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Research Project Summary: Changes in stand structure and composition after thinning and burning in low-elevation, dry ponderosa pine and Douglas-fir forests of northeastern Oregon

• RESEARCH PROJECT SUMMARY CITATION
• SPECIES INCLUDED IN THE SUMMARY
• STUDY LOCATION
• SITE DESCRIPTION
• PREFIRE PLANT COMMUNITY
• PLANT PHENOLOGY
• FIRE SEASON/SEVERITY CLASSIFICATION
• FIRE DESCRIPTION
• FIRE EFFECTS ON PLANT COMMUNITY
• FIRE MANAGEMENT IMPLICATIONS
• REFERENCES

Metlen, Kerry L.; Fiedler, Carl E.; Youngblood, Andrew. 2004. Understory response to fuel reduction treatments in the Blue Mountains of northeastern Oregon. Northwest Science. 78(3): 175-185 [5].

Youngblood, Andrew; Metlen, Kerry L.; Coe, Kent. 2006. Changes in stand structure and composition after restoration treatments in low elevation dry forests of northeastern Oregon. Forest Ecology and Management. 234(1-3): 143-163 [6].

SPECIES INCLUDED IN THE SUMMARY:
Common names are used throughout this summary. For the list of the common and scientific names of species discussed in this summary and to FEIS Species Reviews, see the Appendix.

Most of the area is in a stem exclusion, closed-canopy stage. Most stands are dominated by 70- to 100-year-old, even-aged Columbia ponderosa pines, with smaller Rocky Mountain Douglas-firs that have regenerated after extensive partial cutting. Within this mosaic are scattered large trees and remnants of old, unharvested stands. The study area historically had endemic levels of bark beetles. During the 1990s, the area was affected by increased levels of bark beetles, especially western pine beetle (Dendroctonus brevicomis) and eastern pine engraver (Ips pini) [6]. Grazing is a traditional land use in the study area and occurs on all treatment sites [5]. Indicator species analysis identified species differentially associated with treatment sites [6].

Study sites occur in the following plant communities and probably historically experienced the fire regimes described below:

The researchers implemented 4 treatments with 4 replications of each: thin, burn, thin-burn, and control (no active management). Thinning occurred in summer 1998, and burning was conducted in mid-September 2000. The management goal was to modify stand structure such that mortality from bark beetles was reduced and 80% of the dominant and codominant trees could survive a wildfire occurring under 80th percentile weather conditions. To meet this goal, treatments aimed to reduce total basal area from about 26 m²/ha to about 16 m²/ha. In the thin treatment, all live trees ≥53 cm DBH, which represented late-successional and old stands, were left uncut; conifers within 9 m of dominant Columbia ponderosa pines were removed. The burn treatments aimed to reduce the basal area of Columbia ponderosa pines ≥51 cm DBH by <20%, with similarly sized Rocky Mountain Douglas-firs reduced by ≤30%. For trees ranging from 20 to 51 cm DBH, the burn treatments were designed to kill <30% of Columbia ponderosa pines and 40% of Rocky Mountain Douglas-firs. Targets for the thin-burn treatment were the same as for the thin-only and burn-only treatments; fire behavior was also managed to reduce accumulations of cured tree branches and tops left by thinning [6].

Burns were conducted as both backing and strip head fires under conditions suitable for low flame heights. All burns were "fairly complete," low-intensity fires that consumed 75% of the grasses and exposed <25% of mineral soil [5].

• Treatment effects on trees
• Treatment effects on coarse woody debris
• Treatment effects on understory vegetation

Basal area of dead snags did not differ significantly among treatments, including controls. Mean basal area of snags was 2.8 m²/ha in 1998 (88% Columbia ponderosa pine) and 2.6 m²/ha in 2004 (58% Columbia ponderosa pine). Snag density averaged 46 stems/ha in 1998 and 53 stems/ha in 2004. Burn treatments, with or without thinning, tended to increase snag density relative to other treatments; thinning tended to reduce snags.

Treatment effects on coarse woody debris: All active treatments reduced downed log density relative to that on control sites (P=0.011). Log density was significantly less on burned sites than on thinned sites (P=0.003) and tended to be less on thinned-and-burned sites than on thinned-only sites. Length, volume, and cover of coarse woody debris did not differ significantly among treatments [6].

