Forest dynamics after thinning and fuel reduction in the Pringle Falls Experimental Forest—establishment and early observations of the Lookout Mountain Thinning and Fuels Reduction Study

Abstract

The Lookout Mountain Thinning and Fuels Reduction Study is an operational-scale, long-term management experiment to evaluate the effectiveness of silvicultural treatments to mitigate fire risk and the associated implications for ecological processes and functions. The study is being conducted within the Pringle Falls Experimental Forest, which encompasses about 1019 ha of ponderosa pine (Pinus ponderosa) and mixed-conifer forest originating predominantly from an 1845 wildfire. Study treatments include four levels of overstory thinning, each followed by mastication of understory vegetation and broadcast burning, and an untreated Reference. Thinning treatments target residual tree densities of 50, 75 or 100 percent of the upper management zone (UMZ) and a fourth treatment targeting 75 percent UMZ with gap openings (75 UMZ + Gaps). Treatments were assigned to experimental units in a randomized block design.The study treatments were implemented between 2011 and 2015 in a series of three sales using a combination of conventional timber sale and stewardship contracts. The detection of a pair of nesting northern spotted owls (Strix occidentalis caurina) and litigation complicated implementation. Court rulings determined that the proposed experimental activities addressing research questions stated in the project purpose and need could fall outside of standard practices employed on other National Forest System lands. The nesting spotted owls were accommodated by establishing a no-treatment buffer around the nest site, reconfiguring some treatment unit boundaries, and eliminating of one replication of the 75 UMZ + Gaps treatment.Initial posttreatment observations indicate that the thinning treatments resulted in stands that compositionally shifted to ponderosa pine dominance with increased average stem diameter. Target basal area densities were met within 88.5 to 180.9 percent. Target stand density indices were met within 51 to 193 percent. Thinning and fuel reduction treatments resulted in decreased shrub and subshrub cover and increased diversity of understory plant species. Posttreatment tree mortality from bark beetles occurred at low frequency and is not expected to confound the reduction of landscape-scale risk, the primary objective of the thinning and fuels reduction treatments. Thinning treatments combined with understory burning resulted in a fivefold increase in woodpecker nest occurrence, suggesting a positive response by cavity excavators. Soil heating experiments demonstrated that the occurrence of extreme soil heating is harmful to soil microorganisms and organic matter-derived nutrients. Understory microclimate varied geospatially in relation to physiography and residual overstory density. Presumed future overstory canopy densification arising from crown expansion will likely further mediate understory temperatures through decreasing incoming solar radiation and insolation of nighttime heat loss.Future monitoring will determine the timing of subsequent, repeated understory fuels reduction and longer term dynamics of vegetation composition and structure, wildfire risk, and ecological and biophysical performance.

Citation

Sherman, Lorelle M.; Anderson, Paul D.; Fettig, Christopher J., tech. eds. 2023. Forest dynamics after thinning and fuel reduction in the Pringle Falls Experimental Forest—establishment and early observations of the Lookout Mountain Thinning and Fuels Reduction Study. Gen. Tech. Rep. PNW-GTR-1015. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 168 p. (Online only). https://doi.org/10.2737/PNW-GTR-1015.