Restoring historical forest conditions in a diverse inland Pacific Northwest landscape

Abstract

A major goal of managers in fire-prone forests is restoring historical structure and composition to promote resilience to future drought and disturbance. To accomplish this goal, managers require information about reference conditions in different forest types, as well as tools to determine which individual trees to retain or remove to approximate those reference conditions. We used dendroecological reconstructions and General Land Office records to quantify historical forest structure and composition within a 13,600 ha study area in eastern Oregon where the USDA Forest Service is planning restoration treatments. Our analysis demonstrates that all forest types present in the study area, ranging from dry ponderosa pine-dominated forests to moist mixed conifer forests, are considerably denser (273-316% increase) and have much higher basal area (60-176% increase) today than at the end of the 19th century. Historically, both dry pine and mixed conifer forest types were dominated by shade-intolerant species. Today, shadetolerant tree cover has increased in dry pine stands, while mixed conifer stands are now dominated by shade-tolerant species. Federal managers in eastern Oregon are currently required to retain all live trees >53 cm diameter at breast height in the course of forest management activities because this size class is assumed to be under-represented on the landscape relative to historical conditions. However, we found the same or greater number of live trees >53 cm today than in the late 19th century. Restoring historical conditions usually involves removing shade-tolerant trees that established since Euro-American management significantly altered natural disturbance regimes. We evaluated a wide range of tree morphological and environmental variables that could potentially predict the age of grand fir and Douglas-fir, the most abundant shade-tolerant species found within the study area. We describe several morphological characteristics that are diagnostic of tree age and developed decision trees that predict the approximate age of trees using morphological characteristics that are easy to measure in the field such as height to live foliage or height to dead branches. Information about structural and compositional change over time combined with tree-age prediction tools provides a flexible framework for restoring historical conditions and meeting other resource management objectives.

Widespread severe wildfires under climate change lead to increased forest homogeneity in dry mixed‐conifer forests

“…Furthermore, grand fir is also the most drought-tolerant fir in the Pacific Northwest (Howard and Aleksoff 2000), and it can establish and survive on a range of sites from moist to warm and moderately dry (Lillybridge et al 1995). In the Blue Mountains, as well as in the neighboring Ochoco and Cascade Mountains to the west, infilling of late-seral species (grand fir and, to a lesser extent, Douglas-fir) in formerly ponderosa pine-dominated sites has been demonstrated beginning around the 1880s, likely as a result of widespread fire suppression (Merschel et al 2014, Johnston et al 2018. Grand fir is moderately fire resistant once it reaches maturity (Howard and Aleksoff 2000) and generally only dominates in areas where fire has been excluded (Habeck and Mutch 1973).…”

“…This has increased the continuity of horizontal and vertical fuels thereby causing higher overstory mortality when wildfires do occur (Agee 1998, Agee andSkinner 2005). Fire suppression has also altered forest composition by facilitating the establishment of fire-intolerant tree species such as grand fir (Abies grandis; Larson and Churchill 2012, Merschel et al 2014, Johnston et al 2018.…”

“…Millions of hectares in the west were once covered by fire-adapted dry forests that experienced low-severity, frequent (<35-yr return intervals) fire regimes (Franklin and Johnson 2012). Because of fire suppression and land use (e.g., forest and meadow conversion for cattle grazing and agriculture, commercial timber harvest), many dry forests have shifted from large-diameter, widely spaced trees (interspersed with smaller groups of dense, small-diameter trees) to predominantly high-density forests composed of small-diameter trees (Franklin and Johnson 2012, Churchill et al 2013, Merschel et al 2014, Johnston et al 2018. This has increased the continuity of horizontal and vertical fuels thereby causing higher overstory mortality when wildfires do occur (Agee 1998, Agee andSkinner 2005).…”

Estimating historical forest density from land‐survey data: a response to Baker and Williams (2018)

“…The other three PDE methods did not show a discrepancy in density estimates between 2nn only and 4nn only sampling (see Levine Appendix S1). In an independent study, a comparison of PDEs found the Voronoi density estimate (Williams and Baker 2011) for the Blue Mountains of Oregon "unreliable" and more than twice that independently estimated by either Cottam or Morisita estimators alone (Johnston et al 2018).…”

“…In addition, the 4nn and 2nn sampling have different responses to nonrandom or heterogeneous dispersion (Cogbill et al 2018). In an independent study, a comparison of PDEs found the Voronoi density estimate (Williams and Baker 2011) for the Blue Mountains of Oregon "unreliable" and more than twice that independently estimated by either Cottam or Morisita estimators alone (Johnston et al 2018). The mixing of disparate densities based on MND scaling confounds the calculation of MVA.…”