August 26, 2007
Foraging birds such as the mountain chickadee, red-breasted nuthatch, pygmy nuthatch and yellow-rumped warbler can help spur the growth of ponderosa pine trees in the West, according to a 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, who conducted the study as part of his doctoral research in CU-Boulder’s ecology and evolutionary biology department. All the studied birds, except the warbler, are year-round residents of Colorado ponderosa pine forests.
“More than anything, this study underscores the importance of preserving the ecological communities in the forest, and not just the trees,” said Mooney.
Mooney used mesh netting to exclude birds for three years from ponderosa pine limbs in the Manitou Springs Experimental Forest northwest of Colorado Springs. 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 healthy stands of mature ponderosa were 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.
The activity of the birds also was shown to change the chemical composition of the trees, which may have implications for infestations by damaging insects like bark beetles that have ravaged pine forests in the West, said Mooney. 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 CU-Boulder biology Professor Yan Linhart. The alteration of terpene “flavor” can have wide 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. Nuthatches and chickadees act as tree protectors, helping keep in check insects that can have detrimental effects on forest health.
A paper on the study by Mooney was published in the August issue of Ecology.
Source: University of Colorado.