Middle Moors Scrub Oak Mystery

A visit to the Middle Moors this spring, particularly a walk or drive along Pout Pond or Altar Rock Road, will reveal some strange new patterns on the landscape. This past winter, the Nantucket Conservation Foundation began creating larger brush-cut areas to act as firebreaks. These areas were selected strategically to protect homes from wildfire, and to improve our ability to perform prescribed burns to enhance habitat for rare species of plants and wildlife. The low vegetation in the firebreaks, combined with the sand of the existing roads, act as an excellent obstacle to fire — limiting the areas where it can spread and giving firefighters safer and easier access with firefighting equipment as necessary. The use of water is always limited by how much fire trucks can carry or how quickly they can be re-filled, but the firebreaks do some of the work all by themselves. Fortunately these brush cut areas are also valuable habitat for low growing species that can’t compete with dense scrub oak cover, like the rare lion’s foot plant (Nabalus serpentarius), or the Eastern silvery aster (Symphyotrichum concolor).

But, unless you are just returning to the island for the summer, the huge brush-cut areas are old news. Just a few weeks ago, there was another new pattern that appeared across the landscape of the Middle Moors. Huge areas of vegetation apparently withered over night. “Was it some sort of aerial herbicide spraying,” people began to ask, “or maybe some type of oak disease?” Before theories about aliens or the scrub oak equivalent of crop circles get too far, we thought it might be a good idea to explain what’s really been going on–because it is actually a very interesting natural phenomenon.

Frost Pockets and Radiational Cooling

Cooler heavier air settles overnight in hollows and dips in the landscape on calm nights. These areas are called “frost hollows” or “frost pockets” because they often have temperatures that dip below freezing during the growing season — long after a region’s predicted “Last Frost Date.” The “Last Frost Date” gives gardeners an estimate of when it will be safe to plant their tender veggies or flowering annuals. Of course it’s just an estimate — sometimes the frost comes unexpectedly late in May, though on Nantucket, usually after mid-May you are safe to plant out crops that can’t tolerate frost (temperatures below 32 degrees Fahrenheit, or below 0 degrees Celsius). Having a garden in a low-lying area means that you could be gardening in a “frost pocket,” which could really cramp your gardening style.

To complicate matters further, sometimes the pooling of cold air coincides with clear skies and a lack of insulating cloud cover, typically at night; when that happens, the ground that was was warmed all day by the sun radiates the heat away quickly. The combination of cold pooled areas of air (the “frost hollows”) and the lack of an insulating “cloud blanket” means that temperatures can drop below freezing across both hollow and level areas, even after a warm summery day when people spent the day soaking up the rays on the beach. And that’s what happened in the Middle Moors recently — low-lying areas, as well as areas along the recent brush-cut firebreaks were enveloped in cold air and the leaves were frost bitten overnight.

Research by Harvard Forest scientists Motzkin, Ciccarello and Foster (2002) shows that even level sandplain areas (like much of Nantucket) can radiate enough heat to cool quickly and experience an unexpected frost. In fact, the researchers concluded that these frosts can happen at any time of the year on the flat sandplains, and often happen repeatedly in June. The same phenomenon can occur in the large flat areas of the island’s cranberry bogs — a big problem in June, while the plants are in full bloom. A hard frost at this time can be disastrous for a cranberry crop, so the bog managers run sprinklers overnight that help keep the flowers from being frost killed. The University of Massachusetts Extension has a great fact sheet explains how cranberry growers handle this potentially devastating situation and keep most of their fruit crop intact. The water from the sprinklers actually releases heat and helps keep the tender new foliage and flowers from freezing during this critical time. The Nantucket Conservation Foundation Cranberry Bog crew works hard to ensure that we have a good harvest by keeping a sharp eye on the weather.

The Middle Moors Scrub Oak Mystery

The frost damage that we observed recently here on Nantucket was a result of one or more of these events, on a larger scale than we usually see. The same thing occurred on Martha’s Vineyard around the same time, according to entomologist Paul Goldstein, who has been searching for larvae (caterpillars) of barrens buckmoth (Hemileuca maia), which are usually found in clusters on the freshly emerged leaves of scrub oak. The NCF Science & Stewardship staff was out looking for clusters of the youngest of these spiny creatures which gave us the chance to see the magnitude of the frost damage across the Moors.

Since the scrub oaks and other shrubs had just leafed out when the recent frosts occurred, their tender leaves were especially vulnerable to the cold temperatures, and the cells inside the leaves ruptured with the freezing temperatures, leaving large areas of the scrub oak barrens to wilt and turn brown the next day. So, not a good June for buckmoth larvae over much of the Middle Moors. When we were out there, we noticed a pattern of distinct frost bitten areas in the hollows, but also along the freshly cut firebreaks, so it’s possible that the newly cut areas increased the radiational cooling and added to the areas that would normally be affected by a late frost. It’s also possible that most of the roads simply follow lower ground, and that explains the pattern. It would be very interesting to see aerial photos of the Middle Moors after the frost events, so if anyone snapped some photos while they were flying over the island, please email us. We would love to share an aerial view on the blog!

Scrub Oak as Vital Habitat

Did you know that the scrub oak barrens, which cover vast areas of the island, are actually a rare and valuable plant community in the Northeast? Surprising, but true: these nearly impenetrable thickets provide great habitat for shrubland birds which have been declining elsewhere in the Northeastern U.S. And they are also home (and a tasty, nutritious food source) for the larvae of many rare butterfly and moth species. These much-maligned scrub oak barrens are home to 41% of Massachusetts’ rare butterfly and moth species. Check out this Forest Ecology & Management article by Wagner, Nelson and Schweitzer to learn about a variety of rare insects that just can’t get enough of scrub oak.

So, while a lot of our work at NCF focuses on reducing the amount of scrub oak and other tall shrubs, with the goal of enhancing the even rarer sandplain grasslands, we would never want to eliminate scrub oak shrublands from Nantucket. They’re a vital part of our coastal ecosystem.

And, in case you were wondering, most of the scrub oak that were frost-withered this spring will leaf out again soon, using stored energy reserves in their root systems. In fact, some are already doing so, as shown in the picture below, where the tiny bright red oak leaves have begun to unfurl. The bright red pigments (anthocyanins) in the new leaves are thought to help protect the leaves from environmental stresses such as UV-B light and drought, both very useful adaptations for a dry June on sandy Nantucket.

Frost events like this one, along with droughts, salt spray from large storms, and years with huge caterpillar populations all contribute to the patchy nature of the vegetation on the island and enhance its biodiversity. Keep an eye out for events like this to learn more about the cycles of the natural world. It’s an interesting story.

 

The Nantucket Conservation Foundation is a private, non-profit land trust that depends on contributions from our members to support our science projects, conservation property acquisitions, and land management efforts. If you are not already a member, please join us! www.nantucketconservation.org