Effects of hillslope terracing on productivity and soil properties in semi-arid Ponderosa Pine habitat in Western Montana

L. Cerise¹ ², D.S. Page-Dumroese¹, R. Heinse², P. McDaniel²¹USDA Forest Service, Rocky Mountain Research Station, Moscow, ID 83843

²University of Idaho, Department of Plant, Soil, and Entomological Sciences, Moscow, ID

ConclusionsTerracing did not have a detrimental long-term impact on soil productivity in this study area. Bulk density differences were not beyond the USFS R1 Soil Quality Standards (SQS), and did not have a negative impact on tree growth. Rutting, compaction, and erosion did not exceed R1 SQS on terraced units. Terraced units in the BNF have very similar site conditions as traditional, non-terrace site preparation, and should be thinned and managed to maximize tree production.

Fig. 1. Soil terracing in the Bitterroot National Forest following construction (1970 left). Current terrace conditions in the Bitterroot N.F. Terraced slopes are difficult to see with plant regeneration.( 2009 right)

Fig. 3. Terrace riser/bench, 2009.

Results Benches have lower soil C & N compared to non terrace & risers (Table 1)

Bulk density is significantly lower on benches Soil cations showed no significant difference Root mass(kg/ha) is lower on terrace bench locations from 0-20cm, but is greater at 20-30cm Ponderosa pine productivity (as measured via diameter at breast height) is greater in terraced units (Fig. 4) by a factor of 1.105 cm

Table 1. Soil properties (mean ± standard error). Values in the same row sharing a letter do not differ significantly (p<0.05) according to covariance analysis.

References

Field Sampling/Laboratory Analysis 24 terraced & 24 non-terraced sites. Sites were selected for equal distribution of

Belt rocks & Idaho Batholith granitics Soil was sampled (non-terrace, terrace bench, terrace riser) using large diameter impact coring device (10cm diameter x 30cm long ) (Fig.3) Disturbance monitoring protocol was used to assess compaction, rutting, and erosion. Stand exams were completed at each plot. Lab analyses: C, N, bulk density, texture, organic matter, soil cations, root mass

IntroductionHillslope terracing was done in the Bitterroot National Forest from the 1960’s to the early 1970’s (Behan, 2001) to increase seedling establishment by reducing site competition, facilitate mechanical planting and harvesting, and in some cases reduce erosion (U.S. Forest Service, 1970). The purpose of this study was to identify (1) long-term soil impacts from terracing, and (2) establish whether tree growth and productivity benefited from this drastic and controversial management practice. We measured and analyzed soil physical and chemical properties within terraced plantations, compared to soil conditions in traditional non-terraced machine-prepared sites for timber production.

Study Site Bitterroot N.F. (BNF), SW Montana (Fig. 2) Slope: 25 - 60% Elevation: 1,225m – 2,000m Mean annual precipitation: 450mm - 530mm Mean annual temp: 0°C - 32°C Soil parent material: colluvium/residuum derived from meta- sedimentary rocks or granite formations 2 main soil types: Lithic Haplustepts and Typic Haplustepts (Soil Survey Staff, 2011).

20

20.5

21

21.5

22

22.5

23

23.5

24

23.50

21.26

P. ponderosa mean DBH

Terraced Planta-tions

Non-terraced Plantations

mean dbh (cm)

Behan, R.W. Plundered Promise: Capitalism, Politics, and the Fate of the Federal Lands. Washington, D.C.: Island Press, 2001.USDA Forest Service, Management Practices on the Bitterroot National Forest: A Task Force Appraisal, May 1969—April 1970 (Missoula, MT: 1970).Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Official Soil Series Descriptions. Available online at http://soils.usda.gov/technical/classification/osd/index.html. Accessed [5/28/2011].

Non-terraced Terrace, bench

Terrace, riser

C (Mg/ha) 45.30 a ± 1.457

33.85 b ± 2.051

43.63 a ± 2.059

N (Mg/ha) 0.616 a± 0.344

0.479 b ± 0.477

0.648 a± 0.475

Bulk Density (g/cm³)

0.812 a ± 0.025

0.797 b ± 0.031

0.835 a ± 0.034

% Organic Matter

12.1 a± 0.201

11.4 b ± 0.266

12.3 a± 0.273

Soil Texture%sand / %clay

62% / 9% a± 0.194

71% / 7% a± 0.201

69% / 6% a± 0.198

Fig. 4. Pinus ponderosa mean diameters at breast height

benchriser

Fig. 2. Study location