ENVIRONMENTAL CONSEQUENCES in rocky mountain coniferous ...

During the study, certain phenomena involving nutrient depletion and soil deteri-
oration which I had studied over the past 12 years began to jell into a series of
hypotheses. The first involved "direct nutrient cycling" or the transfer of nutrients
in the tropics on extremely poor soils from dead organic matter via mycorrhizal fungi
almost completely bypassing the soil (Stark 1969). This hypothesis has since been
tested for the tropics (Herrera et a1 . l978), but is of 1 i ttl e importance to the
Rocky Mountains since on young soils nutrients appear to be transferred largely indi-
rectly or from the dead organic litter to the soil solution, and thence to roots or
mycorrhizae. The importance of direct nutrient cycling here is that it led to the
second hypothesis.
The second one involved the concept of the "Biological Life of a Soil," This
term refers to the length of time on the geologic time scale over which a soil is
chemical ly able to support merchantable trees, regardless of cl imate. This concept
acknowledges that a soil can wear out, and that excessive harvest or too frequent
fires can accelerate the rate of "wearing out." The logical question for a nutrient
cycling ecologist now working in western Montana was, "Can these fire treatments being
applied on these soils cause an acceleration of the decline in the long-term produc-
tive capabilities of the soil?" The data being collected on the loss of nutrients
below the root zone in the soil water on 20 treatments and 5 controls, and precipiia-
tion as well as a host of other related measurements were ideal for answering this
question (steel@ 1975). About the same time, a third idea appeared in the author's
mind, that of "nutrient shock," Nutrient shock is the temporary deficiency of one or
more biologically essential nutrients in available form in the soil. The biological
1 ife theory deals with a permanent loss of available and unavailable (total) nutri-
ents. Nutrient shock is the condition which could occur on nutrient-poor
young, thin, or poorly developed soils which had had the vegetation destroyed by too
frequent or intense fires, or by logging. This was a phenomenon which could occur on
the relatively young soils of the Rocky Mountains, It has been observed by the author
in Nevada (unpublished) .
The results of the study are summarized in fig. 1, Fires were classified by
surface soil temperatures as light (300° C or >572' F). No significant nutrient losses occurred with
fires less than 180" C, nor with those ranging from 180-300" C. The nutrient losses
which did occur were largely replaced by precipitation in 1-2 years time. Those fires
greater than 300" C and particularly those greater than 500" C (932" F) caused accel-
erated nutrient losses below the root zone for 2-5 years. In all of the hot fires the
only two elements lost in excess of what was being returned annually in precipitation
were Ca and Mg. Nitrogen was also lost in solution and in smoke, but nutrient losses
in smoke were not quantified in this early study. Nutrient losses below the root zone
in the unburned control soils were 20.8 meq/m2 of surface area, compared to
21.2 meq/m2 for precipitation input. Control nutrient loss just equalled the annual
return of calcium from precipitation, Magnesium showed a slight deficit on control
plots (more routinely lost then added).
By applying the reasoning of the Biological Life of the Soil, we were able to
measure the total nutrient content over a meter square (3 ft3) of surface area to the
depth of the root zone (50 cm, 19.7 inches). This constitutes the "working capital"
in the bank, plus interest (annual additions from weathering). We were also able to
measure how much added nutrient loss occurred for the hottest fires (over the level of
loss by the controls). This was money expended from the bank account for 2-5 years
after each fire over every 50-year fire rotation. A 50-year fire frequency was
selected in the absence of natural or man-controlled fire frequencies for the sake of
calculation, Precipitation inputs cancel 1 ed out control 1 osses.