Dry conifer forest restoration benefits Colorado Front Range avian communities

Fire suppression has increased stand density and risk of severe,
stand-replacing wildfire in lower elevation dry conifer forests of western
North America, threatening ecological function. The U.S. Forest Service’s
Collaborative Forest Landscape Restoration Program (CFLRP) aims to
mitigate impacts to ecological function, while mandating effectiveness
monitoring to verify restoration success. Expected benefits include
improved conditions for biodiversity, but relatively few empirical studies
evaluate restoration effects on biodiversity. We applied the Integrated
Monitoring in Bird Conservation Regions program to survey birds in
relation to CFLRP treatments along the Colorado Front Range in 2015–2017.
We employed hierarchical models to analyze species occupancy and richness
at 292 points nested within 54 1km2 grid cells. Our objectives were to
investigate 1) species occupancy relationships with treatments at local
(point) and landscape (grid) spatial scales, 2) potential mechanisms for
treatment relationships considering species and treatment relationships
with forest structure and composition (i.e., habitat relationships), and
3) treatment and habitat relationships with species richness. The data
supported positive and negative point-level treatment relationships,
suggesting uneven species distributions between treated and untreated
points. At the grid scale, however, we only found positive species
relationships with percent area treated, and accordingly, grid-level
species richness increased with treatment extent. Potential mechanisms for
treatment relationships included treatments generating foraging
opportunities for aerial insectivores by opening the canopy, improving
conditions for ground-associated species by increasing herbaceous growth,
and limiting opportunities for shrub-nesting species by reducing shrub
cover. Landscape-scale patterns suggest CFLRP treatments can benefit avian
communities by generating habitat for open-forest species without
necessarily eliminating habitat for closed-forest species. Our results
provide evidence for a commonly expected but rarely verified pattern of
increased species richness with forest heterogeneity. We suggest
restoration treatments will most benefit forest bird diversity by reducing
canopy cover, encouraging herbaceous ground cover, limiting ladder fuel
species, and encouraging shrub diversity in canopy openings, while
maintaining some dense forest stands on the landscape.