In the Shade
In August, the Northern Research Station’s web features take you into the shade to meet an ecologist interested in understory plants; research exploring how to curb (or not curb at all) an invasive understory plant that shades out oak seedlings; a product that serves as a how-to and why-to for tracking urban tree mortality; and a partnership that is working to make shade work for people and overwintering Neotropical birds in Latin America.
Looking for a shady place to get some fresh air and exercise? The Allegheny National Forest is located within northwestern Pennsylvania; it is a convenient drive for more than one-third of the nation’s population!
Growing up with a tropical jungle in his backyard and the Smithsonian Tropical Research Institute headquarters two blocks away, it is not surprising that Alejandro Royo, born and raised in Panama, developed an affinity for the natural world. After playing in the Cerro Ancon Nature Reserve as a child, he would often do his homework in the air-conditioned library of the Smithsonian. Today, Royo is a research ecologist for the Northern Research Station in Irvine, PA.
“A charismatic biology professor in my first year at Panama Canal College coupled with a field trip to the Nusagandi Field Station deep in the jungle of eastern Panama were key factors in me selecting ecology as a career path,” said Royo. “My particular interest in forest understory plants comes from the often underappreciated fact that the vast majority of plant diversity in temperate forests is comprised, not by trees, but by understory plants.”
Much of Royo’s research has focused on the role deer over browsing plays in diminishing understory plant diversity and changing forest composition. Historically, the common approach to this issue is to consider deer densities as the main factor and reacting to high deer populations by fencing large areas to exclude them or intensifying deer harvests to cull them.
“My work over the last few years proposes a more proactive approach,” says Royo. Specifically Royo and his collaborators are improving understanding of how forage availability throughout the landscape, or the “foodscape,” influences browsing. “By uncovering how deer populations and forage-rich habitat interact, my work provides guidelines for managers that allow them to mitigate browse pressure proactively by managing habitat,” said Royo.
Royo summarizes the most rewarding aspects of his job simply: people and puzzles. “I am a gregarious and curious individual and, therefore, love meeting interesting people and collaborating to tackle fascinating questions,” he said. “My research has taken me from the temperate forests in the Andes of Colombia, to the jungles of the Caribbean, to the hardwood forests of the eastern United States, to the mixed-wood forests of Quebec. Throughout this journey I’ve made dozens of great friends and made scientific discoveries that I hope will help managers sustain healthy forests now and into the future.”
Urban Tree Mortality: A Primer on Demographic Approaches
There is one aspect of urban trees that has not drawn much attention from even their most ardent supporters: mortality. Even in the last decade, as the field of urban forestry exploded, studies addressing urban tree mortality were few and far between in the scientific literature. Research Ecologist Lara Roman of the Northern Research Station’s Philadelphia Field Station is enthusiastic about both the benefits of urban forests to society and the use of mortality data in assessing tree planting programs and evaluating change over time.
With a general technical report titled “Urban tree mortality: a primer on demographic approaches,” Roman and her colleagues created an introduction to demographic concepts and analytical approaches related to urban tree mortality aimed at urban forest researchers and practitioners. Demography is the statistical study of populations, and her intent was to show how the same tools actuaries use in assessing human populations and conservation biologists use to assess wildlife species can be employed to assess tree mortality. “If I plant trees and monitor them, and find that I’ve lost a certain number, knowing more about which trees I’ve lost, where I lost them and when I lost them can help me up my game in future planting programs,” Roman said. “Mortality is a very basic measure of the success of a tree program.”
Data on urban forests begins with data on an individual tree. Repeated observation of a set of individual tree data (called longitudinal data) is essential in delivering mortality and growth rates that can be used to build life tables, survivorship and mortality curves, as well as more advanced analyses such as matrix models and regression analysis.
While cities and non-profit organizations are increasingly implementing “million tree” planting programs, there has been less emphasis on tracking how many of these trees die and, importantly, when they die. “Understanding mortality in urban forests is essential to improving planting programs and ultimately expanding urban tree canopy,” Roman said. “Dead trees have a lot of data to offer.”
How Not to Kill Mountain Laurel
Research Forester Pat Brose of the Northern Research Station’s lab in Warren, Pennsylvania, set out to compare the effectiveness of seven common approaches to killing a native but invasive understory plant called mountain laurel. The results surprised him, and not in a good way. “None of the common approaches have a lasting effect on mountain laurel, which is a worthwhile research finding,” Brose said. “Sometimes failure can be as important as success when it comes to developing appropriate silvicultural treatments for controlling interfering understory vegetation.”
Native throughout the eastern United States from Maine to Florida and from Louisiana to Quebec, mountain laurel is a pervasive problem for forests. Dense thickets of the plant shade out oak seedlings, making regeneration of the ecologically and economically valuable species arduous. It increases the risk for damaging fires, and it is a host for a fungus that causes oak wilt and could potentially help spread the disease. It is well equipped to spread, too. Mountain laurel reproduces by layering, with above ground stems developing roots and then detaching from the parent to become an individual plant; it also produces thousands of tiny seeds.
In an 8-year study on three sites in Pennsylvania, Brose compared the effectiveness of commonly used techniques including cutting, burning, cutting followed by burning, crushing, and two herbicide application methods. He also established a control plot on which no treatments were applied.
The results were underwhelming. Five years after applying the treatments, mountain laurel had sprouted and the thickets were quickly reforming in all treatments. Several treatments resulted in species such as black birch, blackgum, and sassafras regenerating en masse, creating more competition for oak seedlings. Relative to doing nothing, three treatments actually made the situation worse for oak seedlings and three only slightly improved conditions, but Brose found that those benefits appeared to be fading over time and did not expect them to last.
Brose’s quest to defeat mountain laurel did not end with his comparison of mechanical treatments and prescribed fire. In a study published in January 2019, he explored the effectiveness of foliar-applied herbicides, an approach that showed promise but is still being refined.
Growing Coffee and Preserving Bird Habitat
Neotropical migratory birds make up the majority of breeding species in the Northeastern United States, and every year they fly thousands of miles south to Central America and the Caribbean where they overwinter. For decades scientists have noted declines in populations of these birds as tropical forests are cleared and converted to agriculture. A Northern Research Station scientist is part of a diverse partnership that is working to find ways to conserve bird habitat while allowing for increased prosperity of poor rural populations in coffee growing regions.
Typically, growing coffee involves either converting the forest understory from native vegetation to coffee plants, or increasingly, completely clearing large expanses of tropical forests and thus removing habitat for overwintering migratory birds. In desperately poor countries, consideration of bird habitat is often not a high priority. A host of organizations – from the U.S. Fish and Wildlife Service to the Migratory Bird Program of the U.S. Forest Service International Programs group, to universities in the United States and Central America, to local coffee producers—are seeking ways to achieve the dual goals of conserving habitat and allowing for economic development.
As an example, Dave King, a research wildlife biologist with the Northern Research Station, is involved in a project with a group of Honduran coffee producers to develop market-based mechanisms for forest conservation. A “land-sparing” approach in which coffee producers are compensated for conserving forest on their farms by the sale of carbon credits, is showing promise.
King has migrated back and forth to Central American countries for more than 20 years pursuing research related to the birds’ wintering habitat. He is well aware of the age-old conundrum of protecting habitat in regions of widespread poverty.“Solving problems where economic development clashes with habitat conservation is very complex.” said King. “By partnering with a wide range of individuals and organizations we are able to bring together significant expertise and ideas that help us to develop solutions that are responsive to the needs of birds and people.”