There may be a problem with lower Mississippi floodplain forests that have been replanted with common oaks. While the idea was that nature would do the rest, some types of trees may not recover on their own -- and new research suggests that this lower tree diversity may lead to lower bird diversity. "Birds tended to prefer uncommon tree species -- one bird, the yellow-throated warbler, was a virtual specialist on bald cypress," says Aaron Gabbe, who did this work while at the University of Illinois at Urbana-Champaign and is now at the University of California at Santa Cruz. This work is presented by Gabbe and two co-authors in the April issue of Conservation Biology.
Floodplain forests in the Lower Mississippi River Alluvial Valley have been reduced nearly 80% by agriculture, urbanization and flood control. The Nature Conservancy and U.S. Fish and Wildlife Service have been restoring bottomland hardwood forest in the Cache River watershed, which is in the southern tip of Illinois in the floodplain of the Mississippi and Ohio Rivers.
To help guide bottomland forest restoration, Gabbe and his colleagues studied the tree preferences of 13 species of insect-eating birds that breed in forests in the Cache River floodplain. The researchers determined the birds' preferences for 19 tree species in seven forest fragments ranging from about 55 to 6,000 acres. Each study site had three forest types: permanently inundated swamp, seasonally wet swamp and wet floodplain forest.
Gabbe and his colleagues found that many birds preferred to forage in particular types of trees and that birds that were uncommon on the study area tended to be more selective. For instance, yellow-throated warblers strongly preferred bald cyprus, cerulean warblers preferred kingnut hickory, and yellow-billed cuckoos preferred silver maple. Moreover, the three trees preferred by most of the birds (kingnut hickory, bitternut hickory and silver maple) were also uncommon in the study area.
Two of these three most-preferred trees are hickories, which are heavy-seeded and slow to recolonize restored forests. Previous research has shown that restored floodplain forests have few heavy-seeded trees even 50 years after replanting with common oaks. "Forest restored by planting of only common oak species may not be adequate for restoration of a diverse avian community," say Gabbe and his colleagues. They recommend also replanting floodplain forests with the heavy-seeded and rare trees that many birds prefer.
The researchers point out that restoring natural flood cycles may also be critical to restoring bottomland forests. Otherwise, bald cyprus and other water-loving trees can be gradually replaced by upland species. "Ultimately, the diversity of flora and fauna in the floodplain may rely on preserving and restoring natural flood cycles to the river system," says Gabbe.
Gabbe's co-authors are: Scott Robinson and Jeffrey Brawn of the University of Illinois at Urbana-Champaign; and of the University of Illinois in Champaign.
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