The future (and past) of oaks

At the high point of Tucker Hill Road stands a particularly majestic oak tree with a straight tall trunk, ample girth, and a generous crown so high that its spread doesn’t seem to shade the road. It is admired by many, perhaps even the town road crew, since its trunk is not marred by gashes from the snow plow, unlike many roadside trees. This spring the oaks, along with the other deciduous trees, were unfurling their delicate new leaves.

It was then that events took a bad turn. Almost exactly a month ago, in the overnight hours from May 17th to 18th, there was a freak frost. Temperatures fell precipitously to between 18 and 25 degrees F, depending on location. Most forest trees took it in their stride, but not the oaks. Soon the delicate new leaves hung on the branches like limp brown rags. On the hillsides, stands of oak could be picked out by their rusty hue. The Tucker Hill oak did not escape.

Apples and other fruit crops also took a major hit. While their leaves did not appear damaged, the ovary of an apple blossom is far more sensitive. According to Michigan State University, “Freezing temperatures of 28 degrees Fahrenheit will result in about a 10 percent loss and 24 degrees F in a 90 percent loss” of the apple crop.

While the vulnerability of oaks to frost seems puzzling, a look at the past history of oak trees provides an answer.

The evolution of oaks is complex, but in a nutshell (no pun intended), the data shows they existed over 56 million years ago. They were a cold-adapted species and moved between the Eurasian and North American continents via intermittent, northerly land bridges. Starting around 52 million years ago, a period of global cooling took place, forcing the tropical forests that, up till then, covered North America to retreat southward. This allowed oaks to colonize in their stead, spreading southward from the land bridges. In the process, new oak species developed that were adapted to warmer climates. Then came a period of glaciation that killed off northern forests, including the original cold-hardy oaks. Eventually the glaciers retreated, beginning about 20,000 years ago, and southern adapted oaks began to expand northwards. Blue jays are credited with helping oaks move north, as they often carry acorns a mile or more before storing as many as 4,500 per jay underground.

However the re-introduced oaks still bear the marks of adjustment to a warmer climate. One, they do not shed their leaves in the fall — an adaptation to cold winters that protects trees from excess snow load — and two, we now appreciate that their young leaves are not extremely frost-hardy. Trees on the edge of their climactic range are usually more vulnerable to late-season frosts, a fact commonly understood among horticulturists.

Our late-season frost was in itself a very rare occurrence. Lois Paige, who has spent all her 89 years in Thetford Center, had never seen anything like it in her lifetime. And that’s not the end of it. In response to climate change, deciduous trees are now leafing out earlier. A careful study in 2016 of the woods around Walden Pond, where Thoreau made meticulous observations 160 years earlier, found that the average leaf emergence was taking place 18 days earlier than in Thoreau’s time. Against this backdrop there is also the trend of increased extreme weather events. Most reports focus on periods of extreme heat, drought, or flooding, but we now have one example of an unprecedented cold event that blasted oak leaves that were emerging earlier than they used to.

Oaks are keystone species and are critical to the functioning of forests across the Northern Hemisphere. As entomologist  Doug Tallamy notes in his book The Nature of Oaks, they foster a wide diversity of organisms across the tree of life. An oak tree can live for hundreds of years, and in that time it can drop about 3 million acorns, providing an essential food for dozens of bird species (woodpeckers, turkeys, etc.); bears; raccoons; opossums; rodents like squirrels and  mice; and insects like weevils. Their leaves feed the caterpillars of over 500 species of moths and butterflies (compared to maple at around 295 species). Caterpillars contain more protein than beef and are a critical food that birds must feed their young so they grow and develop. In order to fledge, one brood of chickadees consumes between 6,000 and 9,000 caterpillars, which are soft and juicy and easy for baby birds to eat.

The enormous root systems of oaks can spread easily three times wider than their crowns, holding soils against erosion and sequestering large amounts of carbon. The deep leaf litter accumulated beneath them can temporarily trap most of the water from a 2-inch downpour, allowing it to soak into the ground rather than running off and taking precious topsoil away with it. Preventing runoff also stabilizes the waterflow in streams and rivers against scouring that destroys aquatic communities of insects, crustaceans, and fish.

Oaks have also shaped human culture, feeding people and livestock with their acorns and providing wood to build homes, furniture, and ships. Indeed, oaks have proved so valuable to humanity over the centuries that they have been immortalized in legends and myths.

Today our oaks are leafing out again from side-buds, and the prognosis for recovery from the one-time killer frost is good. According to Nancy Patch, the forester for Franklin County,  "As a standalone event this (frost) is not too concerning. Most trees can withstand one to three years of early defoliation. They will re-foliate and replenish the loss of carbohydrates through photosynthesis over the summer.”

However, if there is defoliation three years in a row (which can also happen through spongy moth caterpillar infestation), “all bets are off” according to Patch.

Photo Credit: Li Shen