An answer is often truthful and unsatisfying at the same time. Across the Wintergreen landscape, the skeletal remains of dead oaks litter our lush forest. Each time I venture to sites to analyze the cause of death, the same answer emerges. This Nine Minute Naturalist will venture into the unsatisfying answer of oak decline.

Last week, The Nature Foundation sought answers from an outside source. We walked multiple forest sites featuring dead or dying oaks with Lori Chamberlin, the Forest Health Manager at Virginia’s Department of Forestry. In solidarity with the many homeowners unsatisfied with my diagnosis of oak decline, I too was given no clear easy answers. Oak decline was the prognosis.

Clear answers with simple solutions are preferred by all parties involved. In the cases of the recent forest health problems of emerald ash borer or hemlock wooly adelgid, the cause was obvious and the solution, although expensive, is straightforward. Oak decline is a problem with an assortment of causes and very few good answers. Virginia’s Department of Forestry defines oak decline as “the gradual failure in the health of a tree that results from the interaction among three groups of stress factors: predisposing, inciting, and contributing.” These three groups of factors act as the blueprint to many oak tree deaths in our forest.

Predisposing factors act as the base cause of which the others progress. The primary factor is advanced age, which many trees in our landscape are. The American chestnut removal from the forests at Wintergreen occurred approximately 100 years ago causing the currently mature forest seen today. Many of the trees we see dying out of our canopy are in the 100 year and up age class. Other key factors are poor soils and topography. Much of the dying oaks at Wintergreen are on steep rocky terrain. This terrain often features dry, shallow soils. An old tree on a steep rocky slope is a likely candidate for oak decline.

Spongy moth

Inciting factors are drought, excessive moisture, or defoliating insects. Wintergreen has suffered numerous defoliating attacks from the spongy moths (formerly known as gypsy moth) caterpillars over the past 25 years solidifying this pest as an inciting factor. Long term studies show that oaks show drought response for up to ten years after a drought event. On the flip side, excessive precipitation slows root growth and in extreme circumstances, roots can die due to anaerobic conditions. All these inciting factors cause ramifications lasting well beyond the immediate event.

Contributing factors are secondary insects or diseases that can lead the tree to mortality. Two common factors in Virginia are two-lined chestnut borer and Armillaria root rot. Many of the chestnut oaks dead near Fortunes Ridge were found to have two-lined chestnut borer as a contributing factor.

Two-lined chestnut borer

Oak decline manifests first in crown dieback which generally progresses from the top down and the outside inward. This is a sign of a root system struggling to maintain the health of the tree. The process of death has started and cannot be reversed at this point. In an old, mature stand of oaks, this is a natural process. The process in younger stands is much more outside the normal ecosystem process and is a sign of a multitude of factors converging over a short span. An oak tree located on a steep, rocky slope, having been defoliated by spongy moths’ multiple times and in the midst of a drought is the perfect candidate for the onset of oak decline. Sadly, this describes much of the Wintergreen Forest over the past 20 years.

On a forest wide basis, not much can be done to affect factors such as age, topography or weather patterns but individual homeowners can take positive steps to maintaining healthy oaks. Start by not being a stressor yourself. Avoid damaging tree limbs, trunk, or roots. Prune carefully damaged or disease limbs. Remove invasive vines from the tree and monitor for insect pests and fungus. Consider fertilization and soil amendments as needed. Mulching and proper irrigation can aid in avoiding oak decline. Any questions about oak health can be sent to forestmanage@tnfw.org.