SOIL PROPERTIES AS A FACTOR IN REGENERATION FAILURES IN THE GRAND FIR MOSAIC—PRELIMINARY RESULTS

Mark Sommer
David Adams
Robert Mahler

The grand fir mosaic (GFM) consists of a patchwork arrangement of several forest plant communities located in an elevation range of 4,000 to 5,800 feet above sea level. The grand fir mosaic is found primarily in Idaho's Clearwater and Nez Perce National Forests. The plant communities (habitat type from Cooper and others 1987) are:

  1. Forested stands where the grand fir/wild ginger (Abies grandis/Asarum caudatum) habitat type predominates.
  2. Bracken fern (Pteridium aquilinum) glades that appear to be climax communities.
  3. Alder glades where the Sitka alder/broadleaved montia (Alnus sinuata/Montia cordifolia) habitat type predominates.

As the forested areas are harvested, bracken glade communities can invade and exclude most woody vegetation, including conifers. This occurs despite attempts at natural and artificial regeneration. The pocket gopher (Thomomys talpoides) also invades the harvested areas and rapidly multiplies. Current research indicates bracken fern allelopathy and damage by pocket gophers as major factors in the poor regeneration success (Ferguson and Boyd 1988). The alder glades expand slightly, causing few problems.

The bracken fern produces a large amount of biomass, which dies in the fall and is buried by snow in the winter. When spring arrives, a surface of almost bare soil is revealed. Thus rapid decomposition and mineralization rates may be taking place beneath the snow.

This study compares forest and natural bracken glade soils. It also examines soils from cutover areas where bracken fern has invaded to see if soil properties are changing.

METHODS AND MATERIALS

The study area is located about 15 miles northeast of Headquarters, ID, in the North Fork Ranger District of the Clearwater National Forest. Portions of the area's forests were clearcut about 25 years ago. Within the study area there is uncut forest, natural bracken fern glades, and harvested areas invaded by bracken fern.

A randomized block design was used for sampling. Soil cores were taken from the uncut forest, bracken fern glades, and the bracken-invaded areas. To reduce variability, the cores were taken from areas that had similar relief, parent material, and climate. Fifteen samples from each area were taken to a 1-foot depth. Soil analysis was completed at the University of Idaho's Analytical Services Laboratory and Soil Fertility Laboratory.

RESULTS

Clearwater National Forest soil chemical properties are given in table 1; physical properties are not yet characterized. More thorough soil sampling, in addition to foliar sampling, is planned for 1990. Statistical analysis as well as finalized results will be presented after these data have been received.

TABLE 1 
Clearwater National Forest soil chemical properties means (standard deviations)
Property, units, method Sample area
Uncut forest Bracken invaded Bracken glade
Phosphorus
ug/g
NaOAC		 1.56 (0.75) 2.95 (1.50) 4.25 (1.93)
Potassium
ug/g
NaOAC		 209 (57) 214 (73) 502 (188)
CEC
meq/100 g
NH4OAC		 24.9 (3.8) 30.2 (6.2) 31.6 (3.8)
Exch. Ca
meq/100 g
NH4OAC		 5.6 (1.9) 5.0 (1.5) 3.9 (1.8)
Exch. Mg
meq/100 g
NH4OAC		 0.80 (0.19) 1.12 (0.35) 1.21 (0.46)
Exch. K
meq/100 g
NH4OAC		 0.80 (0.18) 1.15 (0.32) 1.75 (0.51)
Zinc
ug/g
DTPA		 0.73 (0.67) 1.66 (1.14) 2.05 (1.32)
Manganese
ug/g
DTPA		 45.4 (17.3) 28.6 (15.7) 32.0 (14.2)
Copper
ug/g
DTPA		 0.32 (0.09) 0.36 (0.15) 0.41 (0.13)
Extrac. Al
ug/g
KCL		 35 (43) 197 (128) 211 (101)
pH
pH units
1:1 water		 5.93 (0.26) 4.95 (0.38) 4.79 (0.30)
Carbon
percent
CHN analyzer		 4.77 (0.95) 5.18 (1.79) 6.14 (1.53)
Nitrogen
percent
CHN analyzer		 0.28 (0.05) 0.41 (0.12) 0.45 (0.09)

DISCUSSION

The soil chemical properties of the uncut forest and the bracken glade appear to be somewhat different. Soil chemical properties of the bracken-invaded areas differ from both the uncut forest and the bracken glade. However, they seem most similar to bracken glade soils.

The most striking change in the bracken glade-invaded areas is the decrease in pH and increase in extractable aluminum. Within the bracken glades and bracken-invaded areas there are high amounts of composition, mineralization, and nitrification during the winter and early spring. During this time there is little active uptake by plants, and nitrate and cations can be leached from the soil. This process lowers the pH, which could cause aluminum to be released. Bracken fern is also high in phenolic acids, which may aid acidification. Apparently the soil's buffering capacity is not able to neutralize the increased acidity.

The increased amount of decomposition and mineralization could be due to several factors:

  1. The bracken glade community produces a greater annual turnover of biomass than the forest community it replaced.
  2. A change in microclimate where lack of shade in early spring and late fall causes soil to be warmed to higher temperatures.
  3. Warmer and wetter environments than normal for these elevations.

Preventive measures can be based on an understanding of the mechanism that causes this process. In addition, a search for methods of prevention without awareness of the mechanism must continue. Different types of silvicultural practices need to be examined to determine which are least likely to lead to this process.

CONCLUSIONS

  1. The different plant communities lead to formation of different soils on what otherwise appear to be areas with the same relief, climate, and parent material.
  2. When bracken fern invades harvested areas, soil chemical properties can be altered. Thus, in addition to allelopathy and pocket gophers, a change in soil properties may be contributing to the poor regeneration success.

A NOTE ON THE MOSAIC

The introduction stated that the grand fir mosaic (GFM) exists in both the Clearwater and Nez Perce National Forests. However, as more information is gathered, it seems that the GFM is not the same ecosystem in each Forest. For instance:

  1. The "climax" bracken fern glades are almost nonexistent in the Nez Perce National Forest.
  2. After harvest, bracken fern is the primary invader in the Clearwater National Forest, but in the Nez Perce National Forest the primary invader is coneflower (Rudbeckia occidentalis).
  3. Soil chemical properties have not changed after harvest in the Nez Perce National Forest

Because of this, this poster paper was largely focused on one National Forest. The Clearwater was chosen because it is more interesting from a soils standpoint. However, from a forestry standpoint, both Forests are important because regeneration failures in the GFM have been noted in each.

REFERENCES

Cooper, S. V.; Neiman, K. E.; Steele, R.; Roberts, D. W. 1987. Forest habitat types of northern Idaho: a second approximation. Gen. Tech. Rep. INT-236. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 135 p.

Ferguson, D. E.; Boyd, R. J. 1988. Bracken fern inhibition of conifer regeneration in northern Idaho. Res. Pap. INT-388. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 11 p.

Mark Sommer is a Soil Scientist Trainee, Nez Perce National Forest, Forest Service, U.S. Department of Agriculture, Route 2, Box 475, Grangeville, ID 83530. David Adams and Robert Mahler are Professor of Silviculture and Professor of Soil Fertility, respectively, University of Idaho, Moscow, ID.