TheImprovement ofTropical and Subtropical Rangelands

148 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
Air Pollution
Air quality near large urban centers is apt to be marginal at
best, and any land management activity that tends to add significantly
to the pollution is undesirable. Prescribed burnins of extensive
areas can be planned when fire weather forecasters predict airflow
away from urban centers to avoid stagnation and/or mixing
with polluted air layers. Most range burning involves relatively light
fuels, and unless areas are very large, the air pollution-mostly from
particulates-tends to be rather transitory.
Particulate matter from man-made fires can be a principal source
of condensation nuclei necessary for the production of clouds and
precipitation. Forest and grassland fires also yield charcoal, ash, and
other products having a great diversity in form, structure, porosity,
and absorptive capacity. These products are very different from
those produced by the internal-combustion ensine and by burning oil,
rubber, or plastics (Arizona Interagency Ranse Committee, 1977).
Water Quality
Rangeland burning has not seriously affected water quality. Water
quality studies have shown that initial conversion of chaparral
to a gr~shrub range, for example, may temporarily increase nitrates
in runoff to high levels, but these peaks do not last long and
apparently are confined to the first or l!IeCOnd year. Nitrate concentration
then drops to a relatively low level (from 10 to 16 parts per
million), and occasional reburning of the light fuels has little or no
effect on nitrates. No significant increases have been noted in phosphorus,
calcium, or total dissolved salts (Arizona Interagency Range
Committee, 1977).
Erosion
Wildfires often result in massive sheet and sully erosion, but
prescribed range burning is much less of a problem because it is
planned in advance. For example, steep, heavily wooded slopes that
produce heavy sediment loads when burned, usually are not included
in range burning plans. On such slopes, wildfires tend to be more
severe, and leave little residue to provide soil protection.
Even the most carefully planned and executed range burn may
produce sediment. Sediment yield declines rapidly, however, and
even after wildfires, yield drops to near pre-burn levels within 3-5