Forest ecosystem restoration: Initial response of spotted owls to partial harvesting
Highlights
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- We compare habitat selection by spotted owls before vs. after partial harvesting.
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- Most owls used stands scheduled for harvest in proportions less than availability.
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- Most owls used harvested stands in proportion to availability.
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- We model factors that distinguish between used and unused harvested stands.
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- Harvest area, nest distance and midstory basal area influenced probability of use.
Abstract
Conservation planning for spotted owls (Strix occidentalis)
hinges upon retaining late-successional and old-growth forests. This
strategy is to be supplemented over time by creating structural
conditions found in such forests using innovative silviculture in less
well-developed forests. Recent research indicates that spotted owls
often hunt for prey or may nest in relatively young or mid-seral forest
stands that were thinned or partially harvested in previous decades, but
little information has been available to evaluate short-term direct
responses (<5 year) by spotted owls to such practices. We used
selection ratios to compare the frequency of nocturnal use by
radio-tagged northern spotted owls (S.o. caurina) and California spotted owls (S.o. occidentalis)
⩽2 years before and ⩽2 years after 150 forest stands were thinned or
partially harvested within 1200 m of nest sites of 19 owl home ranges in
5 study areas in western Oregon and northern California. We used
logistic regression to investigate habitat and environmental factors
that distinguished between 89 stands that were used and 115 stands that
were not found used by radio-tagged owls for up to 2 years after
treatment via a broad range of partial-harvest or thinning prescriptions
within 2400 m of nest sites. Before harvest, radio-tagged owls
generally used stands scheduled for harvest treatment in proportions
significantly less than availability. After harvesting, selection ratios
increased (n = 4), remained the same (n = 4), or decreased (n = 2)
among 10 owl pairs for which we acquired sufficient telemetry data both
before and after harvesting. Across all owls and all post-harvest
conditions, the overall selection ratio increased after harvesting,
suggesting that many of the harvests were benign or may have resulted in
improved habitat. The probability of use of thinned or
partially-harvested stands increased with harvest-unit size, decreased
with distance from nest sites, and varied with the intensity of harvest
and among forest types as represented by study areas. We found only
limited evidence for a positive effect of retained basal area of large
trees (⩾66 cm diameter at breast height [dbh]), probably because many
treated stands contained no such large trees prior to harvest. We found a
quadratic relationship with retained basal area of mid-story conifers
(10–65 cm dbh), such that harvested stands that contained 25–35 m2/ha
basal area of such mid-story trees were more likely to be used, holding
other factors constant at their means. We also found evidence for a
positive influence of proximity to riparian zones on probability of use
of harvested stands. Although we did not obtain information on prey
abundance or foraging efficiency, our study suggests that judicious
applications of partial-harvest forestry, primarily commercial thinning,
have the potential to improve foraging habitats for spotted owls.
Keywords
- California spotted owl;
- Forest thinning;
- Northern spotted owl;
- Seed tree harvesting;
- Shelterwood harvesting;
- Silviculture;
- Strix occidentalis caurina;
- Strix occidentalis occidentalis;
- Unevenaged management
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