Forests in the western United States are more dense and have more down fuels now than under historic conditions, mostly due to anthropogenic influences such as grazing and fire-suppression. Past overgrazing practices have led to the deterioration of grass covers and allowed tree species, such as ponderosa pine, to become established in former grasslands.
Some wildlife species, however, have benefited from dense vegetation, such as wild turkey, elk, and deer. While these species require management practices that maintain this density to provide nesting, bedding, hiding, and escape cover, this same dense vegetation presents a management problem for reducing the risk of wildfires, and the spread of tree diseases and pathogens.
Managers have recognized this problem and have acted to reduce stem density and fuels by thinning, burning, and/or fuel treatments. Although silvicultural treatments can mimic the effects of fire on structural patterns of woody vegetation, virtually no comparative data exist on how these treatments mimic ecological functions of fire. A nationwide Fire and Fire-Surrogate (FFS) study program was designed in 1998 to conduct research on this issue.
The FFS study evaluates prescribed fire, thinning, and various mechanical treatment methods for treating, removing, or using woody biomass. Site-specific and study-wide evaluations are assessing watershed impacts, soil disturbance, vegetation responses, wildlife changes, ecological consequences, social impacts, economics, and potential effects on wildfire size, severity, and cost. The study design is flexible to address local treatment variations and effects, and plots were installed at 13 locations representative of interior Washington-Oregon, Northern California, Sierra Nevada, Rocky Mountain, southwest ponderosa pine, southern pine, and mixed hardwood-oak forest ecosystems.
The Fire and Fire-Surrogate study incorporates soil moisture measurements into the existing experimental design. The data gathered in this study provides information on the amount of soil moisture available to plants at different rooting depths. Permanent plots have been established consisting of four treatments (control, burn only, cut only & cut and burn). Each treatment consists of 36 permanent plots.
Rocky Mountain Research Station scientists and their partners conducted a soil moisture pilot study near A1 Mountain in northern Arizona. Ten permanent plots from each treatment were selected for installing semi-permanent soil moisture probes. Of the 10 plots, four of them were equipped with 15 cm probes, another four plots with 30 cm probes, while the remaining two plots have both 15 and 30 cm probes. The probes were installed around a "marker" tree in each plot in all four cardinal directions of the tree. Data was collected on a monthly basis from April 2006 to March 2007.
Similar research is being conducted at the Rudd's Tank Site on the Coconino National Forest and the KA Hill Site on the Kaibab National Forest.
Due to high variability between plots in the same treatment areas, the results of this study indicate no significant difference in soil moisture between aspects, treatments, or stand density (basal area). There are a couple of factors that may have contributed to this high variability. Measurements from this pilot study were conducted at only one site (site 32, Powerline), which has a very rocky soil, and the study was conducted during an unusually dry year. Previous studies have shown that hydrological changes in southwestern ponderosa pine forests after silvilcultural treatments only become measurable in wet years.
However, some soil moisture trends are noticeable, even in the wet years. For example at a depth of 30 centimeters, the cut and burn unit had a higher soil moisture content, especially during the monsoon season. This trend became even more interesting when correlated with some of the biological observation. Wildlife biologists, for example, have noticed a higher occurrence of native reptiles in the cut and burn unit during the monsoon season as well.
The cut and burn or burn-only units had the highest soil moisture content throughout the year at either depth. Considering that fire used to be a natural part of the southwestern ponderosa pine ecosystem, this is an important observation. Future studies will look at the fire and fire surrogate treatments at other locations in the southwestern ponderosa pine forests, with different soils as well.