Chickadees, nuthatches and warblers foraging their way through forests have been shown to spur the growth of pine trees in the West by as much as one-third, according to a new University of Colorado at Boulder study.
The study showed birds removed various species of beetles, caterpillars, ants and aphids from tree branches, increasing the vigor of the trees, said study author Kailen Mooney. Mooney, who conducted the study as part of his doctoral research in CU-Boulder's ecology and evolutionary biology department, said it is the first study to demonstrate that birds can affect the growth of conifers.
"In a nutshell, the study shows that the presence of these birds in pine forests increased the growth of the trees by helping to rid them of damaging insects," said Mooney. "From the standpoint of the trees, it appears that the old adage, 'the enemy of my enemy is my friend,' holds true."
A paper on the subject by Mooney was published in the August issue of Ecology. In the study, Mooney used mesh netting to exclude birds from ponderosa pine limbs in the U.S. Forest Service-managed Manitou Springs Experimental Forest northwest of Colorado Springs for three years. The results showed that branches on 42 trees rigged to exclude birds had 18 percent less foliage and 34 percent less wood growth by the end of the study.
Mooney collected about 150,000 insect specimens from the mountain study area, identifying more than 300 separate spider and insect species collectively known as arthropods. The trees used in the study were set up to exclude birds, ants, or both, since ants also can have significant impacts on other arthropods, he said.
"The study indicates that pine canopies are very complex systems with an unexpected level of biodiversity," said Mooney. "Forest managers really need to look at the big picture of ecosystems and not just focus on trees when implementing regulations aimed at encouraging the growth of healthy forests."
The study also has implications for large areas of the West ravaged by forest fires in recent years, he said. A number of once formidable stands of mature ponderosa have been burned and logged and subsequently replaced by smaller pines that offer limited breeding opportunities for cavity-nesting birds like chickadees and nuthatches, which nest and lay their eggs in the holes of large trees and dead snags.
"This is a very rigorous study that essentially shows that even modest little birds like chickadees and nuthatches can help improve the heath of the trees, which are the monarchs of the forest," said CU-Boulder biology Professor Yan Linhart. Linhart was Mooney's doctoral adviser at CU-Boulder and also co-authored a study with Mooney in 2006 in the journal Animal Ecology. The study compared the effects of birds on pine with their effects on dwarf mistletoe, a parasitic plant on ponderosas throughout the West.
Mooney said the activity of the birds also was shown to change the chemical "flavor" of the trees, which may have implications for infestations by damaging insects like bark beetles that have ravaged pine forests in the West. Chemicals in trees known as terpenes, which give vegetation distinctive odors, have been implicated in the resistance of trees to parasites and plant-eating insects, he said.
By removing insects, the birds indirectly altered the terpene composition of pine tissues, said Linhart. The alteration of terpene "flavor" can have wide ranging effects, since terpenes influence decisions that creatures like bark beetles, porcupines and squirrels make when deciding which trees to eat, said Linhart.
"Terpenes act a bit like an immune system by essentially fending off attacks by birds and mammals," said Linhart. "One of the fascinating results of this study is that birds affect how this immune system functions."
The study also showed that chickadees and nuthatches disrupt a mutually beneficial relationship ants have with aphids, which feed on plant tissue known as phloem sap that carries nutrients through the tree, Mooney said. While some ant species "tend" aphid colonies -- protecting them from predators in exchange for their carbohydrate-high "honeydew" secretions - feeding activity by birds can disrupt this relationship, triggering aphid population decreases and increases in tree growth.
"These ponderosa forests have very complex food chains," Linhart said. "In essence the nuthatches and chickadees act as tree protectors, keeping check the insects that can have deleterious effects on forest vigor."
The birds in the study included the mountain chickadee, the red-breasted nuthatch, the pygmy nuthatch and the yellow-rumped warbler. All but the warbler are year-round residents of ponderosa pine forests in Colorado.
"More than anything, this study underscores the importance of preserving the ecological communities in the forest, and not just the trees," Mooney said.
The study was funded by the USFS Rocky Mountain Research Station headquartered in Fort Collins and by CU-Boulder. Mooney was a postdoctoral researcher at Cornell University in 2006-07. Mooney, who received his doctorate from CU-Boulder in 2004, will become a biology department faculty member at the University of California, Irvine, in fall 2007.
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