4 September 2014 Science Briefs

SNAMP Pub #26: Effects of Forest Management on California Spotted Owls: Implications for Reducing Wildfire Risk in Fire-prone Forests

Article Title: Effects of Forest Management on California Spotted Owls: Implications for Reducing Wildfire Risk in Fire-prone Forests

Authors: Douglas J. Tempel, R. J. Gutiérrez, Sheila A. Whitmore, et al.

Research Highlights:


  • We assessed the effects of forest conditions, fuel reductions, and wildfire on a declining population of spotted owls in the central Sierra Nevada using 20 years of demographic data collected at 74 spotted owl territories.

  • Adult survival and territory colonization probabilities were relatively high, while territory extinction probability was relatively low, especially in territories that had relatively large amounts of high-canopy-cover (≥70%) forest.

  • Reproduction was negatively associated with the area of medium-intensity timber harvests characteristic of proposed fuel treatments, but the relationship was weak.

  • Results also suggested that the amount of edge between older forests and shrub/sapling vegetation may positively influence demographic rates of spotted owls.

  • High-severity fire appeared to negatively influence the probability of territory colonization.

  • Life-stage simulation (sensitivity) analyses indicated that the amount of high-canopy-cover forest was the primary driver of population growth and equilibrium occupancy at the scale of individual territories.

Background:

Forest managers are challenged by the need to balance the potentially competing objectives of reducing wildfire risk and protecting threatened species. To address high fuel loads due to decades of fire suppression, policy makers and forest managers have proposed landscape-scale forest treatments to remove surface and ladder fuels and reduce the risk of high-severity fires. However, proposed fuel-reduction measures pose a potential risk to wildlife species associated with older forests because they change forest structure in ways that may negatively affect the species’ ability to survive and reproduce.

For this paper, we assessed the effects of forest treatments and vegetation conditions
on reproduction, survival, and territory occupancy of California spotted owls and used these vital rates to determine the sensitivity of population growth and occupancy to changes in vegetation conditions due to wildfire or timber harvest. We strove to understand the potential direct, short-term impacts of management actions intended to reduce wildfire risk on spotted owls, and to gain insight into the causes of an approximate 30% decline in abundance on our study area over the past two decades.

We analyzed data from 1993-2012 that included reproductive output, mark-recapture histories, occupancy surveys, and aerial photographs (used to classify vegetation cover types and changes). We modeled relationships between vegetation classes and four vital rates (reproduction, survival, territory colonization, and territory extinction) by evaluating the level of support for competing, a priori models and used Akaike’s Information Criterion (AIC) values to rank competing models.

Results:

Results suggested a negative influence of medium-intensity timber harvests on reproduction of spotted owls, but we found only weak support for this effect. For survival, adult males had higher survival rates than sub-adults and females, respectively. Non-juvenile survival and territory colonization were positively related to the amount of high-canopy-cover forest within owl territories, while territory extinction was negatively related to the amount of high-canopy-cover forest. Wildfire had a strong negative effect on territory colonization, but did not affect territory extinction. Sensitivity analyses showed that the amount of high-canopy-cover forest had a strong, positive effect on both population growth rate and equilibrium occupancy within owl territories

Conclusions:


  1. Although owl demographic rates were correlated with several habitat variables, the amount of high-canopy-cover forest was the habitat variable most strongly correlated with population growth and equilibrium occupancy at the scale of individual territories.

  2. Medium-intensity harvests that convert high-canopy forests into lower-canopy vegetation classes could have short-term negative impacts on populations of California spotted owls. Our results further suggest that fuel treatments that occur in lower-canopy-cover forests (<70%) or do not significantly reduce canopy cover in high–canopy-cover forests are less likely to have adverse impacts on spotted owls.

  3. We recommend that fuel treatments focus on ladder fuels and reduction in tree density while maintaining relatively high canopy cover.

  4. We recommend that managers consider the existing amount and spatial distribution of high-canopy-cover forest before implementing fuel treatments within an owl territory, and that treatments be accompanied by a rigorous monitoring program.

Full Reference:
Tempel, Douglas J., R. J. Gutierrez, Sheila A. Whitmore, Matthew J. Reetz, Ricka E. Stoelting, William J. Berigan, Mark E. Seamans, and M. Zachariah Peery. 2014. Effects of Forest Management on California Spotted Owls: Implications for Reducing Wildfire Risk in Fire-prone Forests in Ecological Applications: 24(8), 2014, pp. 2089 - 2106.

Interested persons may request a copy of the paper from Douglas Tempel at: dtempel@wisc.edu

For more information about the SNAMP project and the California spotted owl team, please see: http://snamp.cnr.berkeley.edu/teams/owl.

No Comments Yet

Post a New Comment