Soil DepthSoil Depth
Soil depth is an important factor in forest water relations and shallow soils can cause deduced rates of tree growth. Both of these pictures illustrate shallow to bedrock soils. The picture on the left was taken in the mountains of northern Idaho in a forest dominated by Douglas-fir (Pseudotsuga menziesii). These soils receive little summer precipitation and water for tree growth depends mostly on precipitation from the previous winter and early spring. In a situation like the one in this picture, the soil has limited storage capacity because bedrock occurs at the base of the shovel. The trees have a smaller "bank account" to draw water from during the current growing season, which will likely result in drought, reduced transpiration and photosynthesis, and relatively slow rates of Douglas-fir growth.
The picture on the right shows an extremely shallow to bedrock soil in Michigan's Upper Peninsula. This soil supports trees typical of rocky nobs in Upper Michigan, such as red oak (Quercus rubra), trembling aspen (Populus tremuloides), and white pine (Pinus strobus). Tree growth in this situation is very slow, and such a soil would be very susceptible to erosion if it was possible to run harvesting equipment over such terrain. These soils, in fact, do not support commercial timber production in the Upper Peninsula.