Recently, there has been much concern about oak mortality in Fairfax County. There is no specific pest or pathogen that is attacking and killing healthy oaks in Virginia, but rather a combination of environmental and biological factors that weaken oaks and eventually make them susceptible to native pests and pathogens. The US Forest Service noticed unexplained oak deaths starting in the 1940s and began investigating the causes. After decades of research, they settled on the cause of death as Oak Decline Syndrome. Although it is called Oak Decline Syndrome, this process can affect any tree species.
Oak Decline Syndrome
Oak Decline Syndrome is a process that begins with an initiating stressor – defoliation, drought, flooding, sudden and large temperature changes, excessive heat, construction etc. Before the tree recovers from this stressor it gets hit with another and another. Eventually the tree is too weak to fend off native endemic secondary pests and pathogens or to survive a large stress and dies. The late summer of 2019 saw a lot of oak death throughout Virginia. Locally, it was particularly noticed in the south part of the county. In 2018 and first half of 2019 we had 150% of normal rainfall culminating with a 10,000 year storm on July 8, 2019; then it stopped raining and we started having heat with repeated stretches of 3 or more days of higher than 95 degrees. In mid-August healthy looking trees, primarily chestnut and white oaks, started dying. Researchers at the Smithsonian Research Station in Front Royal examined some oaks that died in 2019 and found they had effectively been dying for a decade or more.
Given oaks’ reproductive strategy of filling forest gaps, this decline and death is probably part of the population dynamics of oaks. This type of decline and death cannot be blamed entirely on climate change, though the environmental stressors that can drive the process have been getting worse and more frequent as a result of climate change. Another part of this issue in Fairfax County is the history of development. Just like people as trees get older they are less resilient and are more affected by stress, so older areas of development with older trees will be more likely to see tree death.
Of course, that this is all part of the natural history of oaks is little solace to a homeowner faced with the cost of removing a large dead oak.
What Homeowners Can Do
Fortunately, there is one thing that a homeowner can do to help their trees that also addresses the three greatest environmental issue we currently face: climate change, the insect apocalypse, and declining bird populations. Turf grass is a stressor of suburban trees that homeowners can control. Trees produce 5 times more root mass under mulch than under turf. By replacing turf with native flowers and shrubs, and mulching with whole leaves and wood chips, homeowners can provide food and habitat for birds, pollinators, and beneficial insects. And sequester more carbon than turf grass does while reducing the carbon footprint of their yards. The leaves and wood chips will also feed the soil fungi that help support our oaks. Replacing turf with living or green mulch, native shrubs and wildflowers, or brown mulch, leaves and wood chips, may not prevent tree mortality, but it is a winning strategy that homeowners can implement. In fact it is a win-win-win-win-win-win strategy – less stress for trees; more carbon sequestration; more habitat for birds, butterflies and bees; and more time for you to enjoy them.
