A Race with the Red Queen

February 26, 2016 by Lauryn Stalter

No disrespect to the ancients, but the best time to plant a tree was not 20 years ago. It might be this year.

A variety of leaves from trees planted in Highland Park. Photo by Taiji Nelson.Whether plant, animal, virus, or bacteria, all living organisms are locked in battle with the pathogens, pests and parasites in their surrounding environment, using the tools and tricks nature's equipped them with to keep them in the game. One of the strongest tools in this arsenal? The ability to adapt, to one-up opponents in a constant arms race.

One huge task that the Parks Conservancy faces is shoring up this arsenal for the trees in our care. The founders of these fine parks left quite the legacy, not only in the consideration that they gave to the design and experience of the parks, but also the impressive diversity of the urban forests.

It's tough to overstate the importance of biodiversity to healthy parks. Voracious pests and sneaky diseases gain a slight foothold within the bounds of parklands and spread like wildfire, faster than we can catch and quarantine them -- even with sharp eyes out at all times. Park trees need their natural defenses as they stand on the front lines of these attacks, especially since they face added stresses of living in the city: polluted water, poor air quality, micro-climates, and human intervention.

London plane tree in Schenley Plaza.A general in this battle, the Parks Conservancy's Director of Park Management and Maintenance Phil Gruszka is a seasoned veteran. Since conducting a study with Dr. Cynthia Morton of the Carnegie Museum of Natural History, he's realized that our legacy park trees are impressive in their biodiversity... but that we have to really work to keep it that way. Their study found that tree stock from major nurseries across the country have actually been whittling down the tree gene pool.

"When London plane trees were first introduced to the United States, one nursery had a tree that did very well. But they couldn't get it to reproduce from seed, so they started getting cuttings to grow out. Then they released it to the trade and named that cultivar 'Bloodgood.'"

Selected for it's superior resistance to the fungus anthracnose, the cultivar (a plant chosen for its particular genetic makeup) Bloodgood has been spread around now for about 30 years, dominating nursery stock of London planes. Plane trees bought from nurseries have identical genetic material to every other plane tree -- they have not been grown from seed from two parent trees. They're clones.

"Today, if I wanted to replace a London plane, I can only buy the cultivar Bloodgood."

But trees from Schenley Plaza and around the park surprised them. "The Schenley Plaza trees were all genetically different, very diverse." The park trees, planted before Bloodgood started to gain popularity, were much more diverse than the current nursery stock. The surviving 100 trees (of the 200 that were planted years ago) were of a strong and diverse population, toughened from years of fighting off pests and disease.

They widened their net. Was this true only for London planes? How much more diverse are our park trees than trees sold around the country? After polling nurseries from various parts of the country, they found that ten common trees used all over the U.S. were clones -- the genetic diversity of trees sold nationwide was actually getting less and less diverse.

Before: An area in Frick Park before dead, invasive and hazardous trees were removed

After: An area in Frick Park before dead, invasive and hazardous trees were removed

Why is this study so important? As we've taken on ecological restoration projects in the parks over the years and established the Park Tree Action Plan with the City of Pittsburgh, TreeVitalize, and Tree Pittsburgh, we've been working diligently to increase biodiversity in the parks. Taking cuttings of our own heterogeneous tree stock, we've started growing new trees around the park and city in our own sort of diversity study, learning as we go about resistant new cuttings that withstand biological threats. This knowledge gives us only a peek at the immeasurable value of Pittsburgh's parks; less mature forests and parks elsewhere are markedly more homogeneous, posing a threat to themselves and surrounding forests against the pests and diseases that have shown an uptick in recent years. New trees are better equipped to keep our parks healthy and beautiful.

All of this is especially relevant right now in Frick Park. The City of Pittsburgh Forestry Department and Frick Park Department of Public Works have been clearing a large stand of dead, dying, and invasive trees from the upper Falls Ravine trail entrance. This project, which drastically changes the appearance of this section of the park, is a long-term solution that will better the health of the park. It's terrible to have to take down so many trees, but it's the perfect example of why planting similar types of trees in big stands is a bad idea. Soon, we look forward to planting a variety of new and diverse trees here to the expanding biodiversity and protect the health of the park.

Wondering about the title of this post? Read more about the Red Queen Effect here.

Ecological Restoration, Trees and Forestry, Regional Parks Master Plan, Frick Park, Schenley Plaza, Volunteering, Schenley Park