Tree Time Part 4: "Now That's Bad Ash."

February 3, 2011 by Melissa McMasters

This morning I had the pleasure of accompanying Phil on an ash-scouting mission in Schenley Park. He's in the preliminary stages of selecting specimen ash trees for treatment against emerald ash borer, and ultimately for seed collection to help preserve the species.

Working from a 2007 inventory of the regional parks' tree population by Davey Resource Group, he has maps of each ash tree and the condition they were in at the time of the inventory. From that information, he can narrow down a list of candidates for preservation and then verify his findings with inspections in the field. Here's a little peek into the process.

We started off in the area near the swimming pool and the Vietnam Veterans pavilion, because this is the area of the park that's most densely covered in ash trees. The below images show a poor candidate for preservation (left) and a good candidate. The tree on the left was listed in the 2007 report as having a structural defect. So even though it's a good size and doesn't appear to have fallen victim yet to the ash borer, it's not a good candidate for preservation because it's not as stable as a tree like you see in the right-hand image.

Bad ash, good ash

Beyond being in a generally healthy state, the trees that are in the best shape to be preserved will also have a relatively large diameter. The larger the tree, the likelier it is to be an older specimen, which means it's likelier to be genetically pure and not a cultivar.

Measuring the diameter
So why don't we want cultivars? Because cultivars have gone through an engineered breeding process rather than a male-female tree reproduction scenario where the gene pool is naturally diversified. A cultivar could either be a hybrid of an ash native to the area and a non-native one, it could be two different natives, or it could be a tree bred from cuttings (meaning that each tree grown from the cutting of one parent tree is genetically identical). In trying to select trees for genetic preservation, we want as large a gene pool as possible, and we want genes from trees that have grown well here in Pittsburgh so their eventual descendents will be well-suited for the area.

Across the street from the swimming pool, Phil flags trees to illustrate how much of this area is populated by ash.

Flagging ash trees

Only two oak trees (at left) stand in this location; the rest of the trees are all ash.

Ash stand

Next we headed up the road a little bit toward the Camp David Lawrence shelter and playground. We encountered another hillside whose trees were solely ash with the exception of one golden rain tree. The difference here, though, was that these trees had already been hit by emerald ash borer. Someone had conveniently drawn a black "D" around one of the emergence holes:

Emergence hole

The pattern of the bark was another telltale sign. Take a look at the two images below.

Good ash, bad ash

The left image is of one of the healthy ash trees near the swimming pool. Its bark has the fissures and diamond pattern characteristic of mature ash trees whose bark has thickened. The right image--the one that provoked Phil to say, "Now that's bad ash"--has been infested with emerald ash borer. You can see that the bark has been stripped off and is losing its definition. Most of the trees in this area looked like this: the product of beetles emerging from the trees and woodpeckers trying to get at the larvae inside.

Our scouting completed, I asked Phil what the next steps were. He'll continue to identify good candidates for preservation, and then return to those trees in April, when they'll be flowering. At that time he can look at the flowers and determine which are males and which are females so he'll be able to preserve a good mix. Then in May, those that have been targeted for treatment will be injected with chemicals that have been shown to protect against emerald ash borer.

The seeds harvested from specimen trees will be given to agencies who are working on a breeding program similar to the one that seeks to reclaim the American chestnut. Similar breeding programs across Pennsylvania will help preserve other species of ash, but in Pittsburgh we're focusing on green and white ash (which make up almost 100% of our ash population). It could be decades before we see results, but putting the work in now will hopefully ensure that the ash tree doesn't disappear from the landscape forever.

Park Threats, Trees and Forestry, Parks Management