No disrespect to the ancients, but the best time to plant a tree was not 20 years ago. It might be this spring.
A variety of leaves from trees planted last fall 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 our parklands and spread like wildfire, faster than we can catch and quarantine them — even with sharp eyes out at all times. Our 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. Phil has been rocking war paint for years now. 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, of the 200 that were planted years ago, were of a strong and diverse population, toughened from years of fighting off pests and disease.
Then, 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 — their genetic diversity was actually getting less and less diverse.
Red oaks with oak wilt in Schenley Park, soon to be cleared.
Why has this study been so important? As the Parks Conservancy has 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 actively worked 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. Our trees are better equipped to keep our parks healthy and beautiful.
This week, a large stand of red oaks — about 50 trees in total — will be cleared from Prospect Drive in Schenley Park. Oak wilt, discovered earlier this year by an observant park user, got a stranglehold on the interlocking root system of the trees, infecting an entire grouping of trees. Left there, the trees are a risk to the health of other park trees. It’s terrible to have to take down so many trees, but it’s something that needs done for the overall well being of the park. And when these trees are replanted in the spring, a variety of new and diverse tree stock will be added to the expanding biodiversity and health of the park.
Wondering about the title of this post? Read more about the Red Queen Effect here.