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Pacific Southwest Research Station

 
Pacific Southwest
Research Station

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General Technical Report

Title: Natural range of variation for red fir and subalpine forests in northwestern California and southwestern Oregon

Author: Coppoletta, Michelle; Meyer, Marc D.; North, Malcolm P.

Date: 2021

Source: Gen. Tech. Rep. PSW-GTR-269. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station

Station ID: GTR-PSW-269

Description: This assessment uses historical observations and datasets, as well as studies conducted in contemporary reference landscapes (i.e., those with active fire regimes and minimal management impacts) to define the natural range of variation (NRV) for red fir (Abies magnifica) and subalpine forests in northwestern California and southwestern Oregon. Comparisons between historical and current conditions in the assessment area suggest that many of the ecological attributes of red fir and subalpine forests are within the NRV; however, some conditions and processes are also approaching or are projected to be outside of the estimated limits of the NRV in the near future.

In contemporary red fir forests, the absence of fire for much of the 20th century has lengthened the fire return interval and fire rotation. Future climate and wildfire projections also indicate an extension of the fire season and an increase in fire size and severity in these forest types. Recent increases in insect- and disease-related tree mortality in red fir forests suggest that the scale and intensity of native insect and pathogen infestations may be increasing to levels outside of the NRV. Red fir forests have experienced an increase in tree densities, particularly in the smallest size classes, and concurrent decreases in the density of large-diameter red fir trees. Simplification of forest structure has occurred at both the stand and landscape scales, with shifts away from a heterogeneous, partially open canopy structure to one characterized by more continuous closed-canopy conditions. These alterations have likely resulted from historical logging, long periods of fire exclusion, and changes in climate.

Subalpine forests of the assessment area are generally within the NRV with respect to function, structure, and species composition. Exceptions include potential changes in forest structure, such as increases in the density of small trees and decreases in large-diameter trees. Fire frequency, fire severity, and insect- and disease-related mortality are also projected to increase and likely exceed the NRV with predicted climate change.

Climate envelope models consistently project a substantial loss in the area currently occupied by red fir and subalpine forests in the assessment area by the end of the 21st century. This suggests that the greatest changes to these high-elevation forest types in coming decades will occur as a direct consequence of climate change and its indirect impact on disturbance intensity and frequency.

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Citation

  • Coppoletta, Michelle; Meyer, Marc D.; North, Malcolm P. 2021. Natural range of variation for red fir and subalpine forests in northwestern California and southwestern Oregon. Gen. Tech. Rep. PSW-GTR-269. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 175 p.