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Research Project Summary: Vegetation response to restoration treatments in ponderosa pine-Douglas-fir forests of western Montana

• RESEARCH PROJECT SUMMARY AUTHORSHIP AND 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

The research stands were primarily ponderosa pine and Douglas-fir in composition, with scattered western larch and lodgepole pine. Measured in basal area, species composition of the pretreatment stands was approximately 60% seral species (primarily ponderosa pine) and 40% shade-tolerant Douglas-fir. These stands regenerated following heavy harvesting in the early 1900s, in which nearly all merchantable-sized trees were removed. Most 2nd-growth trees were ~90 years old, with scattered clumps of regeneration and occasional trees >200 years old. All treatment areas had uneven-aged diameter distributions prior to treatment, with individual tree diameters ranging from 4 inches to 27 inches diameter at breast height (DBH). Typical of ponderosa forests in the region, the study area has been subject to moderate grazing over the last 100 years.

Regeneration of shade-tolerant Douglas-fir was abundant in the understory, while the most abundant undergrowth species included heartleaf arnica, white spirea, common snowberry, pinegrass, and Geyer's sedge.

Vegetation was sampled on modified Whittaker plots randomly located at 10 of the 36 reference points in each unit. Each 66 foot x 164 foot (0.25-acre) Whittaker plot was subdivided into ten, 33-foot × 33-foot (0.025-acre) subplots with two 3.3 foot × 3.3 foot (0.00025-acre) quadrats in opposite corners of each subplot. We identified all understory species on each 0.25-acre plot and sampled DBH of all trees >4 inches DBH. Saplings (trees >4.5 feet tall but <4.0 inches DBH) were tallied in five randomly selected 0.025-acre subplots within each plot. Twelve of the twenty quadrats (0.00025-acre) in each plot were randomly selected to sample understory richness and cover. All sampling locations were permanently marked with metal stakes to ensure accurate plot relocation in future sampling years.

Our "low," "moderate," and "high" burn severity categories correspond well to Burned Area Emergency Response (BAER) severity classifications "lightly," "moderately," and "heavily" burned, respectively. Some of our low-severity areas fell under the BAER "scorch" category, but burn prescriptions were conducted under conditions such that these instances were limited.

The restoration cutting prescription set an average target reserve basal area of 48 feet²/acre. Target stand structures were uneven-aged, with a long-term goal of one-half to two-thirds of the basal area in trees >20 inches DBH. Target species composition over the long-term is >90% basal area composition of ponderosa pine.

Units receiving cutting treatments were leave-tree marked. Low thinning removed most pole-sized trees (ladder fuels), improvement cutting removed most shade-tolerant Douglas-fir from the mid- and upper canopy, and selection cutting reduced overall stand density enough to induce regeneration of shade-intolerant ponderosa pine. A cut-to-length system was used to cut and limb trees on site, leaving nonmerchantable materials in place as a buffer between logging equipment and the soil. Merchantable timber was transported from the stand to a landing area using rubber-tired forwarders. This harvest system, coupled with winter logging, resulted in no significant soil compaction or exposed mineral soil [4].

Prescribed burning was initiated in all burn units using strip headfires to control fire intensity. Firing was slow in the thin-burn units to minimize overstory damage, but was increased in the burn-only units to induce tree mortality. Variation in weather conditions among burn days and in the volume and spatial distribution of fuels within each 22-acre unit led to variation in fire behavior among and within the 6 burn units:

Understory richness increased significantly more in the thin-only and thin-burn treatments than in the burn-only and control. Forb and nonnative species richness increased more in all active treatments, particularly the thin-burn, than in the control. Native species richness increased significantly in the 2 treatments that included cutting. Mean plot-scale (0.25-acre) richness for all species (total), native species, nonnative species, graminoids, forbs, shrubs, and trees are summarized in the following table by treatment and year, with 1 standard error in parentheses. Letters represent significant differences between treatments, within years. See Metlen and Fiedler [6] for methods and details about richness and cover at the smaller spatial scale (0.00025-acre).

Richness of graminoids, shrubs, and tree seedlings was unchanged by treatment. However, broad life form groups can disguise the response of individual species. Species that were strongly affected by treatments were identified using indicator species analysis [1] contrasting treatments within years. Burning treatments created microsites suitable for colonization by seed-dispersed species and increased vigor resulting in greater cover of many sprouting perennial species. Similarly, many species responded to cutting by spreading via rhizomes or stolons. The following table displays species with significant indicator values (P<0.05), their generalized individual responses to treatments, and the observed mechanism for response in this study.

