines the topography; whereas for scales less than 7m, terrain roughness increases rapidly reflecting the stochastic nature of bioturbation associated with large, coniferous trees. Empirical estimates of soil production in the OCR by Heimsath et al (2001, ESPL) reveal that production rates decrease exponentially with depth and the decay constant is 2.68 (1/m). From dozens of soil pits in the OCR, we show that the density of trees roots declines exponentially with depth at a similar rate, 2.57 (1/m). In other words, rates of soil production appear to be well-correlated with root density. Bedrock is often excavated during tree turnover events and we documented that the volume of bedrock incorporated in overturned coniferous rootwads increases rapidly for tree diameters greater than 0.5m (which correponds to a 60-80 yr old Douglas fir tree in Western Oregon). Smaller (and thus younger) trees entrain negligible bedrock when overturned, suggesting that their root systems are unable to substantially disrupt bedrock. Thus, short timber harvest rotation ages (<40 years) may limit the extent to which tree root systems penetrate bedrock and facilitate other biogeochemical weathering processes. Using ground penetrating radar data gathered proximal to trees of varying size, we show that in the absence of tree turnover, root insertion into shallow bedrock is also substantial. Given existing data for forest properties and stand dynamics, we constructed a simple analytical model to determine what fraction of soil on the forest floor spends time within large rootwads as it is exhumed toward the surface under steady erosion. For OCR erosion rates (0.1 mm/yr), our model shows that approximately 100% of the soil surface passes through large roots during erosion and exhumation. For higher erosion rates, this percentage decreases rapidly as the residence time of soil becomes shorter than the time for multiple generations of forest growth to mix and disrupt the landscape surface. Soil/bedrock modification by biological processes may thus control the maximum rate at which bedrock can be converted to soil.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMEP41D..05R
- Keywords:
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- 1824 HYDROLOGY / Geomorphology: general;
- 1865 HYDROLOGY / Soils;
- 1886 HYDROLOGY / Weathering