Historical Northern spotted owl habitat and old-growth dry forests maintained by mixed-severity wildfires

Abstract

Context

Reconstructing historical habitat could help reverse declining animal populations, but detailed, spatially comprehensive data are rare. For example, habitat for the federally threatened Northern spotted owl (NSO; Strix occidentalis caurina) was thought historically rare because low-severity fires kept forests open and habitat restricted to fire refugia, but spatial historical data are lacking.

Objectives

Here I use public land-surveys to spatially reconstruct NSO habitat and old-growth forests in dry forests in Oregon’s Eastern Cascades in the late-1800s. I used reconstructions of forest structure across about 280,000 ha, including 9,605 tree records and 2,180 section-line descriptions. I was able to reconstruct likely NSO nest trees, nest stands, and foraging and roosting habitat, based on modern NSO habitat studies.

Results

Historical nest stands, including sufficient nest trees, were predicted across 22–39 % and foraging and roosting habitat across 11–68 % of the study area, thus neither were rare. More habitat than expected occurred in forests with preceding mixed-severity fires. Early post-fire succession produced foraging and roosting habitat. Mid- to late-succession produced nesting habitat. Late-succession after high-severity fires can also provide NSO habitat. Old-growth forests, covering 76 % of study-area forests, also likely link to preceding mixed-severity fires.

Conclusions

Mixed- and high-severity fires strongly shaped historical dry forests and produced important components of historical NSO habitat. Focus on short-term loss of nest sites and territories to these fires is mis-directed. Fuel treatments to reduce these natural fires, if successful, would reduce future habitat of the NSO in dry forests.