Tree Time Part 1: What’s Eating Our Trees?

Sweetgum 2011 might as well be known as The Year of the Tree around the Pittsburgh Parks Conservancy.  We’re currently hard at work on developing a Tree Action Plan that we’ll unveil at a panel discussion on February 17.  (More info on that event, which will be free and open to the public, coming up shortly!)  One of the major components of the plan is raising public awareness of the threats to our city’s trees, so we thought we’d kick things off with a rundown of what to watch for.

A Dire Confluence
“We have not faced conditions in the past where so many threats of major environmental impact were happening all at once,” says Phil Gruszka, our Director of Parks Management and Maintenance.  Previous widespread killers of trees–such as chestnut blight (first discovered in the U.S. in 1904) and Dutch elm disease (which did most of its damage in the 1930s)–mostly arrived alone.  The multiple pests and diseases we’re facing today make it harder to focus on combating each individual problem, and pose an even greater threat to the biodiversity of our forests.

Emerald Ash Borer

EAB gallery

A serpentine gallery created by an EAB larva in Schenley Park.

First spotted in Pittsburgh in summer 2007, the emerald ash borer has already claimed millions of trees in Pennsylvania and will wipe out around 300 million by the time it’s finished.  According to Phil, this beetle that feeds on the bark of ash trees is the biggest current danger because it will kill virtually any ash tree that hasn’t been treated.  Unlike oak wilt, which tends to affect scattered pockets of trees, EAB will cause complete eradication.

You can detect emerald ash borer infestation by checking ash trees for the presence of D-shaped emergence holes in the bark.  If the bark of an infested tree is peeled back, it will reveal snaking galleries where the larvae have fed on the tree’s tissues.  Another clue is heavy damage to a tree from woodpeckers who feed on the larvae.  Dieback of the tree’s canopy and small shoots at the bottom of the tree are also signs of infestation.

Because the usual timeline for an infested tree to die is two to three years, a lot of the damage to Pittsburgh has already been done.  But we’re hopeful that some trees can be saved, and we’ll tell you more about how in our next post.  For now, check out www.emeraldashborer.info for all you ever wanted to know about the bug.

Oak Wilt
We’ve written a lot about oak wilt in this space (check out our Trees category archive for past posts), and we’ll continue to monitor the parks for areas of infection.  In 2010, the fungus forced clear-cutting of 10 acres of trees in the city parks, and monitoring these areas is a priority of both the Parks Conservancy and the City of Pittsburgh.  While the loss of so many trees at once is unfortunate and makes a dramatic change to the landscape, the good news is that oak wilt is easier to manage than something like emerald ash borer. 

Oak wilt can manifest itself differently in red and white oaks.  Telltale signs of oak wilt in a red oak include the rapid discoloration of leaves, usually beginning in the crown of the tree in late June to early July.  After the color begins to shift away from green, leaves start to wilt from the top of the crown downward, and then leaves begin to take on a bronzed look.  Within a few weeks, many of the leaves have dropped from the tree.  White oaks typically take longer to succumb to the disease.  The visual symptoms are similar, but white oaks often die branch by branch over a few years rather than a few months. 

Here’s a good resource on identifying and controlling oak wilt.

Deer Overbrowse

Deer browse

A deer takes out some of the foliage around the Riverview Chapel Shelter.

With no natural predators, white-tailed deer have experienced significant population growth in Pennsylvania in recent years.  Hungry deer foraging for food eat many of the young plants and trees composing the forest understory, and male deer scrape off the bark of trees in an effort to remove velvet from their antlers.  The presence of deer makes restoration work especially challenging, because young saplings have to be protected to stand a chance at growing into full-size trees.  And because deer like to eat many of the native groundcovers in the area, but don’t like garlic mustard, the deer help this invasive plant to thrive.  Frick and Riverview Parks are particularly affected by deer, but they call all the city’s historic parks home.

Other Threats
This part of the country is holding its breath in hopes that the Asian Longhorned Beetle doesn’t arrive anytime soon.  Like EAB, the larvae of this beetle feed on the inner bark of trees, but ALB discriminates less on the basis of species.  It has most affected maples in the United States, although it also feeds on elms, willows, birches, and horse chestnuts.  Infestations have been recorded in New York, New Jersey, and Massachusetts.  Some estimate that if ALB were to spread across the entire country, it could kill as much as one-third of the U.S. hardwood forest, causing an enormous economic and environmental impact.

Other long-term threats include the hemlock woolly adelgid, gypsy moth, forest tent caterpillar, bacterial leaf scorch, and beech bark disease.  Fortunately, none of these threats are as urgent in our area at this time as EAB and oak wilt.

So…all this potential tree death is pretty depressing.  But we’re firm believers that education is the necessary first step to fighting back.  In our next post, we’ll tell you about our public event in February (the first in a series) as well as some components of the Tree Action Plan.  Stay tuned!

One thought on “Tree Time Part 1: What’s Eating Our Trees?

  1. In addition to a lack of natural predators, there is now more habitat available to deer than ever before, in large part due to habitat fragmentation that has created more edge habitat. When I was volunteering with the parks conservancy, one of the field ecologists tasks was to “soften” the edges around the forests by planting shrubs and native grasses around the edges of wooded areas.

    And the PPC is right on with this task. While we may not be able to control the deer population manually, we can at least plan ahead and design our parks in away that helps keep our forests alive and healthy! Keep up the good hard work, PPC!!!

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