ENVIRONMENTAL CONSEQUENCES in rocky mountain coniferous ...

surface. Use of dozers to pile logging debris has the greatest physical impact on
the surface organic layer--often completely removing it and always exacerbating the
physical disturbance from 1 oggi ng.
The immediate physical changes in the surface organic layer, the removal of the
forest overstory, and microclimatic changes at and near the soil surface, all
contribute to chemical and physical alteration of both the surface organic horizon
and the mineral soil beneath. This is true for a period of years or even decades
after clearcutting and slash disposal. Many of the changes cited in this study were
recorded at the end of 5 years--long enough after treatment to miss any immediate
and transitory effects, but late enough for more subtle and long-term effects to
have emerged. For example, the data in table 4 show an increase in potassium in the
A. horizon during the first year after treatment. The increased amount of this
readily soluble element, however, was largely gone after 5 years. During this time
span, many of the losses or changes in the surface organic horizon exhibited their
influence on measurabl e parameters in the mineral soi 1 . For exampl e, the potassium
content of mineral soil had increased under most treatments by 1977 (table 5).
The mineral soil, immediately after logging and slash disposal, is less severely
impacted than is the surface organic layer. Nevertheless, the mineral soil may be
physically altered. Porosity, bulk density, organic matter content, and related
hydrologic characteristics are often immediately changed (Packer and W i 11 iams 1976,
and this vo~wne). Chemical changes in the mineral soil are most pronounced after
thorough leaching by rainfall or snowmelt water. This water dissolves the soluble
components in the logging slash, the ash layer (if fire was employed after logging),
and the surface organic soil horizon. The water then moves these solutes into the
mineral soil, where they are precipitated and picked up by the ion exchange system,
or where they otherwise alter the chemical characteristics of mineral soil horizons.
The pH of acid soils rises when enough cations are leached from surface
materials. This a1 teration lasted 5 years only under burned piles of slash on the
studied sites (fig. 5). Only here was there a sufficient spike of soluble cations
to have a lasting and measurable effect. It is possible, however, that the lower
pH of the slowly decomposing wood chip mulch will result in a decrease in mineral
soil pH on these sites in future years.
The organic matter content of the surface mineral soil increased during the
logging operation. Afterwards the lack of annual additions through 1 i tter-fall
caused a decline on most treated sites. It is uncertain at this point in time if
organic matter contents will continue to decrease on these treated sites due to
decay rates exceeding the input of plant remains, or if relative stability has been
reached. In general, the content of total nitrogen and, to a lesser extent, phosphorus
will parallel that of organic matter content because almost all of the
nitrogen and about half of the phosphorus are part of organic compounds.
In terms of plant nutrition, the content of available nutrients perhaps was the
most important parameter measured in these soils. It must be kept in mind, however,
that the laboratory techniques used to measure available nutrients provide only
approximations of what is available for uptake by plant roots. They sometimes are
poor approximations, particularly for forest tree nutrition.
The treatments--especial ly the pi1 ing and burning of slash--are the imp1 ied
cause for significant differences in available nutrients on these sites in 1977.
Clearcutting and most slash disposal treatments usually increased available nutrients.
An exception occurred between burned piles, where dozer operations frequently removed
the nutrient-rich surface soil and pushed it into the slash piles. These piles of
logging debris occupied about 18 percent of the clearcut area. Hence, piling caused
a five-fold concentration of nutrients held in the slash and surface debris on these