Derechos, Heat Waves, and Appalachian Forests

The leading edge of a derecho, or “shelf cloud,” often heralds the arrival of strong, damaging winds. (Photo in public domain via NOAA.)

Residents across the Appalachians and much of the Ohio River Valley were surprised yesterday, as an intense weather system called a derecho moved across the region. Taken from the Spanish word meaning “straight,” a derecho is a strong, powerful, long-lived thunderstorm complex given its name due to the storm systems’ movement and straight-line winds.

Derechos most often occur during the early- and mid-summer months when large “bubbles” of high pressure cause heat waves across a large portion of the country. When a stationary boundary between air masses – called a front – sets up along the northern edge of this heat bubble, the right type of upper-level winds can cause this type of large thunderstorm complex to “ride” the frontal boundary, often for hundreds of miles before dissipating. Derechos are infamous for their high winds, as residents of the Appalachians discovered this week as 90 mph winds downed trees, cut off power, and (tragically) caused injuries and several deaths.

So, why is an ecology website mentioning a weather system? Besides being destructive to human populations, derechos can actually cause some significant impacts on the ecology of forest ecosystems. The same trees that downed powerlines and damaged homes this week also fall in forests impacted by a derecho, often in the thousands to tens of thousands in a single event. Downed trees create holes in the forest canopy that let in sunlight, allowing plants tolerant of low-shade conditions to move in and proliferate. In turn, these forest “gaps” let in wildlife species that thrive in open, sun-filled habitats, adding diversity to what might otherwise be evenly-aged forests.

Ecologists have termed extreme wind events like derechos “intense, infrequent forest disturbances,” indicating their low frequency of occurrence in any single spot, as well as their intense nature. When lumping derechos together with other wind events like tornadoes and hurricanes, in fact, biologists have estimated that severe wind disturbance events like a derecho are likely to impact any given patch of forest within northern parts of the Appalachians every 80-200 years. Of course, this frequency can always change given chance and one’s specific location, but the take-home message is that wind disturbance – while being very disruptive and even devastating to human populations – is nevertheless an important process in forest ecosystems.