DETROIT — Successful efforts to restore populations of an endangered bird species in northern lower Michigan have cleared the way for researchers to look at the effects of that restoration on the rest of the ecosystem.
Kirtland’s warblers, which nest in the jack pine forests of Michigan (and secondarily Wisconsin and Ontario), were in danger of extinction in the 1970s, with only about 400 birds in existence. Habitat restoration efforts, according to state of Michigan officials, have involved management by logging, burning, seeding and replanting on a rotational basis to provide approximately 38,000 acres of productive nesting habitat for the birds at all times, and there now are close to 2,000 nesting pairs. Kirtland’s warblers nest on the ground below young jack pines, around the edges of patches of other types of trees and vegetation. The jack pines must be 5 to 25 years old to make suitable homes for the birds.
While habitat restoration helps grow the bird population, Dan Kashian, Wayne State University assistant professor of biological sciences in the College of Liberal Arts and Sciences, and other researchers and wildlife and forest managers have begun to examine how it affects everything else in the forests. Jack pines depend on fire because the cones that contain their seeds require heat to break them open and prepare the ground for seedlings. Those fires once were started mainly by lightning strikes, Kashian said, and created the habitat required to sustain Kirtland’s warblers. Relatively few fires burn the landscape today, making warbler management dependent on large jack pine plantations meant to mimic natural forests.
Large wildfires in jack pines leave behind “stringers” – Kashian’s name for unburned stretches of mature trees and other vegetation – interspersed among the burned jack pine areas. Warbler plantations rarely include those features.
“These unburned patches could be critical for the continuity they provide between the predisturbance and post-disturbance ecosystems. They may have important roles as seed sources and species habitat, and in ecosystem function,” Kashian said. “Biological legacies in general have important implications for the conservation of biological diversity, especially where forest landscapes are heavily disturbed.”
Stringers are prevalent in many regions and ecosystem types, he said, but it has been difficult to incorporate such legacies into prescriptions for managing jack pine-dominated areas without historical data. To do so, he and his colleagues examined the natural range of variability in stringer frequency, size, shape, composition and longevity after formation.
In results recently published in the journal Forest Ecology and Management, researchers examined stringers within 54 wildfire perimeters in the core breeding range of Kirtland’s warblers in northern lower Michigan. They found that stringers were common, persistent and important to creating heterogeneity in jack pine-dominated ecosystems.
Kashian’s work provides baseline data on occurrence, pattern and persistence of stringers in the target area.
“Now that warbler populations have increased, we wanted to give wildlife and forest managers some information, some numbers, so they can preserve some older forest cover when they’re creating these plantations, with the idea that it might help species other than the warbler,” Kashian said. “It’s not warbler habitat, but it’s habitat for something. Some managers have already included stringers in plantations, but without much data to base it on.”
Kashian said more research is needed to determine the ecological importance of structural differences between stringers, and between stringers and adjacent forest; potential effects of stringers on surrounding forest development; plant community differences between stringers and the forest matrix; and the implications of stringers for small mammals, birds and insects.