Coniferous regeneration (<4.5 feet tall) was virtually eliminated from 0.00025-acre quadrats but was still present on 0.25-acre plots. Despite a significant reduction in seedling density at the small scale, regeneration maintained a broad-scale presence similar to that described historically (dispersed and patchy). Western larch regeneration increased significantly in the thin-burn treatment compared to the control, reflecting an adaptation for postfire establishment. Many locally rare, disturbance-adapted understory species also became more abundant after treatment, especially in the thin-burn. While abundance of most species was initially reduced by burning, 2 years after treatment abundance of most species in active treatments was equal or greater than that in the control. The following linked files provide cover and frequencies (by treatment) of all successfully identified species before treatment and in 2004 (2 years after burning and 3 years after harvest):

The majority of understory species at this site were resilient to restoration treatments, especially when given several growing seasons to respond. Many native species that were locally rare prior to treatment benefited from active restoration treatments, especially the thin-burn. However, nonnative species responded similarly and may require specific management strategies to limit their establishment and spread following treatments intended to benefit natives.

Further monitoring will be necessary to determine long-term community trends. However, after 3 years of response, restoration treatments show promise for favoring disturbance-adapted native species, particularly locally rare and short-lived species that may have declined in ponderosa pine forests during a century of fire exclusion.

All active treatments successfully reduced some component of hazardous fuels: surface, crown, or both. Surface fuel accumulations were reduced in all size classes by the burn-only treatment. Although thinning increased fuel loads in the litter+1-hour, 10-hour, and 100-hour classes, burning following thinning (thin-burn treatment) reduced fuel loads below pretreatment levels in most classes. Pre- and immediate post-treatment fuel loadings are summarized below by treatment and size class.

Crown fire hazard was modeled for all treatments using crowning index. Crowning index is defined as the windspeed needed to maintain a crown fire once fire has reached the main canopy. Lower crowning index values (e.g., <25 miles/hour) represent stand structures with relatively greater hazard, whereas higher values (e.g., >50 miles/hour) represent relatively low hazard. The least hazardous post-treatment conditions were achieved by the thin-burn treatment, though all active treatments reduced crown fire hazard. Thinning alone modestly increased crowning index, but when the thin-only treatment was followed by timber stand improvement (TSI) cutting of most saplings, crowning index increased substantially. However, should wildfire occur, potential mortality from crown scorch is elevated in the thin-only treatment due to increased loads of untreated surface fuels.

The Lubrecht FFS study provides evidence that alternative restoration treatments can modify potential fire behavior and differentially impact understory plant communities. Taken collectively, these results suggest that all active treatments promote a more open overstory and diverse understory community ― characteristics commonly associated with historically sustainable conditions. However, a mosaic of treatments (including untreated areas) would likely be needed to increase landscape-scale heterogeneity and improve longer-term sustainability of this ponderosa pine ecosystem.

This Research Project Summary is product #111 of the FFS study funded by the Joint Fire Sciences Program. For more information on the Lubrecht FFS study and links to products generated by this research, visit our website at www.forestry.umt.edu/ffslubrecht.

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. Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Washington, DC: Society of American Foresters. 148 p. [905]

3. Garrison, George A.; Bjugstad, Ardell J.; Duncan, Don A.; Lewis, Mont E.; Smith, Dixie R. 1977. Vegetation and environmental features of forest and range ecosystems. Agric. Handb. 475. Washington, DC: U.S. Department of Agriculture, Forest Service. 68 p. [998]

4. Gundale, Michael J.; DeLuca, Thomas H.; Fiedler, Carl E.; Ramsey, Philip W.; Harrington, Michael G.; Gannon, James E. 2005. Restoration treatments in a Montana ponderosa pine forest: effects on soil physical, chemical and biological properties. Forest Ecology and Management. 213(1-3): 25-38. [54298]

5. Kuchler, A. W. 1964. Manual to accompany the map of potential vegetation of the conterminous United States. Special Publication No. 36. New York: American Geographical Society. 77 p. [1384]

6. Metlen, Kerry L.; Fiedler, Carl E. 2006. Restoration treatment effects on the understory of ponderosa pine/Douglas-fir forests in western Montana, USA. Forest Ecology and Management. 222: 355-369. [61044]

7. Nimlos, T. J. 1986. Soils of Lubrecht Experimental Forest. Miscellaneous Publication No. 44. Missoula, MT: University of Montana, Forest and Conservation Experiment Station. 36 p. [61837]

8. Pfister, Robert D.; Kovalchik, Bernard L.; Arno, Stephen F.; Presby, Richard C. 1977. Forest habitat types of Montana. Gen. Tech. Rep. INT-34. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 174 p. [1878]