Treatment effects on understory vegetation: Total understory cover in the first postfire year was 95.7% on control sites, greater than on thinned-only sites (60.7%) and significantly greater than on burned-only sites (50.5%) or thinned-and-burned sites (42.3%). Species richness on burned sites (with and without thinning) did not differ significantly from that on control sites, but species richness on thinned-only sites was significantly less than on control sites. Cover of forbs was less on thinned than control plots in the first postfire year, perhaps due in part to increased slash on the ground. All active treatments had lower cover of Geyer's sedge than control sites, but the difference was significant only on thinned sites. Prairie Junegrass cover was significantly greater on thinned-and-burned sites than on thinned-only or control sites [5]. The frequency and cover of understory species differed among treatment sites in 1998, before any treatments [6], so differences may not be attributable to treatments alone.

Understory species diversity did not change significantly in any treatment from 1998 to 2004. On thinned sites, the authors note that understory species composition was "remarkably stable and consistent" before and after treatment. Indicator species for thinning units in 1998, before thinning, were also indicators for thinned units in 2004, with similar cover and frequency. Eight nonnative species were found on thinned sites in 2004; the only one with more than trace cover was cheatgrass. On burned-only sites, several perennial species increased in frequency and cover from 1998 to 2004. These included heartleaf arnica, pinegrass, Geyer's sedge, white spirea, and common snowberry. Cover of Idaho fescue decreased. The number of nonnative species on burned-only sites increased, but only 2 of these species (cheatgrass and Japanese brome) had more than "limited" cover in 2004. On thinned-and-burned sites, as on burn-only sites, bluebunch wheatgrass, Geyer's sedge, white spirea, and common snowberry increased, while Idaho fescue decreased. Number of nonnative species increased from 2 to approximately 9. As in other treatments, cheatgrass was the most abundant nonnative species. Forty-nine indicator species were used to characterize the plant communities sampled, based on presence with more than trace cover in most treatments [6].

FIRE MANAGEMENT IMPLICATIONS:
The thin treatments and burn treatments opened stands to some extent and altered tree species composition, but they did not reduce basal area to the target level for increasing resistance to crown fire and bark beetle attack (16 m²/ha). The thin-and-burn treatment did achieve this management objective [6].

Activity fuels from thinning caused variable fire spread in thinned-burned stands because activity fuels occurred in relatively uniform, parallel strips that had higher fuel moisture than areas outside these strips [6].

Treatments caused minor changes in understory vegetation, indicating that "the majority of understory species have adapted to the same disturbance regimes" that the treatments aimed to mimic [6].

REFERENCES:

1. Dufrene, Marc; Legendre, Pierre. 1997. Species assemblages and indicator species: the need for a flexible asymmetrical approach. Ecological Monographs. 67(3): 345-366. [61431]

2. Hann, Wendel; Havlina, Doug; Shlisky, Ayn; [and others]. 2005. Interagency fire regime condition class guidebook. Version 1.2, [Online]. In: Interagency fire regime condition class website. U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior; The Nature Conservancy; Systems for Environmental Management (Producer). Variously paginated [+ appendices]. Available: http://www.frcc.gov/docs/1.2.2.2/Complete_Guidebook_V1.2.pdf [2007, May 23]. [66734]

3. LANDFIRE Rapid Assessment. 2005. Reference condition modeling manual (Version 2.1), [Online]. In: LANDFIRE. Cooperative Agreement 04-CA-11132543-189. Boulder, CO: The Nature Conservancy; U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior (Producers). 72 p. Available: http://www.landfire.gov/downloadfile.php?file=RA_Modeling_Manual_v2_1.pdf [2007, May 24]. [66741]

4. LANDFIRE Rapid Assessment. 2007. Rapid assessment reference condition models, [Online]. In: LANDFIRE. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab; U.S. Geological Survey; The Nature Conservancy (Producers). Available: http://www.landfire.gov/models_EW.php [2008, April 18] [66533]

5. Metlen, Kerry L.; Fiedler, Carl E.; Youngblood, Andrew. 2004. Understory response to fuel reduction treatments in the Blue Mountains of northeastern Oregon. Northwest Science. 78(3): 175-185. [55433]

6. Youngblood, Andrew; Metlen, Kerry L.; Coe, Kent. 2006. Changes in stand structure and composition after restoration treatments in low elevation dry forests of northeastern Oregon. Forest Ecology and Management. 234(1-3): 143-163. [64992]