watching trees grow: The aftermath of the forest tent caterpillars

Watching Trees Grow

The Aftermath of the Forest Tent Caterpillars

August 12, 2010

By Quert Evans

2010 marked a catastrophic outbreak of the forest tent caterpillar throughout much of Northern Michigan. The outbreak began developing around 2006 when many began noticing a few caterpillars crawling around. Nothing real serous, in fact, if you did not know what they were, they went relatively unnoticed. In most instances they were misidentified as the Eastern Tent Caterpillar. Each year, numbers began to increase exponentially. In May and June of 2009, the distinct sound of caterpillar poop falling from the canopy was being reported though out most of the region. Larger blocks of forest lands were becoming visibly defoliated. In 2010 caterpillar numbers exploded and the previously defoliated areas merged together causing defoliation over most of the hardwood forest here in Northern Lower Michigan.

Most forest professionals recognized the insect as a native pest that should run a one or two year cycle then fade away not causing too much damage for another 15 years or so. Starvation, parasitic flies, and bacteria’s would eventually collapse the population. Having been though this before, there seemed to be no real concerns about tree health or our forests, “the trees will recover” was a common reply from many scientist and foresters. Unfortunately, we were wrong about that in many instances.

I first encountered the forest tent caterpillars in the late 1980’s. My memory does not serve me well, but I believe it was 1989 when the entire Jordan Valley was defoliated. This out break did not include the western portions of Antrim County. Defoliation seems to have been contained east on M66. It seemed short and uneventful at the time, an interesting point of conversation for the coffee shop. Many locals don’t even remember the outbreak.

20 years later and after the second event in my lifetime, it turns out that these native bugs can have a devastating effect our northern hardwood forest. I spent the months of June, July and August 2010 inspecting woodlots that have been impacted by the Forest Tent Caterpillar. Some similarities began to pop up. In some areas late spring frost, high spring heat, drought and caterpillars may have combined forces killing thousands of trees. Other woodlots have had as much as 10 percent mortality with more trees stressed to the point where they may not recover. There is also some speculation that woodlots thinned within the past five years are most vulnerable.

STANDS THINNED WITHIN THE PAST FIVE YEARS - Did thinning actually increase stand mortality? Many of the stands I looked at have had some sort of thinning done in them within the past five years. This may be a coincidence, however other foresters have noticed this as well. In one stand there was a fairly distinct difference between a thinned stand and an adjoining un-thinned stand. It is believed that heavily stocked stands retain more heat and thus were less susceptible to below freezing temperatures in the spring. The broken canopy of the thinned stand may have collected colder air causing frost damage forcing some trees to set new leaves. This is normally not a problem for most trees. They have plenty of stored energy to set new leaves. However, this year the new leaves were then quickly eaten by caterpillars and many trees had trouble setting a third set of leaves, especially when defoliated the year before. Stands thinned below 80 sq.ft.BA seemed to be worst hit. Did the low stocking levels increase mortality?

Frost vs. high heat

Is it frost or possibly high heat in the spring that actually stressed the trees the most? One forester speculated that it may have been the 80 and 90 degree spring weather that burned up new leaves in the spring. Weather records indicated long stretches of unusually high temperatures this year and last year.

STANDS WITH NORTH AND EAST FACING SLOPES - In an area West of Kalkaska the North and or East facing slopes seemed to be hit harder than higher elevations. Although this did not hold true all the cases, topography seemed to play a role in the worst hit stands.

LOW ELEVATIONS - Lower lying forests again may not have had the air drainage to deal with cold temperatures. One stand along the Cedar River had over 50 percent mortality. As you moved away from the river valley mortality was not as bad. This stand was thinned two years ago.

SECOND YEAR DEFOLIATION - Common to most of the forests was the fact that most mortality occurred in stands that were in their second year of defoliation. Individual tree mortality and tree dieback is likely to continue for the next few years.

FUNGUS - The MDNR has reported a fungus that has followed the defoliations in several stands in Grand Traverse County. According to the MDNR, this fungus is causing increased mortality.

All of the above explanations are strictly speculative. Combined, these forces appeared to have been a lethal combination for many trees. The fact that caterpillars have had a tremendous impact on our northern hardwood forests is undeniable. The long term effects may not be seen for several years.

FOREST ECOLOGY- The ecologically the influences have been quit profound in most forests impacted by the caterpillars. The lack of tree canopy in the spring allowed scorching sunlight to reach the forest floor. Many of the plants that normally thrive in the under-story were now being replaced by plants rarely seen on the forest floor. Thistle, elderberry, common mullen, polk weed and a bumper crop of blackberry have over come many of the more shade tolerant plants. Also destroyed was much of the newly established sugar maple seedlings and saplings that blanked the forest floor in many stands. This is particularly irritating to many foresters who have worked hard at establishing new seedlings through forest management practices.

(Ground flora changing dynamics on the forest floor)

What are the long lasting effects? One thing is for sure - our forests are very dynamic and ever changing. It is likely that our forest have been through this many times. Some individual trees may have seen this pest eight or ten times in their lifetime. On the positive side, wildlife is likely the prime beneficiary. Dieing and declining trees now provide ample habitats for cavity nesting birds and small mammals. The lush and diverse ground flora is providing food and cover for deer and bear as well as many ground nesting and foraging birds and small mammals.

Here are a few more photos illustrating caterpillar damage.